STRESS DISTRIBUTION ON THE TEMPORO-MANDIBULAR JOINT (TMJ) IN UNILATERAL SAGITTAL SPLIT OSTEOTOMY (USSO): A FINITE ELEMENT ANALYSIS STUDY

Abstract

Introduction: Bilateral sagittal split osteotomy (BSSO) is a standard technique in mandibular orthognathic surgery to correct facial asymmetries. Unilateral SSO was introduced as a less aggressive alternative limited to only one side of the mandible and with a narrow range of movement. Stress distribution on the condyles in USSO has not been addressed to determine symmetry of balance in this asymmetric procedure. Hypothesis/aims: The aims of this study are to evaluate: 1. the stress distribution on and displacement of the contralateral and ipsilateral neck of the condyles and menisci in simulated models of patients treated with USSO following graded movements of the mandible in advancement and setback; 2. the effect of condylar volume variation on stress distribution. The main hypothesis was that minimal stress would be registered on the operated condyle, and that a threshold should exist at which either advancement of setback mandibular movements should not be detrimental to condylar position and function. Methods: This retrospective study was conducted on 8 pretreatment CT scans of patients who presented for the surgical correction of mandibular asymmetry and who qualified for the USSO based on established inclusion criteria. USSO 3D movements (unilateral advancement and setback) in increments of 2 mm (from 4 to 12mm) were simulated in a finite element (FE) model and the corresponding stresses measured at the level of the temporomandibular structures (condyles and their respective discs and glenoid fossae). Stress distributions and displacements were analyzed for the different surgical movements through analyses of variance for group comparisons between advancement and setback and within the USSO variables, and correlation tests for associations among variables. Results: Stress and displacement at the condyle were higher on the non-operated than the operated side: highest on the anterior, posterior and lateral condylar surfaces, at the medial surface of the condylar neck, on the anterior condylar surface during setback, and posterior surface during advancement. The pattern of stress distribution suggested a lateral outward rotation of the condyle on the non-operated side. More displacement (up to 1.3mm) occurred at the non-operated than operated condyle. When condylar volume increased the displacement decreased. A balance of stress and displacement was observed at 6mm of advancement or setback before a new cycle of stress increase and decline. Increase in displacement was associated with increase in stress. Conclusions: 1. This study was the first to contribute FE modeling and findings based on the individual variations among 8 human subjects who underwent USSO surgery. 2. With increased surgical movements, statistically significant increase in stress was observed at the level of the condyles, condylar necks, and menisci on the operated and non-operated sides. 3. The non-operated side disclosed most differences with incremental surgical movements. 4. Individual velocity changes suggested a balance of stress between operated and non-operated sides between 4 and 6 mm of mandibular advancement or setback, which corresponded to the actual amount of surgical movement in the 8 patients under study. 5. This research yielded a clinically testable hypothesis that should be explored in future FE models and direct clinical investigations. Significance: The significance of this research is the determination of the range of the surgical movements with the least and most stress on the temporomandibular structures thus the limits of mandibular displacement using the USSO.