Three Dimensional Finite Element Study Of The Mandible Body Defects Reconstructed With The Plate

Mandible is an important part not only support the facial contours, but also to maintain oral function, especially the chewing function. The defects caused by mandible tumors or mandible trauma, for patients face to significant dysfunction and physiology, psychological barriers, a serious threat to quality of life. Therefore, the reconstruction of the mandible is not only to restore the anatomical shape, but also to restore the necessary physiological functions. The most commen mandibular defects in the clinical is the mandible body defect, and the most widely used in the reconstrution is titanium plate.It can effectivly restore the patient's appearance and protect the basic social functions. However, in the clinical postoperative follow-up, we found that there are so many problems, such as plate loose, fracture, titanium screw off, decline in chewing efficiency. It prompt us to realize that maybe there is a part of stress concentration in the fixition.The finite element analysis used to evaluate the stress distribution in the mandible reconstruction.To explore the internal fixation complication such as plate loosening, plate fracture, titanium screw off and so on,and provide the basis for clinical applications.Part I Reconstruction of the three-dimensional finite element model of dentitional mandible with the reverse engineering softwareObjectiveIn this study, we used CT scanning, combined with Free Form, Mimics and other reverse engineering software, to establish the three finite element model of mandible with dentition.Then give the masticatory loading and analysis the distribution of the stress.Materials and Methods1,The data of the model selected in the department of Oral and Maxillofacial Surgery, Nanfang Hospital, Southern Medical University, from a healthy volunteer, 26 years old male, complete dentition without crowding, it is an individual normal occlusion, ruled out mandibular disease by the X-ray examination.Using 64-slice CT scan,the scan parameters:slice thickness 1mm, tube current of 200mA, the voltage 120kV, scan range from the lower edge of mandibular to the condylar.Received 180 slices with a clear CT bone window images and fromed the images to DICOM format.2,Bound borders and set up conditions:We boundary the condyle is close contact with the articular surface during mastication. Assumptions:This study involved the mechanical properties of biological materials are assumed to be homogeneous, continuous and isotropic, when load to the model,the sections have not mutual sliding, the unit has enough stability.3,Three-dimensional finite element model:The obtained images were inputted into three-dimensional model reconstruction sofewere Mimics 10.0 to renconstruct the mandibular model with dentition.This model included every tooth, cortical bone, cancellous bone.Finally the 3D model was inputted into the ANSYS of the format of IGES,which is a universal finite element analysis software.After surface meshed the 3D finite element modeling was renconstructed with a complete dentition, cancellous bone, cortical bone.4,Three-dimensional finite element model validation:Simulated occlusal loading, the vertical load on the mandibular first molar central fossa,. Loading type is a static load and loading size is the mean bite force 200N which is simulated the half of the largest.Results1,We successful established biomechanical model of the mandible with dentition by using the three-dimensional reconstruction and finite element software Ansys,Mimic. The model have geometric similarity, and vivid shape. The cortical bone were generated 40850 nodes, and 20,070 units, the spongy bone were generated 45,934 node and 27,557 units, the teeth were generated 50184 nodes,.2,After the occlusal loading. the stress distribution is concentrated in the mandibular body,ascending branch, and the leading edge of mandible ramus.And the stress of mandible posterior margin is significant concentration than the force of inferior.There is a uniform stress distribution in the chin, no significant stress concentration.There is no difference of the stress distribution in the contralateral mandible compared with the load side,but lower than the load side during the slightly transmission of the chin. The stress distribution of cancellous bone is coordinated with the stress distribution in the cortical bone. Under the normal occlusion the force along the tooth long axis to the alveolar bone scattered through the mandible, the teeth scattered less stress.Conclusions Using CT scanning technology combined with Freeform, Mimics and other reverse engineering software, can create three-dimensional finite element model of mandible with dentition, realistic shape of the model has good geometry similarity, close to the actual situation and clinical.It can be arbitrary rotation and the scaling can also be observed for a variety of ways to build the model structure and hierarchy, fine mesh elements.Part II Biomechanical analysis of mandible body defect fixation with double cortical screw plateObjectiveThis step is to simulated one side body defect of the mandible, reconstructe d with plate fixation, analysis of the stress distribution and displacement trends, and compared with the normal mandible,provided some reference for clinical work.Materials and Methods1,We established three dimensional finite element modle of mandible body defect used the modle in the first part.Reconstructed the defect with the double-cortical-titanium-screw plate.2,Bound borders and set up conditions:We boundary the condyle is close contact with the articular surface during mastication. Assumptions:This study involved the mechanical properties of biological materials are assumed to be homogeneous, continuous and isotropic, when load to the model,the sections have not mutual sliding, the unit has enough stability.3,The load conditions:three-dimensional finite element model validation: occlusal loading, the vertical load on the mandibular first molar central fossa,. Loading type is a static load and loading size is the mean bite force 200N,which is simulate the half of the largest.4,Observation indexs:To observe the stress distribution after the plate reconstructed the mandible body defects.The stress distribution in the cortical bone, cancellous bone,plate and the double cortical screw.Results1,Finite element analysis of the mandible body defect reconstructed with two-cortical screw fixation model:A left mandibular body defect fixation with double cortical screw model was established in this research, which have geometric similarity, and vivid shape.After molar icon loading:the stress distribution concentrated in the mandibular body, ramus, and mandibular posterior margin.There is a significant stress concentration region in the lower edge,the inferior is greater than the posterior, the stress distribution in the chin is uniform.The left side plate fixation areas contact at the regional and residual stress. Plate stress distribution:the defect is relatively uniform overall force. There is on no significant stress concentration in the double cortical screw peak areas.The stress concentrated at the top of screws Displacement after loading:the most prone to displacement is the anterior of the plate fixation.2,The sum of two stress concentration around the titanium screw in the mandible body defects reconstruction with two-cortical screw fixation. The top of screw take more stress, less stress commitment in the cutting-edge screw. There is no bear for the stress in the screw tip 1/3, both-ends of the stress is greater than the middle screw.The top and the bottom edges of the stress distribution show that stress at the upper edge is greater than that in the lower edge, the stress was more dramatic changes in the upper edge.Eight peaks contacts with the titanium screw were the plate stress concentration area.Conclusions1,The stress distribution of the the mandible body defect reconstructed with two-cortical screw plate model shows:stress distribution focus on the mandibular ramus, anterior ramus, and the lower edge. All the double cortical screw in the situation of fixation are well, no significant stress concentration in the peak of cortical screw.But the top of the screw obviously take more stress concentration.The others stress distribution are similar with the nomal mandible,The two cortical screws in the middle and the plate are the stress transfer.2,Double cortical screw and the plate stress distribution curves show that the most vulnerable parts of the fracture is the plate is in front of mandible angle.The lower edge of the plate is more prone to change than the upper edge,leading to the plate fracture.It can provides a reference in the clinical for prevention the complications.