| Objectives:To establish a three-dimensional finite element model of mandible with temporomandibular joint and full dentition crowns, which will provide a basis for evaluation of the transverse displacement of proximal segment after Bilateral Sagittal Split Ramus Osteotomy(BSSRO) with different lingual split patterns and advancement amounts under loading of masticatory muscle forces groups, and examine the influencing factors that related to mandibular width.Materials and Methods:CT scan datesets of a dry dentate mandible with no cranial and maxillofacial deformities were saved in DICOM format. Scanned images were imported into the reverse transformation and simulation software MIMICS and ANASYS. Then, a 3D model including the cortical and cancellous bone, teeth, temporomandibular joint fossa, temporomandibular joint disc and condylar cartilage were constructed. A BSSRO with two different lingual split patterns was performed on the 3D models, respectively. For pattern I(T1), the lingual split line fractured through the mylohyoid groove towards the inferior border of the mandible as described by Hunsuck. For pattern II(T2), the medial bone cut extended towards the posterior border before bending to the inferior border of the ramus as an Obwegeser split. After mandibular advancement 3mm(A3), 8mm(A8) respectively, parallel to the occlusal plane, fixation between the proximal and distal segment of the mandible was done with rigid internal fixation. Bilateral superior region of the temporal bone and the teeth area were constrainted in all directions to simulate the masticatory muscle forces loading. Evaluate the transverse displacement of proximal segment after Bilateral Sagittal Split Ramus Osteotomy(BSSRO) in different lingual split patterns via finite element method. All the linear displacement data were analyzed using SPSS Statistics, version 19(IBM Corp, Armonk, NY).Results:1. A three-dimensional finite element model of mandible with temporomandibular joints and full dentition crowns was constructed, three-dimensional quadratic tetrahedral elements is densely distributed mainly in the temporomandibular joints, teeth, titanium plates, titanium screws, where the stress concentrated.2. The linear transverse displacement and augmentation of inter-proximal segments was clearly observed in pattern I and pattern II, values increaseimg from T0 to T1A3 and T1A8, loading condition with a mean of 2.99 mm and 4.70 mm; meanwhile, from T0 to T2A3 and T2A8, loading condition with a mean of 2.36 mm and 4.42 mm.3. Under the loading condition,there was no significance with the different lingual split patterns in linear augmentation values changes at A3 or A8(P = 0.058, P = 0.346). But comparison between the increased values was significantly associated with the advancement amount in the same split pattern T1 or T2(P = 0.000, P = 0.000).4. In modles of the two different lingual split patterns with different advancement amounts under loading condition, a displacement color scale of the 3D finite element models all shows the maximum transverse displacement distribution in the cortical layer of the proximal segment, which was measured around the gonial area, regardless of theoretical x-axis direction. The transverse displacement distribution passed through the mandibular gonial area to condyle and coronoid process based on gradient image, beginning from the maximum value to the minimum value and from outward to inward. The early contact was found between proximal segment and distal segment near the border of horizontal osteotomy line and sagittal soteotomy line.5. Comparison of the biomechanical characteristics between the original model and two different lingual split patterns models shows stress concentrated on the fixation area of titanium plate and titanium screws obviously in T1 and T2. Anterior border of ramus, coronoid process, sigmoid notch and mandibular angle were still concentration area of stress, though the amount and scope have changed to some extent.Conclusion:The results of the study indicate that transverse displacements of the proximal segments occur after BSSRO advancement with both T1 and T2 under the loading condition at early postoperative period, transverse augmentation can not identify the difference in transverse augmentation of inter-proximal segment between T1 and T2,but has statistically significant difference with advancement amount in the same split pattern. The masticatory muscle groups has influence on transverse displacement of proximal segment via finite element analysis, but compared to the linear displacement, the transverse displacement was mainly caused by mandibular advancement with rigid internal fixation at early postoperative period. The fulcrum caused by Bone interference between proximal segment and distal segment maybe thought as one of the influencing factors that related to mandibular width. The biomechanical characteristics of mandible has certain change after BSSRO. |
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