Definition and criteria of asymmetric occlusion
Asymmetric occlusal relationships can result from an asymmetry within a single arch or asymmetric skeletal relationships between the maxillary and the mandibular dental arches. People with normal occlusion can have molar asymmetry greater than 1 mm in transverse and anteroposterior directions ( Fig. 83.1 ).
A case of mild asymmetry of malocclusion.
There is a 2-mm midline shift of the mandible to the right, a mild class III premolar and molar relationship on the left side and a mild class II molar and canine on the right side.
The cause of asymmetries in the molar relationship can be multivariate. In some patients, the problem originates with an abnormal dental eruption, premature loss of primary teeth or loss of permanent teeth; however, in other patients, the origin may be primarily skeletal, in which an asymmetric maxilla or, more likely, an asymmetric mandible is present.
Development of asymmetries with dental arches
Asymmetries within the maxillary or mandibular arch can produce asymmetric occlusal relationships.
Ankylosis of primary molars
Ankylosis of primary molars, often seen in the mandibular arch, causes them to remain submerged below the line of occlusion of the adjacent permanent teeth. The continuing eruption of the adjacent permanent teeth leads them to ‘tip’ over the crown of the ankylosed tooth, thereby causing loss of arch length and asymmetric axial inclinations of the adjacent teeth compared to those on the contralateral (unaffected) side of the arch. This leads to an asymmetric molar occlusion. This space loss involves the distal eruption of the permanent tooth anterior to the ankylosed primary molar and a medial tip of the tooth distal to the submerged tooth. The distal eruption of the teeth anterior to the ankylosed tooth can lead to a shift of the dental midline towards the affected side of the arch, compounding the developing arch asymmetry and producing asymmetric canine relationships.
Ectopic eruptions of the maxillary permanent first molars
Maxillary first permanent molars are known for their aberrant behaviour during the eruption, moving more mesially, leading to premature loss of the primary second molar tooth. The early loss of the second primary molar in the maxillary arch leads to a reduction in arch length on the affected side. Loss of arch length can result in a class II molar relationship on that side and a loss of arch length that may contribute to the impaction of the second premolar or crowding in the canine region.
Partial hypodontia
Missing mandibular second premolars can be associated with retained primary molar, which can, in turn, cause asymmetric molar relation by either preventing mesial movement of the erupting permanent molar or allowing mesial tilting of adjacent permanent teeth in the event of shedding ( Fig. 83.2 ).
A case of lower midline shift and asymmetrical malocclusion.
The retained right lower second deciduous molar associated with the missing successor premolar has created a shortage of arch length, leading to a slight shift of lower midline to left and mild crowding of the left premolar. Molar relation and canine relationship on both sides is a class I.
Dental caries
Dental caries is the most common cause of premature loss of the deciduous molar. When space is not managed, the erupting permanent maxillary molars move mesially, leading to the development of class II malocclusion. When it happens unilaterally, the outcome is an asymmetric molar relationship. The mesial tilt of the mandibular molar will cause a class III relationship, and when it happens on one side, it will lead to class III subdivision malocclusion.
Common traits of asymmetrical occlusion
Asymmetric conditions of occlusion that can possibly be managed with orthodontic treatment are:
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1.
Asymmetrical molar relationship
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2.
Asymmetrical canine relationship
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3.
Asymmetrical overjet
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4.
Asymmetrical overbite
Asymmetrical molar relationship
Class II subdivision ( Fig. 83.3 ) and class III subdivision ( Fig. 83.4 ) malocclusions are apt examples of asymmetry of occlusion in the anteroposterior direction. Rarely may one encounter a clinical situation of class III molar relation on one side and class II molar relation on another side. Asymmetrical buccal occlusion leads to shifting of the midline and canine relationship ( Fig. 83.5 A and C).
Class II subdivision malocclusion.
Class III subdivision malocclusion.
Evaluating molar and canine relationship.
Asymmetry of occlusion that has compensations with underlying facial asymmetry of skeletal origin cannot be treated with orthodontic therapy alone. In other situations, mild skeletal asymmetry of occlusion may show compensations, and any attempt to treat such asymmetrical occlusion will compromise the aesthetics and occlusion. Dental compensations might conceal facial asymmetry itself, and if not correctly diagnosed, it tends to be revealed throughout orthodontic treatment, thereby extending treatment time and hindering outcomes.
Asymmetrical canine relationship
The asymmetrical canine relationship usually follows asymmetrical molar relations or could be seen due to partial hypodontia or the presence of severe crowding and dichotomy of teeth. The underlying skeletal deformities affecting one side of the jaw and chin deviations contribute to unilateral malrelationship of the canines in the sagittal direction ( Fig. 83.5 B and D). It can be clinically expressed as unilateral class II or class III or, worse, class III on one side and class II on another.
Asymmetrical overjet and overbite
Asymmetrical overjet is seen in patients with unilateral thumb sucking and children with large unilateral cross-bite associated with functional shift or with underlying skeletal anomalies. The mandible is more frequently affected than the maxilla. Asymmetrical overjet observed with class III malocclusion is often associated with a lateral shift of the mandible, which can be functional or caused by the unilateral excessive growth of the condyle.
While asymmetrical overbite is unusual, it can exhibit as a one-sided open bite.
Clinical evaluation
A detailed clinical and radiological evaluation of the case with asymmetrical occlusion is fundamental to establishing the nature and severity of the problem. Clinical evaluation includes a thorough observation of mesiodistal inclinations of the molars, teeth in the buccal segment in the arch, and their relationship to the apical bases, underlying jaw bones and overall facial skeleton. A comprehensive 3D evaluation with CBCT remains the gold standard ( Fig. 83.6 ) to measure the exact location and severity of the asymmetry; however, CBCT should only be prescribed with caution, keeping in view SEDENTEXCT (safety and efficacy of a new and emerging dental X-ray modality) guidelines on the use of CBCT. In the absence of CBCT, routine OPG X-ray and PA cephalogram are of immense value in differentiating the underlying structures in transverse and vertical dimensions. These are described in the chapter in association with skeletal jaw asymmetries ( Chapter 85 ). Other non-radiation imaging modalities include 3D facial scans with stereophotogrammetry (3dMD) or similar techniques that record face and soft tissue structures, which may prove useful.
CBCT applications in facial asymmetry.
(A) Facial asymmetry as visualised on a volume-rendered CBCT image. Note the chin deviated to left away from mid sagittal reference (MSR) plane. (B, C) Maxillary dental midline deviation caused by ‘Yaw’ of the maxilla.
Prevalence and features of asymmetrical occlusion
The prevalence of molar asymmetry in patients visiting orthodontic clinics has been reported to range from 22% to 43%. , There are only two population-based studies related to occlusal asymmetry, one from Kuwait and the other from Kathmandu, Nepal.
Class II asymmetric relations
The total prevalence of an asymmetric molar relationship in Kuwaiti children was 29.7%, which included 21.3% in class II and 7.9% in class III malocclusion. Among class II, half-step asymmetries were found in 18.9%, and full-step asymmetries were prevalent in 2.4%.
Class III asymmetric relations
Among class III, 7.1% were as half-cusp class III, and 0.8% were full-cusp class III. Interestingly, 0.4% of subjects were a combination of half-step class II on one side and a half-step class III on the other. A study from Nepal has reported a high distribution (16%) of class III malocclusion in their sample population of high school students in Kathmandu Valley, of which 45.5% were a subdivision. Class II malocclusion was observed in 25% of the sample, of which 2% exhibited subdivision.
Asymmetrical canine relationship
The asymmetrical canine relationship usually corresponds to an asymmetrical molar relation in the arches. It can be clinically expressed as unilateral class II or class III or, worse, class III on one side and class II on another.
The study found that 41.4% of Kuwaiti children have an asymmetry of canine relationship, of which 4.2% fall into the moderate to severe category. A detailed analysis showed that 33.4% had half-step class II asymmetries, 3.8% had half-step class III asymmetries, 1.8% had full-step class II asymmetries, 1.2% had full-step class III asymmetries, and 1.0% had a combination of half-step class II on one side and half-step class III on the other.
Features of asymmetric class II subdivision malocclusion
The comparison reveals notable variations in dental arch and facial asymmetries between Angle’s class II subdivision malocclusions and individuals with normal occlusions. Class II subdivision malocclusion is chiefly characterised by dentoalveolar features.
Angle class II subdivision malocclusion signifies a distal positioning of the first mandibular molar on the affected (subdivision) side and a mesial positioning of the first maxillary molar on the same side.
A study by Janson et al. found that the mandibular molars were positioned more distally on the class II side than the maxillary molars’ mesial positioning on that side. This resulted in the mandibular dental midline deviating to the class II side more frequently than the maxillary dental midline deviating to the opposite side.
Observations mentioned above based on lateral and PA cephalometry have lately been reconfirmed using CBCT. Sanders et al. have found that the aetiology of class II subdivision malocclusions is primarily due to an asymmetric mandible that is shorter and positioned posteriorly on the class II side . A mesially positioned maxillary and distally positioned mandibular molar on the class II side are minor contributing factors.
Most class II subdivision malocclusion patients present the mandibular dental midline displaced towards the class II side associated with the maxillary dental midline coincident to the mid-sagittal plane or with a mild deviation.
Li et al. reported that in class II subdivision malocclusion cases, almost one-third of subjects experience functional deviation, resulting in pseudo-asymmetry. This variation is likely due to the disharmonious arch width between the maxillary and mandibular dental arches in the premolar section, leading to the asymmetry of occlusion.
Angle class II subdivision malocclusion phenotype is the outcome of a complex interplay of dental, skeletal and functional factors. The functional deviation occurs due to the disharmony of dental arch widths, while the asymmetric position of the glenoid fossa may account for most of the skeletal asymmetry. A recent study has also shown that Angle’s class II subdivision 2 subjects show a smaller vertical amount of the centric slide than Angle’s class I and class II, subdivision 1 malocclusion.
Management
Asymmetries are among the most challenging clinical conditions in orthodontics, both from the point of quantification and location of the dysplasia and in arriving at the selection and execution of proper mechanics. The goals of treatment outcomes should be based on the patient’s perceptions of the problem and orthodontic needs. The sex and growth status also influence the choice of treatment.
A patient with asymmetrical occlusion needs to carefully be evaluated on occlusion and functional mandibular movements, as well as detailed cephalometric and radiological evaluation. The dentition is examined in all three planes of space.
Each molar should be evaluated for its mesiodistal tip and rotation. A mesial-in, that is mesio-palatal rotation of a maxillary molar, may appear as more class II molar relationship should mesio-lingual palatal cusp remain in the central fossa of the mandible. A mild class II molar relationship will seem more severe when associated with maxillary molar in a rotation.
Dental arch forms should be assessed for symmetry of the dental arches in the study models. Median palatal raphe (MPR) and its projection on the mandibular arch are used as reference landmarks; however, in cases with facial skeletal deformities with severe unilateral cross-bite, MPR may prove a tricky reference plane. The study models can be photocopied on occlusal view or scanned as 1:1 for evaluation. One can use a symmetry grid over the occlusal surface of the dental arch ( Fig. 83.7.i ). The asymmetric occlusion segment should be evaluated for excessive rotation of the teeth, unusual tip and buccolingual position in the arch, such as a cross-bite or the excessive buccal-palatal tip.
