Different Internal Fixation Methods Of Thoracic Vertebral Segments Under Osteoporosis Condition Based On Three-dimensional Finite Element Method

With the gradual increase of the aging society,osteoporosis is becoming more and more common in the elderly,especially postmenopausal women.Due to various factors leading to bone loss and osteoporosis,the risk of fractures has increased.Internal fixation is generally used to fix the type of fracture.However,due to the large amount of bone loss in osteoporosis people,the internal fixation is prone to the risk of loosening,This problem has always existed in the clinic.Therefore,in order to enhance the pull-out force of the pedicle screw,reduce the risk of screw loosening,we have proposed a new type of internal fixation in this research.In addition,To solve the problems of corpse specimens that are difficult to obtain and can’t be reused,Based on computer tomography(CT),We using 3D reconstruction technology for rebuild model,and establishing a finite element model combined with the finite element analysis method,the stability of the new type of internal fixation was studied and verified.The mechanical properties of the thoracic spine were analyzed in this way.The stress distribution of the vertebral body and the internal fixation system and the model’s activity(ROM)were analyzed.Compared with the traditional internal fixation method and double cortical internal fixation method the effectiveness of the internal fixation in clinical practice is verified,which provides a reference for clinical use.The main work studied in this paper includes:(1)According to the needs of this experiment,we have established four finite element models,Respectively is:(A)Intact model: complete osteoporosis thorax T7-T9 segment model;(B)UCPS model: based on the Intact model,the traditional internal fixation method is used for fixing;(C)BCPS model: based on the Intact model,using the double cortical internal fixation method;(D)based on the Intact model,using the triple cortical internal fixation the way.(2)The finite element model is used to mechanical simulation experiments by the finite element software.In the experimental process,according to the anatomy of the thoracic spine,the soft tissue was added,such as ligaments.Then,the four models are divided into surface mesh and volume mesh,and the corresponding material properties were defined.The upper surface of T7 is coupled through a reference point,and a downward focusing force of 150 N is applied to the reference point to simulate the self-weight of the upper body of the human body,and a 5Nm torque force is applied to simulate the physiological activity of the human body in six directions,including forward bend,backward extension,left and right lateral bend,and left and right axial rotation.During the entire process of force application,bound boundary conditions are applied to the lower surface of T9.Finally,the stress distribution,motion displacement and activity ROM of the model were calculated by simulation.The accuracy of the model is verified by comparison with previous literatures.(3)The three-dimensional finite element method was used to analyze the stress of the complete osteoporosis model and the model with three internal fixation methods,and the distribution of the stress and displacement of the vertebral body with three internal fixation methods were compared and analyzed.In addition,the force analysis of the internal fixation system of the three internal fixation models was performed,and the force position of the pedicle system during the six movements was studied.The stability of the model was verified by measuring the vertebral body movement(ROM).The analysis results are compared with the complete osteoporosis model to reach the final conclusion.The finite element model of this test met the requirements from modeling to attribute material assignment.In addition,the validity of the model is verified by comparison with the activity of other people’s finite element models.Through the analysis of vertebral body stress,under the physiological activities of extensor and right bend,the stress is mainly concentrated in the pedicle and the middle part of the vertebral body.The lower stress was reduced by 48.0% and 42.3%,respectively.It shows that implanted internal fixation can increase the stress of the vertebral body and improve the stability of the vertebral body.By comparing the forces of the internal fixation system in the physiological state of the extension and right-hand bending,the TCPS model can improve the maximum stress of the internal fixation in some aspects.From the force cloud diagram,it can be seen that the place where the internal fixation stress concentration occurs generally,The tail of the screw or the area where the pedicle screw is attached to the nail rod.Therefore,in the use of clinical pedicle screws,we must pay special attention to the stress on the tail of the screw to avoid excessive breakage.For patients,it should also reduce the large-scale activities and reduce the internal fixation stress.Comparing the maximum displacement of the vertebral body and the range of motion(ROM),the maximum displacement of the TCPS model is smaller than that of other internal fixation techniques in the same physiological condition at the same position.Compared with the BCPS model,the maximum displacements of TCPS in the six physiological activities of forward flexion,extension,left bend,right bend,axial left rotation and axial right rotation were reduced by 15.0%,31.6%,4.0%,27.0%,36.0%,and 43.2%.For the analysis of activity,compared with the activity ROM of the complete model,the ROM of the three internal fixation models showed a significant decrease in the ROM of the internal fixation in the six physiological states.Compared with the other two models,the UCPS model has significantly improved stability.