Development And Mechanical Study Of Thoracolumbar Kyphosis Orthopedic Pedicle Screw

BackgroundPedicle screw rod system is generally used in the treatment of spinal degeneration,trauma,deformity,tumor and other diseases.The improvement and innovation of its structure and material has been a research hotspot in the field of spinal surgery.Thoracolumbar spine fracture is the most common spinal trauma.Posterior pedicle screw fixation is the mainstream surgical method for thoracolumbar fractures.Thoracolumbar fractures can easily lead to vertebral compression and kyphosis of adjacent segments.However,the principle of pedicle screw reduction of fractures is still to achieve correction by axial stretching or compression with the aid of instruments when installing the connecting rod.Some studies have shown that the commonly used pedicle screw system has poor correction ability for kyphosis of sagittal plane of spine.Some studies have found that the current pedicle screw system has deficiencies in maintaining the height of injured vertebra and cobb angle after operation.In view of the existing problems of pedicle screw in fracture reduction,a pedicle screw system for thoracolumbar fracture reduction and correction was designed.ObjectivesA new pedicle screw which can effectively correct local kyphosis after thoracolumbar fracture was designed and manufactured.The effectiveness and reliability of the screw design were verified by finite element analysis and mechanical test.Research methods1.Design of the new screw.Based on the deeply analysis of the problems of the existing pedicle screw system in the reduction of thoracolumbar fractures,a new type of pedicle screw system is designed,which can automatically restore the anterior column height of the fractured vertebral body through the screw rod combined with locking screw cap,and then correct the kyphosis of the injured vertebral body.2.Finite element analysis.The finite element model of thoracolumbar fractures was established based on the CT data of the case,and the ordinary screw system and the new screw system were assembled on the model respectively.The correction effect of the new screw was observed and compared in the assembly process.Then,by setting the load,the vertebral stress,instrument stress and instrument displacement of the two screw system implantation models under different work conditions just like flexion,extension,lateral bending and rotation were compared to analyze the advantages of new screws’ stress after implantation.3.Mechanical test.To clear the rationality of the design of the new screw’s structure,verify how much axial opening force can be generated during the screw cap locking process,and further verify whether the screw cap will become a mechanical weakness after fixing,three experiments are designed in this part to test the mechanical strength of the screw cap locking,the opening strength test and the dynamic bending fatigue test.Results1.This study designed a new type of thoracolumbar kyphosis correction pedicle screw fixation system,and the screw design has been patented.The new screw consists of screw body,screw tail,two blades with internal thread and screw cap.The feature is that the screw tail and the lower compression block in the screw cap have the corresponding continuous slope with inclination,which can automatically generate axial tension through the combination of two inclined planes and the rod when the upper connecting rod is fixed,restore the anterior column height of the fractured vertebral body,and then correct the kyphosis of the local segment.2.The results of finite element analysis.In this part,the L1 vertebral fracture model was established,and on this basis,two models of pedicle screw system after internal fixation were established by assembling ordinary screws and new screws,respectively.By building the different model,it was proved that the new screw had the ability of kyphosis correction.Through the finite element mechanical analysis,the stress of the injured vertebra in the model after the new screw placement was lower,and the new screw was subjected to more loads than the ordinary screw placement.However,the internal concave structure increased the contact surface between the connecting rod and played a role in dispersing the stress.The new screw has advantages in mechanical bearing and conduction ability,which can effectively reduce the stress of fractured vertebral body and reduce the risk of secondary fracture.3.Mechanical test results.There is no significant difference between the torque of the screw cap and the ordinary screw when locking,and there is no structural failure when the average value is larger.The screw cap structure can withstand the stress intensity required for locking the connecting rod.The maximum axial opening strength of the new screw was125.83 Nm.Through the dynamic bending fatigue test,the anti-fatigue performance of the new screw has reached clinicalConclusionIn this study,a new type of thoracolumbar kyphosis orthopedic pedicle screw was designed.When the connecting rod was fixed,the axial opening force was automatically generated by the combination of two inclined planes and the rod,so as to achieve the effect of opening the anterior column and correcting the cobb angle of kyphosis.The results of FEA and mechanical experiment proved that the screw had advantages in orthopedic ability and mechanical properties compared with ordinary pedicle screw,and the design of the new screw was effective and reliable.