Objectives: This study evaluated if surgical mandibular advancement procedures induced change in muscle main direction and length of moment arms of Masseter (MAS) and Medial Pterygoid Muscles (MPM). Both muscles decrease significantly in size after surgical mandibular advancement and probably loose strength. Compensation can be expected when postoperative change of the direction of MAS and MPM and increase of their moment arms induces major improvement of the biomechanical performance of these muscles.
Patients and methods: The sample, 16 adult patients (8 male, 8 female; mean age 27 years) with mandibular retrognathism was divided in a short-face (SF, n = 8) and a long-face (LF, n = 8) cluster using statistical cluster analysis. Bilateral Sagittal Split Osteotomy (BSSO) was performed in all patients. In LF cases BSSO was combined with Le Fort I osteotomy.
Axial MRI-scans were made preoperatively and 1–5 years postoperatively. Cross-sections were segmented over the length of both muscles with customized software. The centroids of the cross-sections, used to calculate the muscle’s main direction, and the moment arms were also computed with customized software.
Results: Postoperatively, main direction of MAS and MPM was significantly (9°) more vertical only in the LF-cluster. Moment arms changed insignificantly.
Conclusions: The significantly more vertical direction of both muscles in the LF-cluster made MAS and MPM more effective postoperatively. The increased vertical component of the muscle force caused more muscle force to contribute to bite-force. Whether the increased effectiveness of MAS and MPM could make up for their decline in size remains doubtful.
Conflict of interest: None declared.