Studies have evaluated the morphological asymmetry of craniofacial structures to provide better treatment for MA patients. However, functional asymmetry related to masticatory dysfunction is also reported, and there is still limited information concerning the relationship between the asymmetry of skeletal structures and function in these patients. It is important to unravel the mechanism of MA to comprehensively understand the developmental process of masticatory dysfunction and to establish evidence-based occlusal treatment for MA patients.
In the MA group, the volume of both the condyle and the glenoid fossa on the shifted side was significantly smaller than that of the non-shifted side. This result is similar to previous morphological evaluations [9, 18]. The current study also found that the anteroposterior length and surface area of the glenoid fossa on the shifted side were significantly smaller than those of the non-shifted side, with no difference in the mediolateral width of the glenoid fossa. A previous study involving 3D comparison of the shape and size of the TMJ of facial asymmetry patients also reported that the glenoid fossa on the shifted side was significantly smaller on the sagittal view compared with that of the non-shifted side, but with no difference in the coronal view [19]. In the MA group, although the morphology of the condyles and the glenoid fossa differed bilaterally, there was no significant difference in the condyle–fossa ratio, indicating that the occupancy of the condyle within the glenoid fossa is the same bilaterally. Additionally, the measurements of the TMJ spaces were similar bilaterally, consistent with the findings of a previous study [20]. Therefore, in MA patients, as the mandible grows asymmetrically with a smaller condyle on the shifted side, the TMJ structure of the glenoid fossa may also undergo remodelling to maintain the same condyle–fossa relationship. The articular eminence angle was also significantly different between the shifted and non-shifted sides, with the shifted side angle being steeper. This difference in the articular eminence angle may be derived from an adaptation to asymmetrical loading to the TMJ [21]. The discussion on the comparison of 3D condylar movement between the shifted and non-shifted sides in the MA and the control groups can be found in the supplementary. (Additional file: 3).
Previous studies have reported that untreated malocclusion with a unilateral posterior crossbite causes yawing of the mandible towards the crossbite side and posterior displacement of the condyle and glenoid fossa on the crossbite side [22]. This may lead to deviation and asymmetrical growth of the mandible [22]. Our previous study also observed posterior displacement of the glenoid fossa on the shifted side; this could be an etiological factor for asymmetrical development of the mandible in MA patients [16]. With yawing of the mandible caused by posterior displacement of the condyle and the glenoid fossa on the shifted side, outward rotation of the condylar angle may be increased on the shifted side, worsening the symptoms of the mandibular shift. The results of the current study showed that the axial condylar angle and the glenoid fossa angle were significantly larger on the shifted sides. Therefore, it would be reasonable to assume that as the mandible grows asymmetrically with the condyle and glenoid fossa displaced posteriorly towards the shifted side, the condyle on the shifted side would adjust and rotate inwardly along with the glenoid fossa to prevent worsening of the symptoms of the mandibular shift and to compensate for the asymmetrical development of the mandible in patients with MA.
Compared with the control group, the TMJ morphology of both the condyle and the glenoid fossa on the shifted side was significantly different in the MA group. This is consistent with the results of Ikeda et al. [9] but not with those of Kim et al. [18]. The differences may relate to patient selection; for example, the inclusion or exclusion of skeletal Class II patients, in whom the morphology of the condyles and glenoid fossa differs to that of others skeletal Class patients [14]. In the comparison of TMJ spaces between the two groups, the medial, anterior, and superior joint spaces of both sides of the MA group were significantly smaller than those of the control group, as found in previous studies [17, 20]. It has been suggested that the reduction in joint space may lead to severe squeezing of the articular disc within the TMJ, which in turn may increase the biomechanical load on the TMJ and increase the risk of TMJ disorder [17, 20]. Therefore, this may be related to a higher percentage of patients with clicking sounds in the MA group than in the control group when evaluating the clinical signs of TMJ disorder.
A previous study reported that mandibular structural asymmetry (of the condyle, ramus, and mandibular body) was closely related to asymmetrical condylar movement [9]. In the current study, the condylar volume above the glenoid fossa base was evaluated, and this condylar volume asymmetry was negatively correlated with the asymmetry of the CPL in the MA group. The smaller the condylar volume was on the shifted side, the longer was the CPL on the shifted side. The asymmetry of condylar movement may compensate for the asymmetry of the condyle morphology.
The asymmetry of the glenoid fossa volume was negatively correlated with the asymmetry of the SCI angle. The smaller the glenoid fossa on the shifted side, the greater was the SCI angle on the shifted side. Additionally, the asymmetry of the glenoid fossa volume was also found to be negatively correlated with TCI angles on both sides with negative values. Therefore, as the morphological asymmetry of the glenoid fossa increased, the condyles on both sides tended to move towards the shifted side on protrusive movement in MA patients. A previous report comparing magnetic resonance imaging and computed axiography in patients with TMJ disorder also speculated that the 3D condylar movement asymmetry could be caused by functional compensation in the anatomical asymmetry of the TMJ [23]. Moreover, another previous study in patients with unilateral crossbite speculated that there was a relationship between morphological and functional asymmetry because they noted that the articular eminence angle was steeper on the crossbite side than the other side, as was the condylar path angle [24]. Additionally, the current study also showed that the asymmetrical steepness of the articular eminence angle was positively correlated with the asymmetry of the SCI angle, indicating that the steeper the articular eminence angle was on the shifted side, the greater was the SCI angle on the shifted side. Because sliding condylar movements are observed along the anatomical form of the articular eminence [25], the same can be said for MA patients in whom the asymmetrical sagittal condylar angle of condylar movement on protrusion closely follows the morphological asymmetry of the articular eminence angle. Therefore, 3D morphological asymmetry of the glenoid fossa is closely related to the functional asymmetry of condylar movement in MA patients.
The current study in MA patients showed that asymmetrical condylar morphology significantly correlates with an asymmetrical condylar path length, and that asymmetrical glenoid fossa morphology significantly correlates with an asymmetrical sagittal steepness angle of condylar movement. Therefore, the asymmetrical changes in the 3D morphology of the TMJ (both the condyle and glenoid fossa) closely interact with masticatory dysfunction in asymmetrical condylar movements and affect the development of mandibular asymmetry. From a clinical viewpoint, a close relationship between asymmetry of the TMJ structure and condylar movement in MA patients could indicate the need for early treatment of asymmetrical condylar movements that result in a functional shift of the mandible at an early age. Moreover, in MA patients, given that functional asymmetry of condylar movement is reported to remain after surgery [8] and that there is a close relationship between the asymmetry of the TMJ morphology and function, these asymmetries and their reciprocal relationships might affect the stability of the treatment. Further research is needed to follow up MA patients after surgery.
There were several limitations in the current study. The articular disc position, which may affect condylar movement, was not evaluated. Additionally, the TMJ undergoes a remodelling process throughout life, and there may be long-term changes in the masticatory function of condylar movement that is affected by the TMJ morphology. Therefore, longitudinal observation of both condylar movement and TMJ morphology may be necessary.