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Management of anteroposterior dental movements
Management of Anteroposterior Dental Movements Cecile Yoon-Tarlie and Cyril Sadowsky Management of the anteroposterior dimension is one of the most common problems encountered in orthodontic treatment. Numerous methods are available to treat the anteroposterior discrepancy depending on the treatment objectives and the available growth potential. Some situations may require a dental correction such as maxillary molar derotation or distal movements of both the upper and lower molars. If additional arch length is required, the extraction of permanent teeth may serve as an effective treatment modality as long as the anchorage requirements are considered. Other instances may necessitate skeletal correction via growth modification ie, maintaining the maxilla anteroposteriorly during growth. Regardless of the treatment mechanics used, it is important to understand how these different methods can affect the amount of arch length gained with respect to the treatment objectives for incisor and molar position. Once the objectives for incisor and molar position are determined, then the appropriate mechanotherapy can be used. (Semin Orthod 2000;6:58-66.) Copyright © 2000 by W.B. Saunders Company
ne of the most frequent problems encountered during orthodontic t r e a t m e n t is the m a n a g e m e n t of the anteroposterior dimension, specifically correcting the m o l a r relationship. It may require a skeletal a n d / o r dental change d e p e n d i n g on the t r e a t m e n t objectives for the patient and his or her potential growth. In some instances, correction of the malocclusion may simply require derotation of mesiopalatally rotated maxillary first molars; in o t h e r situations, there may be a n e e d to create space for the resolution of crowding. Several m e t h o d s exist to increase arch length. Dental m o v e m e n t s include m o l a r derotation and distal m o v e m e n t of the u p p e r a n d / o r lower molars. Additional space can also be created by removal of p e r m a n e n t teeth. If the decision has b e e n m a d e to extract teeth, specific t r e a t m e n t objectives must be established regarding the a n c h o r a g e requirements of
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From the Department of Orthodontics, University of lUinois at Chicago, Chicago, IL. Address correspondence to Cecile Yoon Tarlie, DDS, MS, Depar~ ment of Orthodontics, University of Illinois at Chicago, 801 S Paulina St, Chicago, IL 60612. Copyright © 2000 by W.B. Saunders Company 1073-8746/00/0501-0007510.00/0 58
the anterior and posterior segments. Regardless of the m e c h a n o t h e r a p y chosen, the potential incisor position anteroposteriorly must be considered, because most of the t r e a t m e n t modalities available to the clinician can positively or negatively influence incisor position. Therefore, based on the objectives of incisor position and molar relationship, an appropriate t r e a t m e n t m e t h o d can be selected.
Distal Molar M o v e m e n t s Maxillary first m o l a r position is an i m p o r t a n t consideration, especially in Class II malocclusions in which they are usually rotated mesiopalatally, j A variety of m e t h o d s can be used for molar derotation including the transpalatal arch, which can p r o d u c e an arch-length gain of 2.1 m m ~ (Fig 1). Equal and opposite m o m e n t s for reciprocal rotation or unequal m o m e n t s for rotation of one m o l a r and unilateral distal m o v e m e n t of the contralateral m o l a r can be applied. The quadhelix appliance can be used not only for m o l a r derotation, but also for transverse expansion of the m o l a r and p r e m o l a r region, allowing an increase in arch length 4 (Fig 2). Extraoral traction (headgear) may be used for m o l a r derota-
Seminars in Orthodontics, Vol 6, No 1 (Mmvh), 2000: pp 58-66
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A nteroposterior Dental Movements
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Figure3. Extraoral traction (cervical headgear). (ReFigure 1. Transpalatal arch. (Reprinted with permission from McNamara JA, Brudon WL. Orthodontic and Orthopedic Treatment in the Mixed Dentition. Ann Arbor, MI, Needham Press, 1993. -~) tion in conjunction with anteroposterior dental and skeletal changes (Fig 3). Maxillary extraoral traction (headgear) can apply a distal force to the maxillary molars, creating arch length. 5 Several authors 5-1° confirm the effectiveness of the h e a d g e a r for m o l a r distalization. Cetlin n advocated maxillary first m o l a r derotation using a transpalatal arch, followed by distal m o l a r movem e n t using the combination of extraoral force (headgear) and an intraoral force with a removable appliance (Fig 4). T h e removable appliance incorporates finger springs mesial to the maxillary first molars that are activated to p r o d u c e 30 gm of distal force while the h e a d g e a r exerts 150 gm of distal force per side. Despite these m o r e
printed with permission from McNamara JA, Brudon WL. Orthodontic and Orthopedic Treatment in the Mixed Dentition. Ann Arbor, MI, Needham Press, 1993. ~) c o m m o n m e t h o d s of gaining maxillary space, Baumrind, 12 in a recent, retrospective, stratified, r a n d o m sample of 48 subjects, questioned the belief that maxillary molars can be m o v e d distally. In conjunction with space gaining in the maxillary arch, lip b u m p e r s are often useful for gaining space in the m a n d i b u l a r arch. u-is T h e lip b u m p e r lies away f r o m the dentition, allowing the mentalis muscle to actively push on the labial aspect of the lip b u m p e r uprighting the molars, while the tongue may cause the m a n d i b u l a r incisors to tip labially and the canines and premolars to e x p a n d laterally ls-21 (Fig 5). In a study by O ' D o n n e l l et al, z2 25 patients ages 10 to 17 years received a m a n d i b u l a r lip b u m p e r for 12
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Figure 2. Soldered quad-helix appliance. (Reprinted with permission from McNamara JA, Brudon WL. Orthodontic and Orthopedic Treatment in the Mixed Dentition. Ann Arbor, MI, Needham Press, 1993. 3)
Figure 4. Maxillary distalizing plate with finger springs to the first molars. (Reprinted with permission from McNamaraJA, Brudon WL. Orthodontic and Orthopedic Treatment in tile Mixed Dentition. Ann Arbor, MI, Needham Press, 1993. 3)
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Yoon-Tarlie and Sadowsky
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Figure 5. Mandibular lip bumper. Occlusal view (A) and facial view (B) with the lip bumper approximately at the level of the gingival margin. (Reprinted with permission from McNamara JA, Brudon WL. Orthodontic and Orthopedic Treatment in the Mixed Dentition. Ann Arbor, MI, Needham Press, 1993. 3) months. T h e lip b u m p e r was placed approximately 2 m m f r o m the gingival margin. The m a n d i b u l a r incisors tipped labially 4.4 ° (mean) with 0.9 m m (mean) of advancement. T h e mandibular molars tipped distally 4.7 ° (mean) with 0.95 m m (mean) of distal m o v e m e n t . As a result, arch p e r i m e t e r increased a m e a n of 4.2 m m with additional arch-width increase. In a prospective, randomized, clinical trial, Davidovitch et al 2a r e p o r t e d that the increase in arch p e r i m e t e r and arch depth was attributed to 45% to 55% incisor proclination, 35% to 50% m o l a r distalization and distal tipping, and 5% to 10% transverse dimension increase. They considered their findings to be m o r e positive than those most previously r e p o r t e d with regard to incisor proclinadon being the p r e d o m i n a n t effect of the lip bumper. They used a lip b u m p e r with an anterior plastic shield placed 1.5 to 2.0 m m labial to the gingival third of the incisor. Location of the lip b u m p e r may also p r o d u c e different effects. Forces on the molars are significantly higher when the lip b u m p e r is placed 4 m m anterior to the incisors when c o m p a r e d with 2 ram, and at a position level with the gingival margin when
c o m p a r e d with the middle of the crown. 24 Also, the acrylic shield lip b u m p e r p r o d u c e d higher resting forces than the wire design. T a n d e m mechanics 95 also employ the technique of simultaneously distalizing maxillary and m a n d i b u l a r molars. T a n d e m mechanics include a maxillary extraoral traction force (headgear) to the maxillary molars, while simultaneously using Class Ill elastics attached to a lower arch wire via sliding hooks placed mesially to active o p e n coil springs. T h e active coil springs lie mesial to a v-stop placed in the arch wire n e a r the first p e r m a n e n t molars, allowing the wire to p r o t r u d e 1.0 m m anterior to the lower incisors. W h e n the wire is ligated directly to the lower incisors, the coils are slightly activated. Regular intraoral adjustments are m a d e placing the wire 1.0 m m labial to the incisors. The Class III elastics are worn to counteract the proclining force on the lower incisors. A n o t h e r m e t h o d for distalizing maxillary molars is the p e n d u l u m appliance, 26which is particularly useful for n o n c o m p l i a n t Class II patients. T h e appliance is able to e x p a n d the maxilla, while simultaneously rotating and distalizing the maxillary first molars. Ghosh and N a n d a 27 evaluated the effects of the p e n d u l u m appliance and r e p o r t e d that the maxillary molars m o v e d distally an average of 3.4 m m (SD 2.1 m m ) with 8.4 ° (SD 8.4 °) o f distal tipping. The m e a n mesial m o v e m e n t of the first premolars was 2.6 m m (SD 1.9 m m ) with 1.3 ° (SD 7.5 °) of mesial tipping. Thus, the premolars m o v e d mesially 0.75 m m for each millimeter of distal m o l a r movement. The maxillary second molars also m o v e d distally 2.3 m m (SD 1.4 m m ) with 12.0 m m of distal tip (SD 11°), while the effect of the third molars was quite variable. Utility arch wires f r o m the molars to the incisors can be used to reciprocally advance incisors while distal tipping the molars. Similarly, the reaction f r o m incisor intrusion with distal molar tipping using an intrusion utility arch wire or o t h e r intrusion mechanics to the first molars and four incisors can be used to gain arch length. It has b e e n suggested that distally tipping the molars can increase arch length 2 m m per side; however, incisor a d v a n c e m e n t and or proclination may occur, yielding additional space. Incorporating loops in the utility arch will allow additional space to be gained. Additional options for distal forces to the molars include repelling magnets 28-s° (Fig 6).
Anteroposterior Dental Movements
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Figure 7. Components of the Wilson distalizing arch. (Reprinted with permission from McNamara JA, Brudon WL. Orthodontic and Orthopedic Treatment in the Mixed Dentition. Ann Arbor, MI, Needham Press, 1993.3) Figure 6. Distalizing magnets. The right side shows before the activation of the magnets, and the left side shows distalization of the upper molars• (Reprinted with permission from McNamara JA, Brudon WL. Orthodontic and Orthopedic Treatment in the Mixed Dentition. Ann Arbor, MI, Needham Press, 1993.~) Gianelly et al 2s,29 reported on eight patients using repelling magnets for which the premolarincisor segments moved mesially 0.25 m m for each millimeter of distal m o v e m e n t of each first molar. Bondemark and KuroP ° reported on 10 patients who u n d e r w e n t similar treatment with repelling magnets attached to fixed appliances for which 4.2 m m (SD 0.9 mm) of distal molar m o v e m e n t was f o u n d with 8.0 ° (SD 3.5 °) of distal tipping. The second molars responded in a similar manner. The u p p e r incisors moved forward 1.8 m m with 6 ° of tipping indicating 0.35 m m of anchorage loss per millimeter of distal molar movement. Thus, magnets proved to be an effective method; however, they can be quite expensive. In addition, the force of the magnets decreases with small amounts of spacing between the magnets as the teeth move, necessitating frequent patient recall. Compressed stainlesssteel coils or nickel-titanium coils on fixed appliances sl,32 and Wilson distalizing mechanics 3s are other approaches to move or tip maxillary molars distally. The Wilson distalizing technique includes a maxillary arch fitted with coil springs compressed on the first molar tube (Fig 7). The pull of the Class II elastics produces a distalizing force through the compressed coil spring (Fig 8). Unlike the techniques using repelling magnets and NiTi coils, the Wilson technique 33 uses the lower arch as anchorage. Additional anchor-
age for the lower arch may include a lingual arch. Muse et al ~4 studied the effects of the Wilson technique and reported an average rate of distal maxillary molar m o v e m e n t to be 0.56 m m per m o n t h (SD 0.37 m m ' with a net distaliza-
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Figure 8. Lateral view of the Wilson distalizing arch before molar distalization (A). Posterior movement of the upper molars (B). Reprinted with permission from McNamara JA, Brudon WL. Orthodontic and Orthopedic Treatment in the Mixed Dentition. Ann Arbor, MI, Needham Press, 1993.3)
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Yoon-Tarlie and Sadowsky
tion of 2.16 m m (SD 1.31 mm). Similarly, Tweed 35 advocated Class II inter-arch elastics to sliding jigs to move molars distally using a biomechanically prepared lower arch for anchorage. Most of the previously m e n t i o n e d appliance mechanisms generate reciprocal forces to the anterior teeth, the effects of which must be considered if treatment objectives are to be satisfied. Specifically, if the incisor position is to be maintained anteroposteriorly, the choice of m e c h a n o t h e r a p y I n u s t be appropriate. It is accepted that incisor advancement is usually easy to achieve, but, unfortunately, it is often a side effect of orthodontic treatment that may not be indicated in a particular patient. It then becomes essential for the clinician to be able to have sufficient research data to support the effectiveness of the m e c h a n o t h e r a p y in satisfying the specific tooth movements desired. Often, the decision for using a particular appliance is based on practitioner preference, without regard for d o c u m e n t e d efficacy.
Leveling the Curve of Spee Many malocclusions include an excessive overbite often accompanied by an increased curve of Spee. Leveling the curve of Spee is usually p e r f o r m e d in the initial stages of treaunent. One of the potential disadvantages of leveling the arch includes incisor advancement and proclination, especially if it is not included as a treatment objective. It is a commonly held belief that to level the curve of Spee, increased arch circumference is required. However, it has been reported 36,a7 that less than 1 m m of arch circumference is n e e d e d to level the curve of Spee. Germane et aP s reported a nonlinear relationship of less than a 1:1 ratio based on a mathematical model for a catenary curve arch form and the Bonwill-Hawley arch form. However, Braun et al ~9 attributed leveling the curve of Spee mainly to a labial crown-tipping m o m e n t resulting in incisor advancement and proclination due to leveling arch wires of r o u n d cross-section. Their study applied a mathematical formula to pretreatmerit data and f o u n d that only 2 m m of incisor advancement (corresponding to 3.2 ° ofproclination) would be required to correct a deep curve of Spee of 9 m m (sum of right and left m a x i m u m depths of the curve to the occlusal plane). Their findings reported 52% less arch circumference
n e e d e d to level the curve than was reported by other studies. A study by Mitchell and Stewart 4° f o u n d that leveling the curve of Spee (3 to 4 mm) in six patients resulted in incisor advancem e n t of 2.1 m m with 1.5 m m of relative incisor intrusion and proclination. On the contrary, a study by C h u n g et a141 f o u n d that 24 of 33 n o n e x t r a c t i o n - t r e a t e d cases e x p e r i e n c e d increased arch circumference in which no significant relationships existed between leveling the curve of Spee and the variables of arch circumference, arch depth, and arch width. A recent study by A1Qabandi et a142 reported that even if initial rectangular leveling arch wires are used, incisor proclination still occurs. It is therefore important to use m e c h a n o t h e r a p y that would minimize lower incisor advancement during leveling of the curve of Spee, if maintaining lower incisor position anteroposteriorly is a treatment objective. To minimize incisor advancement, Class III elastics have been advocated during leveling of the mandibular arch, and the Class III elastics also allow the posterior teeth to be uprightedP 5 Even Tweed-type leveling arch wires were shown to increase incisor proclination. 43 The utility arch wire 44 incorporates incisor intrusion in an attempt to control the axial inclination of the incisors while maintaining their anteroposterior position; however, several studies 43,45 still f o u n d increased incisor proclination. The lower incisors were proclined 5.3 ° (SD 6.7 °) with the Ricketts utility arch wire versus 1.3 ° (SD 6.6 °) with Schudy mechanics, which include Class III elastics and full-arch mechanics. In the Ricketts group, the incisors were retroclined 3.8 ° posttreatmerit, while the incisors in the posttreatment Schudy sample tended to tip labially. It should not be surprising that incisor proclination was evident with the Ricketts utility arch wires, because the intrusive force is placed labial to the center of resistance of the anterior unit. Incisor proclination also occurs with the Mulligan 46 intrusion arch wire, which consists of a continuous arch wire engaging the first molars and four incisors and an off-center gable bend placed mesial to the molar teeth. The segmental approach to incisor intrusion 47 is likely to be more effective with minimal anteroposterior effects on the incisors. Segmental intrusion mechanics consist of two posterior segments and an anterior incisor segment, each with a segmental arch
Anteroposterior Dental Movements
wire, and an additional intrusion base arch wire. The intrusion arch wire is placed in the auxiliary tubes of the first molars and tied with singlepoint contact to the anterior segmental arch wire. T h e intrusion arch wire is tied back, creating a distal force such that the line of action of the intrusive force m o r e closely approximates the center of resistance of the anterior unit. For protrusive incisors, a segmental right and left intrusion arch wire can be used that hooks onto the posterior extensions of the anterior segment, thereby allowing the force to pass t h r o u g h the center of resistance of the anterior u n i t y
Differential Anteroposterior Movement Via Extraction O t h e r m e t h o d s of m a n a g i n g the anteroposterior c o m p o n e n t include the removal of p e r m a n e n t teeth. T h e decision to e m b a r k on an extraction treatment strategy should be based on the specific objectives for tooth m o v e m e n t . This leads to decisions c o n c e r n i n g incisor a n d m o l a r position in the anteroposterior dimension together with the issue of arch-length deficiency. Once the anchorage requirements have b e e n d e t e r m i n e d , the effective m o d e of t r e a t m e n t can be implem e n t e d to accomplish the t r e a t m e n t objectives. T h e r e are essentially three types of anchorage with respect to extraction mechanics: m a x i m u m anchorage, m o d e r a t e anchorage, and m i n i m u m anchorage. M a x i m u m anchorage is defined as maintaining the posterior segments (reactive unit) anteroposteriorly in their original location and allowing the anterior segment to move into the extraction site. Moderate anchorage occurs when 40% to 50% of the extraction site is occupied by the posterior segment moving anteriorly and 60% to 50% by the anterior segment moving posteriorly. M i n i m u m a n c h o r a g e occurs when the posterior segment moves mesially and occupies 50% or m o r e of the extraction site in which the anterior s e g m e n t is essentially maintained anteroposteriorly. T h e vertical dimension must also be considered when planning anchorage. If specific treatment objectives require increasing anchorage, it can be achieved in several ways. Conventionally, as m a n y teeth as possible are i n c o r p o r a t e d into the reactive a n c h o r a g e unit, including the second m o l a r teeth. T h e logic is that the resistance will be distributed over a
63
larger area, which reduces the pressure on the a n c h o r unit. In addition, incorporating an offcenter gable b e n d mesial to the most anterior tooth of the a n c h o r unit during full-arch sliding mechanics can create a large posterior m o m e n t that can e n h a n c e anchorage. 46,4s Several authors 49-53 have r e p o r t e d that without the use of additional appliances such as extraoral traction (headgear), lip bumpers, holding arches, and transpalatal bars to reinforce anchorage, 1.6 m m to 4 m m of mesial m o l a r m o v e m e n t occurred during space closure of the first p r e m o l a r extraction site. Even with the use of auxiliary appliances to control anchorage, 0.0 m m to 2.4 m m of anchorage loss has been reported. 49,51,54,55According to a finite e l e m e n t analysis and theoretical m o d e l p r o p o s e d by Bobak et al, 56 the transpalatal bar is able to control m o l a r rotation; however, it is limited in its ability to prevent tipping, suggesting its ineffectiveness for anteroposterior anchorage. Only with the use of auxiliaries that p r o d u c e distal forces, such as headgears and lip bumpers, is it possible to truly increase anchorage; however, their effectiveness is d e p e n d e n t on patient cooperation. Combinations of extraoral and intraoral m e a n s to e n h a n c e anchorage have shown to be effective in maintaining m o l a r position. 54 A recent article 57 has shown that m a x i m u m a n c h o r a g e can be achieved without the use of distal force auxiliaries. By incorporating an auxiliary utility arch wire tied to the incisors, the intrusive force placed on the incisor region produces a large distal crown/mesial root m o m e n t on the molars to counteract the mesial force p r o d u c e d by space closure mechanics (Fig 9). Mean m o l a r anchorage loss was 0.5 rnm with a range of 0 to 1.4 m m according to the m o d e l analysis and 0.7 m m with a range of 0 to 1.4 m m based on the cephalornetric superimpositions. Similar findings were r e p o r t e d by H a r t et al 4s using unequal m o m e n t s without additional auxiliaries to e n h a n c e anchorage. T h e s e g m e n t e d arch a p p r o a c h 5s,5° to managing a n c h o r a g e uses differing moment-to-force ratios between the posterior (beta) and anterior (alpha) segments. In an a t t e m p t to achieve m a x i m u m anchorage, the moment-to-force ratio of 12 to 13:1 is placed on the posterior segment and 10:1 on the anterior segment, respectively. In other words, root m o v e m e n t of the posterior segment opposes translation of the anterior segment. To achieve m i n i m u m anchorage, the
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ratios are reversed. For moderate anchorage, both the posterior and anterior segments receive the same moment-to-force ratio of 10:1, producing equal and opposite translation. Because the arch is divided into segments, the force systems at the reactive and active units are constant and defined, 59 allowing a more predictable result.
anteroposterior correction, most of the dental m o v e m e n t is the result of mesial movement of the mandibular arch with a minimal a m o u n t of distal m o v e m e n t of the maxillary arch. 6° The net result is not purely in the anteroposterior dimension. Both the transverse and vertical dimensions are affected. Class II elastics tend to extrude mandibular molars and maxillary incisors, resulting in a tipped occlusal plane. If the extrusive effects are not compensated for by sufficient mandibular growth, an opening or backward rotation of the mandible may result. To minimize the extrusive and tipping effects of interarch elastics, the mandibular arch typically has a fully engaged rectangular arch wire, including the second molars, allowing for a more horizontal vector of pull. A recent retrospective study compared the cephalometric records of two groups of patients with Class II Division I malocclusions treated nonextraction with fixed appliances and using Class II intermaxillary elastics. 6~ One group of 30 patients had full fixed appliances and the other group of 96 patients only had a lower utility arch wire from the molars to the incisors to establish "cortical anchorage" (Bioprogressive technique). The lower utility arch was as effective as full-arch mechanics in controlling anchorage. Both groups demonstrated extrusion and mesial m o v e m e n t of the lower molars, as well as proclination of the lower incisors.
Differential Jaw Growth
Conclusion
The majority of the patients u n d e r g o i n g orthodontic therapy are adolescents at an age a r o u n d or near their growth spurt. The possibility therefore exists to improve the anteroposterior relationship via differential growth. Because the mandible generally undergoes greater downward and forward displacement than does the maxilla, an opportunity exists to improve a Class II skeletal relationship. Typically, an extraoral force is applied against the maxillary first molar teeth in an attempt to restrain or redirect maxillary growth while allowing for mandibular differential growth.
Numerous techniques and appliances are available for correcting anteroposterior dental problems particularly involving distal m o v e m e n t of the u p p e r and lower molars. If a nonextraction approach is used, skeletal correction at the appropriate maturity level can be an effective means of molar correction. When crowding exists, the decision to extract teeth must be based on specific objectives for incisor and molar tooth positions that are required to resolve the particular problems of the malocclusion. Anchorage for the extraction patient can then be planned and appropriate m e c h a n o t h e r a p y used to control space closure. It is possible to achieve intraoral maximum anchorage by using appropriate moments and forces. However, when anterior teeth are relied on for anchorage, or when the opposing arch is used for anchorage, objectives
Figure 9. Intraoral maximum anchorage mechanics. This systern consists of an upper .016 × .016 stainlesssteel auxiliary arch wire with a posterior gable bend which causes a distal crown tip of the upper molar (a reaction to the incisor intrusion force). The distal crown tip of the molar counteracts the tendancy toward the mesial crown tip on the same resulting from the mesial force of space closing mechanics. Sliding mechanics occurs on either a stainless-steel .016 round wire or a .016 × .016 stainless-steel continuous arch wire (in an .018 × .095 Edgewise appliance). An off-center gable bend is placed mesial to the second premolar. A transpalatal arch is used for controlling mesiopalatal palatal molar rotation in the horizontal plane.
Correction Via Interarch Elastics Final correction of the molar relationship often includes the use o f i n t e r a r c h elastics. For Class II
Anleroposterior Dental Movements
may be compromised by lower incisor teeth being advanced and proclined. The effects on the lower incisor position, when leveling the curve of Spee with continuous arch mechanics must be considered even if initial rectangular arch wires are used to control proclination. The specific objectives for tooth movements, particularly incisor and molar positions, should be the primary determinant for instituting a particular treatment modality. Appropriate m e c h a n o therapy must be used to achieve a successful treatment outcome.
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40.
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tooth movement, occlusion and space conditions in the lower dental arch. EurJ Orthod 1980;2:257-265. Grossen J, Ingervall B. The effect of a lip bumper on lower dental arch dimensions and tooth positions. EurJ Orthod 1995;17:129-134. Osborn WS, Nanda RS, Currier GF. Mandibular arch perimeter changes with lip bumper treatment. Am J Orthod Dentofac Orthop 1991;99:527-532. Nevant CT, Buschang PH, Alexander RG, et al. Lip bumper therapy for gaining arch length. Am J Orthod Dentofac Orthop 1991;100:330-336. O'Donnell S, Nanda RS, Ghosh J. Perioral forces and dental changes resulting from mandibular lip bumper treatment. Am J Orthod Dentofac Orthop 1998;113:247255. Davidovitch M, McInnis D, Lindauer SJ. The effects of lip bumper therapy in the mixed dentition. Am J Orthod Dentofac Orthop 1997;111:52-58. HodgeJJ, Nanda RS, Gosh J, et al. Forces produced by lip bumpers on mandibular molars. AmJ Orthod Dentofac Orthop 1997;111:613-622. Hass DG. An assessment of tandem mechanics. Angle Orthod 1970;40:234-248. Hilgers JJ. The pendulum appliance for Class II noncompliance therapy.J Clin Orthod 1992;26:706-714. GhoshJ, Nanda RS. Evaluation of an intraoral maxillary molar distalization technique. Am J Orthod Dentofac Orthop 1996;110:639-646. Gianelly AA, Vaitas AS, Thomas WM. Distalization of molars with repelling magnets.J Clin Orthod 1988;22:4044. Gianelly AA, Vaitas AS, Thomas WM. The use of magnets to move molars distally. Am J Orthod Dentofac Orthop 1989;96:161-167. Bondemark L, KurolJ. Distalization of maxillary first and second molars simultaneously with repelling magnets. EurJ Orthod 1992;14:263-272. Jekel N, Rakosi T. Molar distalization by intra-oral force application. EurJ Orthod 1991;13:43-46. Gianelly AA. A strategy for nonextraction Class II treatment. Semin Orthod 1998;4:26-32. Wilson RC, Wilson WL. Enhanced Orthodontics. Book 1 and 2. Denvei, CO: Rocky Mountain Orthodontics, 1988. Muse DS, Fillman MJ, Emmerson WJ, et al. Molar and incisor changes with Wilson rapid molar distalization. AmJ Orthod Dentofac Orthop 1993;104:556-565. Tweed CH. Clinical Orthodontics. Vol 1. St. Louis, MO: CV Mosby, 1966. Baldridge DW. Leveling the curve of Spee: Its effect on mandibular arch length.J Clin Orthod 1969;3:26-41. Garcia R. Leveling the curve of Spee: A new prediction formula. J Tweed Found 1985;13:65-72. Germaine N, Staggers JA, Rubenstein L, et al. Arch length considerations due to the curve of Spee: A mathematical model. Am J Orthod Dentofac Orthop 1992;102:251-255. Braun S, Hnat WP, Johnson BE. The curve of Spee revisited. Am J Orthod Dentofac Orthop 1996;110:206210. Mitchell DL, Stewart WL. Documented leveling of the lower arch using metallic implants for reference. Am J Orthod 1973;63:526-532.
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41. Chung TS, Sadowsky PL, Wallace DS, et al. A three dimensional analysis of mandibular arch changes following curve of Spee leveling in nonextraction orthodontic treatment. IntJ Adult Orthod Orthognath Surg 1997;12: 109-112. 42. A1Qabandi AK, Sadowsky C, BeGole EA. A comparison of the effects of rectangular and round arch wires in levelling the curve of Spee. Am J Orthod Dentofac Orthop 1999;116:522-529. 43. Drake ML, Sinclair PM. Acomparison of the Ricketts and Tweed-type arch leveling techniques. AmJ Orthod Dentofac Orthop 1989;95:72-78. 44. Ricketts RM. Bioprogressive therapy as an answer to orthodontic needs. Part I. AmJ Orthod 1969;70:241-268. 45. Otto RL, Anholm MJ, Eugel GA. A comparative analysis of intrusion of incisor teeth achieved in adults and children according to facial type. AmJ Orthod Dentofac Orthop 1980;77:437-446. 46. Mulligan TE Common Sense Mechanics in Everyday Orthodontics. Phoenix, AZ: CSM Publishing, 1998. 47. Burstone CR. Deep overbite correction by intrusion. Am J Orthod 1977;72:1-22. 48. Hart A, Taft L, Greenberg SN. The effectiveness of differential moments in establishing and maintaining maximum anchorage. Am J Orthod Dentofac Orthop 1989;95:99-106. 49. Baker RW, Guay AH, Peterson HW. Current concepts of anchorage management. AmJ Orthod 1972;42:129-138. 50. Andreasen GF, Zwanziger D. A clinical evaluation of the differential force concept as applied to the edgewise bracket. AmJ Orthod 1980;78:25-40. 51. Ziegler P, Ingervall B. A clinical study of maxillary canine retraction with a retraction spring and with sliding
52. 53.
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55.
56.
57.
58. 59.
60. 61.
mechanics. AmJ Orthod Dentofac Orthop 1992;102:434442. Dincer M, Iscan HN. The effects of different sectional arches in canine retraction. EurJ Orthod 1994;16:317-323. Lotzof LP, Fine HA, Cisneros GJ. Canine retraction: A comparison of two preadjusted bracket systems. Am J Orthod Dentofac Orthop 1996;110:191-196. Paulsen RC, Spiedel TM, Issacson RJ. A laminigraphic study of cuspid retraction versus molar anchorage loss. Angle Orthod 1970;40:20-27. Gjessing P. Biomechanical design and clinical evaluation of a new canine retraction spring. AnaJ Orthod 1985;87: 353-362. Bobak V, Christiansen RL, Hollister SJ, et al. Stress related molar responses to the transpalatal arch: A finite element analysis. Am J Orthod Dentofac Orthop 1997; 112:512-518. Racjich MM, Sadowsky C. Efficacy ofintraarch mechanics using differential moments for achieving anchorage control in extraction cases. Am J Orthod Dentofac Orthop 1997;112:441-448. Burstone CJ. The segmented approach to space closure. AmJ Orthod 1982;82:361-378. Braun S, Sjursen RC, Legan HL. On the management of extraction sites. AmJ Orthod Dentofac Orthop 1997;112: 645-655. Profitt WR, Fields HWJr. Contemporary Orthodontics. 2nd ed. St. Louis, MO: C.V. Mosby, 1993:495-515. Ellen El(, Schneider BJ, Sellke T. A comparative study of anchorage in Bioprogressive versus standard edgewise treatment in Class II correction using intermaxillary elastic force. AmJ Orthod Dentofac Orthop 1998;114:430436.
ne of the most frequent problems encountered during orthodontic t r e a t m e n t is the m a n a g e m e n t of the anteroposterior dimension, specifically correcting the m o l a r relationship. It may require a skeletal a n d / o r dental change d e p e n d i n g on the t r e a t m e n t objectives for the patient and his or her potential growth. In some instances, correction of the malocclusion may simply require derotation of mesiopalatally rotated maxillary first molars; in o t h e r situations, there may be a n e e d to create space for the resolution of crowding. Several m e t h o d s exist to increase arch length. Dental m o v e m e n t s include m o l a r derotation and distal m o v e m e n t of the u p p e r a n d / o r lower molars. Additional space can also be created by removal of p e r m a n e n t teeth. If the decision has b e e n m a d e to extract teeth, specific t r e a t m e n t objectives must be established regarding the a n c h o r a g e requirements of
O
From the Department of Orthodontics, University of lUinois at Chicago, Chicago, IL. Address correspondence to Cecile Yoon Tarlie, DDS, MS, Depar~ ment of Orthodontics, University of Illinois at Chicago, 801 S Paulina St, Chicago, IL 60612. Copyright © 2000 by W.B. Saunders Company 1073-8746/00/0501-0007510.00/0 58
the anterior and posterior segments. Regardless of the m e c h a n o t h e r a p y chosen, the potential incisor position anteroposteriorly must be considered, because most of the t r e a t m e n t modalities available to the clinician can positively or negatively influence incisor position. Therefore, based on the objectives of incisor position and molar relationship, an appropriate t r e a t m e n t m e t h o d can be selected.
Distal Molar M o v e m e n t s Maxillary first m o l a r position is an i m p o r t a n t consideration, especially in Class II malocclusions in which they are usually rotated mesiopalatally, j A variety of m e t h o d s can be used for molar derotation including the transpalatal arch, which can p r o d u c e an arch-length gain of 2.1 m m ~ (Fig 1). Equal and opposite m o m e n t s for reciprocal rotation or unequal m o m e n t s for rotation of one m o l a r and unilateral distal m o v e m e n t of the contralateral m o l a r can be applied. The quadhelix appliance can be used not only for m o l a r derotation, but also for transverse expansion of the m o l a r and p r e m o l a r region, allowing an increase in arch length 4 (Fig 2). Extraoral traction (headgear) may be used for m o l a r derota-
Seminars in Orthodontics, Vol 6, No 1 (Mmvh), 2000: pp 58-66
59
A nteroposterior Dental Movements
'
_..a
//
"\
Figure3. Extraoral traction (cervical headgear). (ReFigure 1. Transpalatal arch. (Reprinted with permission from McNamara JA, Brudon WL. Orthodontic and Orthopedic Treatment in the Mixed Dentition. Ann Arbor, MI, Needham Press, 1993. -~) tion in conjunction with anteroposterior dental and skeletal changes (Fig 3). Maxillary extraoral traction (headgear) can apply a distal force to the maxillary molars, creating arch length. 5 Several authors 5-1° confirm the effectiveness of the h e a d g e a r for m o l a r distalization. Cetlin n advocated maxillary first m o l a r derotation using a transpalatal arch, followed by distal m o l a r movem e n t using the combination of extraoral force (headgear) and an intraoral force with a removable appliance (Fig 4). T h e removable appliance incorporates finger springs mesial to the maxillary first molars that are activated to p r o d u c e 30 gm of distal force while the h e a d g e a r exerts 150 gm of distal force per side. Despite these m o r e
printed with permission from McNamara JA, Brudon WL. Orthodontic and Orthopedic Treatment in the Mixed Dentition. Ann Arbor, MI, Needham Press, 1993. ~) c o m m o n m e t h o d s of gaining maxillary space, Baumrind, 12 in a recent, retrospective, stratified, r a n d o m sample of 48 subjects, questioned the belief that maxillary molars can be m o v e d distally. In conjunction with space gaining in the maxillary arch, lip b u m p e r s are often useful for gaining space in the m a n d i b u l a r arch. u-is T h e lip b u m p e r lies away f r o m the dentition, allowing the mentalis muscle to actively push on the labial aspect of the lip b u m p e r uprighting the molars, while the tongue may cause the m a n d i b u l a r incisors to tip labially and the canines and premolars to e x p a n d laterally ls-21 (Fig 5). In a study by O ' D o n n e l l et al, z2 25 patients ages 10 to 17 years received a m a n d i b u l a r lip b u m p e r for 12
/ ( ,/
/I'/(~ \
/
Figure 2. Soldered quad-helix appliance. (Reprinted with permission from McNamara JA, Brudon WL. Orthodontic and Orthopedic Treatment in the Mixed Dentition. Ann Arbor, MI, Needham Press, 1993. 3)
Figure 4. Maxillary distalizing plate with finger springs to the first molars. (Reprinted with permission from McNamaraJA, Brudon WL. Orthodontic and Orthopedic Treatment in tile Mixed Dentition. Ann Arbor, MI, Needham Press, 1993. 3)
60
Yoon-Tarlie and Sadowsky
/
Figure 5. Mandibular lip bumper. Occlusal view (A) and facial view (B) with the lip bumper approximately at the level of the gingival margin. (Reprinted with permission from McNamara JA, Brudon WL. Orthodontic and Orthopedic Treatment in the Mixed Dentition. Ann Arbor, MI, Needham Press, 1993. 3) months. T h e lip b u m p e r was placed approximately 2 m m f r o m the gingival margin. The m a n d i b u l a r incisors tipped labially 4.4 ° (mean) with 0.9 m m (mean) of advancement. T h e mandibular molars tipped distally 4.7 ° (mean) with 0.95 m m (mean) of distal m o v e m e n t . As a result, arch p e r i m e t e r increased a m e a n of 4.2 m m with additional arch-width increase. In a prospective, randomized, clinical trial, Davidovitch et al 2a r e p o r t e d that the increase in arch p e r i m e t e r and arch depth was attributed to 45% to 55% incisor proclination, 35% to 50% m o l a r distalization and distal tipping, and 5% to 10% transverse dimension increase. They considered their findings to be m o r e positive than those most previously r e p o r t e d with regard to incisor proclinadon being the p r e d o m i n a n t effect of the lip bumper. They used a lip b u m p e r with an anterior plastic shield placed 1.5 to 2.0 m m labial to the gingival third of the incisor. Location of the lip b u m p e r may also p r o d u c e different effects. Forces on the molars are significantly higher when the lip b u m p e r is placed 4 m m anterior to the incisors when c o m p a r e d with 2 ram, and at a position level with the gingival margin when
c o m p a r e d with the middle of the crown. 24 Also, the acrylic shield lip b u m p e r p r o d u c e d higher resting forces than the wire design. T a n d e m mechanics 95 also employ the technique of simultaneously distalizing maxillary and m a n d i b u l a r molars. T a n d e m mechanics include a maxillary extraoral traction force (headgear) to the maxillary molars, while simultaneously using Class Ill elastics attached to a lower arch wire via sliding hooks placed mesially to active o p e n coil springs. T h e active coil springs lie mesial to a v-stop placed in the arch wire n e a r the first p e r m a n e n t molars, allowing the wire to p r o t r u d e 1.0 m m anterior to the lower incisors. W h e n the wire is ligated directly to the lower incisors, the coils are slightly activated. Regular intraoral adjustments are m a d e placing the wire 1.0 m m labial to the incisors. The Class III elastics are worn to counteract the proclining force on the lower incisors. A n o t h e r m e t h o d for distalizing maxillary molars is the p e n d u l u m appliance, 26which is particularly useful for n o n c o m p l i a n t Class II patients. T h e appliance is able to e x p a n d the maxilla, while simultaneously rotating and distalizing the maxillary first molars. Ghosh and N a n d a 27 evaluated the effects of the p e n d u l u m appliance and r e p o r t e d that the maxillary molars m o v e d distally an average of 3.4 m m (SD 2.1 m m ) with 8.4 ° (SD 8.4 °) o f distal tipping. The m e a n mesial m o v e m e n t of the first premolars was 2.6 m m (SD 1.9 m m ) with 1.3 ° (SD 7.5 °) of mesial tipping. Thus, the premolars m o v e d mesially 0.75 m m for each millimeter of distal m o l a r movement. The maxillary second molars also m o v e d distally 2.3 m m (SD 1.4 m m ) with 12.0 m m of distal tip (SD 11°), while the effect of the third molars was quite variable. Utility arch wires f r o m the molars to the incisors can be used to reciprocally advance incisors while distal tipping the molars. Similarly, the reaction f r o m incisor intrusion with distal molar tipping using an intrusion utility arch wire or o t h e r intrusion mechanics to the first molars and four incisors can be used to gain arch length. It has b e e n suggested that distally tipping the molars can increase arch length 2 m m per side; however, incisor a d v a n c e m e n t and or proclination may occur, yielding additional space. Incorporating loops in the utility arch will allow additional space to be gained. Additional options for distal forces to the molars include repelling magnets 28-s° (Fig 6).
Anteroposterior Dental Movements
61
040
BIMETRIC ARCH
OMEGA STOP i
COIL SPRING
.~.,~
ELASTIC HOOK
Figure 7. Components of the Wilson distalizing arch. (Reprinted with permission from McNamara JA, Brudon WL. Orthodontic and Orthopedic Treatment in the Mixed Dentition. Ann Arbor, MI, Needham Press, 1993.3) Figure 6. Distalizing magnets. The right side shows before the activation of the magnets, and the left side shows distalization of the upper molars• (Reprinted with permission from McNamara JA, Brudon WL. Orthodontic and Orthopedic Treatment in the Mixed Dentition. Ann Arbor, MI, Needham Press, 1993.~) Gianelly et al 2s,29 reported on eight patients using repelling magnets for which the premolarincisor segments moved mesially 0.25 m m for each millimeter of distal m o v e m e n t of each first molar. Bondemark and KuroP ° reported on 10 patients who u n d e r w e n t similar treatment with repelling magnets attached to fixed appliances for which 4.2 m m (SD 0.9 mm) of distal molar m o v e m e n t was f o u n d with 8.0 ° (SD 3.5 °) of distal tipping. The second molars responded in a similar manner. The u p p e r incisors moved forward 1.8 m m with 6 ° of tipping indicating 0.35 m m of anchorage loss per millimeter of distal molar movement. Thus, magnets proved to be an effective method; however, they can be quite expensive. In addition, the force of the magnets decreases with small amounts of spacing between the magnets as the teeth move, necessitating frequent patient recall. Compressed stainlesssteel coils or nickel-titanium coils on fixed appliances sl,32 and Wilson distalizing mechanics 3s are other approaches to move or tip maxillary molars distally. The Wilson distalizing technique includes a maxillary arch fitted with coil springs compressed on the first molar tube (Fig 7). The pull of the Class II elastics produces a distalizing force through the compressed coil spring (Fig 8). Unlike the techniques using repelling magnets and NiTi coils, the Wilson technique 33 uses the lower arch as anchorage. Additional anchor-
age for the lower arch may include a lingual arch. Muse et al ~4 studied the effects of the Wilson technique and reported an average rate of distal maxillary molar m o v e m e n t to be 0.56 m m per m o n t h (SD 0.37 m m ' with a net distaliza-
B
Figure 8. Lateral view of the Wilson distalizing arch before molar distalization (A). Posterior movement of the upper molars (B). Reprinted with permission from McNamara JA, Brudon WL. Orthodontic and Orthopedic Treatment in the Mixed Dentition. Ann Arbor, MI, Needham Press, 1993.3)
62
Yoon-Tarlie and Sadowsky
tion of 2.16 m m (SD 1.31 mm). Similarly, Tweed 35 advocated Class II inter-arch elastics to sliding jigs to move molars distally using a biomechanically prepared lower arch for anchorage. Most of the previously m e n t i o n e d appliance mechanisms generate reciprocal forces to the anterior teeth, the effects of which must be considered if treatment objectives are to be satisfied. Specifically, if the incisor position is to be maintained anteroposteriorly, the choice of m e c h a n o t h e r a p y I n u s t be appropriate. It is accepted that incisor advancement is usually easy to achieve, but, unfortunately, it is often a side effect of orthodontic treatment that may not be indicated in a particular patient. It then becomes essential for the clinician to be able to have sufficient research data to support the effectiveness of the m e c h a n o t h e r a p y in satisfying the specific tooth movements desired. Often, the decision for using a particular appliance is based on practitioner preference, without regard for d o c u m e n t e d efficacy.
Leveling the Curve of Spee Many malocclusions include an excessive overbite often accompanied by an increased curve of Spee. Leveling the curve of Spee is usually p e r f o r m e d in the initial stages of treaunent. One of the potential disadvantages of leveling the arch includes incisor advancement and proclination, especially if it is not included as a treatment objective. It is a commonly held belief that to level the curve of Spee, increased arch circumference is required. However, it has been reported 36,a7 that less than 1 m m of arch circumference is n e e d e d to level the curve of Spee. Germane et aP s reported a nonlinear relationship of less than a 1:1 ratio based on a mathematical model for a catenary curve arch form and the Bonwill-Hawley arch form. However, Braun et al ~9 attributed leveling the curve of Spee mainly to a labial crown-tipping m o m e n t resulting in incisor advancement and proclination due to leveling arch wires of r o u n d cross-section. Their study applied a mathematical formula to pretreatmerit data and f o u n d that only 2 m m of incisor advancement (corresponding to 3.2 ° ofproclination) would be required to correct a deep curve of Spee of 9 m m (sum of right and left m a x i m u m depths of the curve to the occlusal plane). Their findings reported 52% less arch circumference
n e e d e d to level the curve than was reported by other studies. A study by Mitchell and Stewart 4° f o u n d that leveling the curve of Spee (3 to 4 mm) in six patients resulted in incisor advancem e n t of 2.1 m m with 1.5 m m of relative incisor intrusion and proclination. On the contrary, a study by C h u n g et a141 f o u n d that 24 of 33 n o n e x t r a c t i o n - t r e a t e d cases e x p e r i e n c e d increased arch circumference in which no significant relationships existed between leveling the curve of Spee and the variables of arch circumference, arch depth, and arch width. A recent study by A1Qabandi et a142 reported that even if initial rectangular leveling arch wires are used, incisor proclination still occurs. It is therefore important to use m e c h a n o t h e r a p y that would minimize lower incisor advancement during leveling of the curve of Spee, if maintaining lower incisor position anteroposteriorly is a treatment objective. To minimize incisor advancement, Class III elastics have been advocated during leveling of the mandibular arch, and the Class III elastics also allow the posterior teeth to be uprightedP 5 Even Tweed-type leveling arch wires were shown to increase incisor proclination. 43 The utility arch wire 44 incorporates incisor intrusion in an attempt to control the axial inclination of the incisors while maintaining their anteroposterior position; however, several studies 43,45 still f o u n d increased incisor proclination. The lower incisors were proclined 5.3 ° (SD 6.7 °) with the Ricketts utility arch wire versus 1.3 ° (SD 6.6 °) with Schudy mechanics, which include Class III elastics and full-arch mechanics. In the Ricketts group, the incisors were retroclined 3.8 ° posttreatmerit, while the incisors in the posttreatment Schudy sample tended to tip labially. It should not be surprising that incisor proclination was evident with the Ricketts utility arch wires, because the intrusive force is placed labial to the center of resistance of the anterior unit. Incisor proclination also occurs with the Mulligan 46 intrusion arch wire, which consists of a continuous arch wire engaging the first molars and four incisors and an off-center gable bend placed mesial to the molar teeth. The segmental approach to incisor intrusion 47 is likely to be more effective with minimal anteroposterior effects on the incisors. Segmental intrusion mechanics consist of two posterior segments and an anterior incisor segment, each with a segmental arch
Anteroposterior Dental Movements
wire, and an additional intrusion base arch wire. The intrusion arch wire is placed in the auxiliary tubes of the first molars and tied with singlepoint contact to the anterior segmental arch wire. T h e intrusion arch wire is tied back, creating a distal force such that the line of action of the intrusive force m o r e closely approximates the center of resistance of the anterior unit. For protrusive incisors, a segmental right and left intrusion arch wire can be used that hooks onto the posterior extensions of the anterior segment, thereby allowing the force to pass t h r o u g h the center of resistance of the anterior u n i t y
Differential Anteroposterior Movement Via Extraction O t h e r m e t h o d s of m a n a g i n g the anteroposterior c o m p o n e n t include the removal of p e r m a n e n t teeth. T h e decision to e m b a r k on an extraction treatment strategy should be based on the specific objectives for tooth m o v e m e n t . This leads to decisions c o n c e r n i n g incisor a n d m o l a r position in the anteroposterior dimension together with the issue of arch-length deficiency. Once the anchorage requirements have b e e n d e t e r m i n e d , the effective m o d e of t r e a t m e n t can be implem e n t e d to accomplish the t r e a t m e n t objectives. T h e r e are essentially three types of anchorage with respect to extraction mechanics: m a x i m u m anchorage, m o d e r a t e anchorage, and m i n i m u m anchorage. M a x i m u m anchorage is defined as maintaining the posterior segments (reactive unit) anteroposteriorly in their original location and allowing the anterior segment to move into the extraction site. Moderate anchorage occurs when 40% to 50% of the extraction site is occupied by the posterior segment moving anteriorly and 60% to 50% by the anterior segment moving posteriorly. M i n i m u m a n c h o r a g e occurs when the posterior segment moves mesially and occupies 50% or m o r e of the extraction site in which the anterior s e g m e n t is essentially maintained anteroposteriorly. T h e vertical dimension must also be considered when planning anchorage. If specific treatment objectives require increasing anchorage, it can be achieved in several ways. Conventionally, as m a n y teeth as possible are i n c o r p o r a t e d into the reactive a n c h o r a g e unit, including the second m o l a r teeth. T h e logic is that the resistance will be distributed over a
63
larger area, which reduces the pressure on the a n c h o r unit. In addition, incorporating an offcenter gable b e n d mesial to the most anterior tooth of the a n c h o r unit during full-arch sliding mechanics can create a large posterior m o m e n t that can e n h a n c e anchorage. 46,4s Several authors 49-53 have r e p o r t e d that without the use of additional appliances such as extraoral traction (headgear), lip bumpers, holding arches, and transpalatal bars to reinforce anchorage, 1.6 m m to 4 m m of mesial m o l a r m o v e m e n t occurred during space closure of the first p r e m o l a r extraction site. Even with the use of auxiliary appliances to control anchorage, 0.0 m m to 2.4 m m of anchorage loss has been reported. 49,51,54,55According to a finite e l e m e n t analysis and theoretical m o d e l p r o p o s e d by Bobak et al, 56 the transpalatal bar is able to control m o l a r rotation; however, it is limited in its ability to prevent tipping, suggesting its ineffectiveness for anteroposterior anchorage. Only with the use of auxiliaries that p r o d u c e distal forces, such as headgears and lip bumpers, is it possible to truly increase anchorage; however, their effectiveness is d e p e n d e n t on patient cooperation. Combinations of extraoral and intraoral m e a n s to e n h a n c e anchorage have shown to be effective in maintaining m o l a r position. 54 A recent article 57 has shown that m a x i m u m a n c h o r a g e can be achieved without the use of distal force auxiliaries. By incorporating an auxiliary utility arch wire tied to the incisors, the intrusive force placed on the incisor region produces a large distal crown/mesial root m o m e n t on the molars to counteract the mesial force p r o d u c e d by space closure mechanics (Fig 9). Mean m o l a r anchorage loss was 0.5 rnm with a range of 0 to 1.4 m m according to the m o d e l analysis and 0.7 m m with a range of 0 to 1.4 m m based on the cephalornetric superimpositions. Similar findings were r e p o r t e d by H a r t et al 4s using unequal m o m e n t s without additional auxiliaries to e n h a n c e anchorage. T h e s e g m e n t e d arch a p p r o a c h 5s,5° to managing a n c h o r a g e uses differing moment-to-force ratios between the posterior (beta) and anterior (alpha) segments. In an a t t e m p t to achieve m a x i m u m anchorage, the moment-to-force ratio of 12 to 13:1 is placed on the posterior segment and 10:1 on the anterior segment, respectively. In other words, root m o v e m e n t of the posterior segment opposes translation of the anterior segment. To achieve m i n i m u m anchorage, the
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Yoon-Tarlie and Sadowsky
ratios are reversed. For moderate anchorage, both the posterior and anterior segments receive the same moment-to-force ratio of 10:1, producing equal and opposite translation. Because the arch is divided into segments, the force systems at the reactive and active units are constant and defined, 59 allowing a more predictable result.
anteroposterior correction, most of the dental m o v e m e n t is the result of mesial movement of the mandibular arch with a minimal a m o u n t of distal m o v e m e n t of the maxillary arch. 6° The net result is not purely in the anteroposterior dimension. Both the transverse and vertical dimensions are affected. Class II elastics tend to extrude mandibular molars and maxillary incisors, resulting in a tipped occlusal plane. If the extrusive effects are not compensated for by sufficient mandibular growth, an opening or backward rotation of the mandible may result. To minimize the extrusive and tipping effects of interarch elastics, the mandibular arch typically has a fully engaged rectangular arch wire, including the second molars, allowing for a more horizontal vector of pull. A recent retrospective study compared the cephalometric records of two groups of patients with Class II Division I malocclusions treated nonextraction with fixed appliances and using Class II intermaxillary elastics. 6~ One group of 30 patients had full fixed appliances and the other group of 96 patients only had a lower utility arch wire from the molars to the incisors to establish "cortical anchorage" (Bioprogressive technique). The lower utility arch was as effective as full-arch mechanics in controlling anchorage. Both groups demonstrated extrusion and mesial m o v e m e n t of the lower molars, as well as proclination of the lower incisors.
Differential Jaw Growth
Conclusion
The majority of the patients u n d e r g o i n g orthodontic therapy are adolescents at an age a r o u n d or near their growth spurt. The possibility therefore exists to improve the anteroposterior relationship via differential growth. Because the mandible generally undergoes greater downward and forward displacement than does the maxilla, an opportunity exists to improve a Class II skeletal relationship. Typically, an extraoral force is applied against the maxillary first molar teeth in an attempt to restrain or redirect maxillary growth while allowing for mandibular differential growth.
Numerous techniques and appliances are available for correcting anteroposterior dental problems particularly involving distal m o v e m e n t of the u p p e r and lower molars. If a nonextraction approach is used, skeletal correction at the appropriate maturity level can be an effective means of molar correction. When crowding exists, the decision to extract teeth must be based on specific objectives for incisor and molar tooth positions that are required to resolve the particular problems of the malocclusion. Anchorage for the extraction patient can then be planned and appropriate m e c h a n o t h e r a p y used to control space closure. It is possible to achieve intraoral maximum anchorage by using appropriate moments and forces. However, when anterior teeth are relied on for anchorage, or when the opposing arch is used for anchorage, objectives
Figure 9. Intraoral maximum anchorage mechanics. This systern consists of an upper .016 × .016 stainlesssteel auxiliary arch wire with a posterior gable bend which causes a distal crown tip of the upper molar (a reaction to the incisor intrusion force). The distal crown tip of the molar counteracts the tendancy toward the mesial crown tip on the same resulting from the mesial force of space closing mechanics. Sliding mechanics occurs on either a stainless-steel .016 round wire or a .016 × .016 stainless-steel continuous arch wire (in an .018 × .095 Edgewise appliance). An off-center gable bend is placed mesial to the second premolar. A transpalatal arch is used for controlling mesiopalatal palatal molar rotation in the horizontal plane.
Correction Via Interarch Elastics Final correction of the molar relationship often includes the use o f i n t e r a r c h elastics. For Class II
Anleroposterior Dental Movements
may be compromised by lower incisor teeth being advanced and proclined. The effects on the lower incisor position, when leveling the curve of Spee with continuous arch mechanics must be considered even if initial rectangular arch wires are used to control proclination. The specific objectives for tooth movements, particularly incisor and molar positions, should be the primary determinant for instituting a particular treatment modality. Appropriate m e c h a n o therapy must be used to achieve a successful treatment outcome.
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