DESIGNING OF KENNEDY CLASS I AND
CLASS II DENTURE
INTRODUCTION Before the designing of a partial denture is commenced,it is essential that all the necessary informationshould be available. Included is a full knowledge knowledge of the clinical conditionof the patient. On that basis it is essential thatpartial denture designing should be undertaken bythe dental surgeon responsible for the patient'scare. It is both unwise and unfair for partial denture designing to be delegated to a dental technicianwho will not be in possession of the necessaryclinical findings.Caution must also be sounded concerning thedesign procedure which is adopted. All too oftenthis has involved scanning through the pages pages of atext book until an example example of design is foundwhere the missing teeth coincide with those in thecase under consideration. A request is then madeto the technician to produce a denture of similardesign. Such a procedure is dangerous since it cannottake cannottake into consi deration the ruling clinical conditionsof the case and may result in irreparabletissue damage.The only approach which is acceptable is that ofdesigning from first principles. This involvesgoing through a series of stages in a step -by-stepmanner. If the order and nature of these stages isunderstood, even the beginner should experienceno difficulty in producing successful partial denturedesigns. DESIGN STAGES There are five stages involved in partial denturedesign: 1. Classify and outline the saddles. 2. Provide support. 3. Provide direct retention. 70
4.
Provide, where necessary, indirect retention.
5. Unite the various components of the denturewith connectors. Classify and outline the saddles Note should be made of the number number of saddleswhich are present, their length and their dispositionin the arch. Each saddle should first beclassified as being either bounded or free-end. Asecond stage in the classification of each saddle isthen to decide on the way that the load borne bythe saddle is to be transferred to the available supportingtissues. This secondary classification of thesaddles is very important since it will influenceprocedures in a number of the subsequent stagesof design and will also play a major part in decidingthe success of the denture.With bounded saddles, the choice lies between: 1. Tooth support. 2. Mucosa support. 3. Combined tooth-and-mucosa support. With free-end saddles, the choice is limited to:' I. Mucosa support. 2. Combined tooth-and-mucosa support. The factors taken into consideration when decidingthe secondary classification of saddles are: 1. An estimate of the response to loading of theavailable supporting tissues. 2. The jaw for which the denture is to be provided(i.e. maxilla or mandible). 3. The length of the saddles. 4.
The number of saddles to be carried by thedenture.
Factor 1
71
Information on the likely response to loading othe abutment teeth and of the mucosa and underlyingbone in the saddle areas will be availablefrom visual, tactile and radiographic
examinationof
the
oral
tissues.
Where
the
abutment
teeth
areperiodontally sound, a bounded saddle may betooth -supported. This is preferred where
possiblesince
it
develops
optimal
masticatory
efficiency.The
second
alternative, that of making making a boundedsaddle boundedsaddle mucosa mucosa -supported, should be consideredwhere the periodontal condition of the abutmentteeth is suspect and where it is judged that the tissuesunderlying the saddle
ould respond w ould
favourablyto
load-bearing. The latter requirement wouldbe met by: 1. The presence under the saddle saddle of a healthymucosa of normal and even thickness. 2. The alveolar bone in the saddle area possessinga dense cortical plate overlying welldevelopedtrabeculae in the cancellous bone. 3. The absence of a history of rapid alveolarbone resorption in the saddle area. The third alternative of choosing a combinationof tooth -and-mucosa support for a bounded saddleis to be deprecated. It introduces the problem ofproviding equitable distribution of loading betweenthe two supporting tissues and is rarelynecessary in the bounded saddle situation.When deciding on the secondary classification offree end saddles, it should be noted that ideallythey should be be mucosa -supported. This avoids therisk of overloading of the abutment tooth and unevenloading of the soft and hard tissues in thesaddle area which tooth-and-mucosa support maycause. The ideal situation may, though, only beattainable by the introduction of undue complexityinto the design. In such cases the compromiseapproach of using combined tooth-and-mucosasupport may be selected. This should only be consideredif the periodontal condition of the abutmenttooth is satisfactory. Factor 2
72
In the mandibular mandibular denture, it is desirable thatbounded saddles be made tooth supported. This isnecessitated by the very limited area of mucosawhi ch is available in the mandible to provide support.If bounded saddles in the mandible are mademucosa-supported, they are liable to be traumaticto the underlying tissues. Where the abutmentteeth of a mandibular bounded saddle are judgedto be unable to carry the load that will be transferredto them by the saddle, it may be necessaryto extract one or more of the teeth until tooth support becomefeasible.In the maxilla, different conditions prevail,since here the hard palate provides provides a sizeable area of mucosa which can be used to help support thedenture. Hence either tooth support or mucosasupport is acceptable for bounded saddles in themaxilla. Factor 3 Where long bounded saddles are present on amaxillary denture, they are often better mademucosa-supported to avoid the risk of overloadingof the abutment teeth. Factor 4 The presence of multiple bounded saddles in amaxillary denture often indicates the desirabilityfor them to be made mucosa-supported to avoidover-complexity in denture design.On completion of the cl assification of the saddlesthey should then be outlined. In the case ofsaddles of the mucosa -supported or tooth-andmucosatooth-andmucosa supported classes, the outline developedshould be that which will provide coverage of themaximum possible area of mucosa. Labially, bu ccallyand lingually, flanges should be extended tolie at the base of the sulcus defined by a functionallymoulded impression. This ensures that theload applied to each unit area of covered mucosawill be of minimal value. The risk of evoking apathological re sponse in the covered
tissues
isthereby
reduced.With
tooth -supported
saddles, the
need
formaximal mucosal coverage to spread the appliedload over the mucosa does not
73
apply. Here, saddleoutline is dictated by aesthetic needs. The extensiondeveloped is that necessary to provide restorationof alveolar form and will be proportional to the degree of alveolar resorption that has occurredin the area.In the case of all classes of saddle, the outlinedecided by application of the above considerationsmay need to be modified by two further criteria: 1. Aesthetics Aesthetic considerations may necessitate the use ofan open -face structure in the anterior maxillaryarea of a denture, even where other factors indicatethe use of a labial flange. 2. Undercuts Note must be made of the presence and extent ofundercuts in areas where flange extensions of saddlesare to be positioned. The flange outline developedmust allow free insertion and withdrawal ofthe denture. ovide suppor t Pr ovide
To be successful, a partial denture must be satisfactoryat the time of insertion and must maintainthat condition throughout its normal period of lifeexpectancy. Providing proper allowance for supportduring the designing of a partial denture isessential if this requirement is to be met. met. Within ayear or two o f the insertion of a partial denture, itis usual to find that some degree of alveolar resorptionhas occurred under the saddles. Unless properprovision has been made for support this mayallow the fitting position of the saddles to change. When this occurs, the artificial teeth carried by thesaddles may lose contact with the opposing opposing dentition, so impairing impairing masticatory masticator y efficiency. A furtherserious consequence of a change in the position ofthe saddles is that
it
may
be
components(e.g.,
accompanied connectors
or
by
achange
clasp
74
arms).
in
position
Because
of of
other
denture
this,these
other
components may become traumatic tothe soft and hard tissues which they contact. Improperlysupported dentures which give rise tothis phenomenon have rightly been termed 'gumstrippers'. To avoid the risk of such consequencesoccurring, the following procedures should beapplied: rovision P rovision
of support for bounded saddles
Where bounded saddles are to be mucosasupported,the whole denture - including thesaddles - should cover the maximum possiblearea of mucosa compatible with other requirements.It should be remembered that it is normallyonly in the maxilla (via use of the hard palate)that sufficient area of mucosa will be available toallow bounded saddles to be made made mucosasupportedw ithout risk of undue trauma to themucosa arising.Where tooth support of bounded saddles is required,this is achieved by the placement of a reston each of the abutment teeth. To ensure that theload borne by the saddle will be transferred fullyto the abutmen t teeth, the rests must provide arigid attachment between the saddle and the teeth.This requirement is met by placing each rest onthe portion of the abutment tooth immediatelyadjacent to the saddle. By their correct use, tooth support ofthe saddles of a pa rtial denture can be achievedwhere this is desired.Additional support for the denture may arisefrom components other than the rests. For example,this will occur where connectors are placed onthe hard palate palate or on the anterior teeth. Used alone, such such auxili ary supportive devices will notensure that the saddles are tooth supported. Theyshould be regarded as an addition to, and not asubstitute for, the use of rests to achieve toothsupport in the saddles. rovision P rovision
of support for free-end free -end saddles
Irrespective of whether the saddle is to be mademucosa-supported, or tooth-andmucosa-supported,it must cover the maximum possible area ofmucosa. In the case
75
of a maxillary denture, buccalflanges should extend to the full depth ofthe sulci, the tuberosities should be fully coveredand the posterior extension lies in the pterygomaxillaryfissures. With a mandibular denture,buccal and lingual flanges should extend to the fulldepth of the sulci, with emphasis on achieving fullengagement of the postero-lingual areas. Posteriorly,the saddles should extend over at least theanterior one-third of the retro-molar pads.With a saddle which is to be tooth-and-mucosa supported,there is an additional requirement for arest to be placed on the single abutment tooth. Itis usually desirable that this rest should be placedon the mesial aspect of the abutment tooth. Thisapplies unless the design includes the use of astress-breaker interposed between the rest and thesaddle. In that case the placement of the rest onthe distal aspect of the abutment tooth is permissible. Provide direct retention Direct retention is one of the means by which apartial denture is provided with resistance to displacement. When a patient who is wearing a partialdenture chews foods, especially those of asticky consistency, forces arise which tend to causea bodily translation of the denture away from thesupportive tissues. Forces giving rise to translationmay also be developed by the action of the oraland facial musculature during activities such asspeech and laughter.If translatory displacement occurs, it can clearlybe a source of embarrassment to the patient. Inaddition, the repetitive unseating and reseating ofthe denture which occurs can cause trauma to both The soft and hard tissues contacted by the denture.T he function of direct rretention etention is to developforces which oppose those tending to cause translatorydisplacement. Where the sum of the retentiveforces equals or exceeds that of the displacingforces,
76
translation of the denture will will be inhibited.The inhibited.The developme nt of direct retention to the levelwhere this is achieved is an essential feature ofpartial denture design. The methods available forachieving direct retention are as follows: 1. Utilisation of the forces responsible forachieving retention in complete dent ures. 2. Frictional resistance to displacement arisingfrom the development of tight contacts betweenthe artificial teeth and the natural abutment teeth. 3. Placement of direct retainers on the abutmentteeth. It should be noted that in a given denture it is sometimes possible to achieve adequate direct retentionby the use of only one of these methods.Most dentures, though, will require the use of acombination of two or more of the availablemethods. ethod M ethod
1 of providing direct retention
Included here are: 1. Use of the physical forces arising from thepresence of a saliva film between the denture andthe underlying soft tissues. 2. The engagement of undercuts in the tissuesunderlying the saddles. 3. Shaping of of the denture so that the environmentalmusculature wil l provide a positive retentiveforce. 4.
The force of gravity in a mandibular denture.
The level to which these forces can be developedin a partial denture is often quite low relativeto that achievable in a complete denture. Thisapplies particularly to the development of thephysical forces which will be limited by the decreasedarea of tissue coverage in a partial dentureand by the presence of the natural teeth which in seal. Thus, in only a very limited number of caseswill this method, used alone, provide adequate retentionin a partial denture. It may be consideredin the case of a maxillary denture which carriesonly mucosa supported free-end saddles. In thecase
77
of all partial dentures, though, this methodwill contribute to the overall retention which isachieved. ethod 2 M ethod 2
of providing direct retention
This method can assist the development of directretention where the partial denture carries one ormore bounded saddles. Maintenance of the forcewill be assisted by the use of porcelain artificialteeth in preferenc preferenc e to the use of acrylic teeth, sincethis will reduce the degree of wear which occurs atthe contact points.Methods 1 and 2 used in combination will provideadequate retention for many maxillary partialdentures which carry only mucosa-supportedsaddles. Success will be aided by the presence ofmultiple bounded saddles. ethod 3 M ethod 3
of providing direct retention
The direct retainers which are used may take theform thefor m of either pre -formed (precision) attachmentsor attachmentso r clasp units.Their use will prove to be necessary necessary in nearl y allinstances where a partial denture carries tooth supportedor tooth-and-mucosatooth-and -mucosasupported saddles.As previously noted, this will apply to nearly allmandibular dentures and to a proportion of maxillarydentures.When considering the placement of tooth-bornedirect retainers, it should be borne in mind thateach saddle needs to be provided with adequatedirect retention. That requirement can best be metby placing a direct retainer on each abutmenttooth and this is done wherever possible. Certainconditions may be encountered, though, where departurefrom that principle is indicated. These areas follows: 1. Where the abutment tooth concerned is acentral or lateral incisor, the placement of a claspunit on the tooth as a direct retainer would generally prove to be unacceptable on aestheticgrounds.
78
2. Where it would involve the placement of aclasp unit as a direct retainer on an abutmenttooth which possesses no usable undercuts. In thatcase an adjacent tooth may offer more favourableconditions for placement of the clasp unit. 3. Where it would involve the introduction ofunnecessary complexity into the design of thepartial denture, with associated increased risk ofcausing caries or periodontal breakdown of theabutment tooth. This arises where two shorttooth -supported saddles 'are separated by a singleabutment tooth.In all instances an attempt should be made tokeep the distance between a saddle and its directretainers to a minimum. This is necessary since the principles of the action of levers shows that themore remote a retainer is placed relative to thesaddle the less effective will it be in providing retentionfor that saddle. Pr ovide, ovide, where necessary, indir ect ect r etention etention Indirect retainers are used to assist direct retainersin resisting displacement of partial dentures infu infunction, nction,
when apatient who is wearing wearing a partial denture chews chews
stickyfoodstuffs, forces arise which tend to cause thesaddles to be displaced away from the underlying tissues. Direct retention is provided to preventthis translatory displacement. Where the denturehas been provided with rests to develop the necessarysupport in the second stage of design it will,however, be found that a second method of displacementis possible. This is by rotation of thedenture about one or more axes developed by thesupporting rests in the manner of a see-saw. Theportion of the denture which lies on the saddleside of the axis will move away from the supportingtissues. At the same time, the portion of thedenture which lies on the opposite side of the axisto that of the saddle will be depressed tissuewards. tissue wards. The procedures involved in this stage of partialdenture design are as follows: rocedure P rocedure
1
79
Examine the cast and note the elements alreadyprovided in the preceding design stages. Whererests have been provided in Stage 2 of design, potential axes of rotation will exist which may bedefined as follows: 1. A line joining any two rests which have beenprovided for support of the denture. 2. Where a tooth-and-mucosa-supported free-endsaddle is present, a line passing along the crestof the ridge underlying the saddle and passingthrough the rest on the abutment tooth.The number and disposition of the potentialaxes of rotation must be noted. The direction in which rotation canoccur about a potential axis of rotation should alsobe noted (i.e. clockwise or counter-clockwise).Where more than one saddle is present on a partialdenture the combined action of the saddles canlead to two-way rotation being possible about agiven axis of rotation. rocedure P rocedure
2
Examine the disposition of rests provided in Stage2 of design and and of any tooth -borne direct retainersprovided in Stage 3 of design, in relation to theaxes of rotation.Decide whether or not these elements will provideresistance to rotation about all the existingaxes. In reaching that decision it should be notedthat resistance to rotation can be provided by: 1. A rest placed on the opposite side of the axisof rotation to that on which the displacing saddleis situated. 2. A tooth-bearing direct retainer placed on thesame side of the axis of rotation as that on which the displacing saddle is situated. When Procedure 2 is applied to dentures whichare being designed for dentitions of the KennedyClass II and Class
III
types it is usual to find thatthe elements already
80
provided render the dentureresistant dentureresistant to rotation about any potential axes ofrotation. In that case no further action is necessaryin Stage
4
of design.
With dentitions of the Kennedy Class I or IVtypes, though, application of Procedure 2 willusually demonstrate the existence of one or moreaxes w here resistance to rotation will not be providedby the elements already placed. When the three axes are examined in turn it willbe found that the elements already present provideresistance to rotation about each of the anteroposterioraxes. rocedure P rocedure
3
Where a need for the addition of indirect retainershas been demonstrated in Procedure 2, a decisionmust now be made on: 1. The type or types to be used. 2. Where they should be placed. Decision 1. As previously noted, resistance torotation about about potential axes may be achieved bythe use of either tooth-borne direct retainers orrests. Where the former are to be used they areconventional in form. Where indirect retention isto be achieved by rests then this action can beobtained beobtained by placement placement of conventional res ts of theocclusal, cingulum or incisal type. Alternatively,certain other elements can be used to achieve thenecessary resting action. For example, certaintypes of connectors when positioned on the hardpalate will serve as indirect retainers of the resttype. The necessary resting effect can also beobtained by the use of elements designed specificallyto act as indirect retainers, e.g. continuousclasps or the Cummer arm. These find applicationparticularly where placement on the anterior teethis required. Decision 2. In all instances, indirect retainersshould be positioned the maximum possible distancefrom the potential axis of rotation. This willensure by lever action
81
that they obtain maximalmechanical advantage. In the case of a maxillarydenture, indirect retainers may be placed on eitherthe hard palate or the teeth. With a mandibulardenture, only tooth -bearing indirect retainersshould be used.When dealing with a potential axis of rotationdeveloped by the placement of two rests, the bestposition for placement of an indirect retainer maybe found as follows: Determine the midpoint ofthe axis between the two rests. From that pointdraw a line at right angles to the axis until the lineintersects the dentition. Where this procedure is applied applied to dentitions ofthe Kenn edy Class IV types, it may be found thatthe line drawn drawn from the midpoint of the axis willfail to interest the dentition. In such a case the indicationis for the placement of bilateral indirectretainers, situated as posteriorly as possible Unite Unite the various components of the denturewith connectorsat the conclusion of the fourth stage of partial denturedesign, the placement of a series of isolatedcomponents will have been planned. In all casesthe saddle areas will have been outlined. In somecases rests will have been placed on teeth to providesupport for the denture along with the possibleplacement of direct and indirect retainers. Thefinal stage in denture design involves the placementof connectors to unite these isolated componentsinto a single entity.Connectors are classified as being either major or minor according to their length or the area oftissue which they cover. Major connectors includethose used to join saddles and they are always rigidin nature. Minor connectors have an auxiliary connectingrole, as where a support/retention unit ona tooth needs to be joined to a major connector. They are also usually made to be rigid,but occasionally an intentional degree of flexibilityis introduced into their structure by the inclusionof a stress-breaker. The normal requirementthat connectors should be rigid stemsfrom the need for a partial denture to be able tofunction as a single unit. When forces are
82
appliedto any part of the denture it is usually deemed desirablethat the denture should be able to transfer the fo rces to all the soft and hard tissues which itcontacts. That aim can best be achieved by the useof rigid connectors.The choice lies between four connectorswhichcan be used in a maxillary denture and fourwhich can be used in a mandibular denture: axillary M axillary
connectors
1. Anterior palatal bar 2. Middle palatal bar 3. Posterior palatal bar 4.
Palatal plate
andibular M andibular
connectors
1. Lingual bar 2. Lingual plate 3. Labial or buccal bar 4.
Modified continuous clasp
It should be noted that in some cases the use oftw oft w o or more of the available range of connectorsmay be desirable in a single denture.The factors that should be taken into considerationwhen selecting the appropriate connector orconnectors to use in a denture are as follows: 1. The patient's history of part ial denture wearing wearing 2. Biological features related to the area of tissuewhich will be covered by the connector. 3. The material of construction which is to beused. 4.
Whether the denture is to be provided forthe maxilla or for the mandible.
Factor 1
83
Where the history reveals that a particular form ofconnector has been successful both in terms ofpatient tolerance and tissue reaction, it is usuallydesirable for the same form of connector to be usedin the replacement denture. Where the converse istrue, change to an alternative form of connectorshould be considered. Factor 2 The use of a bar type of connector relative to theuse of a plate will normally result in an appreciabledecrease in the area of tissue covered by the connector.Since tissue coverage is always accompanied by the risk of evoking a pathological responsein the area concerned, reducing area of coveragecan be beneficial to tissue health. In the case of amaxillary denture, reduced coverage of the palatemay also reduce impairment of the patient's tastesensation. These features favour the choice of abar connector where possible. Against this mustbe. Weighed the fact that plate connectors, by virtueof their greater tissue coverage, offer superiorsupport to the denture than that achievable achievable by theuse of of ba r connectors. The auxiliary supportoffered by plate connectors may be an essentialfeature of some denture designs, as where all thesaddles in a maxillary maxillary denture are to be be mademucosa supported. Factor 3 Choice of the material of construction is closelylink ed to that of the type of connector to be used.To achieve the necessary rigidity in a bar type ofconnector, it must be made in metal. It may becast in a gold alloy alloy or cobalt -chromium alloy or befabricated by wrought work using a gold alloy orstainless steel. Plate connectors can also be madein metal, but here, because of the greater area ofcoverage, it may also be possible to achieve thenecessary rigidity by use of a plastic such as acrylicresin. The use of a plastic in place of a metal forthe construction of a plate connector offers a
84
cleareconomic advantage. In addition, it will decreasethe weight of the denture and thus favour retentionin the case of a maxillary denture.In terms of the preservation of tissue health,however, the use of metal metal for the constr uction ofplates is to be preferred. Tissues which arecovered by a metal plate usually maintain a healthierstate with less pathological reaction than that seenwhere a plastic plate is used. In part, this may bedue to a different different response by the tissues to t hechemical nature of the overlying materials. Themuch greater value for the coefficient of thermalconductivity thermalconductivit y of metals relative to that of plasticsmay also play a part in the observed difference.Yet another factor which may help to explain thedifference in tissue response is the comparativerigidity of the two structures. The modulus ofelasticity of cobalt-chromium alloy, for example, isapproximately sixty times that of acrylic resin.Even a thin lingual plate constructed in cobalt chromiumalloy will be more rigid than the thickesttolerable form of acrylic lingual plate. Duringthe normal functioning of the denture far less flexing of a metal plate will occur than is the case witha plastic plate, and consequently less mechanicalirritation of the tissues is likely to arise.Where bar connectors are to be used, selectionof the appropriate connector system will be influencedby the modulus of elasticity of the metal tobe used in their construction.
The
modulus
ofelasticity
of
cobalt-chromium
alloys
is
approximatelytwice that of gold alloys. As a consequence,where a maxillary denture carries only two short,laterally-positioned bounded saddles, adequaterigidity can often by achieved by the useof a posterior palatal bar as sole connector ifcobalt chromium alloy is used. Where, instead,the same denture is to be constructed in a goldalloy, the use of a combination of anterior andposterior palatal bars will usually be necessary toachieve the same rigidity. Factor 4
85
The general preference for use of bar connectorsrather t han plate connectors connectors wherever possible hasalready been noted. It applies equally to denturesbeing provided for the maxilla or for the mandible.Where a plate connector is to be used in a maxillarydenture, it is desirable that it should be constructed on the principles advocated by Every.Important amongst these principles is that of developing a self cleansing gap of approximately
4
mm between the palatal plate and the palatal
aspects of the gingival margins of the standing teeth .This helps towards the maintenance of thehealth of the periodontal tissues of the teeth.Alternative designs which involve colleting of thepalatal plate around the standing teeth carry agreater risk of producing an inflammatory reactionin the gingival tissues.A simplified version of the 'Every' palatal plateconnector is seen in the 'spoon' form. This can beused where a maxillary partial denture is beingconstructed for a Kennedy Class IV dentition andnot more than two adjacent teeth are to be replaced.As a 'spoon' connector avoids contact with any of the standing teeth it has biological merit,but it suffers the disadvantage of achieving onlylimited retention and stability. The development ofan acceptable level of retention will be favoured bythe presence of a 'V'
shaped palate relative to t o thatof a 'V' shaped palate. It is more successful in
childrenand adolescents than in adult patients, probablybecause younger patients find it easier todevelop control of the denture by tongue action.When selecting the connector to be used to joinsaddles in a mandibular denture, the biological superiorityof the lingual bar should always be bornein mind. Against this, must be weighed the factthat it may give rise to tongue irritation in some patients.The risk of this occurring can be reduced bypositioning the bar as low as possible in relation tothe floor of the mouth. The adopting of a sub -lingual position for the bar may also aid its acceptance. The lingual plate is usually well tolerated bypatients, especially
86
when made in metal and carefullycontoured carefullycontoured to simulate the form of the natural teethwhich it covers. However, the lingual plate crossesthe gingival margins of the teeth it covers and thisalways carries the risk of producing periodontaldamage. The buccal or labial bar is poorly tolerated bymany patient s on both aesthetic and functionalgrounds, and its use should be restricted to thosecases where there is no alternative. This is usuallydue to the pres ence of a retroclined dentition.The modified continuous clasp is rarely selectedfor use. It is indicated where the teeth upon whichit is to be positioned have long clinical crowns,where the health of the gingivae of those teethcontraindicate their being covered, and yet inadequate space is available for use of a lingual bar. ESSENTIALS OF PARTIAL DENTUREDESIGN Design of the partial denture framework should besystematically developed and outlined on an accuratediagnostic cast based on the following prosthesisconcepts: where the prosthesis is supported,how the support is connected, how the prosthesis isretained, how the retention and support are connected,and how the edentulous base support isconnected. To develop the design, it is first necessary to determinehow the partial denture is to be supported. Inan entirely tooth -supported partial denture, the mostideal location for the support units [ rests] is rests] is on preparedrest seats on the occlusal, cingulum, or incisalsurface of the abutment adjacent to each edentulousspace ). edentulousspace ). The type of rest andamount of support required must be based on interpretationof the diagnostic data collected from thepatient. In evaluating the potential support an abutmenttooth can provide, consideration should begiven to: (1) periodontal health; (2) crown and rootmorphologies; rootmorphologies; (3) crown -to-root ratio; ( 4) bone indexarea (how the tooth has respon ded to previous stress); stre ss); (5) location of the tooth
87
in the arch; (6) relationshipof the tooth to other support units (length ofedentulous span); and (7) the opposing dentition. In
a
tooth-
and
tissue-supported
partial
denture,attention
to
these
same
considerations must begiven to the abutment teeth. However, equitable supportmust come from the edentulous ridge areas.In evaluating the potential support available fromthe edentulous ridge areas, consideration must begiven to: (1) the quality of the residual ridge, whichincludes contour, quality of the supporting bone(how the bone has responded to previous stress), andquality of the supporting mucosa; (2) the extent towhich the residual ridge will be covered by the denturebase; (3) the type and accuracy of the impressionregistration; ( 4) the accuracy of the denturebase; (5) the design characteristics of the componentparts of the partial denture framework; and (6) theanticipated occlusal load. Denture base areas adjacent to abutment teeth areprimarily areprimarily tooth suppor ted. As you proceed awayfrom the abutment teeth, they become more tissuesupported. Therefore it is necessary to incorporatecharacteristics in the partial denture design that willdistribute the functional load equitably between theabutment teeth and the supporting tissue of theedentulous ridge. Locating tooth support units ( rests)on rests)on the principal abutment teeth and designing theminor connectors, which are adjacent to the edentulousareas, to contact the guiding planes in such amanner that they disperse the functional load equitablybetween the available tooth and tissue supportingunits will provide designs with controlleddistribution of support The second step in systematically developing thedesign for any removable partial denture is to connectthe tooth and tissue support units. Majorconnectors must be rigid so that forces applied toany portion of the denture can be effectively distributedto the supporting structures. Minor connectorsarising from
88
the major connector make it possible totransfer totransf er functional stres s to each abutment tooththrough its connection to the corresponding rest andalso to transfer the effect of the retainers, rests, andstabilizing components to the remainder of the dentureand throughout the dental arch.The third step is to determine how the removablepartial denture is to be retained. The retention mustbe sufficient to resist reasonable dislodging forces. Retention is accomplished bymechanical retaining elements ( clasps) clasps) being placedon the abutment teeth and by the intimate relationship of the denture bases and major connectors(maxillary) with the underlying tissue. The key toselecting a successful clasp design for any given situationis to choose one that will: (1) avoid direct transmissionof tipping or torquing forces to the abutment;(2) accommodate the basic principles of clasp designby definitive location of component parts correctlypositioned on abutment tooth surfaces; (3) provideretention against reasonable dislodging forces (withconsideration for indirect retention); and ( 4) be compatiblewith undercut location, tissue contour, andesthetic desires of the patient. Location of the undercutis the most important single factor in selecting aclasp. Recontouring or restoring the abutment toothto accommodate a clasp design better suited to satisfythe criteria for clasp selection, however, can modifyundercut location.The relative importance of retention is highlightedby the results from a clinical trial investigating prosthesisdesigns (K. Kapur). A 5-year randomized clinicaltrial of two basic removable partial denturedesigns²one with rest, proximal plate,
and
I-bar(RPI)
design
and
one one
with with
circumferential
claspdesign ²
demonstrated no discernible changes innine periodontal health components of the abutmentteeth with either of of the two designs after 60mon ths. The overall results indicated that the twodesigns did did not di ffer in terms of success rates, maintenance,or effects on abutment teeth. Therefore awell awell -constructed removable partial denture
89
that issupported by favourable abutments and good residualridge s that are both properly prepared and maintainedin a patient who exhibits good oral hygieneoffers the best opportunity for satisfactory treatment.The fourth step is to connect the retention unitsto the support units. If direct and indirect indirect retainersare to function as designed, each must be rigidlyattached to the major connector. The criteria forselection, location, and design are the same as thoseindicated for connecting the tooth and tissue supportunits.The fifth and last step in this systematic approachto design is to outline and join the edentulous area tothe already established design components.
COMPONENTS OF PARTIALDENTURE DESIGN All partial dentures have two things in common: (1) They must be supported by oral structures and (2) They must be retained against reasonable dislodgingforces. In the Kennedy Class I I I partial denture, threecomponents are necessary: support provided byrests, the connectors (stabilizing components), andthe retainers.The partial denture that does not have the advantage of tooth support at each end of each edentulousspace still must be supported. But in this situation,the support comes from both the teeth and theunderlying ridge tissue rather than from the teethalone. This is a composite support, and the prosth esismust be fabricated so that the resilient supportprovided by the edentulous ridge is coordinatedwith the more stable support offered by the abutmentteeth. The essentials²support, connectors,and retainers²must be more carefully designedand executed because of the movement of tissue supporteddenture base areas. In addition, provisionmust be made for three other factors as follows:
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1. The best possible support must be obtained fromthe resilient tissue that covers the edentulousridges. This is accomplished b y the impressiontechnique more than by the partial denture design, although the area covered by the partial denturebase is a contributing factor in such support. 2. The method of of direct retention must take intoaccount the inevitable tissuewards movement ofthe distal extension base(s) under the stresses ofmastication and occlusion. Direct retainers mustbe designed so that occlusal loading will result inthe direct transmission of this load to the long axisof the abutment teeth instead of as leverage. 3. The partial denture, with one or more distalextension denture bases, must be designed tominimize movement of the extension base awayfrom the tissue. This is often referred to as indirectretention and is best described in relation toan axis of rotation through the rest areas of the principal. However,retention from the removable partial denturebase itself frequently can be made to help prevent this movement. Tooth Support The support of the removable partial denture by theabutment teeth is dependent on the alveolar supportof those teeth, the crown and root morphology, therigidity of the partial denture framework, and thedesign of the occlusal rests. Through clinical androentgenographic interpretation, the dentist mayevaluate the abutment teeth and decide whether theywill provide adequate support. In some instances thesplinting of two or more teeth is advisable, either by fixed partial dentures or by soldering two or moreindividual restorations together. In other instances atooth may be deemed too weak to be used as anabutment, and extraction is indicated in favor ofobtaining better support from an adjacent tooth.Having decided on the abutments, the dentist
91
isresponsible for the preparation and restoration of theabutment teeth to accommodate the most idealdesign of the partial denture. This includes the formof the occlusal rest seats. These modifications maybe prepared either in sound tooth enamel or inrestorative materials that will withstand the functionalstress and wear of the component parts of theremovable partial denture. The technician cannot beblamed for inadequate abutment tooth preparation,such as occlusal rest support. On the other hand, thetechnician is solely to blame if he or she extends thecasting beyond, or fails to include, the total preparedarea s. If the dentist has sufficiently reduced the marginalridge area of the rest seat to prevent interferencefrom opposing teeth, and if a definite occlusalrest seat is faithfully recorded in the master cast anddelineated in the pencilled design, then no excusecan be made for poor occlusal rest form on the partialdenture. Ridge Support Support Support for f or the tooth-supp tooth -supported orted removable removable partialdenture partialdenture or the tooth -supported -supported modification spacecomes entirely from the abutment teeth by means ofrests. Support for the distal extension denture basecomes primarily from the overlying soft tissue andthe residual alveolar bone of the distal extensionbase area. In the latter, rest support is effective onlyat the abutment end of the denture base.The effectiveness of tissue support depends on sixfactors: (1) the quality of the residual ridge; (2) theextent to which the residual ridge will be covered bythe denture base; (3) the accuracy and type ofimpression registration; ( 4) the accuracy of the denturebases; (5) the design characteristics of the componentparts of the partial denture framework; and (6) The occlusal load applied.The quality of the residual ridge cannot be influenced,except to improve it by tissue conditioning,or to modify it by surgical
92
intervention.
Such
modificationsare
almost
always
needed
but
are
not
frequentlydone.The accuracy of the impression technique isentirely in the hands of the
dentist.
Maximum
tissuecoverage
for
support
that
encompasses
the
primarystress-bearing areas should be the primary objectivesin any partial denture impression technique. Themanner in which this is accomplished should bebased on a biological comprehension of what happensbeneath a distal extension denture base whenan occlusal load is applied.The accuracy of the denture base is influenced bythe choice of materials and by the exactness of theprocessing techniques. Inaccurate and warped denturebases adversely influence the support of the partialdenture. Materials and techniques that willensure the greatest dimensional stability should beselected.The total occlusal load applied to the residualridge may be influenced by reducing the area ofocclusal contact. This is done by the use of fewer,narrower, and more more effectively shaped artificial teeth. The distal extension removable partial denture isuni que in that its support support is derived from abutmentteeth, which are comparatively unyielding, and fromsoft tissue overlying bone, which may be comparativelyyielding under occlusal forces. Resilient tissues,which are distorted or displaced by occlusalload, are unable to provide support support for the denturebase comparable with that offered by the abutmentteeth. This problem of support is further complicatedby the fact that the patient may have naturalteeth remaining that can exert far greater occlusalforce on the sup porting tissue than would result ifthe patient were completely edentulous. This isclearly evident from the damage often occurring toan edentulous ridge when it is opposed by a fewremaining anterior teeth in the opposing arch andespecially when the opposing occlusion of anteriorteeth has been arranged so that contact exists inboth centric and eccentric positions.Ridge tissues recorded in their resting or non-functioning form are incapable of providing the
93
compositesupport needed for a denture that derives it ssupport from both hard and soft
tissue.
Three
factorsmust
be
considered
in
the
acceptance
of
an
impressiontechnique for distal extension removable partialdentures: (1) the material should record the tissuecovering the primary stress-bearing areas in theirsupporting form; (2) tissues within the basal seat area, other than primary stress-bearing areas, must area covered by the impression should be sufficientto distribute the load over as large an area as can betolerated by b y the border tissue. This is an applicationof the principle of the snowshoe.Anyone who has had the opportunity to comparetwo master casts for the same partially edentulousarch edentulousarch ²one cast having the distal extension arearecorded in its anatomic or resting form and theother cast having the distal extension area recordedin its functional form²has been impressed by thedifferences in the topography. A denturebase processed to the functional form is generallyless irregular and has greater area coverage thandoes a denture base processed to the anatomicor resting form. Moreover, and of far greater significance,a denture base made to anatomic formexhibits less stability under rotating and/or torquingforces than does a denture base processed to functionalform and thus fails to maintain its occlusalrelation with the opposing teeth. By having thepatient close onto strips of soft wax, it is evident thatocclusion is maintained at a point of equilibriumover a longer period of time when the denture basehas been made to the functional form. In contrast,evidence exists that there has been a rapid "settling"of the denture base when it has been made to theanatomic form, with an early return of the occlusionto natural tooth contact only. Such a denture not only fails to distribute the occlusal load equitably butalso al lows rotational movement, which is damagingto the abutment teeth and their investing structures. Major and Minor Connectors
94
Major connectors are the units of a partial denturethat connect the parts of the prosthesis located onone side of the arch arch with thos e on the opposite opposite side.Minor connectors arise from the major connectorand join it with other parts of the denture. Thus theyserve to connect the tooth and tissue support unitstogether. A major connector should be properlylocated properlylocated in relation to gingival and moving tissues andshould be designed to be rigid. Rigidity in a majorconnector is necessary to provide proper distributionof forces to and from the supporting components. A lingual bar connector should be tapered superiorlywith a half half -pear shape in cross section andshould be relieved sufficiently, but not excessively,over the underlying tissue when such relief is indicated. The addition of a continuous bar retainer or alingual apron does not alter the basic design of thelingual bar. These are added solely for support, stabilization,rigidity, stabilization, rigidity, and protection of the anterior teethand are neither connectors nor indirect retainers. The finished inferior border of either a lingual bar ora linguoplate should be gently rounded to preventirritation preventirritation to subjacent tiss ue when the restorationmoves even slightly in function.The use of a linguoplate is indicated when thelower anterior teeth are weakened by periodontaldisease. It is also indicated in Kennedy Class I partiallyedentulous arches when the need need for additionalres istance to horizontal rotation of the denture isrequired because of excessively resorbed residualridges. Still another indication is in those situationsin which the floor of the mouth so closely approximatesthe lingual gingiva of anterior teeth that anadeq uately inflexible lingual lingual bar cannot be positionedwithout impinging on the gingival tissue.Experience with the linguoplate
has
shown
thatwith
good
oral
hygiene
the
underlying
tissue
remainshealth remainshealthy, y, and there are no harmful effects effects to the tissuefrom the met allic coverage per se. However, adequaterelief must be provided whenever a metal
95
componentcrosses the gingival margins and the adjacentgingivae. Excessive relief should be avoided becausetissue tends to fill a void, resulting in the overgrowthof abnormal tissue. The amount of relief used thereforeshould be only the minimum necessary to preventgingival impingement.It does not seem that there are many advantagesto be found in the use of the continuous bar retainerversus the linguoplate. In rare instances when a linguoplatewould be visible through multiple interproximalembrasures, the continuous bar retainermay be preferable for esthetic reasons only. In otherinstances when a single diastema exists, a linguoplatemay be cut out in this area to avoid the displayo f metal without sacrificing its use.Rigidity of a palatal major connector is just asimportant and its location and design just as criticalas for a lingual bar. A U -shaped palatal connector israrely justified except to avoid an inoperable palataltorus that extends to the junction of of the hard andsoft andsoft palates. Neither can the routine use of a narrow,single palatal bar be justified. The combinationanterior-posterior palatal strap-type major connectoris mechanically and biologically sound ifit does not impinge o n tissue. The broad, anatomicpalatal major connector is frequently preferredbecause of its rigidity, better acceptance by thepatient, and greater stability without tissue damage.In addition, this type of connector may providedirect-indirect retention that may sometimes, butrarely, eliminate the need for separate indirectretainers. Direct Retainers for Tooth-supportedPartial Dentures Retainers for tooth-supported partial dentures haveonly two functions: to retain the prosthesis againstreasonable dislodging f orces without damage to theabutment teeth and to aid in resisting any tendencyof the denture to be displaced in a horizontal plane.The prosthesis cannot move tissuewards becausethe rest supports the retentive components of theclasp assembly. There should be no movement
96
awayfrom the tissue tissue and therefore no rotation rotation about a fulcrum because a direct direct retainer secures the retentivecomponent.Any type of direct retainer is acceptable as longas the abutment tooth is not jeopardized by itspresence. Intracoronal Intraco ronal (fri ctional) retainers are ideal for tooth-supported restorations and offer esthetic advant ages that are not possible with extracoronal(clasp) retainers. Nevertheless, the circumferentialand circumferent ialand bar -type clasp retainers are mechanically effectiveand are more econo mically constructed than areintracoronal retainers. Therefore they are used moreuniversally.Vulnerable areas on the abutment teeth mustbe protected by restorations with either type ofretainer. The clasp retainer must not impinge ongingival tissue. The clas p must not exert excessivetorque on the abutment tooth during placement andremoval. It must be located the least distance intothe tooth undercut for adequate retention, and itmust be designed with a minimum of bulk and toothcontact.The bar clasp arm should be used only when thearea for retention lies close to the gingival marginof the tooth and little tissue blockout is necessary. Ifthe clasp must be placed high, if the vestibule isextremely shallow, or if an objectionable space willexist beneath the bar clasp arm because of blockoutof tissue undercuts, the bar clasp arm should not beused. In the event of an excessive tissue undercut,consideration should be given to recontouring theabutment and using some type of circumferentialdirect retainer. Direct Retainers for Distal ExtensionPartial Dentures Retainers for distal extension partial dentures,although retaining the prosthesis, must also be able to flex or disengage when the denture base moves tissuewardunder function. Thus the retainer may act asa stress-breaker. Mechanical stress-breakers accomplishthe same thing, but they do so at the expense ofhorizontal stabilization. When some kind
97
of mechanicalstress-breaker is used, the denture flange must beable to prevent horizontal movement. Clasp designsthat allow for flexing of the retentive clasp arm mayaccomplish the same purpose as mechanical stressbreakerswithout sacrificing horizontal stabilizationand with less complicated techniques.In evaluating the ability of a clasp arm to act as astress-breaker, one must realize that flexing in oneplane is not enough. The clasp arm must be freelyflexible in any direction, as dictated by the stressesapplied. Bulky, half-round clasp arms cannot do this,and neither can a bar clasp engaging an undercuton the side side of the tooth away from the denturebase. Round, tapered clasp forms offer advantages ofgreater and more universal flexibility, less tooth contact,and better esthetics. Either the combination circumferentialclasp, with
its
tapered
wrought-wireretentive
arm,
or
the
carefully
located
and
properlydesigned circumferential or bar clasp can be consideredfor use on all abutment teeth adjacent to theextension denture bases if the abutment teeth areproperly prepared, the tissue support is effectivelyachieved, and if the patient exercises good oralhygiene. Stabilizing Components Stabilizing components of the removable partialdenture framework are those rigid components thatassist in stabilizing the denture against horizontalmovement. The purpose of all stabilizing componentsshould be to distribute stresses equally to allsupporting teeth without overworking any one tooth.The minor connectors that join the
rests
and
theclasp
assemblies
to
the
major
connector
serve
as
stabilizingcomponents.All minor connectors that contact vertical too thsurfaces (and all reciprocal clasp arms) act as stabilizingcomponents. It is necessary that minor connectorshave sufficient bulk to be rigid and yetpresent as little bulk to the tongue as possible. Thismeans Thismeans they should be confined to interdental embrasu reswhenever
98
possible. When minor connectorsare located on vertical tooth surfaces, it is best thatthese surfaces be parallel to the path of placement.When cast restorations are used, these wax-patternsurfaces should be made parallel on the surveyorbefore casting.A modification of minor connector design hasbeen proposed that places the minor connector inthe center of the lingual surface of the abutmenttooth. Proponents of this design claim that it reducesthe amount of gingival tissue coverage and providesenhanced bracing and guidance during placement.Disadvantages may include increased encroachmenton the tongue space, more obvious borders, andpotentially greater space between the connector and the abutment tooth. This proposed variation, however,when combined with thoughtful design principles,ma principles,may y provide some benefit to the periodontalhealth of the abutment teeth and may be acceptableto some patients.Reciprocal clasp arms also must be rigid, and theymust be placed occlusally to the height height of contour ofthe abut abut ment teeth, where they will will be nonretentive.By their rigidity, these clasp arms reciprocate theopposing retentive clasp, and they also prevent horizontalmovement of the prosthesis under functionalstresses. For a reciprocal clasp arm to be placed favourably,some reduction of the tooth surfaces involvedis frequently necessary to increase the suprabulgearea.When crown restorations are used, a lingual reciprocalclasp arm maybe inset into the tooth contourby providing a ledge on the crown on which the clasparm may rest. This permits the use of a wider clasparm and restores a more nearly normal tooth contour,at the same time maintaining its strength andrigidity Guiding Plane The term guiding plane is defined as two or more parallel,vertical surfaces of abutment teeth, so shaped todirect prosthesis during placement and removal.After the most favourable path of placement has beenascertained, axial surfaces of
99
abutment teeth are preparedparallel to the path of placement and thereforebecome parallel to each other. Guiding planes may becontacted by various components of the partial denture²the body of an extracoronal direct retainer, thestabilizing arm of a direct retainer, the minor connectorportion of an indirect retainer ²or by a minorconnector specifically designed to contact the guiding plane surface.The functions of guiding plane surfaces are as follows: (1) To provide for one path of placement andremoval of the restoration (to eliminate detrimentalstrain to abutment teeth and framework componentsduring placement and removal); (2) to ensurethe intended actions of reciprocal, stabilizing, andretentive components (to provide retention againstdislodgment of the restoration when the dislodgingforce is directed other than parallel to the path ofremoval and also to pro vide stabilization against horizontalrotation of the denture); and (3) To eliminategross food traps between abutment teeth and componentsof the denture. Guiding plane surfaces need to be created so thatthey are as nearly parallel to the long axes of abutmentteeth as possible. Establishing guiding planeson several abutment teeth (preferably more than twoteeth), which are located at widely separated positionsin the dental arch, provides for a more effectiveuse of these surfaces. The effectiveness effect iveness of guidingplane guidingp lane surfaces surfaces is enhanced if these surfaces are preparedon more than one common axial surface of the abutment teeth.As a rule, proximal guiding plane surfaces shouldbe about one half the width of the distance betweenthe tips of adjacent buccal buccal and lingual cusps orabout one third of the buccal lingual width of thetooth. They should extend vertically about two thirdsof the length of the enamel crown portion of thetooth from the marginal ridge cervically.
100
In preparingguiding plane surfaces, care must be exerci sed toavoid creating buccal or lingual line angles. Assuming that the stabilizing or retentive armof a direct retainer may originate in the guidingplane region, a line angle preparation would weakeneither or both components of the clasp assembly. A guiding plane should be located on the abutmentsurface adjacent to an edentulous area. However,excess torquing is inevitable if the guidingplanes guidingplanes are used squarely facing each other on a lone standing abutment adjacent to an extension area . Indirect Retainers An indirect retainer must be placed as far anterior from the fulcrum line as adequate tooth support permits if it is to function with the direct retainer to restrict movement of a distal extension base away from the basal seat tissue. It must be placed on a restseat prepared on an abutment tooth that is capableof withstanding the forces placed on it. An indirectretainer cannot function effectively on an inclinedtooth surface, nor can a single weak incisor tooth beused for this purpose. Either a canine or premolar tooth should be used for the support of an indirect retainer, or the rest seat must be prepared with asmuch care as is given any other rest seat. An incisalrest or a lingual rest may be used on an anteriortooth, provided a definite seat can be obtained eitherin sound enamel or on a suitable restoration.A second purpose that indirect retainers serve inpartial denture design is that of support for majorconnectors. A long lingual bar or an anterior palatalmajor connector is thereby prevented from settlinginto the tissue. Even in the absence of a need forindirect retention, provision for such auxiliary supportis sometimes indicated.Contrary to common use, a cingulum bar or a linguoplatedoes not in itself act as an indirect retainer.Because these are located on inclined tooth surfaces,they serve more as an orthodontic appliance than assupport for the partial denture. When a linguoplateor a
101
cingulum bar is used, terminal rests shouldalways be provided at either end to stabilize the dentureand to prevent orthodo ntic movement of theteeth contacted. Such terminal rests may function asthe indirect retainers, but these would functionequally well in that capacity without the continuous bars retainer or linguoplate.
EXAMPLES OF SYSTEMATIC APPROACHTO DESIGN Kennedy Class I, Bilateral, Distal ExtensionRemovable Partial Dentures The Class I, bilateral, distal extension partial dentureis as different from the Class III type as any two dentalrestorations could be. Because itderives its principal support from the tissue underlyingits base, a Class I partial denture made toanatomic ridge form cannot provide uniform andadequate support. Yet, unfortunately, many Class Imandibular removable partial dentures are made from a single irreversible hydrocolloid impression. Insuch situations, both the abutment teeth and theresidual ridges suffer because the occlusal loadplaced on the remaining teeth is increased by thelack of adequate posterior support.Many dentists, recognizing the need for sometype of impression registration that will wi ll record thesupporting form of the residual ridge, attempt torecord this form with a metallic oxide, rubber base,or one of the silicone impression materials. Suchmaterials actually only record the anatomic formof the ridge, except when special design of t heimpression trays permits recording the primarystress-bearing areas under a simulated load. Othersprefer to place a base, which is made to fit theanatomic form of the ridge, under some pressure atthe time that it is in contact with the remainingteeth, thu s obtaining functional
support.
Still
otherswho
believe
that
a
properly
compounded
mouthtemperaturewax will displace only tissue that isincapable of providing support
102
to the denture baseuse a wax secondary impression to record the supporting,or functional, form of the edentulous ridge.Any impression record will be influenced by theconsistency of the impression material and theamount of hydraulic pressure exerted by its confinementwithin the impression tray.
Kennedy Class II Removable Partial Dentures The Kennedy Class II partial denture actually may be a combination of bothtissuesupported and tooth-supported restorations.The distal extension base must have adequate tissuesupport, whereas tooth -supported bases elsewherein the arch may be made to fit the anatomic formof the underlying ridge. Indirect retention must be provided for; however, occasionally the anteriorabutment on the tooth -supported side will satisfythis requirement. If additional indirect retention isneeded, provisions must be made for it.Cast clasps are generally used on the tooth supportedside. However, a clasp design usingwrought wire may reduce the application of torqueon the abutment tooth adjacent to the distal extensionand should be considered.The use of a cast circumferential clasp engaging amesiobuccal undercut on the anterior abutment of the tooth-supported modification space may resultin a Class I lever like action if the abutment teethhave not been properly prepared and/or if the tissuesupport from the extension base area is not adequ ate.It seems rational under these circumstancesto use a bar-type retainer engaging a distobuccalundercut. Should the bar-typeretainer be contraindicated because of a severe tissueundercut or the existence of only a mesiobuccalundercut mesiobuccalundercut on the anterior abutm ent, then a combinationdirect retainer with the retentive arm made oftapered wrought wire should be used. A thoroughunderstanding of the advantages and disadvantagesof various clasp designs is necessary to determinethe type of direct retainer that is to
103
be used for eachabutment tooth.The steps in the fabrication of the Class II partialdenture closely follow those of the Class I partialdenture, except that the distal extension base is usuallymade of an acrylic resin material, whereas thebase for any tooth-supported area is frequently madeof metal. This is permissible because the residualridge beneath tooth -supported bases is not called onto provide provide support for the denture, and later later rebasing is not as likely to be necessary.
References:
Stratton R J, an atlas of removable partial denture design, 1998, Quintessence Publishers, USA, pp 93 ± 17 4, 207 ± 272 John Osborne, George Alexander Lammie, Partial Dentures, Fourth edition, 1985, CBS Publishers, India, pp 256 -274 A.A.Grant,
W.Johnson,
Removable
denture
Proshtodontics,
Second
Edition,1992, Longman Singapore Publishers, Singapore, pp 131 -1 42. Ernest L.Miller, Joseph E.Grasso, Removable Removable Partial Prosthodontics, Second edition, 1989, CBS Publishers, India, pp 137 -150 Alan B. Carr, Glen P. Mcgivney, David T. Brown, Removable partial Prosthodontics, Eleventh edition, 2005, Elsevier Publishers, India, pp 1 45 -164 J. C. Davenport
:Communication between the dentist and the dental
technician: B D J 2000;189: 471 - 475 Filiz KEYF, Frequency of the Various Classes of R emovable Partial Dentures and Selection of Major Connectors and Direct/Indirect Retainers, Turk J Med Sci 2001; 31:445 -449 J. C. Davenport : Indirect retention B D J 2001;190:128 -132
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Rudd and Morrow, Dental Dental Laboratory Procedures ± removable partial dentures, 2nd edition, Mosby company, pp 176-18 4 Singla and Jagmohan, Removable partial denture designing: Forces as primary concerns JIPS 2007;6:179-18 4 J. C. C. Davenport, Davenport, A system of design B D J 2000; 189: 586 ±590
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