The results of this study suggest that condylar sag and luxation after IVRO are associated with the magnitude of setback and temporomandibular joint symptoms. Therefore, it is important that these factors be evaluated during treatment planning for IVRO.
In this study, a decrease in the magnitude of setback was associated with a higher risk of condylar sag and luxation. The association with temporomandibular joint symptoms is supported by the results of a prior study that reported a higher likelihood of condylar luxation in patients with TMD who experienced symptoms (such as clicking sounds and temporomandibular joint pain), compared to patients with simple jaw deformities . These results may be attributed to an increased fragility and loss of elasticity in the joint capsule due to inflammation, which subsequently leads to condylar sag and luxation. The association between condylar sag, luxation and an occlusal cant correction of 2 mm or more may be explained by the development of TMD due to the perturbation of normal mandibular movement by the non-physiological inclination of the maxillary occlusal plane. This was significantly different in univariate analysis, but not in multivariate analysis. Since occlusal cant correction of 2 mm or more may be associated with temporomandibular joint symptoms, it is necessary to increase the number of cases and confirm the association.
While osteotomy line shape was not significantly associated with condylar sag or luxation, a significant relationship was reported in a previous study . This may have been due to the small sample size employed in our study. Although mandibular condylar deformation was a significant explanatory variable in the univariate analysis, it was not retained in the multivariate model. This may be attributed to the finding that the exacerbation of symptoms in patients with TMD often results in mandibular condyle deformity. As a result, mandibular condyle deformation would not be evident prior to the appearance of temporomandibular joint symptoms. Since the multivariate analysis had a strong association and affected other analysis results, the mandibular condyle deformity was excluded. From this, it is necessary to collect and examine a sufficient number of cases for events that may cause temporomandibular joint symptoms and events that may occur continuously for temporomandibular joint symptoms. In the ROC curve, the cut-off value of the magnitude of setback for condylar sag and condylar luxation after IVRO was 3.25 mm; therefore, surgeons should be vigilant for these complications if the magnitude of setback is less than 3.25 mm.
In this study, the medial pterygoid muscle was almost detached from the proximal segment. This facilitated the setback procedure and reduced the potential for possible relapse after IVRO. The complete detachment of the medial pterygoid muscle is typically avoided, as it may result in the complete displacement of the condyle from the glenoid fossa. Retaining some medial pterygoid muscle attachment also lowers the risk of condylar luxation and ischemic necrosis of the tip of the proximal segment [3, 5, 14]. Nevertheless, some studies have reported the absence of complications in cases where the entire medial pterygoid muscle was detached from the proximal segment [13, 15]. In the present study, condylar sag was observed in just over a third (37%) of the cases, and only one case (0.01%) of condylar luxation was documented; the incidence of both complications are lower than those reported by previous studies [5, 10, 11]. Therefore, even if the medial pterygoid muscle is completely detached, the stability of the temporomandibular joint is relatively unaffected. In addition, avascular necrosis of the proximal segment was not observed in the postoperative 1-year follow-up period. This is notable, as the proximal segment was only connected to the mandibular condyle at the joint capsule. This implies that the blood supply and minimal amount of attachment provided by the joint capsule was sufficient to avoid avascular necrosis following IVRO. The adequacy of the blood supply from the joint capsule has been previously reported .
Some limitations are acknowledged in the present study. Firstly, it was a preliminary study involving a small number of patients. Secondly, we investigated the conditions under which condylar sag, and then we estimated the conditions under which condylar luxation occurs. However, by planning and performing the surgery paying attention to the magnitude of setback, the presence of temporomandibular joint symptoms, and mandibular condyle deformation derived from this study, it is possible to prevent condylar luxation, which is difficult to deal with if it occurs. In the future studies, it will be necessary to collect cases from other facilities and clarify the results.