Introduction

Asymmetric malocclusions are common orthodontic problems that are challenging to correct successfully. The early recognition of the asymmetry, the proper localization (skeletal or dental), and treatment planning of the malocclusion, whether compensation or correction will insure the attainment of optimal treatment outcomes. The orthodontic management of a malocclusion presenting with some degree of asymmetry is challenging because of the necessity of asymmetric mechanics on the right and left quadrants of a dental arch to achieve an acceptable correction. Significant undesirable side effects may be associated with the correction of an asymmetric malocclusion and the analysis of the force system in the three dimensions is necessary step of goal‐oriented treatment planning prior to the initiation of orthodontic therapy.

A critical step of planning treatment is to establish precise goals of treatment so the correction of identified problems can be achieved. A goal‐oriented treatment approach allows the analysis of the ideal force system necessary to achieve the treatment objectives and the design of an appliance consistent with these goals. The careful analysis of the desired force system, its equilibrium, and the potential side effects that may arise will improve outcomes and the speed of treatment. The proper assessment of the force system is central to successful outcomes of treatment and clinicians can develop the appropriate biomechanics to achieve their treatment goals. If the nature of the asymmetry is misdiagnosed, the prognosis of successful treatment outcomes is greatly compromised and the results of treatment may be less than optimal.

In this chapter, different treatment strategies for the successful treatment of dental asymmetries are described. Key elements in the differential diagnosis and the analysis of force systems of a number of different orthodontic appliances used in contemporary clinical practice are discussed.

Diagnosis – Problem List

It is extremely important to establish the underlying cause of an occlusal asymmetry so that an appropriate treatment plan can be efficiently implemented. Asymmetric malocclusions may be caused by an underlying skeletal asymmetry, mandibular shifts from initial tooth contact to maximum intercuspal position, or dental asymmetry of purely dental origin (Pirttiniemi 1994; Bishara et al. 1994; Cohen 1995).

Skeletal asymmetries may be due to congenital anomalies such as hemifacial microsomia. Affected individuals have underdevelopment of the condylar‐ramal complex on the involved side along with ear malformations (Figures 10.2.1a–c). Hemifacial microsomia has a variety of clinical presentations with varying degrees of deformation of the mandible on the affected side. A number of classification systems have been described as having various treatment protocols. The type and timing of treatment may depend on the degree of deformation and on the philosophy of treatment. Early treatment may involve the use of an asymmetric functional activator, distraction osteogenesis, or the placement of a costochondral rib graft (Mulliken et al. 1989; Kaplan 1989). Definitive orthognathic surgery may be required (Kaban et al. 1988). Treatment goals include optimizing facial growth and minimizing secondary asymmetric development of the maxilla and canting of the occlusal plane.

Progressive asymmetries may be seen in patients with Parry–Romberg syndrome in which there is a progressive hemifacial atrophy. There is evidence that this disease is of central nervous system origin. There is generally an active, progressive atrophic period followed by relative stability (Cory et al. 1997; Mazzeo et al. 1995).

Unilateral coronal synostosis (plagiocephaly) is associated with a progressive, growth‐related skeleto‐facial deformity involving the frontal bone and orbital rim and secondarily affecting the maxilla and mandible (Loomis et al. 1990; Arvystas et al. 1985). Condylar fracture during childhood has been associated with growth arrest and subsequent asymmetry. As normal facial growth continues, the mandible may progressively deviate toward the affected side. In many cases of early condylar fracture, normal mandibular growth will still occur. Stabilization of the occlusion during initial healing through intermaxillary fixation may be needed. In some cases, compensatory overgrowth of the fracture site occurs, producing an asymmetry with mandibular deviation away from the affected side.

Temporomandibular injury may produce an intracapsular hemarthroses which has the potential to cause joint ankyloses (Skolnick et al. 1994; Proffit et al. 1980). Along with asymmetry, a marked limitation upon opening due to the lack of translation of the condyle on the affected side may be seen clinically. Unilateral condylar or mandibular hyperplasia may result in an asymmetry in which the mandible deviates away from the affected side (Figure 10.2.2a–d). Condylar hypoplasia may also result in a skeletal asymmetry in which the mandible deviates toward the affected side. Compensatory maxillary asymmetry and canting of the occlusal plane may be associated with these skeletal asymmetries (Figure 10.2.3a–d). Temporomandibular disorders, particularly those involving unilateral degenerative joint disease, may be associated with skeletal asymmetry (Figure 10.2.4a and b). Progressive condylar resorption, if occurring bilaterally, is associated with a progressive anterior open bite and increasing mandibular retrognathia (Huang et al. 1997). If the condylar resorption occurs unilaterally, it will be associated with a progressive asymmetry with the affected side becoming more Class II (Figure 10.2.5a and b). Neoplasia and fibrous dysplasia may cause facial and mandibular asymmetry (Arendt et al. 1990) (Figure 10.2.6a and b).

Radionuclide metabolic bone scans with 99‐m‐technetium phosphate may be useful in assessing the metabolic activity of the condyle in cases of hyperplasia, resorption, arthritis, fibrous dysplasia, and neoplasia. While not specific, the bone scan is more sensitive than conventional radiography and may be useful in diagnosis and treatment planning (Bohuslavizki et al. 1996; Matteson et al. 1985).

Asymmetries due to functional mandibular shifts are most often due to centric prematurities on cusp tips and inclines causing a lateral mandibular displacement upon full closure. A common example is seen in young patients with unilateral posterior crossbite (Figure 10.2.7a and b). At rest position, the mandible is symmetric but upon closure deviates to the side in crossbite. This is usually an indication of a maxillary transverse deficiency. It is important to distinguish a functional shift from true skeletal asymmetry. In the case of an asymmetry associated with a unilateral posterior crossbite with a functional shift, maxillary expansion is expected to correct the crossbite and reestablish symmetry of the occlusion. In the case of a unilateral posterior crossbite due to a true mandibular asymmetry, the need for surgical correction is more likely (O’Byrn et al. 1995).

Asymmetric malocclusions can also result from the malposition of a tooth or a group of teeth in the occlusal plane (first order), in the sagittal plane (second order) or the frontal plane (third order), or a combination of these. For example, the presence of a unilateral rotation of a molar can result in a dental asymmetry in the sagittal plane. Also, a difference in molar axial inclination in the sagittal plane will create an asymmetry of the occlusion between the right and left buccal segment of teeth. Similarly, the presence of a unilateral crossbite creates an asymmetric occlusion which will need asymmetric mechanics to be corrected. In this paper, the primary focus will be on the management of dental asymmetries resulting from abnormal molar rotation or incorrect axial inclination of the molars.

Molar Rotation (First Order)

Rotations of the maxillary permanent first molar are usually the result of premature loss of the deciduous molar. A mesial migration with forward tipping of the permanent molar accompanies rotation of the tooth resulting in a significant space loss in the posterior part of the dental arch. This rotation can also be due to ectopic mesial eruption of the molar. During normal development, the crowns of the maxillary molars are facing distally and as the maxilla moves downward and forward. The maxillary molars then upright and their crowns face occlusally (Burstone 1964; Andrews 1972).

The maxillary and mandibular arches must be carefully studied to recognize the presence of a molar rotation. A mesial‐in rotation of a maxillary molar will result in a more Class II molar relationship on that side of the arch (Figure 10.2.8a–d). The malocclusion is evaluated in centric relation and the amount of overjet and overbite is recorded. Any discrepancy between centric relation and centric occlusion is also documented. The buccal occlusion on the right and left side are classified using the Angle classification and, if a dental asymmetry is diagnosed, specific steps will be taken during treatment planning to obtain a symmetric occlusion during the initial stages of orthodontic treatment.

From the occlusal view, the arches are evaluated for symmetry with respect to the median raphe and its projection on the mandibular arch. It is possible to superimpose a grid over the occlusal surface of the dental arch and evaluate the symmetry of the dental arch with respect to the median raphe (Proffit 1993). An easier and more efficient way to evaluate rotation of the molars is to draw a line along the mesial surface of the molar on each side of the arch and observe the point of intersection of these two lines (Figure 10.2.9a). If the right and left molars have the same amount of rotation, these lines will intersect at the median raphe (Figure 10.2.9b). If the right molar is more rotated than the left molar, the lines will intersect on the right side of the arch (Figure 10.2.9c). Once the diagnosis is established and the rotated tooth is identified, orthodontic treatment can be initiated with specific goals designed to correct the asymmetry early during treatment.

Molar Tipping (Second Order)

The presence of a dental asymmetry in the buccal occlusion may also be due to an abnormal axial inclination of the first permanent molar. Abnormal axial inclination of the molar in the second order results from an ectopic eruption pattern or from the early loss of a deciduous molar. During normal development, the maxillary molars have a distal axial inclination. With favorable growth of the facial complex, the molar will erupt with a more mesial axial inclination (Burstone 1964). In cases where the first permanent molar undergoes ectopic eruption, the mesial tipping of the permanent molar as it erupts accounts for a buccal occlusion that is more Class II on that side and a loss of space in the posterior part of the dental arch.

The diagnosis of such a discrepancy in axial inclination between the right and left buccal occlusion can be established by comparing the cant of the right and left posterior occlusal planes on a lateral cephalometric radiographic film taken at 45°. Orthodontic models can also be useful in this evaluation. A line drawn through the cusps of the molar and extended anteriorly will help visualization of the difference in axial inclination between the right and left permanent molars (Burstone 1962) (Figure 10.2.10). Panoramic or periapical radiographs can also be used, but they provide a less precise and reliable method as the two sides are magnified differently in the case of a skeletal asymmetry. Evaluation of the axial inclination of the molars than a cephalometric film taken at 45° or model evaluation may be better by using a cone‐beam computed tomography (CBCT). However, these radiographs are valuable for assessing the distal space available to tip back the molar.

Posterior Crossbite (Third Order)