SPACE CLOSING MECHANICS
CONTENTS
INTRODUCTION THE BASICS OF ORTHODONTIC MECHANICS ANCHORAGE CONSIDERATIONS SPACE CLOSURE IN EDGEWISE AND PRE ADJUSTED EDGEWISE FRICTIONLESS MECHANICS FRICTION MECHANICS SPACE CLOSURE IN BEGG TIP EDGE LINGUAL ORTHODONTICS BIOLOGIC CONSIDERATIONS CONCLUSION
INTRODUCTION Near the end of the nineteenth century, Edward Angle introduced “The Angle System”. His E arch appliance, Pin and Tube, The Ribbon Arch ultimately resulted in the introduction of the edgewise multibanded appliance, the first standardized orthodontic appliance system. Angle“s edgewise employed non-extraction concept
Later Charles H.Tweed held to Angle”s conviction that one must never extract teeth. What tweed began to observe in his patients during retention was discouraging to him. After failures and unsatisfied with the stability following non-extraction treatment, came up with extraction of first four premolars in correction of malocclusion. With extraction as a method of gaining space, space closure became subsequently important.
With all the changes in the appliances that have occurred during the past 100 years, the basic principles underlying how we treat malocclusions remain the same: we exert forces to move teeth in the directions we want. Physics has not changed. The same basic physical principles are common to all orthodontic appliance systems and techniques. Tooth movement occurs in response to controlled force systems placed on the teeth by educated and skilled practitioners
THE BASICS OF ORTHODONTIC MECHANICS
Orthodontic tooth movement results from the application of forces to the teeth. These forces are produced by the appliances (wires, brackets, elastics). Knowledge of the mechanical principles governing forces is necessary for the control of orthodontic treatment. The basis of orthodontic treatment lies in the clinical application of biomechanical concept. Mechanics is the discipline that describes the effect of forces on bodies; Biomechanics refers to the science of mechanics in relation to biologic systems. An understanding of several fundamental mechanical concepts is necessary in order to understand the clinical relevance of biomechanics to orthodontics. They include the following:
Force A force is a load applied to an object that will tend to move it to a different position in space. Forces may be treated as vectors and are conveniently represented as arrows. A force vector is characterized by four features: Magnitude Point of application Line of action Sense Increasing the magnitude of force will increase the amount of tooth displacement initially. However it is unclear how force magnitude is related to the rate of tooth movement, which is biologically controlled phenomenon. (Quinn and Yoshikawa, AJO85)(Lindauer, SIO-2000)
Moment If the force acts at a distance to the applied point, it is moment. Moment is equal to the force magnitude multiplied by the perpendicular distance of the line of action to the center of resistance. (Burstone, AJO-84) M=Fd Couple The only force system that can produce pure rotation (ie: a moment with no net force) is a couple which is two equal and opposite, noncolinear but parallel forces. A. Two equal and opposite, parallel, noncolinear forces form a couple. B. The translational effects of forces cancel each other, but the moments of each force combine. The result is a moment with no net force.
Center of resistance
Is the point at which a free object or body can be perfectly balanced. At this point, resistance to movement is concerned for mathematical analysis. The line of action of the force passes through the center of resistance .Thus tooth will translate, even though the point of attachment to the tooth is at the bracket.
The center of resistance of a single rooted tooth is located about 40% of the distance from the alveolar crest to the apex. For a multirooted tooth, the center of resistance is 1 to 2 mm below the furcation. The center of resistance varies with root length and the degree of periodontal support, which in turn is affected by alveolar bone height. (Bruce Haskel et al, AJO-90) Center of rotation The point around which rotation actually occurs when an object is being moved is center of rotation.
Moment\Force Ratio By varying the ratio of moment to force applied to teeth, the quality of tooth movement can be changed among tipping, crown movement, translation, and root movement. (Burstone, AJO-84)(Kusy and Tulloch, AJO86) Whenever a force is applied at the crown of a tooth, a tendency for the tooth to rotate, tip, or torque is also created. In addition to the force applied, a couple may also be engaged intentionally to partially correct, completely correct, or over correct this tendency. By changing the ratio of the moment from the applied couple to the force applied, the center of rotation of tooth movement can be varied to produce the type of tooth movement desired
Tipping It is the tooth movement with greater movement of the crown of the tooth than the root. The center of rotation of the motion is apical to the center of resistance. It can classified as Uncontrolled tipping Controlled tipping
Uncontrolled tipping A horizontal force at the level of a bracket will cause movements of the root apex and crown in opposite directions. This is the simplest of the tooth movements, but it is often undesirable. Here the center of rotation is just between center of resistance and the root apex. The M\F ratio is 0\1(ie : no counter moment is applied to negate the moment of the applied force)
Controlled tipping Is a desirable type of tooth movement. It is achieved by an application of force to move the crown, as done in uncontrolled tipping, and application of a moment to control or maintain theposition of the root apex. Here the center of rotation is at the root apex and the M\F ratio is 7\1. .
Translation Translation of a tooth takes place when the root apex and the crown move the same distance and in the same horizontal direction. The center of rotation is at infinity. Force alone applied at the bracket will not result in translation. To achieve translation at the level of bracket, a couple and a force are required that are equivalent to the force system through the center of resistance of the tooth .An M\F ratio of 10\1 produces translation
Root Movement Is achieved by keeping the crown of a tooth stationary and applying a moment and force to move only the root. The center of rotation of the tooth is at the incisal edge. Root movement requires large moment. The M\F ratio should be at or above 12\1 .
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Rotation Pure rotation of a tooth requires a couple. No net force acts at the center of resistance, so only rotation occurs. Clinically, this movement is most commonly needed for movement as viewed from the occlusal perspective
COMPONENTS OF THE FORCE SYSTEM
The force system of an orthodontic appliance acts in all three planes and determines the type of tooth movement.
ALPHA MOMENT This is the moment acting on the anterior teeth (also called anterior torque) BETA MOMENT This is the moment acting on the posterior teeth. Tip back bends placed mesial to the molars produce an increased beta moment
HORIZONTAL FORCES These are the mesio-distal forces acting on the teeth which are equal to each other. VERTICAL FORCES These are the extrusive- intrusive forces generated because of the unequal moments. When the beta moment is greater, a intrusive forces act on the anterior teeth. And while alpha moment is greater an extrusive force acts on the anteriors while an intrusive force acts on the posteriors
Material considerations Considerations in the appliance design Force constancy is the consistency of the applied force over the range of activation of the appliance. For tooth movements over large distances, the continuity of the force levels throughout is often desired. Force constancy can be obtained by reducing the loaddeflection rate in one or more of the following four ways:(Stanley Braun and Marcotte,AJO-85) • • • •
By reducing the cross section of the wire Increasing the interbracket distance Incorporating loops in the archwire Using shape memory alloys
Orthodontic space closure
Orthodontic space closure should be individually tailored based on the diagnosis and treatment plan. James Hilgers (JCO, 1987) proposed, extraction therapy can be divided into five phases, each designed to achieve a specific goal. They are: • Initiation phase where anchorage considerations are evaluated • Cuspid retraction and uprighting • Transition and final space closure
• Consolidation •
Idealisation
Charles J .Burstone (AJO, 1982) was one of the earliest to study about space closure procedures. He putforth six goals to be considered for any universal method of space closure. They are
• Differential space closure (anterior retraction, posterior protraction or combination of both) • Minimum patient co-operation (Treatment results must not be altered much by lack of patientco-operation) • Axial inclination control • Control of rotations and arch width • Optimum biologic response, includes rapid tooth movement with a minimum lowering of the pain threshold. In addition, tissue damage particularly root resorption should be at minimum • Operator convenience An appliance, delivering a necessary force with a definite magnitude and direction is required to achieve the above goals. A known force systems must be used to control the active units(teeth being moved) and the reactive units( anchorage teeth)
Idealized objective of space closure Space closure should result in upright well aligned teeth with parallel roots and parallel occlusal plane. Therefore some degree of bodily or even root movement is required. DETERMINANTS OF SPACE CLOSURE The main factors which determine the tooth movement during space closure are: Amount of crowding: In cases of severe crowding, anchorage control is very important to maintain the extraction space for relieving the anterior crowding Anchorage Using the same mechanics for different anchorage needs is very important. Traditional anchorage methods like lip bumpers, headgears, transpalatal arches may be utilized but non compliance methods for anchorage control based on biomechanics can also be used.
Axial inclination of canines The same force and or moment applied to teeth with different axial inclinations will result in different types of tooth movement Midline discrepancies Midline discrepancies should be corrected as early as possible in treatment as it allows the remaining space closure to be completed symmetrically. Using asymmetric asymmetric mechanics can cause in unilateral anchorage loss, skewing of the dental arches, or unilateral un ilateral vertical forces. Vertical dimension Control of vertical dimension is essential in space closure. Undesired vertical extrusive forces on the posterior teeth can result in increased LAFH, increased interlabial gap, and excessive gingival display. Class II elastics may potentate this problem.
ANCHORAGE CONSIDERATIONS IN SPACE CLOSURE
Anchorage refers to the resistance to displacement displacement offered by an anatomic anatomic unit unit in effecting tooth tooth movement . .
CLASSIFICATION OF ANCHORAGE GROUP A ANCHORAGE This category describes the critical maintenance of the posterior tooth position. 75% or more of the extraction space is needed for anterior retraction GROUP B ANCHORAGE This category describes relatively symmetric space closure with equal movement of the anterior and posterior teeth to close the space. This is the least difficult of the space closures. GROUP C ANCHORAGE This category describes non critical anchorage, where 75% or more of the space closure is achieved through mesial movement of the posterior segment; this could also be described as critical anterior anchorage
FRICTIONLESS MECHANICS
• Burstone T loop • Opus loop
• Double key hole loop • Tear drop loop • K-SIR appliance • Three Piece Mechanics • Rickett s intrusion and retraction arch • Poul Gjessing Spring • Lingual Lever Arm
Burstone T Loop
• One of the main principles of the segmental arch technique is considering the anterior segment and posterior segment as one large tooth respectively. The right and left buccal units are connected by a transpalatal arch forming one big posterior unit. • The basic configuration of the TMA loops consists of a .O17X.O25"TMA wire.
• In the passive state there are no moments or forces acting on it. In its active state it applies a force fo rce system on the teeth, The activation of a spring requires r equires forces and moments to engage the spring in its brackets and tubes
• Neutral position: • The neutral position in an activated loop is found by applying the activation moments and without any horizontal forces. In other words the ends are twisted to bring the each attachment to its horizontal position. in this position the spring has zero horizontal force • The horizontal force is got by pulling the spring open from this position. • Differential moments are obtained by the principle of off center V bends which results r esults in unequal moments. the closer the V bend is 0 the tooth the higher the moment. The segmented T loops approximated a V bend. bend . Clinically the spring needs to be positioned at least 1-2mm closer to one side than another to obtain ob tain a moment differential. .
SYMMETRIC SPACE CLOSURE (group B anchorage)
The center position of the spring can be found by: distance = (interbracket distance -activation)/2
With the use of a vertical tube at the canine a 90 degrees gingival bend at the calculated distance eases placement and monitoring throughout space closureThe T loop is placed in the molar auxiliary tube and then inserted into the canine bracket. The distal end is pulled back until it is the desired length which results in the desired activation.
• To check the remaining activation the spring is removed from the canine tube and the remaining activation at neutral position is checked. The activation is the distance the spring must be pulled to be inserted into the tube.
• During the tipping phase the anterior and posterior segments tip towards each other which is corrected during the root movement phase. When the Occlusal plane regains parallelism reactivation is indicated.
MAXIMUM POSTERIOR ANCHORAGE: (Group A anchorage) • The T loop is positioned closer to the posterior segment (1-2 mm off centering) is sufficient .Activation of 4 mm is necessary. This reduces the horizontal forces without altering the moment differential. The force system acting on the anterior segment favors tipping. The moment difference remains as the space closes and the spring deactivates. • The spring must be re activated when 2 or less mm of activation remains. • Because the beta moment is greater than the anterior moment a vertical intrusive force acts on the anterior teeth which can exaggerate the tipping tendency and steepen the occlusal plane. Similarly the posterior occlusal plane can be steeped by the extrusive force. Maintaining adequate horizontal force helps to reduce this effect
• A High pull headgear can also be used to control the posterior occlusal plane. • It is likely that root correction will be required at the end of space closure. The nature of the root correction will depend on the specific needs of the case. En masse anterior and posterior root correction, anterior root correction or separate canine root corrections are possibilities.
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MAXIMUM ANTERIOR ANCHORAGE :( Group C anchorage) This is the most difficult of all space closures. The increased alpha moment has a tendency to deepen the overbite. The loop must be placed 1-2mm closer to the anterior teeth. Care must be taken that the wire segment achieves full bracket engagement because play can reduce the moment differential. Space closure with tipping of the buccal segments will occur. The activation must be around 4mm and should be activated every 2mm. The major side effects are loss of anchorage and extrusion of the anteriors. Class III elastics or protraction headgear may help in the protraction of the upper buccal segments. For mandibular molars class II elastics may help
CONTINUOUS ARCH T LOOP SPACE CLOSURE
• Segmental T loop space closure principles can also be applied to space closure on a continuous arch. • The force system is not as well defined a the segmental but careful use of the alpha and beta moments helps to achieve comparable results especially for group B and C anchorage cases. For group A cases high pull headgear is necessary to control tooth position
• T loops one on each side are made using preformed arch wires. 017 X .025 TMA or .016 X .022 Stainless steel arch wire. The activations given are for TMA wires and the Stainless steel wires activation is reduced by half. • The T loops are made 6-7mm tall and 10mm wide and are positioned distal to the cuspids. • Desired alpha and beta moments are placed anterior and posterior to the T loop vertical legs. Recommended beta activations for A, B, and C anchorages are 40 degrees, 30 degrees and 20 degrees
• After the activations are placed the loops should be open approximately 2mm before placing in the mouth. the wire is inserted into the molar auxiliary tube and ligated to the anterior teeth. The T loop bypasses the premolar brackets and is not inserted in them. • For TMA loops the activation can be 3mm distal to the molar tube which gives it a range of force of 250-300gms. • The patient should be monitored but no further activations are necessary for 2-3 months. Too frequent reactivation can prevent root movement and cause excessive tipping
CORRECTION OF THE SIDE EFFECTS Tipping of the anterior and posterior teeth into the extraction space Increase the alpha and beta moments . Flaring of the anterior teeth Reduce the alpha moment or increase the distal activation Mesial in rotation of the buccal segments Mesial out rotation of the palatal arch, archwire or lingualarch. Excessive lingual tipping of the anterior teeth Increase the alpha moment
T-LOOP POSITION AND ANCHORAGE CONTROL: Effect of Spring Positioning on the Force System • The components of the force system produced by aT-loop are the alpha moment, the beta moment, the horizontal force, and the vertical force. • For off-centered positioning, the magnitude of the alpha moment, the beta moment, and the horizontal force was dependent on both the activation and the position. The horizontal force ranged from approximately 340 gm at full activation to 0 gm at zero activation. Eccentricity had a small but statistically significant effect on the horizontal force magnitude. The horizontal force increased with increased eccentric positioning by approximately 6 to 8 gm/mm off centering. The alpha and beta moment • magnitudes also increased with activation.
• The moments were also dependent on the spring position, with the moment increasing for the side closer to the Tloop and decreasing for the further side. • The vertical forces (extrusive or intrusive forces) increased with greater off centering. • The vertical forces on anteriorly positioned springs (extrusive forces on the anterior teeth) were dependent on the spring position and independent of the springs activation. • The vertical forces increased approximately 26 gm/mm off centering toward the anterior (extrusive to the anterior teeth). The vertical forces produced by posteriorly positioned springs (intrusive forces to the anterior teeth) were dependent on the spring position and activation. The vertical forces increased approximately 24 gm/mm of off center positioning (intrusive to the anterior teeth).
• The rate of decay of the force applied by a spring is called the load-deflection rate, and it averages 33 gm. per millimeter in the Burstone's T loop. • The low load-deflection rate is important in this spring, since it enables the orthodontist to deliver optimal magnitudes of force. The two important characteristics of the force system: • (I) The required moment-to-force ratio to give us the needed center of rotation • (2) the change of the moment-to-force ratio as the tooth moves • It is desirable to maintain a constant center of rotation during retraction of the anterior teeth. This necessitates a relatively constant M/P ratio where is small. • On the other hand, we should purposely increase the M/P ratio on the posterior teeth to enhance anchor age during space closure.
• With the introduction of beta-titanium wire (TMA), it has been possible to simplify the design so that a T loop by itself will have a relatively low load-deflection rate and a large maximum springback. The heavier base arch which fits into the auxiliary tube of the first molar is important, since it allows positive orientation of the spring and, more significantly, it is capable of withstanding, without permanent deformation, the higher moments that are needed for anchorage control. Furthermore, the use of a • heavier base arch tends to increase the moment-to-force ratio on the anterior teeth, since any bending in the occlusally positioned part of the spring tends to minimize this ratio. • The retraction spring is not centered but is positioned mesially. It is used to enhance the moment-to-force ratio (better anchorage control) on the posterior segment during space closure and give better axial-inclination control over the anterior teeth. • To aid the clinician in achieving the proper angulation, templates are used. Rather than to measure the angles, it is more expeditious to duplicate the shape of the spring from
• Anchorage control during en masse space closure • In patients with Group A arches, where little anterior displacement of the posterior teeth is allowed, two stages of space closure are planned- en masse controlled tipping followed by en masse root movement. The calibrated mechanism is a composite TMA spring comprising two different cross sections of wire, a 0.018 inch round T loop welded to a 0.017 by 0.025 inch base arch • Anchorage control with the TMA retraction spring is accomplished in a number of ways. The forces that are used are relatively low in magnitude. The initial forces are under 200 Gm, with an average force of about 150 Gm. during the retraction period. • The low load-deflection rate allows the clinician to determine accurately the magnitude of this force and an error of 1 mm. would produce an error of only 33 Gm.
EN MASSE TRANSLATION FOR GROUP B ARCHES
The spring is placed centrally between the two auxiliary tubes for two reasons. The most important is that it allows the same rate of change of the moment-to-force ratio in both the alpha and the beta positions. Furthermore, it is simpler to place a symmetrical angulation in the spring.
POSTERIOR PROTRACTION FOR GROUP C ARCHES
• This spring is designed to encourage posterior protraction by utilizing the following principles: (l) the loop is placed off center; this produces a more constant center of rotation in the beta position. By contrast, in the alpha position, the moment to force ratio rapidly increases so that if these teeth move at all, they will tend to move forward rather than posteriorly. (2) The force is kept under 300 Gm. to minimize anterior retraction or root movement
• This spring is designed to encourage posterior protraction by utilizing the following principles: (l) the loop is placed off center; this produces a more constant center of rotation in the beta position. By contrast, in the alpha position, the moment to force ratio rapidly increases so that if these teeth move at all, they will tend to move forward rather than posteriorly. (2) The force is kept under 300 Gm. to minimize anterior retraction or root movement. • . The second strategy that can be used for displacing posterior segments forward uses a symmetrically placed attraction spring with the use of either Class Il or Class III elastics. By using intermaxillary elastics during space closure, one can minimize some of the side effects that would be evident if the same elastics were used after all space is closed, and the entire arch must be displaced. In lieu of elastics, protraction headgear may also be considered.
CANINE RETRACTION
• The force system that is used for retraction of the canine is similar to that for en masse space closure. The composite retraction spring is used in Group A arches, and the attraction spring is employed in Group Band C arches. The difference lies in rotational control of the canine, which is achieved with a nonsliding mechanism. Antirotation bends are placed in the retraction assemblies to prevent the canine from rotating as it retracts. It is also possible to use an arch wire to prevent rotation. •
The TMA composite 0.017 by 0.025 inch retraction assembly is used with antirotation bends .Not only are bends and twists placed in the area of the spring, but a toein bend is placed immediately in front of the first molar. The angulation and distal activations are usually identical to those used for en masse space closure.
Certain design features have been incorporated into retraction springs to optimize the force system: 1. The material used is beta-titanium, which simplifies the design and allows for direct welding of materials. TMA has excellent spring-back properties with good forrnability. The wire cross sections are kept as small as possible, limited by the moments needed rather than the force. In one spring, a composite spring uses a heavier-base arch in order to ensure that an adequate beta moment is produced. 2. Additional wire that is placed into an attraction spring or loop is critical. Additional wire should be placed as far apically as possible to increase the activation moment-toforce ratio. Indiscriminant placement of wire will reduce the moment-to force ratio. The T loop design is employed to enhance this ratio.
3. The loop centricity affects the rate of change of the moment-to-force ratio in the alpha and beta positions. If equal rates of change are required, loops should be centrally placed. Where greater moment-to-force ratio constancy is required, loops should be displaced off center in the direction of those teeth (segment) where constancy is needed. 4. The large interattachment distance between the auxiliary tube on the first molar and the vertical tube of the canine allows sufficient room for the large activations required. In addition, it adds to the accuracy of determining the force system, since small errors in the shape or geometry of the spring will not radically change the forces produced
OPUS LOOP RAYMOND SIATKOWSKI(AJO-97)
• The OPUS loop was designed to deliver an inherent M/F ratio sufficient for enmasse space closure via translation of teeth of average dimensions with no bone loss. • Because its inherent M/F ratio is high enough no preactivation bends is needed before insertion • The neutral position is the passive position of the spring as it sits before insertion. • Simple cinch back activations can take care of the tooth movement thresholds to meet anchorage objectives.
No closing loop design previously has been capable of delivering at constant M/F of 8.0 to 9.1 mm most having inherent M/F of 4-5 mm or less. To achieve net translation, orthodontists have had to add residual moments to the closing loop arch wire with angulation bends (gable bends) anterior and posterior to the loop, a posterior gable bend and angulations within the loop, or a posterior gable bend and anterior wire-bracket twist (anterior root torque).
Adding these residual moments has several disadvantages:
1. The teeth must cycle through controlled tipping to translation to root movement to achieve net translation (lower Young's Modulus materials go through fewer of these cycles for a given distance of space closure). 2. The correct residual moments are difficult to achieve precisely in linear materials. 3. The resulting ever-changing PDL stress distributions may not yield the most rapid, least traumatic method of space closure. If a closing loop design capable of achieving inherent, constant M/F of 8.0 to 9.1 mm without residual moments were available, en masse space closure with uniform PDL stress distributions could be achieved. Such a mechanism would be less demanding of operator skill to apply clinically and might provide more rapid tooth movement with less chance of traumatic side effects
The opus loop achieves a M/F ratio of 8-9.1mm without addition of activation bends in the loop or archwire itself. Therefore its neutral position is the same as the inactivated position before it was tied into the brackets. Having the loops neutral position accurately allows known forces systems to be applied to the teeth via simple cinch back activations.
• The ascending legs at an angle of 70 degrees to the plane of the brackets • The apical helix is on the leg ascending from the anterior teeth, (that ascent must begin within 1.5mm posterior to the most distal bracket of the anterior teeth being retracted) • The spacing between the ascending legs especially the apical loops legs must be 1mm or less All these dimensions are critical to the performance of the loop. • Clinically comfort bends are not necessary. • The apical horizontal leg is 10mm long,
• Being free of residual moments, the design can produce a true rest period when deactivated and therefore could be used with future technology to produce intermittent force systems during space closure Wire bracket play numbers as given in the figure shows that it is important that sufficient lingual twist exists in the arch wire engaging the incisors so that bracket wire play is reduced for axial control of the incisors.
• It is appropriate to begin with a straight wire and bend the arch wire in a torquing turret to achieve incisor axial inclination control by inducing wire twist ("lingual root torque") just enough to eliminate labiolingual wire-bracket play in the incisor brackets. •
The amount of such twist is dependent on the wire/bracket sizes and slot torque used
• A torquing turret has been designed for use with TMA wire.
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Maximum incisor twist is appropriate for posterior protraction
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The advantage of having the opus loop formed in 17X25 TMA is that it provides a relatively long range of activation;
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unfortunately it is difficult to bend the wire with sufficient incisor torque to reduce the wire play.
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It is difficult to contour the loop for comfort on one side without altering the other side also and a large stock of wires is necessary for preformed wires. This can be over come by having:
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An anterior wire of Niti alloy with two separate 17X25 TMA posterior segments, which are attached by a Forestadent cross tube
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This bialloy has the following advantages: – Infrequent activations – Ease of comfort bending – Incisor axial inclination control
• The OSTEOLOGIC graphic form is the theoretical explanation for the mode of action of the opus loop arch wire. • It relates the orthodontic force systems to the stresses in the PDL rather than the strains. •
It examines the rate of tooth movement as the loop deactivates
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When a force system is applied on a tooth initially after a quiescent period, the initial rate of tooth movement corresponds to no 2 on the diagram.
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This model is valid only for uniform stress on the PDL as produced by translation and not tipping followed by up righting.
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These arch wires by definition are activated far less than the systolic blood pressure at which hyalinization is supposed to occur.
The various possible activations of the opus loop cinch back as a function of time is shown in the figure. Group B anchorage Curve 1: anteriors retract Curve 2: posteriors protract Group A anchorage Curve 2: anteriors retract Curve 3: posteriors little change Group C anchorage Curve 4: anteriors no change Curve 1 posteriors protract
DISADVANTAGES OF THE OPUS LOOP
• Although less so than with other closing loop designs, Opus loops do have the potential to steepen the cant of occlusal plane in the maxillary arch and flatten it in the mandibular arch. • Although steepening occlusal plane can be useful for overtreatment of Class III relationships (and flattening occlusal plane for Class II relationships), that potential should be monitored for possible intervention. •
Such intervention could be reducing maximum activation force levels or using an occipital headgear with short and high outer bows to generate a moment tending to flatten maxillary occlusal plane.
• For the most severe anchorage required to achieve treatment goals, second molars, if available, could be included with the posteriors and/or a Combi headgear used.
The configuration for posterior protraction
• The closing loop arch wire generates the moments required and some of the protraction force. • Most of the protraction force is generated by the large anterior moment and by the intermaxillary elastics to a rigid rectangular arch wire in the opposing arch. • Intermaxillary Niti closed coil springs capable of delivering 150 gm force can be substituted for the elastics. The potential exists for changing occlusal plane in the opposing arch. Should such cant changes begin to be observed, the intermaxillary force can be reduced.
• In group C anchorage cases, class III elastics with a force of 150gms/side from the opposite arch which has a rigid rectangular stainless steel archwire can be used. • Another alternative is to use TP 256 torquing auxiliary which when overlaid over the closing loop provides an additional protraction force to the posteriors. It has the following advantages: – The clinician is free to continue treatment in the lower arch – Undesired vertical forces from the elastics are not a problem – Posterior arch width increases are not a problem when using a TMA wire
DOUBLE KEY HOLE LOOP
Introduced by John Parker .Made of round edge rectangular wires,usually 0.019*0.025 dimension. These are used in extraction cases, and also when there are slight spaces in non extraction cases. They are used along with Asher face bow, which is an appliance that connects directly to the anterior teeth. It is used in intrusion and retraction of anterior teeth.
Concept behind using • To allow the operator the luxury of complete space closure with one set of arch wires • To allow a reasonably happy medium between severe tipping and sliding mechanics • Allow the operator to select how the space will be closed from front backward or from back forward and how much of which.
TEAR DROP LOOP
TEAR DROP LOOP (R.G. ALEXANDER- JCO-83)
After the maxillary canines have been retracted on the .016" round wire with the power chain, an .018" * .025" stainless steel closing-loop archwire is placed. This archwire is bent in an ideal i deal arch form with large, teardropshaped loops just distal to the maxillary twin lateral bracket. Omega stops are not used, but the wire extends through the first molar tubes.
• Before placing the archwire in the mouth, the portion of the archwire distal to the closing loops is reduced approximately .001" in the anodic polisher, so that part of the wire can slide through the brackets easily during activation. It is activated by placing a #442 plier on the archwire distal to the molar tube, pushing it distally 1-2mm to open the closing loop, and bending the end 45 degrees gingivally to produce a stop • The patient is seen every four to five weeks, and the maxillary closing loops are activated 1mm at each appointment. This method of retracting the four incisors as a unit allows more torque control than if all six anteriors were retracted together. Complete space closure should be accomplished in six to eight months.
K-SIR APPLIANCE (Varun Kalra, JCO-98)
Separate canine retraction has the disadvantages of increased treatment time and the creation of an unesthetic space distal to the incisors. The rationale for separate retraction in pre edgewise technique is that molar anchorage is conserved. However, Burstone and Nanda have demonstrated molar anchorage control, using non-frictional loop mechanics for en masse retraction of the anterior teeth , that compares favorably with that of conventional edgewise sliding mechanics.
An appliance for simultaneous intrusion and retraction of the six anterior teeth should ideally control: Magnitude of forces and moments Moment-to-force ratio Constancy of forces and moments Friction From a practical standpoint, the appliance should: Be easy to fabricate and adjust Be comfortable for the patient . Require a minimal amount of patient cooperation . Be cost-effective
• The K-SIR (Kalra Simultaneous Intrusion and Retraction)archwire is a modification of the segmented loop mechanics of Burstone and Nanda. • It is a continuous.019" ' .025"TMAarchwirewith closed7mm ' 2mmU-Ioops at the extraction sites • To obtain bodily movement and prevent tipping of the teeth into the extraction spaces, a 90° V-bend is placed in the arch wire at the level of each U-Ioop This V-bend, when centered between the first molar and canine during space closure, creates two equal and opposite moments to counter the moments caused by the activation forces of the closing loops A 60° V-bend located posterior to the center of the interbracket distance produces an increased clockwise moment on the first molar which augments molar anchorage as well as the intrusion of the anterior teeth
To prevent the buccal segments from rolling mesiolingually due to the force produced by the loop activation, a 20° antirotation bend is placed in the archwire just distal to each U-Ioop Activation A trial activation of the archwire is performed outside the mouth This trial activation releases the stress built up from bending the wire and thus reduces the severity of the V-bends After the trial activation, the neutral position of the each loop is determined with the legs extended horizontally In neutral position, the U-Ioop will be about 3.5mm wide. .
The archwire is inserted into the auxiliary tubes of the first molars and engaged in the six anterior brackets. It is activated about 3mm, so that the mesial and distallegs of the loops are barely apart The second premolars are bypassed to increase the interbracket distance between the two ends of attachment. This allows the clinician to utilize the mechanics of the off center V-bend
When the loops are first activated, the tipping moments generated by the retraction force will be greater than the opposing moments produced by the V-bends in the archwire. This will initially cause controlled tipping of the teeth into the extraction sites. As the loops deactivate and the force decreases, the moment-to-force ratio will increase to cause first bodily and then root movement of the teeth. The archwire should therefore not be reactivated at short intervals, but only every six to eight weeks until all space has been closed. The archwire is typically in place for four to five months
INTRUSION AND RETRACTION UTILITY ARCH
• The utility arch has multiple uses in various stages of orthodontic treatment. This auxiliary archwire has been developed according to biomechanical principles described by Burstone and refined for incorporation into Bioprogressive therapy • Although it is a complete arch extending across both buccal segments, the utility arch engages only the first molars and the four incisors. It originally was developed to provide a method of leveling the curve of Spee in the mandible, but it has been adapted to perform many more functions than just lower incisor intrusion.
• With an .018" appliance, the recommended wire for the mandibular arch is .016" × .016" or .016" × .022" Blue Elgiloy (not heat-treated). For most maxillary arches, .016" × .022" Blue Elgiloy is recommended. With an .022" appliance, .019" × .019" Blue Elgiloy can be used in either arch. • The usefulness of a retrusion utility arch in retracting and intruding incisors is obvious in cases of upper incisor flaring. However, this type of mechanics is also helpful in retracting the four anterior teeth as a unit, particularly in the maxilla. The retrusion utility arch can close interproximal spaces while intruding and aligning the upper anterior teeth and correcting midline discrepancies
• The retrusion arch originates in the auxiliary tube on the molar, and 5-8mm of wire should protrude anteriorly before a posterior vertical step of 3-4mm is placed. The vestibular segment extends anteriorly to the interproximal region between the lateral incisor and the canine. At this point, a 90° bend is placed . • A loop-bending plier is then used to place a loop in which the anterior leg crosses behind the posterior leg. After a 58mm anterior vertical step, another right-angle bend then carries the wire across the anterior teeth. A gentle anterior contour is placed in the wire to simulate the arch form
As with the intrusion utility arch, there are two possible types of activation.
• First, a Weingart plier can be used to grasp the extension of the utility arch posterior to the auxiliary tube. The wire is pulled 2-3mm posteriorly and then bent upward at a 90° angle. Care must be taken that this protruding end of the utility arch does not impinge on the gingiva or cheek. • Second, an occlusally directed gable bend in the vestibular segment can be used to produce intrusion.
THREE PIECE MECHANICS BHAVNA SHROFF (AJO-95)
THREE PIECE BASE ARCH MECHANICS (BHAVNA SHROFF ET AL, AJO-1995) A three-piece base arch is used to intrude the anterior segment . A heavy stainless steel segment (0.018 ´ 0.025 or larger) with distal extensions below the center of resistance of the anterior teeth is placed passively in the anterior brackets. The distal extensions end 2 to 3 mm distal to the center of resistance of the anterior segment. The intrusive force is applied with a 0.017 ´ 0.025 TMA tip-back spring The design of this appliance enables low-friction sliding to occur along the distal extension of the anterior segment during space closure . The application of a light, distal force delivered by a Class I elastic to the anterior segment is used to alter the direction of the intrusive force on the anterior segment. This appliance design allows the application of the intrusive force to get true intrusion of the incisors along their long axes.
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An intrusive force perpendicular to the distal extension of the anterior segment and applied through the center of resistance of the anterior teeth will intrude the incisor segment It is possible to change the direction of the net intrusive force by applying a small distal force. The line of action of the resultant force will be lingual to the center of resistance and a combination of intrusion and tip back of the anterior teeth will occur. This the line of action of the resultant force can be made parallel to the long axis of the anterior teeth if an appropriate distal force is combined with a given intrusive force. To obtain a line of action of the intrusive force through the center of resistance and parallel to the long axis of the incisors, the point of force application must be more anterior and as close to the distal of the lateral incisor bracket as possible If the intrusive force is placed farther distally and an appropriate small distal force is applied intrusion and simultaneous retraction of the anterior teeth occurs because of the tip back (clockwise) moment created around the center of resistance of the anterior segment consisting of four incisors. The distal force used in this intrusion retraction system is of very low magnitude and is used to redirect the line of action of the intrusive force. One advantage of this system is the low magnitude of force applied on the reactive or anchorage unit.
• This segmented approach to intrusion and retraction is clinically advantageous because it allows simultaneous control of tooth movement in the vertical and anteroposterior planes. • The low load deflection rate of this appliance delivers a constant intrusive force, and the levels of force can be kept low. • The design of this appliance allows the clinician to deliver a well-controlled, statically determinate force system in which only minimal chairside adjustments are required.
POUL GJESSING SPRING
PG-SPRING
POUL GJESSING SPRING (POUL GJESSING,AJO-85) The spring design, made from 0.016 by 0.022 inch stainless steel wire The predominant active element is the ovoid double helix loop extending 10 mm apically. It is included in order to reduce the load/deflection of the spring and is placed gingivally so that activation will cause a tipping of the short horizontal arm (attached to the canine) in a direction that will increase the couple acting on the tooth. Height is limited by practical considerations, so that a double loop is necessary to incorporate sufficient wire. The gently rounded form avoids the effect of sharp bends on load/deflection and, through the use of the greatest amount of wire in the vertical direction, reduction of horizontal load/deflection is maximized. At the same time, minimizing horizontal wire increases rigidity in the vertical plane.
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The smaller loop occlusally is incorporated to lower levels of activation on insertion in the brackets in the short arm (couple) and is formed so that activation further closes the loops.
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The mesial and distal extensions of the looped wire segment are angulated both in the vertical and in the horizontal plane. When the spring is in place, but prior to activation of the driving force (neutral spring position, F = 0 gm), static antitip and antirotation couples will be exerted to the canine.
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The distal driving force is generated by pulling the distal, horizontal leg through the molar tube. A desirable force level of approximately 160 gm is obtained when the two sections of the double helix are separated 1 mm. During the activation the force is matched by an additional couple (activation couple) arising from the double-helix loop which, in theory, acts as four lever arms.
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Incorporation of a segment of a circle ("sweep") in the distal leg of the spring is an adjustment with the purpose of eliminating undesirable ß moments acting at the second premolar bracket and tending to move the root apex too far mesially.
This study deals with considerations of importance in optimizing the biomechanical system related to canine retraction, the purpose being to develop a canine-retraction spring which
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Promotes translation sagittally and horizontally through an antitip moment-to-force ratio of approximately 11:1 and an antirotation moment-to-force ratio of approximately 4:1, both being relatively constant over a certain range of activation;
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Results in a low load-deflection ratio during generation of retraction forces in the range of 50 to 200 gm;
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Results in no adverse interaction between antitip and antirotation moments during activation;
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Could be used in both 0.018 and 0.022 inch edgewise systems; and
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Have limited dimensions and allow for faciolingual adjustments without altering the above-mentioned characteristics.
MODIFIED LINGUAL LEVER ARM
Modified Lingual Lever Arm Technique - GERHARD ET AL(JCO-93) The original lingual lever made from .032" stainless steel wire soldered to a bonding pad or a band. A hook is bent in the wire 20mm from the pad The lever arm is adapted to the palatal vault and bonded to the lingual surface of the tooth to be moved (usually a cuspid or premolar) at the same height as the bracket on the buccal side. Two elastic chains or superelastic closed coil springs are used as a power source; one is stretched buccally between the cuspid or premolar bracket and the molar tube at crown level, and the other is stretched palatally from the lever arm to an extension soldered on a transpalatal bar
• An .016" round or .016"X.022" stainless steel guiding archwire should be used. Because translation will occur, tooth movement can be accomplished without significant loss of force due to bracket friction. • With superelastic coil springs, a constant moment-to-force ratio can be obtained over a long deactivation range
• Advantage over conventional space-closure mechanics • Ability to apply palatal root torque, which enables translation within the spongiosa. • Patient tolerence good compared to quad helix or transpalatal bar.
FRICTION MECHANICS
Friction is defined as a force that retards or resists the relative motion of surfaces in contact. James Bednar (JCO, 90) reported that in orthodontic sliding mechanics, friction is determined by the type of arch wire, the type the bracket, and the method of ligation. ADVANTAGES OF FRICTION MECHANICS (Row et al,AJO,2001) Complicated wire configurations are not needed, making initial wire placement less time consuming. This can enhance patient comfort and permit more delegation to assistants. Better rotational control and arch dimensional maintenance. Bennet and Mclauglin (JCO,90) advocates that, most efficient method of closing spaces is sliding mechanics with a standard appliance prescription, closing 0.5-1.5 mm of space per month with gentle forces.
DISADVANTAGES (Row et al, AJO-2001) • Anything that adds friction slows the movement of teeth along the arch wire. Hence, the lack of efficiency compared to frictionless mechanics should be accounted. •
More tipping and extrusion when compared to frictionless Classical Laws of Friction (Tidy, AJO-89) 1. Proportional to the forces normally acting on the contact 2. Independent of the area of contact 3. Independent of the sliding velocity
CONTROLLED SPACE CLOSURE IN PRE ADJUSTED EDGEWISE SYSTEM
• The most significant distinction between the mechanics of standard edgewise. and preadjusted appliances was observed during space closure. With standard edgewise appliances, rectangular archwires did not effectively slide through the posterior bracket slots because of the 1st-,2nd-, and 3rd-order bends. The closing loop arch activated in the office by opening the closing loop and moving the archwire through the posterior bracket slots • The level bracket slot alignment of the new appliances allowed archwires, for the first time, to move more effectively through the posterior slots when the patient was not in the office. As a result, many orthodontists discontinued use of closing loops and began using various forms of sliding mechanics- for example, placing hooks in the anterior sections of straight archwires and tying elastics or springs to them from molar brackets
CLOSING LOOP ARCHES
Closing loop arches had several disadvantages: 1. Extra wire-bending time 2. Poor sliding mechanics 3. Tendency to run out of space for activation (after two or three activations, the omega loop contacted the molar bracket and the archwire had to be adjusted or remade) 4. High initial force levels They also had advantages: 1. Precise control of the amount of loop activation (often as little as Imm), limiting the amount of initial tipping 2. Adequate rebound time for uprighting between appointments (with minimal activations, loops closed quickly with little tipping)
Sliding mechanics had these advantages: 1. Minimal wire-bending time 2. More efficient sliding of archwires through posterior bracket slots 3. Sufficient space for activations But sliding mechanics at first also had disadvantages:. 1. No established guidelines on amounts of force to be used during space closure 2. Tendency for initial overactivation of elastic and spring forces, causing initial tipping and inadequate rebound time for uprighting
SLIDING MECHANICS WITH LIGHT FORCES (Bennett and Mclaughlin, JCO-90) ACTIVE TIE BACK USING ELASTOMERIC MODULES To maximise the advantages and minimise the disadvantages of sliding mechanics, force levels during space closure must be reduced. Instead of springs or overactivated elastics. So they attached single elastic module to anterior archwire hooks with ligature wires extended forward from the molars. These “ elastic tiebacks”, when activated 2-3mm generate about 100150gm of force.
ACTIVE TIE BACK USING NITI COIL SPRINGS •
If large spaces are to be closed or if there are infrequent adjustment oppurtunities, instead of elastomeric modules, niti springs can be used. • Samuels et al (AJO-98) has recommended that the optimal force for space closure is 150 gm when using niti coil springs as the force for space closure. The 150 gm springs were found to be more effective than 100 gm springs, but no more effective than 200 gm springs. This, confirmed with findings of Rudge et al (AJO-93) that niti springs produce more consistant space closure than elastomeric modules. These studies suggest the use of light closed coil niti springs to give up a force of 150 gm.
In first premolar extraction cases , lower labial segment does not procline during the levelling stage with PEA the use of laceback ligatures conveys no difference in the AP or vertical position of the lower labial segment. Furthermore the use of laceback ligatures creates a statistically and clinically significant increase in yhe loss of posterior anchorage (IRVINE et al,JO-04)
Force decay with elastomeric chains is rapid in the first 24 hours and is affected by environment and temperature. Force decay did not occur to the same extent with niti springs. (Nattrass et al, EJO-1998)
• If the arches have been properly leveled, such light force allows for effective space closure; there is little tipping with subsequent binding of the archwires, and leveling is maintained .019" x .025" archwires with .022" slots provide optimum rigidity, but adequate freedom for the wires to slide through the slots. • Round wires and smaller rectangular wires provided less precise control of torque, curve of Spee, and overbite. • Hooks of .024 " stainless steel or .028 " brass are soldered to the upper and lower archwires
• Group movement and sliding mechanics are combined for gentle, controlled space closure, so that about .5mm of incisor retraction and .5mm of mesial molar movement can be seen each month. The tiebacks are replaced every four to six weeks. • Alternative force delivery systems have been evaluated, but has been found to have disadvantages. An elastic modular chain gave variable force, was difficult to keep clean, and sometimes fell off. Elastic bands, changed daily by the patient, relied on sometimes inconsistent cooperation. Pletcher-type wire coils delivered excessive force that caused tipping and binding, and they also proved unhygienic.
Inhibitors to Sliding Mechanics • Proper alignment of bracket slots is essential to eliminate frictional resistance to sliding mechanics. The common procedure is to use .018" or .020 " round wire for at least one month before placing .019"'.025" rectangular wires. Leveling and aligning continues for at least a month after insertion of the rectangular wires, and that space closure cannot proceed during that period.
• Therefore the rectangular wires are tied passively for at least the first month, until leveling and aligning is complete and the arch wires are passively engaged in all brackets and tubes Conventional elastic tiebacks are than placed ,In some cases, this phase takes three months.
There are three primary sources of friction during space closure • First-order or rotational resistance at the mesiobuccal and distolingual aspects of the posterior bracket slots is produced by rotational forces on the buccal aspects of the posterior teeth. The most effective way to counteract this resistance is to apply intermittent lingual elastic forcesone month from cuspid to first molar, the next month from cuspid to second molar. • Second-order or tipping resistance at the mesio-occlusal and distogingival aspects of the posterior bracket slots is caused by excessive and overactivated tieback forces, which lead to tipping of the posterior teeth, inadequate rebound time to upright these teeth, and a resultant binding of the system. The importance oflight forces (50-150g) and minimal activation length (to provide time for uprighting) cannot beoveremphasized.
Third-order or torsional resistance occurs at any of the four areas of the bracket slot where the edges of the archwire make contact. Like tipping resistance, this is produced mainly by excessive and overactivated tieback forces, which cause the upper posterior lingual cusps to drop down and the lower posterior teeth to roll in lingually
ENMASSE RETRACTION OF ANTERIOR TEETH WITH HEADGEAR
: Advantages are as follows: • Anterior headgear may have the advantage of retracting anterior teeth with minimum strain on posterior anchorage. • The adjustability of the outer bow in relation to the premaxilla's center of resistance, provides effective desired movements. • Intrusion and torque control are achieved in the course of anterior segment retraction.
• Guray et al (AJO-80) moved maxillary canines distally with extraoral forces applied by means of directly bonded tubes. They mentioned that this movement was followed by the spontaneous retraction of anterior teeth • Faran desribed the use of “J” hook head gear for canine retraction.
• Roth, in his straight wire technique, has used the "Asher" type face-bow for retraction of maxillary anterior teeth, to help control suitable root inclination. • In all these systems, extraoral forces were critically important for force application and anchorage control. For this reason, the headgear cooperation is essential.
Carlos et al (AJO-80) in their study regarding “J” hook head gear for canine retraction with high, medium, low pull head gear advocated ,the use of high pull head gear with ”J” hook produced least tipping effect when compared to the other two.
• In the Preadjusted Appliance System, the lack of cooperation can cause irreversible harm to the patient. • The first step on the en masse retraction of the six anterior teeth is to consolidate them as a "multirooted tooth." Banding of these teeth should be performed first. • In cases of arch length discrepancies, the canines are first moved distally with "Anterior Headgear" and then the previously stated process is applied. • Canine tubes: Any brand of tubes that are produced for molars are modified. The tubes are placed distogingivally when applied to the canines
• Face-bow: The inner bows of the widely used Kloehn-type face-bow were shortened and modified for the canine tubes. Outer bows were shortened to the distal level of the canines •
Headgear system: Consists of a Hickam-type variable pull headgear The intensity of the force applied is 128 gm on each side 16 to 18 hours of usage every day is advisable.
• Bracket system: Consists of 0.018 * 0.022-inch slot Roth system edgewise brackets
SPACE CLOSURE IN BEGG MECHANICS
It is performed in II stage treatment All six upper & lower anteriors & II premolars have modified ribbon arch brackets placed. 0.016 inch SS wire with anchor bend mesial to molar is given. This is to prevent binding of these anchorage bends in the molar buccal tubes as the arch wire slide distally & the extraction space from being closed. On each side , a light rubber elastics that exerts a force of 60 to 70 gms is stretched from the distal end at the arch wire ( or buccal molar hook ) to the intermaxillary hook on the same arch wire, mesial to canine.
The crowns of the six anterior teeth are simultaneously tipped back by the force from the horizontal space closing elastics to close the first premolar extraction space rapidly without causing much mesial movement of first molar. The anchorage bend in the arch wire prevent the anchor molar from tipping mesially & also gives the roots of the molar to resist the mesial pull of the elastics During the space closing procedures, the crowns of the six anterior teeth are tipped back until the space are closed . At the same time crowns of the secnd premolars become tipped mesially. For tipping only less force required than for bodily movement.
SPACE CLOSURE IN TIP EDGE
• Space closure is performed in stage II. It is the shortest of all three stages. • For space closure Cl-II or CL – III elastics are neeeded to maintain the desired anterior tooth relationships. Horizontal elastics are engaged from the molar hook to the intermaxillary circle in each quadrant to close any existing posterior space.022 inch SS wire is uesd for this process
BREAKNG MECHANICS
When protraction of posterior teeth is desired during stage II, then uprighting springs ( side winder uprighting springs) are placed on the anterior teeth, Turning them into a large anchor unit that can drag the posterior teeth forward using strong horizontal forces.
LINGUAL
ORTHODONTICS
Retraction/Consolidation Mechanics Lingual mechano therapy techniques for retraction and consolidation have followed conventional appliance procedures, using either sliding mechanics, closing loop arches, or combinations. Proper anchorage preparation should receive additional attention, with the initial loss of posterior occlusion resulting from the bite plane opening. It has become standard practice to place a transpalatal arch and/or headgear on all extraction cases.
Buccal segments should be leveled and aligned prior to initiating retractions Retraction of the maxillary anterior segment as a unit has become more frequent, as the cosmetically conscious lingual patient may object to the spacing with cuspid retraction. Cuspid retraction, when indicated and permitted by the patient, has most often been accomplished on .016" stainless steel or .016" × .022" stainless steel. Posterior segments should be ligated as a unit and the cuspids firmly ligated with double-over steel ligatures. The maxillary anteriors may be ligated lateral to lateral with either steel ligature or elastic chain. A three-unit elastic chain is then placed from the ball hook on the cuspids to the most mesial tooth in the posterior unit.
Once cuspid retraction is complete, the cuspids are ligated as part of the posterior unit and anterior retraction is initiated on rectangular wire (e.g., .017" × .025" TMA) with closing loops The anteriors must be firmly ligated with a double-over tie at this time. One method used to insure complete seating of the retraction archwire and to unify the anterior segment is to place both metal and elastic ligatures. Steel ligature wire (.009") is first figureeight tied to the segment. The archwire is then placed and secured with double-over tie elastic ligatures
BIOLOGIC CONSIDERATIONS
Effects of Overly Rapid Space Closure Space closure typically occurs more easily in high-angle patterns with weak musculature than in low-angle patterns with stronger musculature. The rate of closure can be increased, particularly in high-angle cases, by slightly raising the force level or using thinner archwires. However, more rapid space closure tan lead to loss of control of torque, rotation, and tip. Loss of torque control results in upper incisors being too upright at the end of space closure with spaces distal to the canines and a consequent unaesthetic appearance. The lost torque is difficult to regain. Also, rapid mesial movement of the upper molars can allow the palatal cusps to hang down, resulting in functional interferences, and rapid movement of the lower molars causes "rolling in"
• Reduced rotation control can be seen mainly in the teeth adjacent to extraction sites, which also tend to roll in if spaces are closed too rapidly • Reduced tip control produces unwanted movement of canines, premolars, and molars, along with a tendency for lateral open bite. In high-angle cases, where lower molars tip most freely, the elevated distal cusps create the possibility of a molar fulcrum effect • In some instances, excessive soft-tissue hyperplasia occurs at the extraction sites ,this is not only unhygienic, but it can prevent full space closure or allow spaces to reopen after treatment. Local gingival surgery may be necessary in such cases.
Problems During Space Closure
Since forces are directed from the first molars to anterior hooks on the arch wire, small spaces occasionally open between the first and second molars. This can be managed in one of three ways:
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The first and second molars can be tied together before beginning space closure K-2 elastic can be extended from the second molar to the archwire hook, in addition to the elastic or wire tieback to the first molar .or the tieback can be extended to the archwire hook from the second molar instead of the first molar The third method is particularly effective after the extraction sites and all other spaces have been closed.
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A damaged lower premolar or first molar bracket, either from careless use of biting sticks during bonding or from improper diet, can hinder space closure. Local thinning of the archwire can allow space closure to resume, but it, is better to replace the bracket
• Interference from opposing teeth sometimes restricts lower arch space closure, particularly if bracket placement was incorrect or a full-unit Class II molar relationship existed. Correcting the band or bracket position usually solves the problem. Occasionally a green stone must be used to selectively remove individual wings of lower brackets, or one or two lower brackets must be removed for a few weeks until the interference is eliminated. • As spaces close, the distal ends of the archwires will protrude more and more, and these protruding wires will tend to become bent gingivally by chewing forces. The archwires should be shortened whenever they extend more than 2mm from the molar tubes, both for patient comfort and for ease of archwire removal. Anything such as a ligature wire or an erupting molar that could restrict the steady distal emergence of thearchwire should be eliminated.
• It is not usually advisable to increase force levels above those provided by conventional elastic tiebacks. However, in some low-angle adult cases, force can be increased after two months if space closure is not occurring with normal force applied to fully level brackets. • Certain tissue factors can hinder full space closure with any kind of mechanics. Soft tissue build-up can result from poor plaque control or overly rapid space closure. The alveolar cortical plate, mesial to the lower first molars, tends to narrow after extraction of the second premolars, especially in lower-angle situations.
• Retained roots, ankylosed teeth, and bone sclerosis are other possible factors to be considered.
Inhibiting factors of space closure
1. Inadequate leveling, resulting in archwire binding 2. Posterior torque such that torquing and sliding cannot occur simultaneously 3. Blockage ofthe distal end ofthe main archwire by a ligature wire 4. Damaged or crushed brackets that bind the main archwire 5. Soft tissue resistance from build-up in extraction sites , 6. Cortical plate resistance from a narrowing of the alveolar bone in extraction sites 7. Excessive force, causing tipping and binding 8. Interferences from teeth or the opposing arch 9. Insufficient force
ROOT RESORPTION Light intermittant forces during closing spaces allows the resorbed cementum to heal and prevent further resorption .(Steadman, Angle-42) Root resorption is the same, irrespective of the treatment modality. Be it Begg or edgewise, it is accepted that extensive tooth displacement, torque movements and jiggling forces are responsible for resorption (REITAN-Graber and Swain)
CONCLUSION
