Stress Analysis Of Thoracic Vertebrae With Bone Reduction Based On Three-dimensional Finite Element Method

Osteoporosis is a common disease among middle-aged and elderly people.With the increase of age,the prevalence of osteoporosis is also increasing.However,due to the lack of awareness and incomplete medical diagnostic equipment,the best treatment period is often missed.Therefore,it is of great practical significance for guiding clinical practice to study the force exerted on the vertebral body during the specific period of osteoporosis and the holding force of the osteoporotic vertebral body on the internal fixation system.Based on the three-dimensional finite element method,the normal bone mass model and osteoporosis model were studied in this paper.The normal bone mineral density and osteoporosis thoracic vertebral model were established by computer finite element method,which could provide a reliable and effective reference for the further study of the biomechanical stability of posterior transpedicular screw.The stability of two different bone mass models under the same roof placement was compared on the established models,and the effect of moderate bone loss on the biomechanical stability of posterior thoracic screw was discussed.The stress distribution of vertebral body,intervertebral disc and internal fixator and the change of screw pull-out force were analyzed to provide reference for clinical use of pedicle screw.The main research work of this study includes:(1)Four models were reconstructed in this study,namely(A)Normal thoracic T7-T9 segments;(B)Osteoporotic thoracic T7-T9 segments;(C)Normal model with internal fixation system;(D)Osteoporotic model with internal fixation system.(2)Mechanical experiments were carried out on the model,that is 150 N force was added to the upper surface of the four models to simulate the weight of the upper body of the human body,and 5Nm motion torque was added to the model according to the right-hand rule to simulate the six motion states of the forward,backward extension,left and right lateral bending,left and right center rotation.The stress distribution,maximum displacement and maximum movement of the model were calculated respectively.Established with more than 870000 units and 190000 nodes.The finite element model of normal thoracic vertebral segments was compared with the finite element model reported in domestic and foreign literatures and experimental data.(3)Stress analysis of normal human skeleton and osteoporotic skeleton was carried out by three-dimensional finite element method,and internal fixation system was simulated.The displacement,motion angle,maximum stress and the holding force of osteoporotic skeleton were compared with those of normal skeleton and internal fixation screw,to verify whether the screw rod internal fixation system has the capability of reducing bone mass to this extent.Model A and model C were the control group,model B and model D were the experimental group.Compared with the six motion states of the normal model and the osteoporosis model,the maximum displacement of the osteoporosis model is more than 20% higher than that of the normal model,and the biggest change is that the maximum displacement of the osteoporosis model is 58.46% higher than that of the normal thoracic model in the extended motion state.The maximum displacement of osteoporosis model implanted with internal fixation system was in six motion states.Compared with the normal model implanted with internal fixation,the change of maximum displacement was the smallest in the right bending motion state,with a change rate of 14.24%.In addition,the change rate of forward bending and backward extension was significantly higher than that of left and right central rotation.Compared with the two models of normal bone mineral density,the motion displacement of the implant system decreased by 58.07%~83.37%.This shows that the implant system has an obvious fixation effect on the vertebral model.Comparing the maximum stress of the four models,the maximum stress of the osteoporosis model increased in varying degrees under the four motion states of flexion,extension,left lateral bending and right central rotation.Comparing the two models,the maximum stress of the osteoporosis vertebral body implantation model in four motion states except right lateral bending and flexion was equal.With a small increase,the change rate of the minimum motion displacement 4.07% higher than that of the extended motion.For the patients with moderate osteoporosis,the stress and displacement have a great change compared with the normal patients.With the loss of bone mass,the holding force of vertebral body to screw decreases.The stress distribution of vertebral body and pedicle screw during the extraction of pedicle screw was analyzed in this experiment.It was also proved that the bone mineral density had a significant relationship with the pull-out force of pedicle screw.Bone mineral density should be measured before the pedicle screw-rod internal fixation system was implanted to refer to the necessity of whether it is suitable for the implantation of the internal fixation system.The pull-out force test results show that when the screw is loosened by external force,the stress in the tail of the screw is the strongest.The stress from the tail end of the screw to the joint of the vertebral body and the tail of the pedicle screw will first have the risk of fracture.