FORCES ACTING ON RESTORATIONS
CONTENTS
Introduction
Force
Force on dental structure
Stress
Types of stress
Mechanical properties of material
Biomechanics for restorative dentistry
Stress analysis and design of dental structures a) Finite Finite – elem element ent stress stress anal analysis ysis b) Photoelasticity
Stress in the periodontal membrane
Stress patterns of teeth
On anterior teeth
On posterior teeth
Occlusal considerations in restoring teeth
Forces eerted during occlusion ! mastication and their resolution
forces acting on amalgam restorations
"lass i
"lass ii
Forces acting on inlay restoration
Forces acting on composite restoration
Forces acting on posts
Forces acting on a cast metal and porcelain restorations
"onclusions
FORCES ACTING ON RESTORATIONS INTRODUCTION:
#esign of any structure re$uires a means to predict the stress that %ill develop in the the struc structu ture re unde underr the the anti antici cipa pated ted appli applied ed loads loads&& In many many resp respec ects ts the the desig design n of structu structures res for the oral environ environmen mentt is among among the
most most demanding demanding because because of the
compleity of the functional and parafunctional loads that must be accommodated and because of esthetic and space limitations& limitatio ns& In spite of these t hese special conditions ho%ever all dental tissues and structures follo% the same la%s of physics as any other material or structure& By necessity these studies involve the application of physics and engineering to the oral cavity and its surrounding structures& 'll structural analysis and design re$uire (no%ledge of the forces that %ill be applied and the mechanical properties of the materials that must %ithstand these forces& Since Since most most restor restorativ ativee materi materials als must must %ithstan %ithstand d forces forces in servic servicee either either during during mastication or fabrication& Those mechanical properties are important $uantities of force stress strain strength hardness and others can help identify the properties of a material& FORCE
The general concept concept of force is gained through the muscular action of pushing or pulling on an ob*ect& +hen there is a tendency to change the position of rest as the motion of a mass it is said that a force is applied&
' force al%ays has a direction and the direction is often characteristic of the type of force&
If the body to %hich the force is applied applied remains at rest the force causes causes the body to deform&
,nits of force are the pound or the (ilogram or -e%ton&
FORCES ON DENTAL STRUCTURES :
One of the most important applications of physics in dentistry is in the study of forces applied to teeth and dental restorations& There are numerous reports in the dental literature literature that describe describe the measurement measurement of biting forces on teeth& The maimum maimum forces reported have ranged form .// to .00/ - 102 to 22/ lb)&
.
-umerous instruments have been used to ma(e this measurements including strai strain n gauge gaugess and and telem telemetr etric ic devi device cess smal smalll enoug enough h to be incor incorpor porate ated d into into denta dentall restorations& NORMAL BITING FORCES :
3periments conducted on adults have sho%n that the biting force decreases form the molar molar region region to the incisors incisors&& Studie Studiess have have reveal revealed ed that four patien patients ts developed developed biting forces on the first and second secon d molars that varied form 45/ to 6// 6/ / - 166 to 756 lb) %ith the average average being 282 282 - 17.9 lb)& The average average force force on the bicuspids bicuspids cupids and incisors %as .66 ./6 and 722 - 182 09 and 42 lb) respectively& In a similar investigations of the biting forces in children 964 boys and girls %ere studied& "hildren form 8 to 79 years of age %ere included and it %as %as observed that there %as an increase in force form .42 to 050 - 124 to 777 lb) as age increased %ith the average yearly increase being in the order of ..&. - 12 lb)& The average biting forces in persons %ith normal and modified occlusion %ere measured& measured& #ata indicate indicate that the %hen the bite %as raised
/&2 mm the measured measured
forc forces es %ere %ere gener general ally ly highe higher r appr approa oach ching ing t%ice t%ice the the valu values es obtai obtaine ned d %ith %ith norma normall occlus occlusion& ion& This observa observation tion may be eplained eplained by the fact that the force force on teeth teeth are determined by muscular effort and this effort is controlled by the nervous system& Thus some force – regulating mechanism %as operating and it probably eists in case of malocclusion& The maimum force measured %ill depend on the type of food& FORCES ACTING ON THE TEETH : FORCES AND RESPONSES :
The forces %hich act on the teeth and cause them to move %ithin their periodontal tissues vary in magnitude duration fre$uency and direction&
The responses by the teeth to the forces depend on such factors as the shape and length of the roots the characteristics of the fluid content of the periodontal space the composition and orientation of the periodontal fibres and the etent of the alveolar bone&
The responses by the teeth %ill also depend on the consistency of the bolus being che%ed che%ed and the muscul muscular ar forces forces being used used to crush crush it& This This %ill also apply to parafunctional clenching and che%ing %ith or %ithout a foreign body bet%een the teeth& It is therefore therefore difficult difficult to assess assess %hat is a normal response response to a force on a tooth tooth and %hat is potenti potentiall ally y harmful& harmful& 's a result result of these forces forces a tooth tooth can be 4
displaced in one of si directions : ; apically mesiodistally or buccolingually and each one producing a rotation or a translation&
The result is li(ely to be a combination of all directions leading to an omnidirectional move moveme ment nt&&
The The same same prin princi ciple ple of move moveme ment nt %ill %ill appl apply y to the opposin opposing g tooth tooth
involved& OMNIDIRECTIONAL AND UNIDIRECTIONAL RESPONSES :
These omnidirectional tilting and rotations of teeth %ill reach a limit %hen an e$ual and opposite resistance is reached and the periodontal receptors cause a refle arrest arrest of the muscle muscle force& force& +hen +hen the force is removed removed the teeth %ill recover recover their their positions due to the elastic recovery of the compressed periodontal tissues&
This is
referred to as i)
'lveolar bone support
ii)
'd*acent te teeth su support
iii)
?ori@o ?ori@onta ntall muscl musclee activ activity ity on both both buccal buccal and lingual lingual surfac surfaces es of the the teet teeth& h&
These 4 variable factors may lead to an unidirectional movement of a tooth or teeth %hen they %ill become become repositioned repositioned&& Teeth %ill continue continue to move unidirecti unidirectionally onally until positions positions of stability are are reached& reached& The opposing forces forces are then then e$ual to the movin moving g forc forces es&&
Thus Thus mail mailla lary ry inciso incisors rs %ith %ith poor poor peri periodo odont ntal al suppo support rt and and
incompetent incompetent lips %ill drift drift for%ards& for%ards& This for%ard for%ard drift %ill continue continue until the teeth teeth are shortened or are prevented from moving further by an appliance and by treatment of the periodontal brea(do%n& STRESS •
+hen +hen a forc forcee acts acts on a
body body tendin tending g to produ produce ce deform deformat ation ion a resis resista tanc ncee is
developed to this eternal force application& •
Stress is the internal reaction to the eternal force&
•
Both the applied force and stress are distributed over a given area of the body and so the stress in a structure is designated as the force per unit area& Force Stress Stress A ;;;;;; ;;;;;;;;; ;;;
'rea
0
•
'rea over %hich the force acts is an important factor of consideration especially in dental restorations in %hich areas over %hich the force applied often are etremely small& small& Since Since stress stress at a constant constant force is inversel inversely y propor proportion tional al to the area area the smaller the area the larger the stress& 'nd vice versa&
•
Technically stress is the internal resistance of the body in terms of force per unit area and is e$ual and opposite opposite in direction direction to the force force 1eternal) 1eternal) applied& applied& This eternal eternal force is also (no%n as load&
TYPES OF STRESSES :
#epending upon the nature of the force all stresses can be divided into 4 basic types %hich are recogni@ed as > i&
Tension
ii&
"ompression and
iii&
Shear
7) Tension Tension : esults esults in a body body %hen it is sub*ec sub*ected ted to . sets of forces forces that are directe directed d a%ay from each other in the same straight line& F
F .) "omp "ompre ress ssion ion : esu esults lts %hen %hen the the body body is sub*e sub*ect cted ed to . sets sets of forc forces es in the same same straight lien and directed to each other& F
F 4) Shear : Is Is a result of of . forces forces directly directly parallel parallel to each each other& other&
S
F
F
Tensile Stress : -
Is caused by a load that tends to stretch as elongate a body&
-
The molecules ma(ing up the body must resist being pulled apart& 2
Compressie Compressie Stress : -
Produced by a load that tends to compress the body&
-
The molecules resist being forced more closely together&
S!e"r Stress : -
' stress that tends to resist a t%isting motion or a sliding of one portion of a body over another&
-
The molecules resist sliding of one body past another&
-
' forc forcee appli applied ed to a dent dental al resto restora rati tion on may may be reso resolv lved ed in the the struc structur tures es as a combination of compressive tensile and shear stresses&
Comple# Stresses :
+hen +henev ever er forc forcee is appl applie ied d over over a body body comp comple le as mult multip iple le stre stress sses es are are produced& They may be a combination of tensile shear or compressive stress& These multiple stresses are called comple stresses& MECHANICAL PROPERTIES OF A MATERIAL :
The mechanical mechanical propertie propertiess of a material material describe describe its response to loading& loading& It is common to simply describe the eternal load in terms of a single dimension 1direction) as compression tension or shear combination of these can produce Torsion 1T%isting) or Fleion 1transverse bending)& +hen a load is applied the structure undergoes deformation as it bonds are compressed compressed stretched stretched or sheared& sheared& The load deformation deformation characteristi characteristics cs are only useful information if the absolute si@e and geometry of the structure involved are (no%n& Therefore it is typical to normali@e load and deformation as stress and strain&
Stress is load per unit cross sectional area&
Strain is deformation per unit length& #uri #uring ng load loadin ing g bonds bonds are are gene genera rall lly y not comp compre ress ssed ed as easil easily y as they they are are
stretc stretched hed&&
Theref Therefore ore materi materials als resist resist compressio compression n more more readil readily y and are said to be
stronge strongerr in compressi compression on than than in tension& tension&
Mater Materials ials have differe different nt proper propertie tiess under under
different different directions directions of loading&
?o%eve ?o%ever r increased increased loading loading finall finally y
produces some irreversible strain as %ell 1plastic strain) %hich causes permanent 8
deformation& deformation& The point point of onset of of plastic plastic strain is called called the elastic elastic limit& limit& "ontinuing "ontinuing plastic strain st rain ultimately leads to failure by fracture& The highest stress before fracture is the ultimate strength& The total plastic tensile strain at fracture is called the elongation& The slope of the linear portion of the stress strain curve is called the modulus modulus of elasticity youngCs modulus or the stiffness of the material& T%o of the most useful mechanical properties are the modulus of elasticity and elastic elastic limit& ' restorative restorative material material generally generally should be very stiff so that under load its elastic elastic deformation deformation %ill be eternally eternally small& 'n eception eception is "lass D composite %hich should be less stiff to accommodate tooth fleure& If the stress is %ell beyond the elastic limi limit t then then the resul resultin ting g defo deform rmat ation ion is prim primar arily ily plas plasti ticc stra strain in and and at some some point point ultimately results in failure& Often it is convenient to determine the elastic limit in a relative manner by compar comparing ing the onset onset of plastic plastic deform deformati ation on of differ different ent materi materials als using using scratc scratch h or indentation tests called hardness tests& The energy that a material can absorb before the onset of any plastic deformation is called its resilience resilience and is described as the area under the stress;strain stress;strain curve up to the elastic elastic limit& The total energy energy absorbed to the point of fracture fracture is called the toughness toughness and is related to the entire area under the stress strain curve& Time;dependent responses to stress or strain also occur& #eformation %ith time in response to a constant stress is called creep 1strain relaation)& #eformation overtime in response to a constant strain is called stress relaation& BIOMECHANICS FOR RESTORATI$E DENTISTRY :
Teeth are are sub*ected sub*ected to many forces during during normal use& The interactions interactions bet%een bet%een the applied forces the shape and structure of teeth the supporting structures and the mechanical properties of tooth components and restorative materials are all included in the sub*ect of biomechanics& Biomechanical ,nit : The standard biomechanical unit involves the 7& esto estora rati tive ve mate materi rial al .& Tooth Tooth struct structur ure e and and 4& Interf Interface ace bet%e bet%een en the restor restorati ation on and tooth tooth
9
The importance of considering three structures in the biomechanical unit is to detect stresses that that may cause un%anted fractures fractures or debonding& The restorative material material may be strong enough to resist fracture but the interface or tooth structure may not be& STRESS TRANSFER :
-ormal tooth structure transfers eternal biting loads through enamel into dentin as compression& compression& The concentrated concentrated eternal eternal loads loads are distributed distributed over a large internal volume of tooth tooth structure and and the local stresses stresses are lo%er& lo%er& #uring this process process a small amount of dentin deformation may occur %hich results in tooth fleure& ' restored tooth tends to transfer stress differently than an an intact tooth& 'ny force on the restoration produces compression tension or shear along the tooth restoration interface& Once enamel enamel is no longer continuous its resistance resistance is much lo%er& Therefore most restorations are designed to distribute stresses onto sound dentin rather than to enamel& enamel& The process process of stress transfer transfer to dentin becomes becomes more complicated complicated %hen the amount of remaining remaining dentin is thin and the restoration must bridge a significant significant distance to seat onto thic(er dentin 1Einers or bases)& TOOTH FLE%URE :
Tooth fleure has been described as either a lateral bending or an aial bending of a tooth during occlusal loading& This fleure produces the maimal strain in the cervical region and the strain appears to be resolved in tension or compression %ithin local regions causing the loss of bonded class D restorations in preparations %ith no relative grooves& Moreover one current hypothesis is that tensile or compressive strains produce microfractures 1called 'BF'"TIO-S) in the thinnest region of enamel at the "3& Such fractures predispose enamel to loss %hen sub*ects to tooth brush abrasion and!or chemical erosion& This process may be (ey (ey in the formation of "lass D defects& PRINCIPLES OF BIOMECHANICS :
Stre Stress ss tran transf sfer er and and the resul resultin ting g defo deform rmat ation ionss of struct structur ures es are are princ princip ipal ally ly governed by : 7& The elastic elastic limit limit of the materi materials als .& The rati ratio o of the the elastic elastic modu moduli li involv involved ed 4& Thic(n Thic(nes esss of the the struct structur ures es
6
Materi Materials als %ith a high elastic elastic modulus modulus transfe transferr stress stresses es %ithout %ithout much much strain& strain& Eo%er modulus materials undergo dangerous strains %here stresses are concentrated unless there is ade$uate thic(ness& STRESS ANALYSIS AND DESIGN OF DENTAL STRUCTURES
The mechanical properties of a material used in a dental restoration must be able to %ithstand the stresses and strains caused caused by the repetitive forces forces of mastication& The design design of dental dental restoratio restoration n is partic particula ularly rly import important ant if the best advanta advantage ge of a material material is to be ta(en& It is necessary necessary to use designs that do not result result in stresses or strains that eceed the strength properties of a material under clinical conditions&
Stresses Stresses in dental structures structures have been studied by such techni$ues as brittle coatings coatings strain gauges t%o and three;dimensional photoelasticity and finite element analysis& Stress analysis studies of inlays cro%ns bases supporting restorations fied bridges complete dentures partial dentures and implants have been reported&
"& T'o Dimension"l Dimension"l P!otoel"sti(it) :
The procedure for t%o;dimensional models is to prepare a transparent plastic or other isotropic isotropic model of the restoratio restoration n or appliance& appliance& This model model is usually larger than than the actual actual si@e& The material material becomes becomes ais atropic %hen %hen stressed and and so the behaviour of light is affected by the direction it ta(es& 's a result of the applied stress the plastic model ehibits double refraction because of its an isotropic structure&
The light from the source passes through a
polari@er %hich transmits light %aves parallel to the polari@ing ais or plane polari@ed light& The plane plane polari@ed polari@ed light is converted converted to circularl circularly y polari@ed polari@ed light by a $uarter $uarter %ave plate and this polari@ed beam is split into t%o components travelling along the direction direction of principal principal stress in the model& #epending #epending on the state of stress in the model model the t%o beams travel at different rates& 'fter the light emerges emerges form the model model it passes through a second $uarter – %ave plate %hich is crossed %ith respect to the first and an analy@er analy@er that is most fre$uently fre$uently perpendicula perpendicularr to the polari@er& polari@er& The interference interference pattern pattern may be recor recorded ded photograp photographica hically lly %hich %hich is the isochroma isochromatic tic fringe fringe patter pattern& n&
These These
isochro isochromat matic ic fringes fringes or dar( dar( liens liens repre represent sent locati locations ons %here %here the differ differenc encee in the principal stresses is a constant&
The magnitude of the stress can be determined by
identification of the order of the isochromatic fringes& The fringe fringe order order multiplie multiplied d by a constan constantt and divided divided by the thic(ness thic(ness of the model gives gives the value of the differenc differences es in the principal principal stresses& stresses& 'reas in the model model 5
%here the fringer are close together are under higher stress gradients than areas %here there are fe%er fringes and areas containing fringes of higher order are under higher stress than these having fringes of lo%er order& ' t%o dimensional photoelastic model of a second molar %ith a gold cro%n is analy@ed& The elastic modulus of the plastics used to represent the gold dentin and and bone had the same relativ relativee values as the actual actual materials& materials& The cro%n %as %as luted to the tooth %ith dental stone and a layer of silicone rubber simulating the periodontal membrane separated separated the tooth tooth from the bone& bone& ' force of .88 - 18/ lb) %as %as applied 4/ degrees degrees to the ais of the tooth at a single site on the mesial cusp and the isochromatic fringes %ere photographed& ?igh stresses stresses are apparent under the contact contact and in the bone at the tip of the mesial root 1seven fringes) fringes)&& "onsiderably "onsiderably lo%er lo%er stresses occurre occurred d in the bone *ust under the distal root and at the crest of the ridge on the mesial side& The effect of various configurations of the proimal margins %as studied by t%o; dimensi dimensiona onall photoel photoelasti asticity city on the stress stress distributi distribution on in "lass "lass II inlays& inlays&
Eight Eight field field
isochromatic fringes for rounded shoulder and shoulderless models under a 002 - load %ere analy@ed& The load %as applied at 4 other locations : i)
't th the groove in in th the re restoration
ii)
On the cusp
iii) iii)
't the the *un *unct ctio ion n of of the the rest restor orat atio ion n and and the the toot tooth h The maimum shear stress %as determined at nine critical areas to% in the
restoration t%o in the tooth and five at the *unction of the restoration and the tooth& The study sho%ed that the chamfer and rounded type of preparations are the optimum designs in proimo;occlusal posterior restorations since they demonstrated the lo%est lo%est stress stress %hen loaded loaded verticall vertically& y&
The maimu maimum m fringe fringe order order for the rounded rounded
shoulder %as 7/ %hereas %hereas that for the shoulderless preparation %as 79& It %as also sho%n that rounding the aiogingival line angle in the shoulder geometry reduced the stress concentratio concentration n factor by upto 2/G& The gingival gingival area of the proimal proimal shoulder %as %as the area of high stress and etra retentive features such as pins or grooves should not be placed in this area& FINITE ELEMENT STRESS ANALYSIS :
The finite element is a ne%er method than photoelasticity and offers considerable adva advant ntag ages es&&
In this this metho method d a finit finitee numb number er of discr discret etee struc structur tural al elem elemen ents ts are are 7/
interconnect interconnected ed at a finite number number of points or nodal nodal points& These finite finite elements elements are formed %hen the original structure is divided into a number of appropriately shaped sections %ith the sections retaining the actual properties of the real materials& The information needed to calculate the stresses and displacement in the model is 7) The total total number of nodal nodal points points and element elements& s& .) ' numbering numbering system for identifying identifying each each nodal nodal point and and element& element& 4) The The elas elasti ticc modu modulus lus and and Pois Poisson sonCs Cs ratio ratio for for the the mater materia ials ls asso associ ciat ated ed %ith %ith each each element& 0) The coor coordina dinates tes of of each each nodal nodal point point 2) The type type of of bounda boundary ry constr constrain aints ts 8) The evaluatio evaluation n of the forces forces applied applied to the eternal eternal nodes& nodes& ' first molar %ith an amalgam restoration %as ideali@ed by an aisymmetrical model model and analy@ed analy@ed by the t%o;dim t%o;dimens ension ional al finite elemen elementt method& method&
The model model is
divided into a number number of triangles& triangles& The smaller smaller triangles triangles are located in areas areas of greater interes interest& t&
The ability ability of various various types and thic(ne thic(ness ss of cement cement bass to support support the
amalga amalgam m %as studied& studied& The plots plots of maimum maimum tensile tensile stress stress start start at the centre centre of the cavity and etend to%ard the cavity %all& The stress induced in the amalgam restoration restoration %as from four to five times higher %hen the amalgam %as supported by . mm Hinc Oide – 3ugenol cement base as compared compared %ith an e$ual thic(ness thic(ness of @inc phosphate phosphate cement base& base& +hen the stresses stresses induced in the amalgam by a @inc phosphate base of . mm are considered in relation to those induced by a dentin floor alone one can see that replacement of dentin by @inc phosphate to a depth of . mm does not result in any significant increase in the tensile stress stress induced induced in the amalga amalgam& m& The @inc oide oide eugenol eugenol cement cement base unli(e unli(e the @inc phosphate cement bar does not function as
rigid material and induces a larger
displacement& In comparison %ith @inc phosphate cement base the @inc oide eugenol material does not have ade$uate ade$uate mechanica mechanicall properties properties to support a restoration& restoration& 3ven thin layers layers 1/&2 mm) of @inc oide eugenol cement caused significant changes in the stress induced n the amalgam& amalgam& Therefore Therefore the study indicates indicates that the fracture of amalgam amalgam is influenced more more by the the modu modulu luss of elas elasti tici city ty 1Sti 1Stiff ffne ness ss)) of the the base base mate materi rial al than than by the the compressive compressive strength strength of the base& base& 'n ideal situation situation %ould be to have have a cement base base %ith a modulus of elasticity elasticity e$ual to that of the restorative restorative material& material& 'lso a subse$uent subse$uent 77
study found that tensile and shear stresses occurring in the cement base %ere of sufficient magnitude to eceed the strength of some cements& The stress distribution in porcelain fused to metal and porcelain *ac(et cro%ns %as conducted conducted using a finite element method& method& #esign parameters parameters of rounding rounding of shoulders avoidance of sharp notches minimum thic(ness of metal copings and minimum labial bul( of porcelain %ere incorporated into the model of an upper central incisors& ' load of 000 - %as applied at the incisal third of the lingual surface and at the middle third of the lingual surface&
Dertical loading and loading 4/ degrees to the vertical %ere used&
Since fracture is probably initiated by tensile failure at the periphery the tensile stress at the boundary is of special importance&
+ith vertical loading at the incisal third the highest tensile stresses %ere found tat the labial third and on the lingual surface near the load decreasing to%ard the incisal edge& Eo% stresses stresses %ee observed observed at the margin and on the lingual surface surface belo% belo% the load& The surface surface stress %as %as nearly the same %hether %hether a gold or -i;"r base base alloy %as used> the use of -i;"r caused a slight decrease in surface stress&
+hen the direction of the loading %as changed to 4/ degrees degrees from the vertical vertical high tensile stresses %ere observed near the lingual margin that %ould be of sufficient magnitude to fracture the cement in this area&
STRESS IN THE PERIODONTAL MEMBRANES :
'lthough limited measurements have been made on the periodontal membrane of animals the actual stress in the membrane has not been determined eperimentally& ?o%ever the stress to be epected epected has been calculated& In one case it %as %as assumed that the periodontal periodontal membrane %as incompressible incompressible %hereas %hereas in another it %as assumed to be approimately that of %ater& In both cases the root of the tooth %as assumed to be a cone and the elastic modulus of the membrane %as ta(en as 7&02 M-!m .& +hen the force %as applied to the center o the tooth ais the stress distribution %as uniform %ith respect to the longitudinal ais of the tooth and the pressure %as greatest at the ape& If the loading %as transverse the maimum stress occurred near the apical third of the root on the same side as the compression force&
7.
STRESS PATTERNS OF TEETH
3very tooth has its o%n stress pattern and every location on a tooth has special stress patterns& ecogni@ing them is vital prior to designing a restoration %ithout failure potential& A& STRESS
BEARING
AND
STRESS
CONCENTRATION
AREAS
IN
ANTERIOR TEETH :
i)
The *unction bet%een the clinical cro%n and clinical root bears shear comp compon onen ents ts of stre stress ss toge togeth ther er %ith %ith tens tensio ion n on the the load loadin ing g side side and and comp compre ress ssio ion n
at the the
non; non;lo load adin ing g
side side
duri during ng ecu ecurs rsiv ivee
mand mandib ibul ular ar
movements& ii)
The The inci incisal sal angl angles es espe especia ciall lly y if the they y are are s$ua s$uare re are are sub* sub*ec ectt t tensi tensile le and and she shear ar stress in normal normal occlusion& occlusion& Massive Massive compressive compressive stresses stresses %ill be present present in edge;to;edge occlusion and if the incisal angles are involved in a disclusive mechanisms these stresses are substantially increased&
iii) iii)
The The aia aiall angl angles es and and ling lingua uall marg margin inal al ridg ridges es %ill %ill bear bear conc concen entr trat atio ion n shea shearr stresses& stresses& In addition addition on the loading side tensile tensile stresses stresses are present and on the nonloading side compressive stresses are found&
iv)
The slopes slopes of the cuspid cuspid %ill bear bear conc concent entrat rated ed stresse stressess 14 14 type types) s) especi especiall ally y fi fi the cuspid is a protector for the occlusion or part of a group function during mandibular ecursions&
v)
The The dista distall surf surfac acee of a cus cuspid pid ehib ehibit itss a uni uni$ue $ue stres stresss patt patter ern n as a res result ult of the the ante anteri rior or comp compon onen ents ts of forc forcee conc concen entr trat ating ing comp compre ressi ssive ve load loadin ing g at the *unction of the anterior and posterior segments of the dental arch and microlatera microlaterall displacement displacement of the cuspid during ecursive ecursive movements& movements& Both of these factors %ill lead to tremendous stress concentration %ith resultant abrasive activity there&
vi)
The The lingua linguall conca concavit vity y in upper upper ante anteri rior or teeth teeth bea bears rs subs substa tant ntial ial com compr pres essiv sivee stresses during centric occlusion in addition to tensile and shear stresses during protrusive mandibular movements&
vii) vii)
The The inci incisa sall edge edgess of lo% lo%er anter nterio iorr tee teeth are are sub* sub*eected cted to compr ompres essi sive ve stresses& stresses& In addition addition tensile and shear shear stresses stresses are present during during protrusive protrusive mandibular mandibular movement& movement& The incisal incisal ridges of upper anterior anterior teeth teeth %ill have 74
these same stresses during the mid;protrusive and sometimes at the protrusive border location of the mandible& B& STRESS
BEARING
AND
STRESS
CONCENTRATION
AREAS
OF
POSTERIOR TEETH :
i)
"usp "usp tips tips espe especi cial ally ly on the func functio tiona nall sid side e bear bear comp compre ressi ssive ve stre stress sses es&&
ii) ii)
Marg Margina inall and and cross crossin ing g ridge ridgess bear bear trem tremen endo dous us tens tensile ile and and comp compre ressi ssive ve stre stress sses es&&
iii) iii)
'ia 'iall angl angles es bear bear tensi ensile le and she shear stre stress sses es on the the non; non;fu func ncti tion onal al side side and compressive and shear stresses on the functional side&
iv) iv)
The The *unct *unctio ion n bet% bet%ee een n the the clini clinica call root root and the the clini clinica call cro% cro%n n duri during ng func functi tion on 1espec 1especial ially ly lateral lateral ecurs ecursion) ion) bears bears treme tremendou ndouss shear shear stresse stresses s in additio addition n to compression compression on the occluding contacting contacting side and tension on the non;contracting non;contracting side&
v)
'ny occlusa usal facial ial or lin lingua gual concavity ity %ill ill ehibi ibit compr ompreessiv sive stress conc concen entr trat ation ion espe especi ciall ally y if it has has an opposi opposing ng cuspa cuspall elem element ent in stat static ic or functional occlusal contact %ith it&
C& *EA+ AREAS OF TOOTH :
+ea( areas in the tooth should be identified and recogni@ed before any restorative attempt in order to avoid destructive loading& They are > i)
Bifur ifurccation ion and trifurc urcation ion area&
ii) ii)
"eme "ement ntum um sho shoul uld d be eli elimi mina nate ted d as a com compo pone nent nt of of a cav cavit ity y %all %all&& The The *unc *uncti tion on bet%een the cementum of the dentin is al%ays irregular so the dentin surface should be smoothed flat after cementum removal&
iii) iii)
Thin Thin den denti tin n brid bridge gess in dee deep p cavi cavity ty pre prepa para rati tion ons& s&
iv) iv)
Subpu Subpulpa lpall floo floors rs in in root root cana canall trea treate ted d teet teeth& h& 'ny 'ny stre stress ss conc concen entr trat ation ion ther theree may may split the tooth interceptally&
v)
"rac "rac(s (s or or cra@ cra@in ing g in ena ename mell and! and!or or den denti tin& n& Both Both sho shoul uld d be tre treat ated ed pas passi sive vely ly in in any restorative design& They may act as as shear lines leading to further spread& SOME APPLIED MECHANICAL PROPERTIES OF TEETH:
7& 'lthough the the follo%ing follo%ing figures figures are averages averages they provide provide an idea idea about the princip principal al mechanical properties of tooth structure& It must be understood that these figures can differ from one location on a tooth to another and from one tooth to another&
70
a) "ompre "ompressiv ssivee streng strength th of enamel enamel support supported ed by vital vital dentin is usually usually 48;0.// 48;0./// / psi& b) "ompressive strength of vital dentin is 0/;2//// psi& c) Modu Modulu luss of resi resili lien ence ce of enam enamel el supp suppor orte ted d by vita vitall dent dentin in is 8/;6 8/;6/ / inch inch – lbs!cubic inch& d) Modulus of of resilience resilience of vital dentin dentin is 7//;70/ 7//;70/ inch – lbs!cubic lbs!cubic inch& e) Modulus Modulus of elastici elasticity ty of enamel enamel supported supported by vital dentin dentin under compre compressio ssion n is 9////// psi& f) Modulus Modulus of of elastic elasticity ity of vita vitall dentin dentin is 75// 75///// /// psi& psi& .& In general general %hen enamel enamel loses loses its support support of dentin dentin it loses loses more more than 62G of its strength properties& 4& Tensile strength strength of dentin is is about 7/G less less than its compres compressive sive strength& strength& 0& Tensi Tensile le streng strength th and and comp compre ressi ssive ve streng strength th of enam enamel el are simi simila lar r as long as the the enamel is supported by vital dentin& 2& Shear Shear strength strength of dentin dentin is almost 8/G less less than its compress compressive ive strength strength and this is very critical in restorative design& 8& There is is minimal minimal shear strength strength for for enamel enamel %hen it it loses its dentin dentin support& support& 9& +hen +hen the dentin dentin loses loses its vitality vitality there there is a drop of almos almostt 0/;8/G 0/;8/G in its strength strength properties& $ALE E%PERIMENT :
The original eperiment involved preparation of occlusoproimal cavities %ith different different crossing dimensions dimensions at the marginal marginal and crossing crossing ridges %ith a standard standard depth& The teeth %ere %ere then sub*ected sub*ected to measured occlusal occlusal loads& loads& The load that split the tooth tooth %as recorded and compared to the control %hich %as the load that split a round tooth& Eate Eater r the the same same epe eperi rime ment nt %as %as repe repeat ated ed by seve severa rall inve invest stig igat ator orss usin using g more more sophisticated e$uipment than that used by vale& The results %ere consistent& ' summary of their findings brought to the closest round figures is as follo%s : i)
By cros crossin sing g one one marg margina inall ridg ridgee at at the inter intercu cuspa spall dist distan ance ce the there re is almo almost st 7/G 7/G loss of a toothCs resistance to splitting&
ii) ii)
By cro crossi ssing ng t%o t%o mar margin ginal al ridg ridges es at at the the inte interc rcus uspa pall dista distanc nce e ther theree is alm almost ost 72G 72G loss of a toothCs resistance to splitting&
iii) iii)
By cros crossi sing ng one marg margin inal al ridg ridgee at 7!4 the the inte interc rcus uspa pall dist distan ance ce ther theree is almo almost st 4/G loss of a toothCs resistance to splitting& 72
iv) iv)
By cro crossi ssing ng t%o t%o marg margina inall ridge ridgess by 7!4 7!4 the the inter intercu cuspa spall dista distanc nce e ther theree is alm almost ost 42G loss of a toothCs resistance to splitting&
v)
By cros crossin sing g one one mar margi gina nall ridge ridge at J the inter intercu cuspa spall dista distanc nce e there there is alm almos ostt 0/G 0/G loss of a toothCs resistance to splitting&
vi)
By crossin crossing g t%o t%o margina marginall ridge ridgess at at J the intercu intercuspal spal distanc distance e there there is is almo almost st 02G 02G loss of a toothCs resistance to splitting&
vii) vii)
By cros crossin sing g a crossin crossing g ridge ridge at the inte interc rcusp uspal al dist distan ance ce there there is almo almost st ./G ./G loss of a toothCs resistance to splitting&
viii) viii)
By crossi crossing ng a crossin crossing g ridge ridge at 7!4 7!4 the inter intercus cuspal pal dista distance nce there there is almost almost 42G 42G loss of a toothCs resistance to splitting&
i) i)
By cros crossi sing ng a cross crossin ing g ridge ridge at J the inte interc rcus uspa pall dista distanc nce e ther theree is almo almost st 02G 02G loss of a toothCs resistance to splitting& OBTAINING RESISTANCE FORM FOR TOOTH STRUCTURES :
7) To best resist resist masticatory masticatory forces forces use floors floors or planes planes at right right angles to the directio direction n of loading to avoid shearing stresses&
.) If possible possible %alls %alls of prepar preparati ations ons should should be parall parallel el to the directi direction on of the loading loading forces in order to minimi@e or avoid shearing stresses&
4) Intrac Intracoro oronal nal and intraradic intraradicular ular cavity cavity preparat preparation ionss can be done in bo bo or cone cone or inverted truncated cone shapes&
Thus from the above figures it is possible to deduce that the inverted truncated cone shapes %ill have a higher resistance to loading than the bo shapes and the bo shapes %ill have have a higher resistance resistance than the cone shapes& shapes& Therefore Therefore if conditions conditions and
78
re$uirements allo% cavity preparations should be prepared in an inverted truncated cone shape& 0) #efinite #efinite floors floors %alls and and surfaces surfaces %ith line line and point angles angles are are essential essential to prevent prevent micromovements of restorations %ith concomitant shear stresses on remaining tooth structures&
2) Incr Increa easin sing g the the bul( bul( of a resto restora rativ tivee mate materi rial al or leav leavin ing g suffi suffici cient ent bul( of tooth tooth structure structure in critical critical areas is one of the most practical practical %ays of decreasing stresses stresses per unit volume& Eoad – '
Eoad '
7/ stress units!mm4 7 stress unit ! mm4 8) #esig #esigni ning ng the the outlin outlinee form form %ith %ith mini minima mall epos eposur uree of the the resto restora ratio tion n surfa surface ce to occlusal loading %ill definitely minimi@e stresses and the possibility of mechanical fracture in the restoration& 9) unctions unctions bet%een differen differentt parts of the preparatio preparation n especially especially those those acting as fulcra fulcra should be rounded in order to minimi@e stress concentration in both tooth structure and restorations and to prevent any such sharp components from acting as shear lines for fracture future& 6) ete etenti ntive ve feat featur ures es must must leav leavee suffi suffici cien entt bul( bul( of tooth tooth stru struct ctur uree to resis resistt stre stress sses es resulting from displacing forces& OCCLUSAL CONSIDERATIONS IN RESTORING TEETH
The %ay %e occlude teeth affects the periodontium the temporomandibular *oints throat muscles muscles tongue tongue chee(s chee(s lips nerves and so son& The occlusion occlusion of the restored teeth should hence establish healthy relations bet%een the dentition and rest of the stomatognathi stomatognathicc system& ' clinician must have ade$uate ade$uate (no%ledge (no%ledge about the principles principles of occlusion %hich enables him to diagnose cases that need modifications ! alteration of occlusion occlusion %ith or %ithout the use of various various restorative restorative materials& materials& Before Before initiating any restorative care thorough occlusal eamination should be carried out& 79
The (ind of occlusion occlusion a patient patient should have must must be *ustified by the principle principless of physiology& The occlusion affects almost every part of stomatognathic system mainly : a) The pulp pulp of the tooth tooth is a very very sensitive sensitive orga organ& n& IT react reactss immediat immediately ely to abnor abnormal mal occlusal forces& ?ence occlusion should not be detrimental to pulp& b) The proimal relations of the occlusion should prevent food impaction bet%een teeth& c) The cusp;fos cusp;fossa sa relation relationship ship should should be such that the ade$ua ade$uate te forces forces eerte eerted d during during functional movement aids in optimum health of the periodontium& 3ach tooth should be restored follo%ing the principles of occlusion so as to achieve optimum functions of the neuromusculature *oints and the supporting structures of the teet teeth& h&
-e% -e% resto restora ratio tion n shoul should d not introdu introduce ce any any prem premat atur uree conta contact ctss and and cuspa cuspall
interferenceCs& POSTERIOR RESTORATIONS :
'll posterior restorations should be planned (eeping in mind the basic principles of occlusion&
Prio Priorr to cutt cuttin ing g a toot tooth h its its oppo opposi sing ng occl occlus usal al surf surfac acee shou should ld be eam eamin ined ed&& Malpositioned opposing supporting cusps and ridges should be recontoured in order to achieve optimal occlusal contacts in the restored tooth&
,se articulating paper to register the centric holding spots and ecursive contacts so that these mar(ed areas can either be ecluded form the outline form or properly restored&
Plunger cusps and over erupted teeth should be reduced removing all the cuspal interferenceCs so as to improve the plane of occlusion and decrease the chances of fracture of ne% restoration as a result of occlusal forces&
+hen carving for occlusion attempt to establish stable centric contacts of cusps %ith opposing surfaces that are perpendicular to occlusal forces should be made&
Occlusal contacts located on a cuspal incline or ridge slope are undesirable because these create a deflective force on the tooth and hence should be ad*usted until the resulting contact is stable&
i& AMALGAM RESTORATIONS :
Sufficient bul( of amalgam is mandatory %hen restoring a cavity %ith amalgam so as to %ithstand the load of occlusion& 76
'de$uate thic(ness of amalgam should be provided at the marginal ridges in order to support the opposing supporting cusps&
'malgam restorations are carved follo%ing the cuspal inclines&
In case of large restorations %here there are no cuspal planes to guide carving the operator should follo% a cautious approach : •
Buccal and lingual cusp tips should be placed in lines *oining those of ad*acent teeth&
•
The level of central fossa and the marginal ridge should be carved similar to that of ad*acent teeth&
•
The bucco;lingual %idth of the occlusal surface is (ept narro%er than the original buccolingual %idth of the tooth&
•
In case both the %or(ing cusps on more than . cusps are restored preferably the occlusal table is (ept narro%ed&
This narro%er occlusal table leads to : •
eduction of force : +hen the occlusal table is made narro%er lesser force is applied over the same to undergo masticatory functions& Force is transmitted to all structures underlying the occlusal table %hich include the restoration the tooth structure and the periodontium&
•
edu educt ctio ion n of the the effe effect ct of forc forcee : The directi direction on in %hich %hich the the appl applie ied d forc forcee is transmitted is governed by muscular activities and the area on %hich the force is applied& applied& ?o%ever ?o%ever the effect effect of the force may be modified modified by altering the surface surface at %hich the force is applied thus altering the direction of resolved components&
•
eduction eduction of tor$ue : The tendency tendency to rotate may be reduced reduced by altering altering the point of application application of the force relative relative to the fulcrum& fulcrum& The point of application application of the force may be altered by modifying the occlusal table %hich indirectly depends upon the design of the cavity and the restoration&
ii& CAST METAL RESTORATIONS :
Simila Similarr to amalgam amalgam restor restoratio ations ns before before prepar preparatio ations ns of any tooth tooth evalua evaluate te the occlusal occlusal contacts contacts of the teeth in centric centric occlusion occlusion and in ecursive ecursive movement movements& s& 's part of this evaluation decide if the eisting occlusal relationships can be improved %ith the cast metal restorations&
75
The The cusp cuspal al inter interfe fere renc nces es are are depi depict cted ed in mandi mandibu bular lar %or( %or(in ing g move moveme ment nts s non %or(ing movements and protrusive movements&
The opposing opposing occlusa occlusall surfac surfaces es should should be eamin eamined ed an he malposi malposition tioned ed cusps cusps plunger cusps and over erupted teeth should be recontoured&
Premat Premature ure contact contactss or cuspal cuspal interf interfere erence ncess from from the teeth teeth opposing opposing the re$uir re$uired ed restoration should be removed&
The remaining tooth structure and the length of clinical cro%n dictates us to choose the type of cast restoration&
ANTERIOR RESTORATIONS :
The resin composites and the glass ionomer cements are mainly used in anterior restorations& restorations& Though these teeth teeth do not come under direct occlusion occlusion ho%ever ho%ever they they do ta(e part part in various movements of the mandible& The restoration should be carved and finished maintaining the contacts and the cervical curvature of these restorations& The lingual area is carved to maintain the anatomy of cingulum and the lingual marginal marginal ridges& Patient Patient is as(ed to protrude and the interferenc interferences es are chec(ed chec(ed and removed&
Similarly the relationship of lips %ith the labial surfaces of restored teeth are chec(ed and the over;contouring if any is removed&
The gingival etension of the material is ta(en care of to maintain the gingival health&
Role o, Cont"(t Are"s :
Kood Kood resto restora rativ tivee dent dental al proc proced edur ures es must must repr reprodu oduce ce the prope properr cont contac actt area areas& s& estorations %ith contact areas %hich are flat open improperly placed rough or poorly polished %ill lead to failure&
' slight slight frictio frictional nal moveme movement nt of teeth teeth al%ays al%ays occurs occurs bet%ee bet%een n the interp interpro roima imall surfaces of teeth during physiologic movement> and %ith time the contact point becomes broad resulting in a %ider contact area& IF the teeth remained in contact %ith each other merely by contact points they %ould eventually eventually be forced out of the dental dental arch in either a buccal buccal or lingual lingual direct direction& ion& +herea +hereass %ith a %ider contact contact bet%een teeth this is not li(ely to occur& The opposing interproimal surfaces of restorations must be hard in order not to flo% flatten %ear or become abraded %ith use& ./
Rel"tions!ip -et'een toot! 'e"r "n. restor"tie m"teri"ls :
Occlusal Occlusal forces lead lead to %ear of enamel& The %ear is ho%ever ho%ever very slo% if occlusal occlusal forces are appropriately transmitted to underlying bony tissues&
The pattern of %ear varies individually depending upon various factors& factors& -on;uniform ear of opposing teeth is $uite common %hen one teeth is restored %ith a restorative materi material al %hose %ear resistan resistance ce is differ different ent as compar compared ed to that that of enamel& enamel& Dery Dery rarely the %ear resistance of a restorative material e$ual the %ear resistance of enamel&
't present no restorative material is available %hich %ears at the same rate as enamel or as enamel and dentin at later stages&
#ifferential %ear can result in locali@ation of occlusal loads %ith subse$uent failure of restor restorativ ativee materi materials als or develop developmen mentt of deflec deflective tive contact contactss %ith mandibu mandibular lar repositioning and an effect on a distant tooth&
?ypothetically if t%o restorative materials %hich %ear at a slo%er rate than the natural teeth are placed so as to oppose each other in a dentition undergoing %ear the restorations %ill produce occlusal interferences at a later stage&
-on;%earing
materials opposing each other can lead to natural teeth %ear during
contact in lateral and protrusive movements&
"onversely "onversely if the materials materials %ear faster than the teeth the opposing cusp might over erupt erupt into the %orn materia material& l& I- lateral lateral ecurs ecursion ion this cusp might then then come in contact %ith an opposing cusp and if %ea(ened by previous caries can lead to fracture&
Compens"tion ,or O((l/s"l *e"r :
Occlusal interferences can develop through differential %ear patterns and unmatched compensatory mechanisms&
The clinician can shape and regulate the form of occlusal surfaces of teeth and restorations so that he can determine surfaces of teeth and restorations %hich contact during activities such as mastication s%allo%ing and bruism&
The advantage of this approach are : ;
.7
•
The direction of stresses through the strongest portions of the restorations an the remaining tooth structure can be arranged&
•
The The effe effect ct of occl occlusa usall inter interfe fere renc nces es deve develop lopin ing g from from diff differ eren entia tiall %ear %ear can can be minimi@ed&
•
It is possib possible le to main mainta tain in the the part partial ially ly resto restore red d denti dentiti tion on by mean meanss of peri period odic ic ad*ustment&
Since %ear defects are not repaired automatically the dentist should replace and maintain the configuration of teeth in accordance %ith the functional activities& FORCES E%ERTED DURING OCCLUSION 0 MASTICATION AND THEIR RESOLUTION
Darious types of forces are eerted on teeth during movement of mandible and also during mastication& Since the tooth surfaces are curved or at at an incline these forces forces are not only vertical but other types types of forces may also be eerting n these surfaces& The tooth in turn counteracts these forces %ith the help of periodontal membrane and alveolar bone& If the surfaces are flat and perpendicula perpendicularr to the force of mastication mastication only vertical vertical forces forces %ould %ould ta(e part& But in curved surfac surfaces es other other forces forces are also also set up and the resulting forces might not be eerted along the long ais of the tooth& This phenomenon can be understood by studying the resolution of forces on inclined planes& The cuspal planes are ta(en as inclined planes& +hen a force acts perpendicular to a fied hori@ontal surface the resolving force reacts reacts perpendicular perpendicular to the surface %ith %ith an e$ual and opposite force& force& If the surface is tilted at an angle to the hori@ontal it still reacts at right angle to the surface&
F Surface
F
Su Surface
Thus the reaction force no longer opposes the applied force in direction nor is e$ual to its magnitude& ?ence the forces forces are not in e$uilibrium %hen applied on inclined planes& The e$uilibrium can be maintained if more than one force is eerted on tooth or the forces are resolved in both directions& ..
Forces acting on inclined planes& 'B is a tangent dra%n at inclined plane or the contact contact bet%een . cusps& 'ngle LαC represents the angle made %ith the hori@ontal '" by the tangent 'B of the cuspal contact& M is the force of mastication and - is the resolving force& M is perpendicular to the hori@ontal '" and - is perpendicular to the incline plane tangent 'B ? is the hori@ontal component of the resolving force %hich maintains the e$uilibrium& 's the angle L αC decreases i&e& incline plane decreases - and ? become shorter and finally merge %ith M i&e& e$ual to @ero& The effect effect of friction friction bet%een cusps cusps also plays an important important role& role& Friction Friction is the resistance to a sliding motion of one body over another and the coefficient of friction is the force of friction over normal force& Many a times t%o or more inclined surfaces surfaces %ith slopes facing facing each other of one tooth contact the buccal and lingual cusps of the opposing tooth or the buccal and lingual cusps and marginal marginal ridges& ridges& This condition condition accounts accounts for the proper balance balance in occlusion occlusion and in case the contact is not normal it may account for displacements of the restoration or the fracture of the teeth& The effect so produced is is termed as %edging effect& effect& The hori@ontal components of the normal force are responsible for this %edging effect& effect& These hori@ontal hori@ontal components components set up by inclines are e$ual e$ual and opposite and tend to push the inclined surfaces apart& FORCES ACTING ON THE TOOTH : A& In (entri( o((l/sion
a b c are forces %hich acts at 4 contact points&
ab the result resultan antt of force forcess a and b& ab ab is the ab and c are the . ad*acent sides of the parallelogram passing through a given point as sho%n& The resultant is represented by diagonal passing through the same point i&e& Dabc&
?c is the hori@ontal hori@ontal componen componentt of force force c& ?ab the hori@ontal component of force a and b and ? c should be e$ual for achieving achieving e$uilibrium that is %hy abc abc and Dabc are e$ual&
B& D/rin1 C!e'in1 :
+hen mandible moves form lateral to centric occlusion the resultant of forces acting is not vertical but inclined medially&
.4
+hen tough food is compressed or all cusps are in intimate contact at the 4 points the forces forces a and and b are are decr decrea eased sed and and c is incr increa ease sed d %ith %ith resul resulta tant nt chan change gess in hori@ontal and vertical vertical components& Since during che%ing ?c is greater than ? ab and the net resultant is ?abc& So the hori@ontal component component is along the direction of c&
By using triangle of vector addition the resultant is represented by abc abc&
The resultant abc palatally on the maillary maillary teeth and buccally buccally on abc is a thrust inclined palatally the mandibular teeth %hose hori@ontal component is ?abc&
MECHANICAL FUNCTIONS OF THE MARGINAL RIDGES : Role o, M"r1in"l Ri.1es :
The marginal marginal ridges ridges play play an import important ant role role in %ithstan %ithstanding ding and dissipa dissipating ting the occlusal stresses&
The correct form of marginal ridge compatible %ith the ad*acent tooth and also %ith its o%n surrounding is important during carving of posterior restorations&
The absence of marginal ridge or marginal ridge %ith improper height can lead to altered dissipation of forces subse$uently damaging the underlying periodontium&
23 Norm"l M"r1in"l Ri.1e :
Forces Forces 7 and . act on marginal marginal ridges of teeth ' and B respectively& respectively& The hori@ontal hori@ontal component of 7 ?7 and the hori@ontal component of . ?. counteract each other& The vertical component D7 and D. are resolved normally by the underlying tissues&
43 No m"r1in"l ri.1e
Toot Tooth h B has has no marg margin inal al ridge ridge&&
Forc Forcee 7 and and . are are acti acting ng on toot tooth h a and and B
respectively& respectively& The hori@ontal hori@ontal component component of . ?. is missing missing in the the tooth B because because force . is mainly directed to%ards tooth '&
?ori@ontal component ?. %ill drift the tooth ' apart and the vertical component D 7 and D. of both the forces forces 7 and . %ill help help the food impact impact vertically& vertically& The vertical vertical force D. %ill be more than re$uired there may occur slight tilting of the tooth B& This This %ill %ill furt furthe herr dete deteri rior orat atee the the resol resolut ution ion of forc forces es and and lead lead to furt further her food food impaction&
53 A M"r1in"l Ri.1e 'it! " 'i.er o((l/s"l em-r"s/re3
.0
Inspite of putting optimal pressure pressure on marginal ridges of tooth ' and B the forces 7 and . act on ad*acent teeth& teeth& The force . %ill put pressure on tooth ' and force 7 %ill put pressure on tooth B& This %ill lead to drifting of both the teeth& The vertical components of forces %ill %edge the food is bet%een the t%o teeth&
Similar effect is seen %hen one marginal ridge is higher than other&
63 No o((l/s"l em-r"ss/re
In totality the vertical component of forces 7 and . %ill be more concentrated than hori@ontal components& Though there %ill to be any any vertical impaction of food the continuous impact of higher concentration of vertical component of forces may lead to changes in alveolar bone after sometime& $ERTICAL LOADS AND DISTRIBUTION OF STRESSES :
's the load is applied over the teeth stresses are distributed& i)
Parallel to to the long ais and
ii)
Perpendicular to to th the lo long a ais
The force or the load is applied applied at different different areas at a time and the stress distribution depends upon various factors& a) If the cros crosss – secti section on of that that area area is cons constan tant t stre stress ss distr distribu ibuti tion on is prac practi tica call lly y uniform& b) If there is variation in cross;section 1such areas are normally termed as prisms)> here stress varies form point to point being inversely proportional to area& c) If change change of cross;section cross;section area area is abrupt> abrupt> greater concentr concentration ation of stress stress occurs occurs at that point&
In vertical loading there there %ill be shearing stresses in prism prism in any plane& This haring stress increases to a maimum at 02 o and then then decreases decreases to @ero @ero at 5/o& Therefore materials that are %ea(er in shear than in compression or tension replace in planes at 02o to the ais&
The modulus of elasticity of the material is an important property and should be ta(en care of& If a cavity is restored restored %ith gold gold inlay or porcelain porcelain the modulus modulus of elasticity elasticity varies bet%een the the tooth and the restorative material& +ith the vertical force eerting on both the compression compression %ill be the same for the restoration restoration and the tooth but since gold!porcelain is much stiffer they %ill be highly stressed since S A d3& .2
S 1Stress) A S 1,nit strain) 3 1Modulus of elasticity)
+hen the force is applied perpendicular to the prism ais the resultant resolution is (no%n as beam& beam& Beam can be supported supported form both the ends ends 1simple beam) beam) and may be supported form one end 1cantilever beam)&
3ample of simple beam : MO# preparation 3ample of "antilever beam : MO ! #O preparation The retention of the restoration depends upon these beams although the strength and the deflection of the material also play part& Moment of Force A Force Perpendicular #istance
The bonding moment is at the aiopulpal line angle %hich tends to rotate the restoration out of the cavity&
Kingival retention %ith a moment e$ual to F E is re$uired to counteract this moment& The total retentive force 1) is e$ual to F E ! l +here l is the depth of the aial %all&
If %e ta(e depth of gingival %all 1d) into account then and d %ill be in the same direction direction so their moment moment of force is @ero& Therefore Therefore the depth depth of the gingival %all does not ta(e part in retention&
In MOD Prep"r"tion :
In MO# preparation preparation the force 1F) is divided e$ually on both the sides& The mesio distal distance 1E) is also divided into t%o& The moment of force force at the midpoint is : F ! . E ! . A FE ! 0 If this moment of force is divided into t%o 1because it is actually acting on both the ends) then the moment of force : FE
7
FE
;;;;; ;;;;; A ;;;;; ;;;;; 0
.
6
Since the beam forces a concave do%n%ard curvature bet%een the load and the fied end therefore by sign convention this end moment is ta(en as negative& By e$uation l A FE ! 6 So A FE ! 6 l The negative sign is used only in vector form and in magnitude only positive sign is used& .8
If %e ta(e depth of gingival %all 1d) into account then and d %ill be in the same direction so their moment of force remains @ero&
It is presumed in MO# preparations preparations that the length of the aial %all 1l) is (ept e$ual on booth the ends& ends& If there there is mar(ed mar(ed discre discrepan pancy cy bet%een bet%een the t%o ends ends the end result may not be the same as is described earlier& Therefore preferably the length of the t%o aial %alls should be the same&
In Ceri("l 0 Gin1i"l Restor"tions :
It has been establish established ed that certain certain forces forces act on the cervic cervical al reign reign %hich could could destabili@e the restoration and even lead to crac(s at the cemento;enamel *unction&
The forces acting on inclined planes of the occluding cusps conse$uently lead to transverse stresses& These transverse stresses try to bend the tooth gingivo;occlusally& Since the teeth are firmly held in alveolar soc(et these rotations are minimum and counteracted&
In cases %here a cavity is cut on the cervical surfaces depending upon the height of the aial %all the deflectiv deflectivee force is increased& increased& If the restorative restorative materials materials are not adhesive in nature a gap can be created at the cervical surface of the restoration on buccal side and occlusal surface on the lingual side&
Force 1F) 1F) is applied at incline plane plane perpendicular perpendicular to the tangent tangent of the planes& The hori@ontal hori@ontal component component 1?) acts approimately approimately at the centre of the tooth& The vertical vertical component component 1D) is constant& constant& The deflection deflection is mainly by the hori@ontal hori@ontal component component %hich depends upon the height of the aial %all 1E) and the depth of the occlusal 1d 7) and cervical %alls 1d .)&
APPLICATION OF STRESSES AND THEIR DISTRIBUTION IN INDI$IDUAL RESTORATIONS RESTORATIONS :
7) "las "lasss I res restor torati ation on a) If rest restor ored ed %ith %ith amal amalga gam m
It is recommended to converge the side %alls occlusally and to (eep the floor flat&
In case the floor is not (ept flat the forces %ill rotate the restoration on both the sides& 'nd also since since the remaining remaining dentin dentin %ill be less at the centre centre and if the restoration is deep the forces might fracture the tooth&
b) "ast restorations : .9
Movement ! rotation of the cast restoration is easy if the pulpal floor is not (ept flat&
Since the %alls are diverging occlusally the chances of rotation are much more&
c) "omp "ompos osite itess or KI" KI"
These adhesive materials counteracts such rotational forces&
.) "las "lasss II res resto tora rati tion on
Stresses %hich tend to rotate the restoration mostly act on marginal ridges&
Stresses also is more at aiopulpal line angle hence this aiopulpal line angle should be %ell rounded thereby decreasing stress concentration and increasing the bul( of the material at this point&
In MO# restorations bending of the occlusal portion is caused by the difference bet%een the total masticatory force and the support given by the pulpal floor of the cavity&
Kingival retention and rounding of the aiopulpal line angles are re$uired as in MO and #O cavity&
In cases %here the opposing cusps occlude in such a %ay that one contact point is on a proimoocclusal restoration %hile the other is on tooth structure there is a tendency tendency to %edge the t%o t%o apart& To prevent prevent this %edging %edging the occlusal occlusal loc( is used even though occlusal surface is not involved by caries&
4) "lass "lass III and and "lass "lass ID estor estorati ations ons :
Since these lesions are not in direct contact %ith opposing teeth only transverse stresses play part in dislodging ! rotating the restoration
In such restorations restorations there is tendency to rotate about an aial approimatel approimately y parallel to the long ais ais of the tooth& tooth&
's incisa incisall rete retenti ntion on canno cannott be made made due to thin thin
labiolingual si@e so lingual loc( is placed placed on lingual surface& It should be as close to the incisal edge as possible and still be in dentin to reduce the stress in this lingual loc(&
In maillary teeth force of mastication ahs labial component %hich provides the seating effect on the restoration& In case the labial enamel is not intact the chances chances of dislodgment dislodgment of the restoration restoration %ill increase& increase& In mandibular mandibular teeth the component component of the masticatory force is from the labial to the lingual so chances of dislodgement of restoration are more&
0) "las "lasss D rest restor orat ation ionss : .6
'nalysis indicates that physical forces putting on occlusal surfaces could result in displacement of the restoration&
#uring occlusion the vertical stresses on the teeth led to transverse stresses and this component of stresses tends to rotate the cervical restoration&
The mandibular teeth bend more than maillary teeth&
' gap is evident evident on the cervical cervical ! occlusal %all of the cavity and if the depth of these %alls is less the restoration may come out&
.5
FORCES ACTING ON AMALGAM RESTORATION CLASS 7 I :
By definition "lass I cavity preparations are placed in pit and fissure lesions that occur in one more of the follo%ing locations : '& Occlusal Occlusal surfaces surfaces of of molars molars and premola premolars rs B& Occlusal Occlusal .!4 of the buccal buccal and lingual lingual surfaces surfaces of molars molars "& Eingual surfaces surfaces of the upper anterior anterior teeth teeth 1usually the central central and lateral lateral incisors) incisors) #& 'ny other usually usually located located pit or fissure fissure involved involved %ith decay& decay& Me(!"ni("l pro-lems in Cl"ss I restor"tion "n. t!eir sol/tions3
'& 'll "lass I cavity preparations %ill have a mortise mortise shape i&e& each %all and floor floor is in the form of a flat plane meeting each other at definite line and point angles& -
The seat of the restoration is placed at a distinct right angle to the direction of stresses&
-
It is advantageous to have a mortise shape preparation in an inverted cone shape to minimi@e shear stresses that tend to seperate the buccal and lingual cuspid elements i&e& to prevent the splitting of the tooth& So %henever the anatomical and cariological cariological factors allo% the cavity preparation should be an inverted cone shape&
B& +hen a caries cone penetrates deeply into dentin removing undermined and decayed tooth structures can lead to a conical 1hemispherical in cross;section) cavity preparation& Mechanically t%o problems can occur if a restoration is inserted into such a cavity preparation& 7& If the occl occlusa usall load loading ing is appl applie ied d centr centric ical ally ly the the resto restora ratio tion n may may act act as a %edg %edge e concentrating forces at the pulpal floor and leading to dentin bridge crac(ing and an increased tendency for tooth splitting& .& If the occlusal occlusal loading loading is applied applied eccentri eccentrical cally ly the restorat restoration ion %ill have tenden tendency cy to rotate laterally for there %ould be no lateral loc(ing %alls in definite angulation %ith a floor& 'lthough these these lateral lateral movements movements are microscopic microscopic they they occur fre$uently fre$uently enoug enough h to encou encoura rage ge micr microle olea( a(ag agee aroun around d the the resto restora ratio tion n pred predisp ispos osing ing to a recurrence recurrence of decay& decay& These movements movements can can also lead to fracture fracture of marginal marginal tooth structure and even to splitting of lateral %alls& To solve solve thes thesee probl problem ems s flat flatte ten n the pulpa pulpall end end of the the cavi cavity ty prep prepar arat ation ion&& ?o%ever if accomplishing this at a deep location incurs increased ris( of involving the pulp chamber pulp horns or recessional lines containing remnants of pulp tissues ma(e 4/
the pulpal floor at more than one level& One level %ill be the ideal depth level 17&2 mm) mm) and the others %ill be the caries cone1s) level1s) dictated by the pulpal etent of the decay& decay& The shallo% shallo% level creates creates the flat portion portion of the pulpal floor at definite definite angles angles to the surrou surroundin nding g %alls %alls ade$ua ade$uatel tely y resist resisting ing occlusa occlusall forces forces and lateral laterally ly loc(ing loc(ing the restoration restoration %ithout %ithout impinging on pulp tissues& tissues& eiterating eiterating the other other level1s) is 1are) 1are) only necessitated by the caries etent& creating one or more concavities or cones in the pulpal floor& "& +hen a cavity %all comes in contact %ith a marginal ridge the %all should be divergent divergent pulpo;occlusa pulpo;occlusally lly ma(ing an obtuse obtuse angle %ith the pulpal pulpal floor& This design allo%s for maimum bul( of tooth structure supporting the marginal ridge and avoids undermining of the marginal ridge creating more mechanical and biological problems& #& If cariogenic conditions do not dictate other%ise the %idth of the cavity should be limited limited to to 7!2 the intercuspal intercuspal distance distance 1not less less than 7&2 mm)& This minimi@es minimi@es loss of tooth structure in this critical cross;section of the tooth& This %idth %ill also facilitate easy carving of the restoration and minimi@e the possibilities of occlusal interferenceCs& 3& 'll cavosurface cavosurface angles angles should be right angled to create a butt;*oint butt;*oint %ith the marginal amalgam& amalgam& This configuration configuration allo%s allo%s marginal amalgam amalgam to %ithstand stresses stresses %ith the least possibility of failure& F& 'll line and point angles or any *unction bet%een different details in the cavity preparation should be rounded but definite& This design has all the advantages of the mortise shape %hile avoiding stress concentration in the tooth structure and restorative materials that may occur from sharp angulations& K& Occluding forces %ill tend to move marginal amalgam and tooth structures from position 7 to position .&
's vital tooth structures are more deformable than set
amalgam the displacement %ill not be e$ual thereby creating a gap bet%een them& This places the marginal amalgam under intolerable tensile loading %hich may lead to amalgam failure if the amalgam is in thin cross sections i&e& acute angled marginal amalgam amalgam %ill fracture& fracture& B If marginal amalgam amalgam is right angled angled it can be stand induced induced stresses from occlusal loading %ith less possibility of failure even if the stresses are tensile in nature&
CLASS II AMALGAM RESTORATION
By definition definition "lass II cavity cavity preparation preparation is proimal preparatio preparations ns of molars and premolars& 47
Resist"n(e Form :
The fundamental concept of resistance form is based on the reaction %ithin the restoration and the remaining tooth structure to occlusal loading& The ob*ective of a cavity preparation design is to establish the best possible configuration that can cope %ith the distribution and magnitude of the stresses in both structure structure and the restoration restoration %ithout %ithout failure& failure& To design such a configurati configuration on one must first comprehend the nature of loading and of resistance to such loading& A3 O((l/s"l Lo".in1 "n. Its E,,e(t :
#uring centric and ecursive movements of the mandible both restoration and the tooth structure structure are periodical periodically ly loaded both separate separately ly and *ointly& This brings about about different different stresses stresses patterns depending depending on the actual morphology morphology of the occluding area area of the both the tooth in $uestion and opposing contacting cuspal elements& For the purpose of this discussion one can classify these loading situations and their induced stress patterns in the follo%ing %ay& 7) ' smal smalll cusp cusp contac contacts ts the fossa fossa a%ay a%ay from from the rest restor ored ed proim proimal al surfac surface e in a proimo occlusal restoration at centric closure& 's sho%n due to the elasticity elasticity of the dentin 1in young teeth) a restoration %ill bevel at the aio;pulpal line angle 1provided the proimal part of the restoration is self; retained)& This creates tensile stresses at the isthmus portion of the restoration shear stresses at the *unction of the main bul( of the proimal part of the restoration and self retained parts and compressive stresses in the underlying dentin& .) ' larg largee cusp cusp cont contac acts ts the the foss fossaa ad*a ad*ace cent nt to the the rest restor ored ed pro proim imal al surf surfac acee in a proimo;occlusal restorations at centric closure either in the early stages of moving out of centric or at the late stages of moving to%ard it& 's sho%n the large cusps %ill tend to separate the proimal part of the restoration from from the occlus occlusal al part& This This crates crates tensile tensile stresse stressess at the isthmus isthmus portion portion of the restoration even fi the proimal portion is self – retained& This loading situation %ill deliver compressive forces in the remaining tooth structure apical to the restoration& 4) Occludi Occluding ng cuspal elements elements contact contact facial facial and lingual lingual tooth structur structuree surrou surroundin nding g a proimo;occlusal or proimo;occluso;proimal restoration during centric and ecursive movements& "oncent "oncentrat rated ed shear shear stress stresses es %ill occur at the *unctio *unction n of the surroun surrounding ding tooth structu structure re and corres correspond ponding ing floors floors %ith a tenden tendency cy to%ards to%ards failur failuree there& there& 4.
This This
loadi loading ng situa situatio tion n can can be unila unilate tera rall or bilat bilater eral al depen dependi ding ng on the the mand mandibu ibula larr movement it is the most deleterious to tooth structure especially on the orbiting side if there is interference during lateral ecursion& 0) Occl Occlud udin ing g faci facial al elem elemen ents ts cont contac actt faci facial al and and ling lingua uall part partss of the the rest restor orat atio ion n surrounded by tooth structure during centric and ecursive movements& This This arrang arrangeme ement nt %ill induce induce tensile tensile and compre compressiv ssivee stresse stressess in the restor restorati ation on %hich %ill be transmitted to the surrounding tooth structure& 2) Occl Occlud udin ing g cusp cuspal al elem elemen ents ts cont contac actt faci facial al or ling lingua uall part partss of the the rest restor orat atio ion n completely replacing facial or lingual tooth structure during centric or ecursive movements& The tensile stresses %ill be induced at the *unction of the occlusal and facial and!or lingual part of the restoration in both occluding situations& 8) Occl Occludi uding ng cuspa cuspall elem elemen ents ts cont contac actt a resto restora rati tionC onCss marg margina inall ridge ridge1s) 1s) or part part of a marginal ridge during centric or ecursive movements 1assuming the restoration is loc(ed occlusally) there %ill be concentrated tensile stresses at the 1*unction of the occlusal occlusal and facial facial or lingual parts of the restoration at full intercuspation intercuspation and to end from that position) at the *unction of the marginal ridge and the rest of the restoration& This %ill be true if its an area of advance contact during mandibular closure& 9) "uspal elemen elements ts occlude occlude or disclude disclude via the facial facial or lingual groove groove of a restora restoration& tion& There %ill be tensile stresses at the *unction of the occlusal and facial or lingual parts of the restoration at full intercuspation and to and from that position& 6) "usp "uspss and and cros crossi sing ng ridg ridges es are are part part of the the rest restor orat atio ion n in cent centri ricc and and ecu ecurs rsiv ivee movement& Both %ill be sub*ected sub*ected to compressive compressive stresses stresses during such positions and movement& Besides Besides tensile tensile stresses could concentrate concentrate at their *unction %ith the main restoration restoration specially during contacting ecursive movement& 5) 'ial 'ial portions portions of the restorat restoration ion during during centri centricc occlusi occlusion on and ecursi ecursive ve movement movement contacts: +henever these portions are in contact %ith opposing occlusal surfaces there %ill be induced compressive and shear stresses %hen they are not reciprocating 1one side not in contact %ith occluding surfaces %hile other aial portion)& The aial surfaces %ill be stressed in a slight s light tensile and shear pattern at their *unction * unction %ith the main bul( b ul( of the restoration& 44
7/)estoration is not in occluding contact or is in premature contact during centric occl occlus usion ion or ecu ecurs rsiv ivee move moveme ment nt of the the mand mandibl ible& e&
The The firs firstt situa situati tion on is not
conducive to function insofar as the restoration %ill not be involved %ith direct loading from the opposing occluding teeth& 'fter a period of time ho%ever the tooth %ill supraerupt rotate and or tilt establishing contact %ith the opposing cuspal elemen elements& ts&
,suall ,sually y this ne%ly ac$uire ac$uired d location location %ill not be the most favorable favorable
position for the restoration tooth or the remainder of the gnatho stomatic system eith either er mech mechan anic ical allly or biol biolog ogic ical ally ly&& It is safe saferr to buil build d the the rest restor orat atio ion n to predetermined contacting areas %ith opposing teeth %hich %ill lead to predictable physiologic stress patterns in the tooth structure and restoration& "onversely any portion of the restoration occluding prematurely %ill tremendously eaggerate the same same types of stresse stressess normal normally ly induced induced in that that area area of the restora restoration tion&& Besides Besides additional additional shear components components of stress stress could be precipitated precipitated there& there& This too could could lead lead to loca locali@ li@ed ed or gene genera rali li@e @ed d gnat gnatho ho stoma stomatic tic distur disturba banc nces es %ith %ith even eventua tuall mechanical and biological failures& -eedless to t o say pre;eisting premature p remature contacting areas should be eliminated before restorative restorative treatment& treatment& This is done for many reasons reasons but primarily primarily because cavity cavity preparation increases the susceptibility of remaining tooth structure to fracture failur failure& e&
Beside Besides s the restor restoratio ation n should should be built built to the predet predeterm ermine ined d occlusa occlusall
position even fi the preeisting tooth structures %ere not& Seve Severa rall facto factors rs must must be acco accomm mmod odate ated d in desig designin ning g "las "lasss II prepa prepara rati tions ons for for amalga amalgam& m& Occlusa Occlusall loading loading is dynamic dynamic and cyclic cyclic in nature nature %hich is a far more more destructive destructive type of loading than than static loading& 'malgam 'malgam is least resistant resistant to tensile stress and most most resistant to compressiv compressivee stress& Tooth structure structure particular particularly ly %hen interrupted by a cavity preparation is least least resistant to shear shear stress& Therefore "lass II cavity preparations for amalgam restorations should be designed to resist cyclic loading %hile minimi@ing tensile loading in the amalgam and shear loading in the remaining tooth stricture& B3 Desi1n ,e"t/res ,or t!e prote(tion o, t!e me(!"ni("l me(!"ni("l inte1rit) o, t!e restor"tion : 1. Isthmus :
In the isthmus i&e& the *unction *unction bet%een the occlusal part of a restoration restoration and the proimal facial or lingual parts potentially deleterious tensile stresses occur under any type of loading& 40
Most mathematical mechanical and photoelastic analyses of these stresses reveal three things : 7) The fulcrum fulcrum of bending occurs occurs at at the aio;pul aio;pulpal pal line angle .) Stresses Stresses increase increase closer to the surface surface of a restoration restoration a%ay from from that fulcrum fulcrum and 4) Tensile stresse stressess concentrate concentrate at the marginal marginal ridge ridge area of of a "lass II restorati restoration& on& Materials tend to fail therefore starting from the surface near the marginal ridge and proceeding internally to%ard the aio;pulpal line angle& These problems problems may may be solved by applying applying common common engineering engineering principles& principles& ' theoretical solution might be : 7) to increase increase amalgam amalgam bul( at the aio;pulpa aio;pulpall line angle angle thereby thereby placing placing the surface surface stresses stresses a%ay from the fulcrum& fulcrum& ?o%ever ?o%ever its actually results results in increased increased stresses %ithin the restorative restorative material and a deepened deepened cavity cavity preparation preparation dangerously dangerously close to pulp anatomy& Therefore such a solution solution in and of itself is %holly unacceptable& .) 'nother solution solution might might be to bring the aio;pulp aio;pulpal al line angle angle closer to to the surface surface in an effort to reduce tensile stresses occurring near the marginal ridge& ?o%ever this too is unacceptable in that the conse$uent diminished bul( of amalgam %ould no longer ade$uately resist compressive forces& 4) ' combina combinatio tion n of the t%o solution solutionss i&e& increasin increasing g amalgam amalgam bul( near the margina marginall ridge %hile bringing the aio;pulpal line angle a%ay from that stress concentration area and closer to the surface can be achieved simply by slanting the aial %all to%ard the pulpal floor& a) The The obtus obtusee aia aiall pulpa pulpall line line angle angle thereb thereby y crea create ted d not only provid provides es grea greate terr amalgam bul( in the marginal ridge area of the restoration but also reduces tensile stresses per unit area by bringing this critical area of the preparation closer to the surface surface of the restora restoration tion&&
Furthe Furthermo rmore re this design design featur featuree improv improves es
accessibility to the proimal facial and proimal lingual parts of the cavity during preparation procedures& This is the first design feature& b) Secondly if the aio pulpal line angle is rounded structural pro*ections or sharp *unctions that may concentrate stresses at the isthmus %ould be avoided& This second feature %ill also improve the visibility for the facial and lingual gingivo; aial corners of the preparation proimally as %ell as increase the amalgam bul( at the fulcrum&
42
c) Thirdly Thirdly by slanting slanting the aial aial %all bul( bul( is improve improved d by increa increased sed depth depth rather rather than increased increased %idth& %idth& Increasing Increasing the %idth at the isthmus isthmus portion portion only increases increases the surface area receiving deleterious occluding stresses& 0) 's a fourth design design featur feature e the pulpal pulpal and gingival gingival floor floorss at the isthmus isthmus should be perfectly flat in order to resist forces at the most advantageous angulation& 2) The fifth design design feature feature is that every every part of the preparation preparation 1occlusal 1occlusal facial facial lingual or proimal) proimal) should be self;retentiv self;retentive& e& If every every part of the restoration restoration is loc(ed in tooth structure independently independently from other parts there %ill be minimum stresses at the *unction of one part %ith another i&e& the isthmi& This can be achieved in amalgam preparations by retentive grooves internal boes and undercuts& 8) Sit Sith h one one shou should ld avoi avoid d as much much as poss possib ible le plac placin ing g or leav leavin ing g any any surf surfac acee discontinuities such as carved developmental grooves scratches etc at these critical areas areas in the restorat restoration& ion& These These can precipi precipitat tatee and accentua accentuate te stresses stresses leading leading to fatigue failure& Fina Finally lly by chec chec(i (ing ng occl occlusi usion on to elimi elimina nate te prem premat atur uriti ities es in the the resto restora ratio tion n immediate overloading and failure can be avoided& 2. Margins :
'malgam has good compressive strength %hen it has sufficient bul( 17&2 mm minimum)& minimum)& ?o%ever ?o%ever frail feather feather edged edged margins of amalgam amalgam %hich %ill occur %hen the cavosurf cavosurface ace angles angles of prepar preparatio ations ns are bevelle bevelled d %ill fractur fracturee easily& easily& Occludi Occluding ng forces %ill cause amalgam at the bevel to bend %ith maimum tensile stresses occurring as a result result of elastic elastic deformat deformation ion of the tooth tooth structure structure beneath beneath the bevel& bevel& Margina Marginall ecess of amalgam %ill similarly fracture leaving a ditch around the restoration that %ill enhance enhance recurrence recurrence of decay& decay& So for the margins margins of these p reparations reparations four four design features should be observed > create butt *oint amalgam tooth structure at the margins leave no frail enamel at the cavosurface margins remove flashes of amalgam on tooth surfac surfacee ad*ace ad*acent nt to amalga amalgam m margi margins ns and and as practic practicall ally y as possible possible the interfa interface ce bet%een amalgam and tooth structure should not be at an occluding contact area %ith opposing teeth either in centric or ecursive mandibular movements& 3. Cuspal and Axial angles :
The follo%ing are the design features for these parts of a restoration& a& 'malgam 'malgam bul( in all three dimensions dimensions should should be atleast atleast 7&2 mm 48
b& 3ach portion of the amalgam should be completely immobili@ed %ith retention modes& c& 'malgam 'malgam should be seated seated on a flat floor floor or table table in these areas& areas& d& 'malgam 'malgam replacing replacing cusps cusps or aial angles angles should should have a bul(y bul(y connection connection to the the main part of the restoration %ith similar design features as for the isthmus areas& C3 Desi1n ,e"t/res ,or t!e prote(tion o, t!e p!)siome(!"ni("l inte1rit) o, rem"inin1 toot! str/(t/re :
In addition to design features in the restoration there are also certain design feat featur ures es in the the tooth tooth stru structu cture re %hic %hich h enha enhanc ncee resis resista tanc ncee of the resto restore red d tooth tooth to deleterious stresses& Retention ,rom :
In order to design a cavity preparation that %ill hold a restorative material it is necessary to (no% the possible displacements that can happen to such a restoration the forces forces that can cause cause them and the fulcrum fulcrum of these these movements& movements& There are are four such displacements for a "lass II proimo;occlusal restoration& '& Proimal #isplacement of the 3ntire estoration : In analy@ing the obli$uely applied force <'= into a vertical component
In order order to preven preventt such such a
displac displaceme ement nt proim proimal al self;r self;rete etentio ntion n in the form form of facial facial lingual lingual and!or and!or gingiva gingivall grooves grooves are re$uired& re$uired& ?o%ever ?o%ever shear stresses stresses %ill be induced at the *unction bet%een bet%een the amalga amalgam m of the main restora restoration tion and that that in the grooves& grooves& Theref Therefore ore it is to be 49
understood that these grooves are prepared only %hen there is complete assurance that there %ill be sufficient dentinal bul( to accommodate them and that they %ill not impinge on the aial angle or on the pulp anatomy& "& Eate Eatera rall ota otatio tion n of the esto estora ratio tion n 'rou 'round nd ?emi ?emisph spher eric ical al Floor Floorss 1Pul 1Pulpa pall and and Kingival) 's in "lass I cavity preparations this displacement can be prevented by definite point and line angles and ledges %here indicated& #& Occlusal displacement : The can be prevented by directing occlusal loading to seat the restoration and by inverted truncated cone shaping of (ey parts of the preparation& 'lthough the magnitude of these four displacements is minute they are repeated thousan thousands ds of times times per day& day&
This This can definit definitely ely increase increase microle microlea(a a(age ge and initiat initiatee
mechanical and biological failure of the restoration and surrounding tooth structure& Ther Theref efor ore e prope properr loc(i loc(ing ng of the the resto restora rati tion on into into the the tooth tooth shoul should d be eer eerci cise sed d to minimi@e these ha@ards& To repeat every part of the cavity preparation should be self;retaining if possible i&e& i&e& independe independent nt in its retent retention ion from the rest rest of the cavity& cavity&
This minimi@ minimi@es es shear shear
concentratio concentration n areas at the *unctions of different parts of the restoration restoration %ith less failure to be epected as a result& FORCES ACTING ON INLAY RESTORATION
The cavity should have such retention form that the restorations %ill be firmly held in place the cavity should also have resistance form that the restoration %ill %ithstand the stress %ithout being dislodged& 'n understanding of the materials used in constructing an inlay together %ith a (no%ledge of correct manipulation is also an important factor in the success or failure of an inlay 1inlay is not only a part of mechanical structure replacing lost teeth but it is also intimately related to the vital tissues it is the medium through %hich mechanical and physical forces are translated into physiological functions and biological reactions in living tissues& The other preparation features that %ill help solve the mechanical problems of cast restorations are as follo%s :
46
'll the line and point angles should be definite but not angular so they can be easily reproduced in a casting and to avoid stress concentration in the casting and the tooth structure& The roundness must be be substantial for "lass "lass D materials& The aial aial %all should should slant to%ard to%ard the pulpal pulpal floor floor as part of the taper& taper& This This together %ith rounding of the aio;pulpal line angle can reduce stresses at the isthmus area& eduction of tooth structure should follo% the original anatomy of the tooth to crea create te even even redu reduct ctio ion n %ith %ith minim minimum um tooth tooth involv involvem ement ent and and even even physi physiolo ologic gic distribution of forces applied on the restoration and remaining tooth structure& Maimum reduction should be at the occluding surfaces especially the parts of the tooth surfaces that are in contact during static and dynamic relations of the mandible& 'n average of 7 mm should be cleared for metallic casting in the inclined planes of the cusp cusps& s&
This This reduc reductio tion n should should be 7&2 mm for for cast cera cerami mics cs&&
The The reduct reduction ion of the
occluding inclined planes should be cut in a concave form to accommodate maimal bul( of the casting %here stresses are at their maimum& The internal parts of the cavity preparation should be mortised to preserve the resistance and retention features of the preparation 1and to assure one path for the preparation)& The internal boed up portion should sho uld occupy the maimum dimensions of the cavity preparation preparation as practically practically as possible& This %ill necessitate necessitate ma(ing the cavity %all in different planes& 't least the internal planes are fied in their angulation 1almost right angle) %ith the ad*acent floors or %alls& Since the retention of an inlay and its resistance to displacement are primarily mechanical problems a group of the principles of retention is based on understanding the forces of mastication and the analysis of strains %hich are present in the restoration& It has been stated that %hen a force is applied at right angles to a surface its effe effect ctiv iven enes esss %ith %ith the the dire direct ction ion of forc forcee and and that that is propo proport rtion ional al to its magni magnitud tudee li(e%ise the opposing forces are e$ual and opposite in direction& 'nother la% states that if the force is applied at an angle to the surface other than right angle the magnitude of %hich depends n the angle of application ad that the reacting force is neither e$ual nor opposite in direction& Eateral or tangential forces may cause displacement of the restoration unless ade$uate resistance and retention have been incorporated in the preparation& Frictional retention can be achieved by the action of dentin and enamel %alls grasping the restoration 1intracoronal retention)& 45
-o% let us consider the forces applied at right angles to the flat surface of a restoration& P/lp"l Floor "n. Gin1i"l Se"l :
7) ' typical proimooc proimoocclusal clusal cavity cavity %ill have have t%o such surfaces surfaces to vertical vertical forces forces – the pulpal and gingival %alls& If the forces are perpendicular to these surfaces the opposing forces are e$ual and opposi opposite te then there there is no tenden tendency cy to displa displace ce the filli filling ng&&
Floor Floorss positio positione ned d
perpendicular to these lines of force absorbs the stress over a broad area of tooth& .) It is only %hen %hen the pulpal pulpal %all is flat flat and the t%o vertic vertical al %alls %alls are paralle parallell to each other that the maimum retention form is obtained& +hile these above illustration refer to simple bo type cavity preparation the same principles hold good %hen the force is applied at right angles to the occlusal surfaces of proimo occlusal inlay& 4) In a toot tooth h %ea( %ea(en ened ed by ete etens nsiv ivee cari caries es the the resi resist stan ance ce form form is obta obtain ined ed by etracoronal etension of the preparation in the form of etra long reverse bevel in capped cusps or by partial or complete coverage of facial or lingual surfaces& 0) If the dentin dentin of the pulpal %all %all is compressed compressed elasticall elastically y under vertical vertical forces forces if the compression compression is conical then the gingival portion of the filling %ould rotate out of the cavity %ith the aiopulpal line angle acting as the fulcrum& Beca Becaus usee of the the added added rete retenti ntion on obtain obtained ed by the pulpa pulpall ete etensi nsion on and and if the the diagonal diagonal force is applied applied to the casting %hich %hich is LEC shaped& shaped& It %ill have a tendency tendency to straigh straighten ten out so this causes causes the metal metal to move move out lateral laterally ly at the gingiva gingivall area& To resist this lateral spreading at the gingival %all provision is made for the depression of the %all and creating the gingival groove %hich restores the retentive form to a certain etent&
A#iopro#im"l *"lls 8F"(i"l 0 Lin1/"l& :
"ompressive forces resulting from vertical pressure have an important bearing on the reten retention tion of of the inlay& inlay&
This This bears bears on the relat relations ionship hip of the the buccal buccal and and lingual lingual
proimal %alls& -o% %hether they should flare aioproimally or be parallel to each other 1that is the part of the %all lying %ithin the dentin)&
0/
There are 4 different relationships of %all ' to %all B in the gingivoocclusal direction& 7) The %alls %alls are are paral parallel lel to each each other other&& .) +alls +alls are are %idel %idely y diverg divergent ent&& 4) #iverge #ivergence nce not eceed eceeding ing 2o from the vertical plane& +hen forces are applied at an angle other than right angle force is resolved in . %ays one of %hich %hich reacts in its effectiven effectiveness ess at right angle to the surface& surface& This force is not opposite in direction nor is it e$ually magnitude to the original force& The tendency in a tooth is for the cusp of the opposing tooth to slide do%n the inclined inclined plane or for an inlay to be pushed out of the cavity in a hori@ontal plane& +hen a vertical force is applied to a proimal etension the filling is rotated occlus occlusopr oproim oimall ally y out of its cavity& cavity&
The rotation rotation point point of fulcrum fulcrum being gingival gingival
marginal marginal %all& These forces forces are al%ays al%ays effective effective unless counteract counteracted ed by an opposing move moveme ment nt&&
This This can can be achi achiev eved ed by prop proper erly ly prep prepar ared ed occlu occlusa sall loc( loc( by prop proper er
preparation of gingival %all pulpal %all and lastly by the proper contour and contact point& Slice : Slice preparation provides eternal support of %ea(ened tooth or areas sub*ected to high stresses stresses during during function& function&
It increase increasess the resista resistance nce and retentio retention n form by
eposing a larger amount of tooth structure to the frictional grasp of the restoration& O((l/s"l Doe T"il : Tensile stresses developed by this is one of the strongest means of
resisti resisting ng the displac displaceme ement nt of an inlay& inlay&
"linic "linical al precauti precaution on demand demandss that that by lingual lingual
inclined planes %hich etend into the isthmus of the occlusal bloc( be on sound cusps %ith a sufficient amount of supporting dentin& If these are are lac(ing there is li(elihood of fracture of one or both the cusps %henever inlay is sub*ected to hori@ontal forces& -o% the buccal and lingual aial %alls instead of flaring from the aial line angles to the cavosurface margin in a continuous plane are no% changed into t%o narro%er narro%er but parallel parallel planes and t%o smaller smaller diverging planes& planes& It is evident that in this type of preparation it is possible to retain the retentive form of the preparation even if the %alls diverge in a continuous plane %hen stress is applied to the occlusal surface the reaction reaction of the opposite forces forces %ill tend to dislodge dislodge the filling& filling& So retention retention in this type of preparation is by placing a gingival groove in the gingival %all and by adding an
07
occlus occlusal al loc(& ?ence ?ence effort effort is made made to parall parallel el at least part of the buccal buccal and lingual lingual proimal %alls that lie in dentin& Seco Second nd meth method od of resi resist stin ing g hori hori@o @ont ntal al disp displa laci cing ng forc forces es is by the the prop proper er preparation of gingival %alls& The properly prepared gingival groove assist in preventing the lateral lateral displac displaceme ement nt of an inlay& inlay&
But because because of the inherent inherent %ea(nes %ea(nesss of the
gingival groove the possible fracture to this %all of the tooth structure bet%een the groove an the cavosurface angle so many operators prefer the in%ard beveling of the gingival %all forming an acute angle bet%een the aial and gingival %alls& P/lp"l *"ll : Third method of obtaining opposing movements to hori@ontal displacing
force is by establishing resistance resistance into pulpal %all& The pulp %all %hich is flat offers no resistance to hori@ontal displacement %hen it is prepared %ith t%o inclined planes it %ill prevent the lateral displacement of the inlay& 'nother modification is placement of grooves parallel to the long ais of the tooth at the aial aial angles& angles& Such prepara preparation tion resist resist hori@ontal hori@ontal displacem displacement ent of the inlay& This %ill also resist rotary displacement because of the frictional resistance of the dentin at this point of the cavity& In addition to increased mechanical retention resulting from slight modification of cavity preparations it is essential that suitable gold alloys be used and casting made of such alloys be properly properly heat treated treated in order that their maimum physical properties properties are made available& 'iopulpal Eine 'ngle : This line angle is slightly rounded to dissipate the stresses& Kingival Bevel : 4/;02o to have sliding lap fit *oint cement tooth interface& "ertain forces collectively act on a cemented restoration mainly in the same direction as the path of %ithdra%al& Some of the factors pertaining to these forces are : 7) Magn Magnit itud udee of the the disl dislod odgi ging ng forc forces es : Forc Forces es that that tend tend to remo remove ve a ceme cement nted ed restoration along its path of %ithdra%al are small compared to those that tend to tilt it& Kenerally Kenerally eceptionall eceptionally y stic(y food stuffs stuffs act as a pulling force& force& The $uantum $uantum of vertical vertical and obli$ue forces forces also tend to dislodge dislodge the restoration& restoration& The magnitude magnitude of the dislod dislodgin ging g forc forces es depe depend ndss on the stic( stic(in ines esss of food food occl occludi uding ng and and late latera rall
0.
movement forces of the *a%s and the surface area and teture of restoration being pulled& .) Stre Stress ss "onc "oncen entr trat ation ion : Stre Stresse ssess are are not not unif unifor orm m thro through ughou outt the the ceme cement nt but but are are concentrated around the *unction of the aial and occlusal surfaces 1aio pulpal line angle)& This may may eplain the retentive retentive failure of the cast restoration& The strength of the cement is less than the induced stresses& FORCES ACTING ON DIRECT TOOTH COLOURED RESTORATIONS
For any proimal restoration in anterior teeth there are t%o possible displacing forces& forces& The first is a hori@ontal hori@ontal force displacing displacing or rotating rotating the restoration restoration in a labio; proimo lingual or linguo proimo labial direction& It has its fulcrum almost parallel to the long ais ais of the tooth tooth being loaded& loaded& The second second is a vertic vertical al force force displacing displacing or rotating the restoration proimally1sometimes facially or lingually)& The latter has a loading loading arrang arrangeme ement nt simila similarr to occluso occluso;pr ;proim oimal al 1occlu 1occluso;f so;faci acial al or occluso occluso;li ;lingua ngual) l) restorations restorations in posterior posterior teeth& The amount of teeth teeth depends upon the location location etent and type of occluding contacts bet%een the upper and lo%er teeth during function& The mechanical picture can be summari@ed as follo%s : 7& In ante anteri rior or teet teeth h %ith %ith norm normal al overb overbite ite and over over*e *ett during during centri centricc clos closur uree of the mandible 1from centric relation to centric occlusion) mainly the hori@ontal forces %ill be in action& Those forces if loading the proimal restoration directly %ould try to move it linguo;proimo labially 1for the upper restoration) and labio;proimo lingually lingually 1for the lo%er one)& one)& The magnitude magnitude of the hori@ontal hori@ontal force component component at this stage of mandibular movement is not very substantial and the vertical one is almost nil& In protrusive and lateral protrusive movements of the mandible directly loade loaded d pro proim imal al resto restora ratio tions ns in ante anteri rior or teet teeth h %ill %ill be sub*ec sub*ected ted to subst substan antia tiall hori@ontal as %ell as vertical displacing forces especially in restorations replacing the incisal angel& The results of this loading loading are rotational forces 1previously 1previously described) as %ell as forces rotating the restoration labially and proimally 1for the upper) or lingually and proimally 1for the lo%er)& .& If anterior anterior teeth teeth meet meet in edge;t edge;to;e o;edge dge fashion fashion at centric centric occlusion occlusion loading loading of the proimal restoration involving incisal angles 1"lass ID) %ill %il l be similar s imilar to t o any "lass II proimo proimo;oc ;occlu clusal sal restora restoration tions s i&e& i&e& vertica verticall displac displacing ing forces forces %ith %ith very very limite limited d hori@ontal hori@ontal components& components& This loading %ill %ill continue during during all centric centric closures closures and
04
ecursion ecursion movements movements of the mandible& ?o%ever ?o%ever if the incisal angle is intact 1"lass 1"lass III) these displacing forces %ill be minimal& 4& If the upper upper and lo%er lo%er anterior anterior teeth teeth meet meet such that that the lo%ers lo%ers are labial labial to the uppers uppers in centric occlusion 1'ngleCs "lass III) there %ill be the same type of loading conditions mentioned in 17) ecept the hori@ontal loading %ill tend to rotate or displace restorations labio proimo lingually 1for uppers) and linguo;proimo labially 1for 1for lo%ers)& lo%ers)& #uring #uring ecursi ecursive ve movement movements s if teeth teeth are in contac contactt and there there is a possibility of retrusive mandibular movements the loading %ill be much less than that described in 17) %ith its hori@ontal displacement capability eactly the reverse to that described in 17)& 0& In occlusio occlusions ns %ith deep anter anterior ior overbite overbite and and normal normal or no over*et over*et the hori@ontal hori@ontal type of loading loading %ill be greatly greatly eagger eaggerate ated& d& The vertical vertical displace displacemen ment t although although present %ill be minimal by comparison& 2& In occlusio occlusion n %ith anterior anterior open bite bite or severe severe over*e over*et t or any other conditi condition on that creates a no;contact situation bet%een upper and lo%er anterior teeth during centric occlusion and ecursive movements of the mandible proimal restorations %ill not be loaded directly either vertically or hori@ontally& 8& In cases cases %hen the proim proimal al restora restoration tion of an anteri anterior or tooth tooth is a part part of a mutuall mutually y protective occlusion i&e& an incisor and the ad*acent cuspid are involved in an anterior lateral lateral disclusion mechanism mechanism the teeth and restoration restoration %ill be part of that disclusion mechan mechanism ism %ith ecess ecessive ive hori@onta hori@ontall and vertical vertical loading loading forces forces&&
This This situati situation on
should be properly diagnosed so that the tooth preparation can be designed and prepared accordingly& It should be understood un derstood that none of these loading forces %or( separately& separately& They %or( %or( together together and simultane simultaneously& ously& ?o%ever ?o%ever they they may may differ differ in magnitude at different stages of mandibular movement& It should be mentioned here that a restoration replacing part or all of the incisal ridges of an anterior tooth %ill have the same pattern of loading as mentioned in 17) – 18) but %ith increased intensity& intensity& Eoss of the incisal incisal angle of a tooth tooth i&e& conversi conversion on from a "lass "lass III to a "lass ID represents a ma*or complication in the mechanical problems of anterior tooth restorations& This loss %ill lead to definite direct loading of the restoration loss of the incisal %all %hich %ould normally accommodate one of the t%o possible main internal retentive modes for the restoration definite vertical loading %ith its se$uelae and and the place placeme ment nt of marg margins ins on the the incis incisal al ridge& ridge&
This This furt furthe herr epo epose sess the the
restoration to the maimal loading possible in anterior teeth and it is %ith the 00
minim minimal al tooth tooth struc structur turee to be used used for for resis resista tanc ncee and! and!or or rete retenti ntion on agai agains nstt such such loading& The structu structure re of anterio anteriorr teeth teeth themse themselve lves s have have a compar comparativ atively ely differ different ent stress stress pattern as a result of occlusal loading from that of posterior teeth& The uni$ue shape as %ell as the mechanical structure and function of these teeth is very important to comprehend before designing a cavity and!or tooth preparation for a direct tooth colored restoration& The follo%ing is a summary of these these uni$ue features : a) 'nteri 'nterior or teeth teeth have thei theirr maima maimall bul( gingiva gingivally& lly& They They taper taper incisall incisally y %ith the least bul( at the incisal ridge& So resistance to stress fractures %ill be maimum at the gingival end and decrease incisally& b) Forces are directed hori@ontally and vertically on anterior teeth as mentioned mentio ned %ith the force force analyse analysess on restorati restorations ons for these teeth& teeth&
These These forces forces accumu accumulate late
maimal shear stresses at the *unction of the clinical root %ith the clinical cro%n and maimum tensile stresses at the incisal ridges especially their corners 1incisal angles)& c) The labial labial enamel enamel plate plate is much thic(er thic(er than the lingual lingual or proima proimall ones %ith maimal thic(ness of enamel usually at the incisal ridge& d) The inciso incisors rs may be involved involved in a disclu disclusion sion mecha mechanism nism of the mandib mandible le %ith loading similar to that of the cuspid but to a much lesser etent& e) "ervical "ervical portions portions of anterior anterior teeth teeth %hen they they are affected affected %ith %ith a "lass D lesion lesion or cavity preparation %ill have a stress pattern similar to posterior teeth and the stress pattern pattern is governed governed by the same factors as in posterior posterior teeth& In addition the deeper the overbite is the more induced the stresses are at these cervical areas& f) 'S mentioned mentioned previously previously loss loss of an aial aial angle angle incisal incisal angle angle or or tooth structur structuree at the nec( of the tooth %ill dramaticall dramatically y reduce that toothCs ability ability to resist loading %ithout fracture failure& Idea Ideall lly y a resto restora ratio tion n made made of tooth tooth color colored ed mate materi rial alss should should not not be loaded loaded directly directly i&e& there should be intervening tooth structure bet%een bet%een the occluding occluding tooth and the restoration& restoration& This situation situation can only be achieved by four intact intact %alls surrounding surrounding the resto restora rati tion& on&
,nfor ,nfortun tunat atel ely y this this is usual usually ly not the case& case&
That That is %hy %hy the clinica clinicall
performance of tooth colored materials differs d iffers from one situation to another sometimes dramatically&
02
'nterior 'nterior teeth have their their maimal maimal bul( gingivally& gingivally& They taper incisall incisally y %ith the lest bul( at the incisal incisal ridge& So resistance resistance to stress fracture fracturess %ill be maimum at the gingival end and decrease incisally& Forces are directed hori@ontally and vertically on anterior teeth as mentioned %ith the force analyses analyses on restoration restorationss for these teeth& teeth& These forces forces accumulate accumulate maimal maimal shear stresses at the *unction of the clinical root %ith the clinical cro%n and maimum tensile stresses at the incisal ridges especially their corners 1incisal angles)& The labial enamel plate is much thic(er than the lingual or proimal ones %ith maimal thic(ness of enamel usually at the incisal ridge& The incisor may be involved in a disclusion mechanism of the mandible %ith loading similar to that of the cuspid but to a much lesser etent& Idea Ideall lly y a resto restora ratio tion n made made of tooth tooth color colored ed mate materi rial alss should should not not be loaded loaded directly directly i&e& there should be intervening tooth structure bet%een bet%een the occluding occluding tooth and the restoration& This situation can only be achieved by four intact %alls surrounding the resto restora rati tion& on&
,nfor ,nfortun tunat atel ely y this this is usual usually ly not the case& case&
That That is %hy %hy the clinica clinicall
performance of tooth colored materials differs form the situation to another sometimes dramatically& FORCES ACTING ON POSTS
'n endodontically endodontically treated treated tooth has been structurally structurally compromised compromised by caries caries and its removal prior restorations and finally endodontic preparation and filling& It should be emphasi@ed again that posts are only used for retaining the restorative material in the remaining tooth structures and by no means %ill they reinforce or improve the strengths of these tooth structures& Because the retention of posts is accomplished by various means it might be epected that different stresses are associated %ith post installation& +ith posts retained by the cement alone the main potential for installation induced stresses is the build up of hydrostatic hydrostatic bac( pressure& pressure& This potential potential %ith parallel parallel – sided post is circumvented circumvented by means of longitudinal vents along the posts %hich provide an outlet for the pressure& Tapered post are self;venting and conse$uently there is no pressure build up& 3ndodontic posts provide a protection function through their ability to distribute the forces forces of mastication mastication to the remaining remaining tooth structure structure&& ?o% %ell this protection protection is achieved depends upon post design embedment depth and diameter&
08
MECHANICAL ASPECTS OF POST9RETAINED RESTORATIONS AND FOUNDATIONS : A3 Stressin1 C"p"-ilities o, Posts :
The follo%ing features and factors of posts and the involved tooth %ill govern the stress patter induced in the surrounding tooth structures due to the use of posts as retentive means : 7& Type of Posts : Parallel sided posts %ill have the tendency to evenly distribute the forces it receives at and around its cavity end onto the root canal %alls if these forces are applied parallel 1a) to the post ais 1vertical occlusal loading& IF the forces applied are at a right angle 1b) or obli$ue 1c) to the post ais the induced stresses in the root canal %alls %ill be unevenly distributed i&e& there is a great possibility of stress concentration due to uneven thic(ness of the root canal %alls around the post 1root taper) %hile the post remains remains the same diameter& diameter& This leads to a thin sectioned sectioned %all at the very very apical end of the post& On the contrary contrary taper sided posts posts and combination combination type posts %ill concentrate concentrate stresses due to apical loading 1a) in the root canal %alls resulting from its %edge shape& Eateral loading on and around cavity ends of the post ho%ever %ill induce evenly distributed stresses in the root canal %alls for the taper of the post %ill correspond %ith the root and root canal taper leading to an even thic(ness of %alls occlusoapically& .& Method of Inserting root canal posts : #uring insertion of a post into the root canals highly threaded posts can induce ten times the amount amount and etent of stresses as smooth sided posts& Serrated surface posts %ill induce about one and a half to t%o times the stresses that are induced by smooth surfaced posts& This can be eplained by the cemented cemented techni$ue utili@ed by the serrated and smooth surfaced posts& 4& Bul( of dentin in root canal %alls : -aturally the bul(ier bul( ier that the dentin surrounding a root canal post is the less %ill % ill be the induced stresses per unit volume during the post insertion and functional use of the post retained retained restoration& restoration& It has been estimated estimated that a minimum of . mm of dentinal root canal %all should surround a post so that the stresses induced there %ill not lead to dentinal failure in the form of crac(s and gross fracture& 0& Eength of clinical root involved %ith the root canal post : 09
'lth 'lthou ough gh the the toot tooth h to rece receiv ivee a root root cana canall post post shou should ld be non; non;vi vita tall and and endodontically treated the clinical cro%n portion of the tooth is much more dehydrated than the clinical root portion as the dentin portion of the root still receives some fluids from the ad*acent ad*acent periodontal ligament& The more dehydration dehydration that there is the less %ill be the modulus of resilience and elasticity of the dentin and conse$uently the less %ill be the dentinCs ability to absorb and resist stresses %ithout failure& 2& Ferrule or embracing features of the restoration : Post;c Post;core ore and do%el do%el coping coping foundat foundations ions for endodon endodontica tically lly treate treated d teeth teeth %ill al%ays induce stresses in the root canal %alls and remaining tooth structures %hich can only be counteracted by embracing the buccal and lingual cuspal elements of the tooth and!or banding 1circumferential embracing) the tooth at its most apical part of the clinical cro%n 1i&e& area area of maimum stresses)& Such bracing is referred to as the Ferrule effect effect&& The Ferrul Ferrulee featur featuree of the restora restoration tion should should involve involve at least least . mm of cro%n length to counterac counteractt stresses induced induced by the post& ,sing less than than . mm of cro%n tooth structure the counteracting Ferrule effect %ill be reduced& The closer this embracing feature is to the *unction bet%een the clinical cro%n and the root the more effective it %ill be& This is the ma*or protecting protecting feature feature against against induced induced stresses in a restoration restoration for endodontically treated teeth& 8& Eateral Eoc(ing Mechanisms for the post and restoration : Because most premade posts are rounded in cross;section there is a great tendency for the post and and the restoration retained retained by the post to rotate under torsional forces& This rotational rotational tendency tendency can induce unnecessary unnecessary stresses stresses in remaining tooth structures& structures& The presence of a method to loc( the post and the restoration against such rotation 1e&g& a lateral pin internal boes opposing %alls etc) %ill drastically reduce the effect of torsional forces& 9& Presence of a pulp chamber %ith pronounced %alls : +all +a llss of the the pulp pulp cham chambe ber r espe especi cial ally ly if they they are are oppo opposin sing g each each other other %ill %ill increase the frictional retention of the foundation or restoration minimi@ing the retention demands on the pot and thereby minimi@ing stresses in the root canal %alls& 6& Presence of intact marginal or crossing ridges : These ridges ridges %ill act as a binder bet%een the buccal and lingual cuspal elements elements resulting n better distribution and resistance of induced stresses& 5& Proimity of the post to the root canal filling : 06
oot canal fillings should not be involved in the mechanical problems of the posts& For this reason there should sho uld be a space bet%een the apical api cal end of the post and the occlusal occlusal end of the root canal canal filling& filling& IF the post approimat approimates es the root canal canal filling forces can be transmitted to that filling %hich mechanically is made of very %ea( materi materials als and lead lead to profou profound nd straining& straining& This This can move move the post in an undesirabl undesirablee direction direction and it may induce unnecessa unnecessary ry stresses stresses in the remaining tooth tooth structure& structure& In addi additi tion on the the dire direct ct or indi indire rect ct load loadin ing g of the the root root cana canall fill fillin ing g may may chan change ge its its relationship to the surrounding %alls and apical anatomy resulting in endodontic failure& 7/& Presence of flat planes in the remaining tooth structures at a right angle to occluding forces : Flat planes in the form of tables gingival floors and ledges etc %hich %ill be able to receive and resist occluding forces before arriving to the post are the second ma*or feature feature used to reduce reduce induces stresses stresses in the remaining remaining tooth structure& structure& Besides Besides partially protecting the post from direct loading these flat planes %ill protect a very %ea( subpulpal floor from being directly loaded& 77& Presence of lateral %alls in the remaining tooth structure : 3tra or intracoronal aial %alls that %ill receive and resist laterally applied forces on the restoration before they arrive at the post %ill drastically reduce stresses I the remaining %ea(ened tooth structure primarily in the root canal %alls& 7.& The root post portion relative to the cro%n post portion : The ideal ratio is to have the root portion of the post t%ice as long as the cro%n portion i&e& a ratio of .:7&
Eess than that especially less than a ratio of 7:7 %ill
definitely concentrate intolerable stresses on the lateral %alls of the root canal ad*acent to the apical end of the post& 74& ?ydraulic pressure during post cementation : If there are no lateral vents in the post or if the post diameter is very close to that of the post channel channel diameter the semi;li$uid semi;li$uid cement mi during the cementation cementation of the posts may eert tremendous amounts of hydraulic pressure that eceed the elastic limit of the surrounding dentin or prevent complete seating of the post& 70& Surface teture and shape of the root end of the post : Kreater post surface roughness and!or the presence of a chisel %edge or irregular conf config igur urat atio ion n on the the root root end end of the the post post incr increa ease sess the the poss possib ibil ilit itie iess of stre stress ss concentratio concentration n on the root canal %alls& %alls& The concentration concentration of these stresses stresses %ill increase increase %ith increasing proimity of the pot to the involved root canal anatomy& 05
72& The length of the post relative to the entire length of the root : Kenerally spea(ing the more that the root canal length is involved %ith a post the more evenly distributed and the better resisted the stresses %ill be in the root canal %alls& On the other hand the apical one third of root canals usually have a very limited thic thic(ne (ness ss of dentin dentin %alls& %alls&
By plac placing ing the tip of the the root root pot there there %ith %ith atte attenda ndant nt
possibilities of substantial stresses being concentrated at that tip catastrophic failures become inevitable&
's a rule from one half to t%o thirds of the root canal should
encapsulate the post if the forces transmitted by the post are to be ade$uately dissipated& 78& Shape of the post in cross section relative to the shape of the post channel : ' post post shoul should d have have a circ circum umfe fere renc ncee that that coin coinci cide dess %ith %ith the post post chan channe nel& l& #iffer #ifferenc ences es e&g& rounde rounded d post in an oval post channe channel l %ill concentr concentrate ate stresse stressess at isolated locations in the root canal %all possibly eceeding the local brea(ing point of the dentin& 79& Eoose post in the post channel : ,nconfined movements of a post %ithin a root canal can eaggerate stresses in the root canal %alls upto the fracture point of dentin& 76& Post ending apically at the *unction of the clinical root %ith the clinical cro%n& This specific location is an area of appreciable stress concentration in normal sound sound teeth& teeth& +ith +ith root canal canal thera therapy py the the strength strength of the the area area is decreas decreased& ed& If in restoring a tooth the apical end of the root canal post is placed at this *unction a %hen the clinical cro%n is far longer than the anatomical cro%n three problems %ill be concentrate concentrated d at these locations& locations& Eess strength strength than normal normal 1due to a decrease decrease in bul( resulti resulting ng from from the postCs postCs taper) taper) above above normal normal stress stress concen concentra tration tion result resulting ing from from a reduced reduced cro%n root ratio and maimum stresses stresses from the apical end of the post as it is in effect the end end of a level& These stresses may approach the failure level of the dentin& 75& "entral Slitting of Posts : Eength %ise slitting of a post involving one half or more of its length %ill ma(e the post elastically collapsible in a lateral direction& If such a post is a threaded type and during threading threading into the root ecessive stresses stresses are induced induced at the post dentin interface instead of these stresses being consumed in detrimental deformation of dentin they may be consumed in part to partially close the central slit& The rigidity of the t%o parts of the post at this area %ill (eep the post engaged in dentin for retention and their elasticity %ill reduce the stress concentration in that dentin& ./& Thread numbers and patterns : 2/
"ontinuous threads from one end of a post to another create more stresses than interrupted interrupted threading& threading& The greater greater that the spacing spacing is bet%een threads threads the less %ill be the attendan attendantt stresses& stresses& The sharper sharper that the threads threads are are the less less %ill be the stresse stresses& s& "ircumferentially interrupted threading creates less stresses than continuous threading& The %ider and more fre$uent that the interruptions are the less %ill be the stresses& Interruptions 1cross cuts) further serve to facilitate escape of debris during post insertion& The more etended that the threads are laterally the more the surface interfacial contact %ith dentin %ill be and conse$uently the higher the stresses& FORCES ACTING ON A CAST METAL AND PORCELAIN RESTORATIONS BIOMECHANICAL PRINCIPLES OF PREPARATIONS:
The design and preparation preparation of a tooth for a cast metal or porcelain restoration restoration are governed by : 7) Preser Preservati vation on of tooth tooth structu structures res&& .) etenti etention on and and resist resistanc ancee forms forms 4) Struct Structura urall durabilit durability y of the restorat restoration ion 0) Marg Margin inal al inte integr grity ity 2) Preser Preservati vation on of the the periodo periodontiu ntium& m& ' restoration can meet its functional biological and esthetic re$uirements if it remains remains firmly attached attached to the tooth& Its capability capability for retention retention and resistance resistance must be great great enough enough to %ithsta %ithstand nd the dislodging dislodging forces forces it %ill encounter encounter in function function&&
'n
estimate as to the prevailing occlusal forces can be had by noting the degree of %ear on the other teeth firmness of the opposing teeth thic(ness of the supporting base and the bul( of masticatory muscles& RETENTION AND RESISTANCE :
If a restoration does not remain firmly attached to the tooth it cannot meet its functional biological and esthetic re$uirements&
Its capability for retention and resistance must be great enough to %ithstand the dislodging forces it %ill encounter in function&
Some estimate estimate of the prevailing prevailing occlusal forces in an individual individual patient can be made by noting the degree of %ear on the other teeth the firmness of the opposing teeth the thic(ness of the supporting bone and the bul( of the masticatory muscles&
27
It is the geometric form that determines the orientation of the tooth;restorations interfaces to the direction of forces encountered& This in turn determines %hether the cement in a given area %ill be sub*ected to tension shear or compression&
'll cements cements ehibit their greates greatestt strength under compres compression& sion& They are %ea(est %ea(est under tension %ith the value for shear strength lying in bet%een&
+her +heree a part part of the the resto restora rati tion on is pulle pulled d dire direct ctly ly from from the tooth tooth sepa separa rati tion on is prevented only by the relatively %ea( tensile strength and adhesive properties of the cement&
If the applied force is parallel %ith the cement film movement at the cement;tooth and cement;metal interfaces is more effectively impeded by the minute pro*ections of cement into the surface irregularities than %hen the force is tensile in nature&
Movement %ithin the cement film itself is resisted by its relatively greater shear strength&
' force directed at an angle to%ard the restoration has one component parallel %ith and and one one comp compone onent nt perp perpend endic icula ularr to the the *oine *oined d surfa surface ces& s&
Thus Thus the the ceme cement nt is
sub*ected to a combination of shear and compression and movement is resisted ore effectively than if the forces %ere purely tensile or shear in nature&
' compressive force perpendicular to the cement film can produce no movement of the restoration relative to the tooth unless it is great enough to crush the cement or deform the structures& Such forces are seldom encountered encountered in function&
etention and resistance can be maimi@ed by shaping the preparation so that as much of its surface as possible %ill eperience compression and shear %hen the restoration is sub*ected to an unseating force&
RETENTION :
It is the ability of the preparation preparation to impede removal of the restoration along its path of insertion&
,nder this condition the cement bond sub*ected to tension and shear&
' restoration can eperience %ithdra%ing forces along its path of insertion during mastication of stic(y foods&
There are 0 factors under the control of the operator during tooth preparation %hich influence retention& i)
#egree of taper 2.
ii)
Tot Total su surface ar area of of th the ce cement film
iii) ii)
'rea of cement und under shear
iv)
oughn ghness of th the too tootth su surface
i& De1ree o, T"per :
The more nearly parallel the opposing %alls of a preparation the greater %ill be the retention& Thus retention decreases decreases as taper increases&
?o%ever in order to avoid undercuts and to allo% complete seating of the restoration during cementation the %alls must have some taper&
'n overall taper or angle of convergence of 8 degrees is considered as appropriate i&e& approimately 4 degrees being produced on each surface eternal or internal by the sides of a tapered instrument&
ii& Tot"l S/r,"(e Are" o, Cement Film :
The greater the surface area of cement film or the of the preparation the greater the retention of the restoration&
The total surface are of preparation is influenced by the si@e of the tooth the etent of coverage by the restoration and features such as grooves and boes that are placed in the preparation&
iii& Are" /n.er s!e"r :
More important for retention than the total surface area is the area of cement that %ill eperience shearing rather than tensile stress %hen the restoration is sub*ected to forces along the path of insertion&
To decrease the failure potential it is essential to minimi@e tensile stress&
For For the shea shearr stren strengt gth h of the ceme cement nt to be utili utili@e @ed d the prep prepar arat ation ion must must have have opposing %alls i&e& t%o surfaces of the preparation in separate planes must be nearly parallel %ith each other an the line of dra%&
To obtain the greatest area of cement under shear the direction in %hich a restoration can be removed must be limited to essentially one path&
Thus the addition of parallel sided grooves limits the path of %ithdra%al to one direction thereby reducing the possibility of dislodgment&
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The length and %idth of the preparation is an important factors in retention : a long preparation as %ell % ell as %ider preparation has greater retention than does a shorter sh orter or a narro%er preparation&
i& S/r,"(e Ro/1!ness :
'dhesion of dental cements depends primarily on pro*ections of the cement into micros microscopi copicc irregul irregulari aritie tiess on the surfaces surfaces to be *oined& *oined& Theref Therefore ore prepare prepared d tooth tooth surface should not be highly polished&
RESISTANCE :
it is the ability of the preparation to present dislodgment of the restoration by forces directed in an apical obli$ue or hori@ontal direction&
+here there is effective resistance much of the cement film %ill be placed under compression although some parts %ill be still be sub*ected to tension and shear&
If the cement film is disrupted by the restorations sliding or tipping on its preparation the smallest fraction of a millimeter the restoration is doomed through percolation of fluids dissolution of the cement and recurrent caries&
esistance to sliding and tipping must be designed into a preparation by forming %alls to bloc( the the anticipated anticipated movements& movements& The more nearly nearly perpendicul perpendicular ar it lies to the force the greater is the resistance provided by the supporting surface because the cement %ill be compressed and failures are less li(ely to occur form compression than shear&
Leer"1e "n. Resist"n(e :
The strongest forces encountered in function are apically directed and can produce tension and shear in the cement film only through leverage&
Eeve Eevera rage ge prob probab ably ly the the pred predom omin inan antt fact factor or in the the disl dislod odgm gmen entt of cem cemente ented d reiterations occurs %hen the line of action of a force passes outside the supporting tooth structure or %hen the structures fle&
If the line of action of force passes %ithin the margin of a restoration there %ill be no tipping of restoration restoration&& The margin margin on all sides of the restoration restoration is supported by the preparation& The tor$ue produced merely tends to seat the cro%n further&
If the line of action of force passes outside the margins of restoration the occlusal table of the restoration is %ide even a vertical force can pass outside the supported 20
margin margin and produce produce destruct destructive ive tor$ue& tor$ue& This can also also occur occur in cro%ns cro%ns on tipped teeth&
' force applied to a cemented cro%n at an obli$ue angle can also produce a line of action %hich %ill pass outside the supporting tooth structure&
Prep"r"tion Prep"r"tion Len1t! "n. Resist"n(e :
The ability of a restoration to resist tipping depends not only on the preparation but also on the magnitude of the tor$ue
If t%o cro%ns of une$ual length on t%o preparations of e$ual length are sub*ected to identical forces the longer cro%n is more li(ely to fail because the force on it acts through a longer lever arm&
Resist"n(e "n. Toot! *i.t! :
' %ide preparation has greater retention than a narro%er one of e$ual height&
T"per "n. Resist"n(e :
The resisting area decreases as the preparation taper increases&
The The %all %allss of a shor short t %ide %ide prep prepar arat ation ion must must be (ept (ept near nearly ly para paralle llell to achie achieve ve ade$uate resistance form&
Rot"tion "ro/n. " erti("l "#is :
+hen a cro%n is sub*ected to an eccentric hori@ontal force movements of tor$ue occur around a vertical as %ell as hori@ontal ais&
It is possible for a full cro%n on a cylindrical preparation to rotate enough to brea( the cement bond before may compressive resistance is encountered&
Keometric forms such as grooves or <%ings= increase resistance by bloc(ing rotation around a vertical ais&
P"t! o, Insertion :
The path of insertion for posterior full and partial veneer cro%n is usually parallel %ith the long ais of the tooth&
' tipped tooth must be handled differently& 22
If the path of insertion on a tipped tooth parallels the long ais the cro%n %ill be prevented form seating by those parts of the ad*acent teeth %hich protrude into the path of insertion&
The correct path of insertion for such a tooth is perpendicular to the occlusal plane&
'll negative taper or undercut must be eliminated or it %ill prevent the restoration form seating&
O((l/s"l Re./(tion :
It should reflect the geometric inclined planes underlying the morphology of the finished cro%n&
'void creating steep planes %ith sharp angles since these ca increase stress and hinder hinder complete complete seating seating of the casting casting&& To diminish diminish stress stress round the angels angels and avoid deep grooves in the centre of the occlusal surface& FORCES ACTING ON PARTIAL $ENEER CRO*N
Since vertical occlusal occlusal forces forces have hori@ontal resultants resultants displacing forces have a tendency to tip or rotate a restoration usually the tipping is in the lingual direction and rotation occurs mesiolingually or distolingually& Observe the proimal groove of a force LPC directed lingually %hich is applied at the incisal incisal edge& It %ill have a tendency tendency to tip the casting out of the cavity cavity turning turning on the fulcrum LfC& The resistance to the displacement is offered by a rib of L'a %hich lies in the aial groove LabC also by that part of the aial %all lying lingually to the aial groove and encompasses area e and s& It is observed that the lingual %all of the mesial groove does not furnish any resistance to lingual displacement& Since the plane of this %all lies in the tipping path of arc L"C for this reason the incisal edge is usually prepared in a plane and not %ith a groove& +hen force P is applied mesiolingually to the marginal ridge of the upper central the tendency is to rotate the restoration out of the mesiolabial %all of the cavity point LFC actin acting g as the centr centree of rota rotatio tion& n&
Obvio Obvious usly ly then then the resist resistan ance ce to this this rota rotatio tiona nall
displacement is furnished by the distoproimal groove and that portion of the proimal surface lying %ithin the arcs 7 and .&
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'nalogous resting forces are present %hen acting forces are in a distolingual direction then the distolabial %all act as to point of rotation the mesio proimal %all and groove furnish the resistance to displacement& There There is a hori@o hori@ontal ntal force force LPC applied applied distall distally y to the incisal incisal area& area& This This has a tenden tendency cy to tip the casting casting mesially mesially rotati rotating ng on point point LFC& LFC& This displac displacing ing force force is resisted by the proimodistal groove and that portion of the proimodistal surface lying bet%een 7 and .& The same general displacing forces are present in the posterior partial cro%n as in the anterior partial cro%n but the ability of the preparation to resist displacement is more favourable than in the anterior teeth& The occlusal forces forces may be occlusal hori@ontal or any one component of force indicated by LPC& Minimum problems eerts %hen the force is vertical for the resistance is e$ual and opposite& +hen the force force tends to displace the casting lingually it does so along the paths 7 . and 4 %ith its rotation center at point F& esistance in this displacing force is furnished by the ribs of gold lying %ithin the aial groove and by that portion of the proimal and surface etending lingually from the proimal groove and lying %ithin the areas 7 . and 4& In addi addition tion the the occlu occlusal sal surface lying to plane . – 4 offers resistance %hen force LPC is applied in mesiolingual direction the tendency is to rotate the casting mesiodistally %ith the rotation centre being point F the mesiobuccal %all& The resistance is this displacing force is developed by the rib of gold lying into distoproimal groove and by the portion of the casting coming in cont contac actt %ith %ith the the proi proima mall surf surfac acee lying lying bet% bet%ee een n the area areass 7 and .&
'dditional 'dditional
resistance to displacement is offered by the occlusal inclined planes 4 0 and 2& +hen force P is applied in the distal direction the tendency is to rotate the casting occlusally %ith its dislodgment along the areas 7 and . %ith F serving serving as the point of fulcrum& The resistance to this displacing force is furnished by the rib of gold lying in the mesio proimal proimal groove and also by the buccal buccal and mesioproima mesioproimall %alls lying %ithin the areas 7 and .&
FULL CO$ERAGE CRO*N 8ANTERIOR PORCELAIN AC+ET CRO*N& :
's mentioned earlier the anticipated forces place don the restoration cannot be ta(en lightly& 29
placed at right angles to the In(is"l Re./(tion : In (eeping %ith the rule that planes are placed applied forces forces the incisal incisal edges of the upper anterior anterior teeth slopes lingually lingually %hereas %hereas that of the lo%er teeth slopes labially& The incisal reduction should be ade$uate to ensure clearance in protrusive movements of the mandible and permit satisfactory esthetics and enhance optimal function& ;.;2o
parall parallelis elism m is desire desired d %ith %ith proim proimal al reduct reduction& ion&
'fter 'fter the the removal removal of
enamel the labial gingival termination is made at or *ust above the crest of the labial gingiva& 'nother retention area area is immediately belo% the cingulum& More than any other restoration porcelain *ac(et cro%n depends on its tooth preparation& Tooth support is more critical to fracture resistance of the restoration than is the bul( of porcelain& porcelain& The crescent crescent moon fracture fracture seen on the labial cervical cervical region region is a direct result of inade$uate preparation length& Incisal reduction recommended recommended ranges from 7&2 to . mm& For esthetic result it is best to reduce the incisal edge by . mm to the level of depth orientation grooves& 'ny greater reduction %ill increase the stress on the facial surface %hich can result on the facial facial half moon fracture& fracture& The plane of the reduced incisal incisal surface should should parallel %ith the the form former er incis incisal al surf surfac acee and more more impor importa tantl ntly y perp perpen endic dicul ular ar to the forc forces es of mastication& Failure to create this near incisolingual bevel 102o") %ill produce ecessive stress at the shoulder& shoulder& 1Shoulder 1Shoulder /&2 – /&9 mm) Plane Plane of shoulder shoulder is perpendicular perpendicular to long ais ais of the tooth& tooth& If it is at obtuse obtuse angle possibi possibiliti lities es of fracture fracture at the cervical cervical region is more& The The mesi mesioa oai ial al and and dist distoa oai ial al %all %allss are are more more favo favour urab able le for for deve develo lopi ping ng parallelism to frictional resistance& The buccal and lingual surfaces due to their natural contour do not afford the same opportunity for paralleling %alls& The occlusal planes are repr reprodu oduce ced d at a lo%e lo%err leve level& l&
Thes Thesee plane planess help help consid consider erab ably ly to resi resist st stab stabili ility ty or
displacement displacement&& +hen necessary necessary and %here indicated indicated additional additional resistance resistance form may be obtained by placing pins grooves or boes in any available surface %here the length of this surface is ade$uate& If an occlusal force is directed LPC buccally the lingual portion of the cro%n tends to be dislodged occlusally and buccally %ith the point of rotation situation at LFC this displacement is resisted by that lingual surface %hen it lies outside the arc 7& On the other hand an occlusal force P. directed lingually is li(ely to dislodge the cro%n lingually since the buccal %all of the preparation lies %ithin the tipping path of the arc ?.& 26
+hen such a condition prevails either in the buccal or lingual %all resistance to displacement can be developed by placement of . proimal grooves at K in the mesial and distal surface as sho%n in the figure and it is evident that an occlusal force P directed mesially mesially %ill not dislodge the cro%n since the distal %all of the preparation preparation lies outside the tipping path of arc 7& CONCLUSION
Optima Optimall functi functiona onall capaci capacity ty and stabili stability ty of occlusa occlusall relatio relationshi nships ps are ma*or ma*or considerations in every phase of restorative dentistry& The first phase ob*ective of a cavity preparation design is to establish the best possible configuration that can cope %ith the distribution and magnitude of stresses in tooth structure structure and restoration restoration %ithout failure& failure& To design such a configuration configuration one must first comprehend the nature of loading and resistance to such loading& estoration not only mechanically replace the lost part but acts as a medium through %hich physical physical and mechanical mechanical forces forces are transmitted transmitted to the tooth and investing tissues& 3ach tooth ahs its o%n stress patterns& ' thorough (no%ledge in dental materials is necessary to understand the physical properties including their response to stress& Before any restorative procedure al%ays chec( location of the tooth in the arch and the patients patients occlusal relationshi relationship& p& The functional functional non functional functional cuspal elements elements shou should ld be note noted d bye bye eam eamin inin ing g the the invo involv lved ed teet teeth h duri during ng stat static ic and and func functi tion onal al mandibular mandibular movements movements The operator operator can then recogni@e recogni@e the nature of stresses stresses that can be epected in the remaining tooth structure especially the occluding ones& From From these these inform informati ationC onCss obtaine obtained d during during the patient patient evalua evaluation tion the operat operator or must envision the restoration restoration replacing replacing lost tooth structure structure being sub*ected sub*ected to functional functional loading and then try to plan the best tooth preparation to both retain this restoration and ma(e it resistant to these loads&
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