Application Of USSO To Treatment Of Asymmetric Mandibular Prognathism—Study On The Preoperative Simulation,Postoperative Stability And Variation Of Stresses

Objective:Unilateral mandibular sagittal split osteotomy(USSO)and bilateral mandibular sagittal split osteotomy(BSSO)can be used to treat the mandibular deviation caused by unilateral condylar hyperplasia(UCH).In this study,the sagittal split of the affected mandible was simulated by computer-assisted surgery to analyze the position of the contralateral condyle.The stability of bite and joint symptoms after unilateral sagittal split of mandible were evaluated,and the stress changes in joint area before and after the operation were also studied.Methods:This study was a clinical retrospective study,from August 2016 to June 2018,thirty-one patients with asymmetric mandibular prognathism caused by UCH were selected from the School of Stomatology,China Medical University,among them,sixteen patients were treated by USSO.Fifteenth cases were treated by BSSO,which could not be treated by USSO.The deviation of midline,over-bite,over-jet were measured preoperatively in all patients.Digital surgery was used to simulate USSO of the affected mandible and measured the position change of condyle in three-dimensional space,and the changes of the torsion angle of the condylar internal and external poles and the long axis angle of the condylar neck in non-operative side.Patients treated by USSO were followed up for more than 6 months to evaluate the occlusal parameters(the deviation of midline,over-bite,over-jet)before operation,immediately after operation,1 month and 6 months after operation,and the joint symptoms of left and right sides before operation and 6 months after operation.A comparative study of maxillary and mandibular skull overlap was made in 1 patient before and 6 months after operation.We established a three-dimensional finite element model of a patient preoperatively and 6 months after operation and investigate the stress distribution in the TMJ before and after the operation.Results:Between the USSO group and the BSSO group,the movement of the condyle on the unaffected side was statistically significant in the sagittal and coronal planes.In the USSO group,the condylar process moved slightly downward and laterally,but all the head of the condyle moved in the articular fossa.In USSO group,there were significant differences in midline deviation(P<0.05),over-bite(P<0.05)and over-jet(P<0.05)before and immediately after operation.There was no significant difference between immediately after operation and 1 month after operation(P=0.875,P=0.569,P=0.987)as well as immediately after operation and 6 months after operation(P=0.075,P=0.05,P=0.246).There was no significant difference in joint symptoms before and after operation(x~2=0.437),and there was consistency(Kappa=0.361,P=0.003)between them.The pre-operative stress of TMJ on the unaffected side was significantly higher than that on the affected side,and the bilateral joint stress was basically balanced after USSO.Conclusion:Digital surgery has certain guiding significance for USSO implementation.The position of condyle may not change obviously in three dimensional direction after USSO,but can slight change in the inner pole and the outer pole of condyle and the angle of long axis of condyle neck.The occlusion can reach a stable state after operation,and the joint symptom has no obvious aggravation or relief after operation;Compared with preoperative,the stress distribution of bilateral temporomandibular joints tends to be balanced.