A Biomechanical Analysis of Muscle Force Changes After Bilateral Sagittal Split Osteotomy.

A basic procedure affecting maxillofacial geometry is the bilateral sagittal split osteotomy. During the surgery, the bony segments are placed in a new position that provides the correct occlusion. Changes in the geometry of the mandible will affect the surrounding structures and will have a significant impact on the functioning of the masticatory system. As a result of the displacement of the bone segment, the biomechanical conditions change, i.e., the load and the position of the muscles. The primary aim of this study was to determine the changes in the values of the muscular forces caused by mandible geometry alteration. The study considered the translation and rotation of the distal segment, as well as rotations of the proximal segments in three axes. Calculations were performed for the unilateral, static loading of a model based on rigid body mechanics. Muscles were modeled as spring elements, and a novel approach was used to determine muscle stiffness. In addition, an attempt was made, based on the results obtained for single displacements separately, to determine the changes in muscle forces for geometries with complex displacements. Based on the analysis of the results, it was shown that changes in the geometry of the mandibular bone associated with the bilateral sagittal split osteotomy will have a significant effect on the values of the masticatory muscle forces. Displacement of the distal segment has the greatest effect from -21.69 to 26.11%, while the proximal segment rotations affected muscle force values to a less extent, rarely exceeding 1%. For Yaw and Pitch rotations, the opposite effect of changes within one muscle is noticed. Changes in muscle forces for complex geometry changes can be determined with a high degree of accuracy by the appropriate summation of results obtained for simple cases.

Mandibular condyle displacements after orthognathic surgery-an overview of quantitative studies.

The repositioning of bone segments during orthognathic surgeries often results in mandibular condyle positional changes and can also affect jaw muscles, soft tissues and the temporomandibular joint (TMJ). Condylar displacements are considered as one of the factors of bone remodeling and further skeletal relapse. The quantitative approach is commonly used in comparative analyses and evaluations of the relationships between examined factors. The aim of this study is the overview of the current literature including quantitative analysis in the research of mandibular condyle positional changes as a consequence of orthognathic surgeries. Thirty articles were included in the overview. Most of the articles present a comparative and evaluative analysis of treatment results concerning different surgical approaches, fixation methods or types of skeletal defects. The correlation between condylar displacements and bone remodeling, skeletal relapse and TMJ dysfunctions were considered. The most frequently repeated study variables were: short-term changes, Class III malocclusion, yaw rotation, 3D cephalometry measurements. Quantitative data might be useful in the evaluation of patterns and range of condylar displacements for specific treatment conditions. Available literature concerning the analysed topic is characterized by great heterogeneity with regards to the purpose and methodologies of the studies. More systematic approaches and long-term considerations are needed in future research.

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.