Changes in muscle length and orientation after orthognathic surgeries using a bilateral sagittal split osteotomy as an example

Purpose: The aim of the present study was to analyze the changes in the angular positions and lengths of the mandibular elevator muscles due to the displacement of bone segments after bilateral sagittal split osteotomy. Additionally, the impact of changes in mandibular geometry on the values of occlusal forces and mandibular condyle loading was considered. The combined geometric and force analysis makes a valuable contribution to the operating conditions of the system affected by the changes. Methods: The considerations were based on elementary principles of analytical geometry and the analysis was performed for two craniofacial geometries. Results: For the rotation of the proximal segment, the greatest differences in angular position concern the masseter muscle during roll rotation (11.7). Significant changes in muscle length occurred during pitch rotation and amounted to 3.7 mm. Translation of the distal segment by 10 mm changed the angle of the pterygoid muscle by 30.2 in the coronal plane and 18.7 in the sagittal plane, simultaneously changing its direction to that of the opposite. Posterior translation (10 mm) caused an elongation of the muscle by 4.7 mm and anterior translation caused a shortening by 2.6 mm. For the mandible with elongated geometry, lower values of occlusion forces and increased reaction forces in the condyle were observed. Conclusions: The analysis revealed significant changes in the orientation and length of the masticatory muscles, and thus, their potential impact on the functioning conditions of the masticatory system.