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Treatment Strategies for Midline Discrepancies Ravindra Nanda and MichaeIJ. Margolis

Midline discrepancies are among the most complex and commonly seen problems in clinical orthodontics, Skeletal, soft tissue, and dentoavleolar asymmetries may be present alone or in combination in a patient who has noncoincident midlines. Due to the multifactorial etiology of midline discrep- ancies, a thorough understanding of the components that may contribute to the problem is essential for a correct diagnosis. The treatment mechanics that are derived from the treatment plan must be biomechanically oriented in order to achieve the desired results without the introduction of adverse side effects. This article discusses strategies to correctly diagnose and treat patients with significant midline asymmetries. (Semin Orthod 1996;2:84-89.) Copyright © 1996 by W.B. Saunders Company

O cclusal asymmetries are commonly encoun- tered problems that pose both diagnostic

and treatment difficulties in orthodontic pa- tients. Of all occlusal asymmetries, midline dis- crepancies are the most obvious from the pa- tients' perspective. Midline discrepancies may be isolated, or may occur in concert with other occlusal asymmetries, particularly molar occlu- sion asymmetry, or the angle subdivision maloc- clusions. Midline asymmetries may have a variety of origins including skeletal asymmetry, dentoal- veolar changes, and functional shifts of the mandible.

Anecdotal clinical evidence would suggest a correlation between occlusal and skeletal or facial asymmetry, however, a number of studies have failed to demonstrate any strong interrela- tionships. Letzer 1 found no relationship between occlusion and skeletal asymmetry when using posteroanterior (PA) radiographs to assess skel- etal position. A]avi 2 and Rose 3 found no relation- ship between occlusion and facial or mandibular asymmetry in Angle Class II subdivision malocclu- sion patients. It is also possible to have normal

From the Department of Orthodontics, University of Connecticut, School of Dental Medicine, Farmington, CT.

Address correspondence to Ravindra Nanda, BDS, MDS, PhD, Department of Orthodontics, University of Connecticut, School of Dental Medicine, Farmington, CT 06030-1725.

Copyright © 1996 by W.B. Saunders Company 1073-8746/96/0202-000355. 00/0

occlusal relationships in the presence of facial asymmetry, this being the result of compensa- tions by the dentition. Skeletal asymmetry has been described in subjects with normal occlu- sion in studies byVig 4 and Shah. 5

Database for Diagnosis of Midline Asymmetry

In each patient an appropriate database for detection of midline asymmetries should be assembled to aid in making an appropriate diagnosis of the nature, extent, and location of the midline asymmetry. This should include a detailed facial and intraoral examination, intra- and extraoral photographs or video, 6 dental models tr immed to centric relation occlusion, an occlusogram, a lateral cephalogram, a posteroan- terior cephalogram, a 45 ° cephalogram, a pan- oramic radiograph, and a submental vertex radio- graph.

The facial and intraoral examination aids in the visualization of the facial and dental mid- lines, as well as their interrelationship. Axial inclinations of the incisors and their relationship to the facial midline should be noted at the clinical examination. The clinical examination should also include a functional analysis of mandibular movements in an attempt to deter- mine both centric relation (CR) and centric occlusion (CO) and to record any significant discrepancies between them. Anterior and poste

84 Seminars in Orthodontics, Vol 2, No 2 (June), 1996: pp 84-89

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Treatment Strategies for Midline Discrepancies 85

rior crossbites should alert the clinician to the possibility of a functional shift and a significant CO-CR discrepancy. Clinical pho tographs or a video should record any observations made at the clinical examination, but are not an ad- equate substitute for a comprehensive clinical examination.

Dental models, correctly t r immed to centric relation, allow for an examinat ion for dental abnormalit ies that may contr ibute to midline discrepancies. These may include missing or p remature loss of teeth with resultant drifting of teeth, tooth size discrepancies, crowding, and posterior occlusion discrepancies. Once a dental midline discrepancy is noted, the clinician should first seek the most obvious cause, eg, asymmetric crowding. Axial inclinations of incisors and pos- terior teeth should be examined clinically, and evaluated on the dental models. It is impor tan t to differentiate midline and molar discrepancies caused by t ipping of teeth f rom those midline discrepancies occurr ing with upr ight teeth. Skel- etal asymmetry is significantly more likely in the latter case. In patients where drifting of perma- nen t teeth is suspected because of axial inclina- tion differences, the pat ient a n d / o r parents should be quest ioned regarding early loss of pr imary teeth. An occlusogram, a 1:1 representa- tion of the occlusal view of the maxillary and mandibular arches, or iented transversely on the midpalatal raphe, is an excellent aid for the identification and t rea tment p lanning of mid- line discrepancies. The occlusogram is helpful in graphically de termining the location and extent of the asymmetry, but is also a valuable aid in t rea tment p lanning as it represents an easily adjustable and quantifiable paper version of a wax set-up.

Radiographs, particularly the posteroanter ior (PA) cephalogram, are invaluable aids in the diagnosis of midline discrepancies. The PA cepha- logram should be taken whenever facial or mid- line asymmetries are detected on clinical exami- nation. A tracing, combined with construct ion of skeletal and apical base midlines, as well as horizontal reference lines will assist the clinician in de termining the extent and location of mid- line asymmetries. A panoramic radiograph is useful for the detection of missing or ectopically erupt ing teeth which may contr ibute to a mid- line discrepancy and also gives an adequate screening view of the condyles to help detect

gross skeletal le f t / r ight asymmetries. The pan- oramic radiograph can also be used in determin- ing axial inclinations of poster ior teeth. The lateral cephalogram, though not specifically taken for the purpose of detect ing asymmetry, may be helpful for assisting in the detect ion of mandibu- lar asymmetry, particularly in the vertical plane of space, when significant differences in the lower mandibular bo rde r may be noted. Forty- five degree cephalograms provide the best views of the axial inclinations of the posterior teeth, and as such are helpful in de te rmin ing whether the presence of molar asymmetry is related to t ipping of these teeth in the anteroposter ior plane of space. Finally, a submental vertex radio- graph is an excellent way of conf i rming and quantifying mandibula r asymmetries.

Diagnosis of Midline Asymmetry

Because midline asymmetries may occur in the presence or absence of skeletal or facial asymme- try, an accurate diagnosis of the p rob lem is critical to formulat ing an appropr ia te t rea tment plan. The goal of the diagnosis is to localize and quantify the extent o f the asymmetry. There are at least six impor tan t midlines that must be de te rmined and these include the facial midline, the skeletal midline, the maxillary and mandibu- lar apical base midlines, and the maxillary and mandibular dental midlines.

Clinically, a de terminat ion of the location of the facial midline must be made based on mid- line soft tissue structures including the intercan- thus point, nasal base, nasal tip, phi l t rum, and chin midpoint . 7 These can be de te rmined most accurately f rom a superior view of a supine pat ient 's face with the aid of a piece of dental floss s tretched f rom above the forehead to below the chin. A true frontal view of the face is bet ter achieved if the pat ient is in a standing position. An inferior view of the mandible often helps in the location of the chin midpoin t in cases of mandibular asymmetry. Excellent clinical photo- graphs or a video can also aid in de te rmin ing soft tissue midlines, but a careful clinical examina- tion must be considered the gold standard in the evaluation of facial midlines.

The skeletal, apical base, and dental midlines are best de te rmined with the use of PA radio- graphs. It is essential that the PA radiograph be taken in centric relation to eliminate the possibil-

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86 Nanda and Margolis

ity of a functional shift of the mandible. A skeletal midline can be constructed by connect- ing the root of the crista galli, the midpoint of the medial orbital rims and the anter ior nasal spine. The maxillary and mandibular apical base midlines are then de te rmined by taking the midpoin t of a line connect ing the centers of resistance of the central incisors and projecting these midpoints to the occlusal plane. Maxillary and mandibular dental midlines are also deter- mined f rom the PA radiograph with the help of centric relation dental models.

Finally, assessment of molar occlusion asymme- try must be per fo rmed . Data f rom the clinical examination, the dental models, and the radio- graphs are used to assess axial inclinations of poster ior teeth and are combined to de te rmine the extent of molar occlusion asymmetry. In asymmetric Angle Class II subdivision malocclu- sions, maxillary molars should be evaluated for forward crown tip on the Class II side, a condi- tion that may result f rom premature loss of p r imary second molar teeth. In these patients, or thodont ic mechanics should tip the molar crown distally. Combinat ions of midline and molar asymmetry often imply skeletal or apical base asymmetry and are problems that may be beyond the scope of correct ion by conventional or thodont ic t rea tment alone and that may re- quire a combined or thognathic surgical ap- proach where these asymmetries are significantly great.

Determinat ion of Treatment Plan

Once the appropr ia te diagnosis has been made it is impor tan t to formulate a t rea tment plan f rom which a mechanotherapeu t ic plan can be de- vised. The first decision in p lanning t rea tment and mechanics is the selection of a t rea tment midline. This midline represents the final goal. The t rea tment midline may coincide with ei ther the uppe r or lower dental midlines, or in certain instances both uppe r and lower midlines may have to be moved to make them coincident with the facial midline. I f the dental midlines are coincident while the uppe r and lower soft tis- s u e / o r skeletal midlines are not and this is due to growth, pathology, or trauma, the determina- tion of a t rea tment midline should be assessed along with surgical alternatives. Trea tment plan-

ning of midline problems caused by tipping of incisors should first de te rmine whether the prob- lem is in the uppe r or lower arches or both. Indiscriminate movemen t of midlines to arbi- trarily align them could result in undesirable results.

Apical base midline asymmetries require care- ful at tention dur ing the t rea tment planning process. Apical base discrepancies are often asso- ciated with asymmetric left and right molar occlusion. If the apical base discrepancy is up to 2.0 mm, it is advisable to select ei ther the uppe r or the lower midline, whichever is closest to the facial midline, as a t rea tment midline. For larger apical base discrepancies, both uppe r and lower midlines may need correction.

Apical base midline discrepancy may be pres- ent with or without t ipping of the incisors. If both apical base midline discrepancy and t ipped incisors are involved, the t rea tment mechanics should make adjustments for the t rea tment of the two separate problems. Apical base midline discrepancy alone, often presents with upright incisors. It is very difficult to translate incisors because of problems with mechanical design and geometr ic shape of the arch in the incisor area. The midline correct ion for apical base problems is primarily achieved by changing the axial incli- nations of the incisors. The latter may not result in the most satisfactory a r rangement , but it helps to achieve an improved posterior occlusion.

A t rea tment plan for the correct ion of an asymmetric molar relationship should be a prior- ity in the p rob lem list for correction, regardless of whether it is associated with the dental mid- line problems. As previously ment ioned, apical base midline problems are often associated with asymmetric molar relationships. Clinical exami- nation, lateral cephalograms and panoramic ra- diographs are all useful in the diagnosis of the asymmetry, but for modera te to severe discrepan- cies, 45 ° cephalograms are essential to quantify and clearly visualize the extent o f the problem. First it should be de te rmined whether the prob- lem is due to an alteration of the axial inclina- tions of the poster ior teeth. This latter type of discrepancy is easiest to correct by upright ing the buccal segments at the beginning of the t rea tment so that symmetric mechanics can be used for the remainder of the treatment. If the axial inclinations of the teeth on both left and

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Treatment Strategies for Midline Discrepancies

H o m e I T O C I I ndex

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Figure 1. Diagrammatic representation of a cantile- ver appliance activation to move midlines to one side. The cantilever is inserted into the auxiliary tube of the molar and is hooked to the distal end of the anterior segment. This method would tip the incisors.

r ight sides are correc t a long with the asymmetric mo la r re la t ionship , t h e n the maxi l lary mola r on the Class II side should be t ipped back a n d then the roots upr igh ted .

Determinat ion of the Treatment Mechanics

An appropr i a t e b iomechan ica l p lan is i m p o r t a n t in o rde r to achieve the goals of the t r e a t me n t p lan to correc t the mid l ines a n d the mola r relat ionships. Appropr i a t e use of b iomechan i - cally o r i en t ed appl iances will min imize undesi r - able side effects. The fol lowing r e c o m m e n d e d mechan ics can be used to correc t m i d l i ne a nd asymmetr ic mo la r problems.

1. Bracket p l acemen t : In pa t ients with apical base mid l ine discrepancies , the incisor brack- ets can be a ngu l a t e d at the t ime of b o n d i n g in

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Figure 2. (A, B, and C) Frontal and buccal views before the start of the treatment showing midline discrepancy and an asymmetric molar occlusion. The treatment plan was to move the maxillary midline to the right side. Panel (D) shows occlusal view with only unilateral extraction of a first premolar on the right side. Panels (D, E, and F) show a segmental unilateral retracuon of the cuspid which was followed by the use of a cantilever as shown in Figure 1 to move the midline to the right side. Panels (G, H, and I) show frontal and lateral views at the end of the treatment with overcorrected midline, a Class II and a Class I molar occlusion the result of asymmetric extraction.

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88 Nanda and Margolis

such a way that placement of a straight wire would change the axial inclinations of the incisors toward the desired midline. This method results in tipping of the incisors.

2. Cantilevers: Figure 1 shows the mechanics of using a cantilever. It is ideal for uprighting tipped incisors. It can also be used to change axial inclinations in patients with apical base discrepancies. The use of cantilevers is ideal because the side effects are minimal and the application of the force is localized. The cantilever also delivers small forces due to the long interat tachment distance and a low load deflection rate.

3. Asymmetric mechanics: Asymmetric mechan- ics such as retraction on only one side in extraction patients is sometimes an option used to correct midlines. However, caution is advocated if the anchorage is critical. Segmen- tal mechanics are ideal for asymmetric retrac- tion because it allows differential use of mo- ments and the anterior segment can be brought back without friction and any adverse effect on the contralateral side. Closing loops on a continuous wire can also be asymmetri- cally activated to correct midlines but caution should be exercised by keeping the force values low to minimize loss of anchorage.

4. Asymmetric extraction pattern: This usually involves atypical extraction patterns such as unilateral single arch extraction, unilateral extraction on opposite sides in two jaws, extraction of three premolars and other com- binations. This extraction pattern also allows asymmetric mechanics while maintaining the anchorage on the opposite side (Fig 2).

5. Varying the timing of extraction: Often the canines are blocked out on the side to which the midline has shifted. In this clinical situa- tion, in extraction patients, to move the inci- sors to the opposite side it is advisable to first extract on the side to which the incisors are being moved. Once the midline is corrected, the bicuspid can be extracted on the side of the blocked out cuspid or the crowding. This method preserves the anchorage and also ensures that the midline is corrected at an earlier stage of the treatment.

6. Asymmetric intra-arch mechanics: The use of elastics with a continuous wire is often the most popular method to place asymmetric forces to correct the midlines. This method is

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satisfactory for minor midline problems re- stricted to tipped teeth. Because teeth slide along a wire at the anterior arch form curva- ture, this method often results in skewing of the arch. Asymmetric palatal arch: If a patient has a Class I molar relationship on one side and a Class II on the other, the Class II molar can be tipped back by an asymmetric palatal arch activation. Figure 3 shows that activations can be placed in the palatal arch to deliver a tip forward movement on the Class I side and a tip back movement on the Class II side (Figs 3 and 4). A highpull headgear with a short outer bow high above the center of resistance of the molar may be used to move the molar roots distally.

Adverse Mechanics

Some inappropriately used mechanics to correct midline problems often result in undesirable side effects which may result in a camouflage of the discrepancies for a brief period of time. The use of oblique elastics in the anterior region is one example. The force delivery system accom- plishes midline correction by tipping of the incisors as well as steepening of the anterior

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Figure 3. Diagrammatic representation of asymmet- ric palatal arch activation. The wire delivers a distal force on the Class II molar side and a mesial out rotation on the contralateral side with a mesial force. The mesial force can be minimized by increasing the number of teeth in the buccal segment and also by using a headgear.

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Treatment Strategies for Midline Discrepancies 89

Figure 4. (A a n d B) Buccal view of a pa t i en t with Class I m o l a r o n o n e side a n d Class II o n the opposi te side with a large overjet. T r e a t m e n t was nonex t r ac t ion . Occlusal view before t r e a t m e n t (C) a n d distal m o v e m e n t of the bucca l s e g m e n t o n the Class II side (D) is p e r f o r m e d with a palatal a rch act ivat ion as shown in Figure 3. T h e s e g m e n t was la ter u p r i g h t e d with a h igh pull headgear . (E a n d F) show buccal occlusion at the e n d of the t r ea tment .

occlusal plane on both sides. Once the elastics are removed, the discrepancy may return.

Similarly, the simultaneous use of Class II and Class III elastics is often advocated. This proce- dure may be of limited value in some patients. The force system tips uppe r and lower occlusal planes of left and right sides in different direc- tions and may result in a malocclusion that may be more severe than that being corrected. The correction of the midline in these situations may also be temporary. The latter methods also assume that the midline p rob lem in these pa- tients has been the result of discrepancies in both the uppe r and the lower arches.

Summary

Correction ofmidl ine discrepancies and asymmet- ric molar relationships need a careful diagnosis, treatment plan and biomechanical plan in order to achieve predictable results. Some biomechanically

sound methods to correct dental asymmetries have been presented.

References 1. Letzer GM, Kronman JH. A posteroanterior cephalomet-

ric evaluation of craniofacial asymmetry. Angle Orthod 1967;37:205-211.

2. Alavi DG, BeGole EA, Schneider BJ. Facial and dental arch asymmetries in Class II subdivision malocclusions. Am J Orthod 1988;93:38-46.

3. Rose JM, Sadowsky C, BeGole EA, et al. Mandibular skeletal and dental asymmetry in Class II subdivision malocclusions. AmJ Orthod 1994;105:489-495.

4. Vig PS, Hewitt AB. Asymmetry of the human facial skeleton. Angle Orthod 1975;45:125-129.

5. Shah SM, Joshi MR. An assessment of asymmetry in the normal craniofacial complex. Angle Orthod 1978;48:141- 148.

6. Sarver DM, Johnston MW. Orthognathic surgery and aesthetics: Planning treatment to achieve functional and aesthetic goals. BrJ Orthod 1993;20:93-100.

7. Arnett GW, Bergman RT. Facial keys to orthodontic diagnosis and treatment planning--par t II. Am J Orthod 1993;103:395-411.