Biomechanical Modeling Of The Mandible And Temporomandibular Joint And Researches On Mandibular Angle Osteotomy

BACKGROUNDWe, East Asian nation think that women with "oval face contour outline" is attractive. Mandibular angle is an important anatomic structures to determine the width and shape of the lower face,while Female mandibular angle hypertrophy makes the appearance of a lack of tender, elegant, grace and other oriental women-specific temperament. Mandibular angle contouring surgery caused by Prominent mandibular angle,in recent years, become more and more popular in our East Asian country. In 1989, Baek et al defined the concept of Prominent mandibular angle. The incidence of mandibular angle deformity in Oriental populations is relatively high, mostly bilateral simultaneous. The clinical features of the mandibular angle Prominence is the prominent mandibular angle droop down, extend toward anterior-posterior, the angle of prominent mandibular angle becomes smaller, the distance from the mandibular angle to the ear lobe is large, even more than 3cm. The normal mandibular angle in eastern population is 110 to 120 degrees, in women are smaller than men. Prominent mandibular Angle accompany by masseter muscle hypertrophy commonly, as well as hypertrophy of the buccal fat pad and facial subcutaneous fat hypertrophy and other factors affecting lower face width. Facial contouring surgery is the application of craniofacial surgical techniques to achieve the purpose of transfiguring the facial contours. The mandibular osteotomy is the most common procedure in facial contouring surgery. Traditional mandibular angle osteotomy is difficult to expose and saw because of the intra-oral approach. Recently, Liu have developed a drilling-chisel method for mandibular angle osteotomy, and deem that this method has more advantages than traditional methods, Should be widely applied as well.In clinical application, The drilling-chisel method for mandibular angle osteotomy have been validated that have the advantages with controllable osteotomy line design with convenient, simple, minimally invasive, less intra-operative hemorrhage and postoperative complications. Before accepting this method, however, many surgeon remain concerned about that the impact may damage temporomandibular joint and other parts of the mandible, so that the popularization and application of this surgical procedures were limited.In the clinical application and relevant researches on the drilling-chisel method for mandibular angle osteotomy, there are still a few questions.â‘ Biomechanical mechanism of the drilling-chisel method;â‘¡Biomechanical status of the whole process of the drilling-chisel method;â‘¢Beliability of the drilling-chisel method.For in-depth, detailed study on biomechanics of the drilling-chisel method for mandibular angle osteotomy during surgery, three-dimensional finite element analysis method can be very good to achieve this goal.OBJECTIVEThrough this research project, to explore the methods and means of establishing a three-dimensional finite element model of mandible and temporomandibular joint with high similarity;by simulating of the drilling-chisel method for mandibular angle osteotomy, to understand the biomechanical principles;propose and analysis the approaches to improve and optimize the drilling-chisel method.METHODS1 Comparison of different modeling methods of mandibular FEA model 1.1 The direct modeling method to establish three-dimensional finite element model of mandible by MimicsDirect method establish the nodes and elements in accordance with the geometry of the structure system of the object. Based on a young female volunteer head CT scanning images, using contour extraction and threshold segmentation to extract the mandible, remove bone and soft tissue nearby. Using the three-dimensional reconstruction and Remesh modules, obtained a smooth and continuous bone surface element model,inported into Ansys software to create a three-dimensional finite element model of mandible. Mimics FEA module through reading grayscale information of CT images of bone tissue and assigned material parameters for each element, we can simulate the heterogeneous nature of bone tissue material properties, and established Model-1.1.2 Indirect modeling method to establish three-dimensional finite element model of mandible by SolidworksBy indirect method, first establish the physical model of the object, and then the solid model is automatically meshed to form a finite element model. According to the same original CT image, First used an image processing software for image segmentation to separate the mandible from other tissues, and the point cloud format file was entered into reverse engineering software Solidworks. software divided surface based on the curvature domain to generate patch surface, adjusted the surface characteristics line to eliminate distortions in surface; wire frame model generated surface model in Ansys software, surrounded part of the surface model generated solid model;Finite element analysis software ANSYS meshed entities to generate the final three-dimensional finite element model (model-2). Bone tissue material parameters assigned the same value of model-1.1.3 Comparison of Mimics model and Solidworks ModelFor the comparison and analysis the two modeling methods, three-dimensional finite element model of mandible by the direct modeling method using Mimics and three-dimensional finite element model by indirect modeling method using Solidworks, adopted the same boundary conditions, losded the same level of anterior-posterior impact load at the mental symphysis; calculated in the same solver and compared the the magnitude and distribution of Von Mises stress of two models.2 Biomechanical Study on the drilling-chisel method for mandibular angle osteotomy2.1 The establishment of three-dimensional finite element model of mandible and temporomandibular jointBased on a Youth female volunteer's head CT scanning images, by the indirect method, we established the three-dimensional finite element model of mandible (model-3)comprising glenoid fossa, temporomandibular joint disc and masticatory muscles and ligaments of the mandible. Link element simulated the masticatory muscles and ligaments, defined the mandibular condyle,articular fossa of temporal bone and the articular disc as the contact element, constrainted the nodes in all directions of displacement of fossa surface and top of link elements..2.2 The drilling-chisel method for mandibular angle osteotomy operation modeling and biomechanical analysisAccording to clinical procedure, through Boolean operation, we modified the model-3 to simulated drilling holes on mandible, then the drilling-chisel method for mandibular angle osteotomy model (Model-4) was established. Applying the impact loads at the lower edge of osteotomy line simulated osteotomy procedure. Based failure criterion to determine the failure load, observed the distribution and magnitude of Von Mises stress during the procedure.3 Biomechanical optimization of the drilling-chisel method for mandibular angle osteotomy3.1 Non-uniform drilling method for mandibular angle osteotomyFollowing the principle of minimize the failure load, we designed a new drilling fashion which increased the number of drilling at both ends of osteotomy line. Other conditions same as with the model-4,then we established Model-5 and compared the results with model-4's.3.2 Mandibular angle buccal cortex removal method According to local anatomical measurement and the optimization principles, combined with the actual surgical operation, we designed a optimized method to remove buccal cortex of mandibular angle of model (model-4) and non-uniform drilling osteotomy model (Model-5), other conditions same as with the model-4,then we got the model-6 and model-7. All the calculation results of model-4,5,6,7 are compared with each other and selected the mandibular angle osteotomy optimization model.3.3 The contribution of constrain condition to the mandibular angle osteotomyBased on model-7, loaded the axial Pre-tension on the link elements to simulated the max muscle contraction force, we established model-8; constrained the nodes on the posterior border of the mandibular ramus over the osteotomy line, we established Model-9; and applied all of two constrain conditions on Model-7,we established Model-10. The computing results of Model-7,8,9,10were compared with each other.RESULTS1 Comparison of different modeling methods of mandibular FEA model5.1 The direct modeling method to establish three-dimensional finite element model of mandible by MimicsWe established the mandibular three-dimensional finite element model (model-1), comprising cortical bone and cancellous bone,214354 nodes,147219 tetrahedron elements.5.2 Indirect modeling method to establish three-dimensional finite element model of mandible by SolidworksWe established the mandibular three-dimensional finite element model (model-2), comprising cortical bone and cancellous bone,79538 nodes,73999 tetrahedron elements.5.3 Comparison of Mimics model and Solidworks ModelIn the peak load, the von Mises stress value of each node on corresponding parts of each model analysised by paired t test, and the result showed that the differences between the two groups of data has no significant (p=0.761). 2 Biomechanical Study on the drilling-chisel method for mandibular angle osteotomy2.1 The establishment of three-dimensional finite element model of mandible and temporomandibular jointWe established a three-dimensional finite element model of uniform drilling mandibular angle osteotomy (Model-4).2.2 The drilling-chisel method for mandibular angle osteotomy operation modeling and biomechanical analysisThe results showed that the failure load of mandibular angle is 2426N; von Mises stress mainly distribute near the lower half of the osteotomy line, while conduction to the mandibular symphysis; the value of von Mises stress in left TMJ articular disc is larger than right's, the stress values in most regions is between 0.56-7.54MPa.3 Biomechanical optimization of the drilling-chisel method for mandibular angle osteotomy3.1 Non-uniform drilling method for mandibular angle osteotomyWe established the three-dimensional finite element model of non-uniform drilling method for mandibular angle osteotomy (Model-5) and loaded. The computing results showed that the failure load of mandibular angle was 2018N; Von Mises stress distribution in the mandible showed that stress concentrated at the lower half of the osteotomy line and the mandibular symphysis, the red area showed the peak value of stress is smaller than Model-4.3.2 Mandibular angle buccal cortex removal methodWe established the three-dimensional finite element model of mandibular angle buccal cortex removed uniform drilling method for mandibular angle osteotomy (Model-6)and mandibular angle buccal cortex removed non-uniform drilling method for mandibular angle osteotomy(Model-7), loaded as well. The computing results showed that the failure load of mandibular angle of Model-6 was 1854N and Model-7's 1447N; the distribution of von Mises stress of Model-6, and Model-7 were more concentrated than in Model-4 and Model-5,while it was more concentrated in Model-7 than in Model-6.3.3 The contribution of constrain condition to the mandibular angle osteotomyThe computing results of Model-8,9,10 showed that the failure load of mandibular angle of Model-7 was 1183N and Model-9's 1028 N,Model-10's 876N;von Mises stress distribution of Model-8 were more concentrated than the model-7 with a smaller stress concentration zone of mandibilar corpus, stress in Model-9 concentrated in the region near the osteotomy line and the upper area, the conduct direction same as the osteotomy line. In Model-10, von Mises Stress concentrated in a narrow region around osteotomy line.CONCLUSIONAccording to biomechanical principles and clinical experience, using the mechanical simulation experimental method, established the model of simulation surgery, we analysised the mandibular angle osteotomy. To minimize the failure load, we designed a series of 3D finite element model of mandibular angle osteotomy. Computing and analysis these models, we believe that non-uniform drilling method is better than uniform methods; the mandibular angle buccal cortex removed non-uniform drilling method for mandibular angle osteotomy is superior to the method that without removing the buccal cortical bone of mandibular angle and mandibular angle buccal cortex removed uniform drilling method for mandibular angle osteotomy.Based on the mandibular angle buccal cortex removed non-uniform drilling method for mandibular angle osteotomy model, Constraints imposed by the maximum muscle contracting force alone or constraints at the posterior border of the mandibular ramus and also applied them all can significantly reduce the failure load, but to impose constraints at the posterior border of the mandibular ramus approach are more favorable and reliable.