Condylar morphology can be changed as a result of adaptive remodeling to functional or pathological stimulis even after growth has ceased18. Knowledge of the condyle is essential for correctly assessing the TMJ status and treatment planning. TMJ morphology and the position of the condylar head varies in diverse sagittal and vertical skeletal patterns11. Our research specifically evaluated the difference between the morphological bony changes of the condyle of Class II hyperdivergent female and their association with dentoskeletal characteristics.
Most studies evaluated condylar size in some specific plane19, 20. By conducting 3D measurements, we found that the condylar volume of changed groups was significantly smaller in comparison to the normal group. Since function affects form18, larger condyle seemed to be more resistant to stimulations, while smaller condyle may provide unreliable support for stimulations and thus prone to pathological changes21. Flattening of the condyle may be the first adaptive alteration to degenerative changes of the TMJ22. Resorption of the head probably result in occlusal changes23. Osteophyte formation was considered to widen the condylar surface in an attempt to stabilize and improve the loading capacity24. Surprisingly, our study showed no significant difference among condylar volumes of the three groups. We speculate that condyle may diminish after adaptive alteration no matter what changing form is.
Our results indicated that condylar volumetric reduction was accompanied with shorter mandibular length, ramus height, posterior facial height, reduced posterior to anterior facial height ratio, but was not associated with anterior facial height, lower face height or mandibular length (Fig. 9A). Overall, the mandible displayed a clock-wise rotation with condylar volumetric reduction, but the maxilla relatively stayed the same. Ahn et al. found the a similar trend that posterior facial and ramus height were significantly different between those with or without degenerative diseases25. Nonetheless, Gidarakou et al. found a reduction on both SNA and SNB angle in TMJ degenerative patients26. The increase of the alveolar height of posterior teeth as well as the mandibular anterior teeth may result from compensational extrusion of the teeth after clockwise rotation of the mandible.
We speculate that when mandible rotated clockwise, the component of occlusal force that made the lower posterior teeth tip mesially would be greater. But the upper posterior teeth were less impacted since they were more upright in the alveolar bone. The mesially tipped lower posterior compensated for the possible deterioration of the sagittal occlusal discrepancy. Meanwhile, lower incisors protruded in order to have occlusal contacts with upper incisors. But the compensation was insufficient for the increased overjet caused by the retrusive mandible. In addition, muscular activity should be taken into consideration. Tongue pressure on the lower incisors increased when the mandible retruded, while the labial-lingual muscle strength on the upper incisors were almost in balance. Thus in our study, condylar volume was positively correlated with ∠U1-L1 and negatively correlated with ∠IMPA and overjet, but not correlated with ∠U1-SN (Fig. 9B).
No previous study has established the interrelationship between condylar bony changes and dentoskeletal features in the transverse dimension. Our results implied that condylar volume had no correlation with transverse measurements of upper posterior teeth. We hypothesized that when mandible rotated clockwise, the retropositioned lower arch occluded with wider posterior upper arch. Thus the occlusal force on the lower molars was more buccally inclined. Meanwhile, forces of tongue muscle pressure on lower molars relatively increased. As a consequence, buccal inclination and arch width of the lower first molars increased (Fig. 9C). The basal bone inclination frequently aligned with the inclination of the mandibular molars27. Thus the width between the buccal alveolar crests in the lower first molar section also increased.
Our study established a comprehensive correlation between condylar volume and dentofacial deformities, which would be useful for indicating potential pathological changes of TMJ. However, the cause-and-effect relationship remains unclear. A homogeneous population may have racial, gender and skeletal differences affection on the results, this need to be clarified in future studies. Besides, it was not possible to take CT scans on more subjects to enlarge the sample size due to ethical reasons, so the results of this study would be better interpreted and supported by new studies.