Evaluation of the stress distribution on the inferior alveolar nerve in mandibular advancement and counterclockwise rotation surgery with THREE-dimensional finite element analysis

Introduction: Sagittal split osteotomy is a frequently used treatment for the correction of hypoplasia and hyperplasia of the mandible. Temporary or permanent hypoesthesia of the inferior alveolar nerve is a common complication of this procedure. Especially in cases where the mandible is advanced, the tension on the nerve is higher. This condition affects nerve conduction. Accordingly, it affects the duration of post-operative hypoesthesia. Although it is known clinically that the tension in the nerve increases as the amount of mandibular advancement increases, there is no study in the literature on inferior alveolar nerve tension in cases where counter clock-wise rotation of the mandible is performed. The aim of this study is to compare the tension in the inferior alveolar nerve in models with different amounts of advancement and counterclockwise rotation. Materials and methods: For this experimental study, a 3D finite element model of the mandible was created from 0.5 mm serial axial sections from two-dimensional (2D) computerized tomography (CT) images using 3D DOCTOR. The precise organic properties of cortical and cancellous bone structures are considered isotropic, homogeneous and linearly elastic. The organic properties of the inferior alveolar nerve are considered viscoelastic and homogeneous. In the study, a total of 6 models were used. Different amounts of advance in the linear direction to 3 of these models; The other 3 were rotated counterclockwise in combination with different amounts of advancement. First model 6mm, second model 9mm, third model 12mm linear advancement; The fourth, fifth and sixth groups were rotated 10 degree counterclockwise with 6.9 and 12 mm advancement, respectively. Then, the tensile stress of the inferior alveolar nerve in each model was calculated. Finite element solver software was used to calculate the stresses in each model. Stress lines were calculated and drawn in the inferior alveolar nerve tissue and fixation devices. By comparing the tension values in the nerve with each other, the effect of orthognathic surgery on the risk of nerve damage was evaluated. Results: In Model 1, Von Mises stress value: 1724, Pmax stress value: 2864, Pmin stress value: -1009, has occurred as a result of 6 mm linear advancement. In Model 2, the Von Mises Stress value: 2585, Pmax stress value: 4296, Pmin stress value: -1513 has occurred as a result of 9 mm linear advancement. In Model 3, Von Mises stress value: 3443 is: 3447, Pmax stress value: 5728, Pmin stress value: -2018 has occurred as a result of 12 mm linear advancement. In Model 4, Von Mises stress value: 1739, Pmax stress value: 2867, Pmin stress value: -1029 has occurred as a result of 6 mm linear advancement and 10 degrees counterclockwise rotation movement. In Model 5, Von Mises stress value: 2600, Pmax stress value: 4299, Pmin stress value: -1532has occurred as a result of 9 mm linear advancement and 10 degrees counterclockwise rotation movement. In Model 6, Von Mises stress value: 3461, Pmax stress value: 5731, Pmin stress value: -2036, has occurred as a result of 12 mm linear advancement and 10 degrees counterclockwise rotation movement. Von mises values are listed as M6>M3>M5>M2>M4>M1. Von mises values are listed as M6>M3>M5>M2>M4>M1. Minimum principal stress values are listed as M1>M4>M2>M5>M3>M6. Conclusion: The main reason for the stress on the inferior alveolar nerve after SSRO is the amount of advancement rather than the rotational movement. When the amount of mandibular advancement increases by 2 times, it is seen that the stresses on the nerve increase approximately 2 times. Key words: mandibular advancement, counter-clockwise rotation, inferior alveolar nerve, finite element analysis