| Background and Objective:The spinal pedicle screw fixation system for spinal stabilization has become common and widely accepted in spine surgery.Currently,the most popular orthopedic materials are metals such as titanium and its alloys because of its good biocompatibility.The conventional pedicle screw-rod system is made from the titanium-alloy TI-6A1-4V,which is biocompatible,corrosion resistant,bendable,and stiff enough to provide sufficient spinal stability.However,the elastic modulus of TI-6A1-4V is much higher than the cortical bone,the high stiffness of titanium alloy is closely related to postoperative complications such as screw loosening,poor spinal bone fusion and accelerated degeneration of adjacent segments.Recently CFRP has been successfully used in orthopedic implants.Due to its radiolucency and elastic modulus comparable to those of the normal spine,many have suggested its potential for use in specialized spine surgery.Given this,the aim of this study was to design and develop a new type of lumbar pedicle screw that can improve the biomechanical environment after spinal surgery based on CFRP materials.Methods:1.In the first part of the experiment,we aim at detecting the biomechanical property of CFRP materials.A batch of lumbar pedicle screws was trial-produced with CFRP materials and compared with Ti6A14 V screws through a three-point bending test and finite element simulation,so as to make a preliminary evaluation of the biomechanical strength of the CFRP screws.2.In the second part of the experiment,we designed three models of CFRP screw to reinforce the biomechanical property based on the results of the first part of the experiment,namely: a conical screw design,reinforce the inner diameter of the screw,and TI-CFRP composite screw retained as titanium alloy thread at the tail.The conical screw consists of two different sub-design schemes: v0.2 type with a complete cone at the screw shaft and v0.3 type with a 2-stage design for the screw rod section.The TI-CFRP composite screw contains 6 different sub-design schemes,namely CFRP material full coverage type(Cover),2 turns of thread of titanium alloy retained at the tail of screw(2rds),3 turns retained(3rds),4 turns retained(4rds),5 turns retained(5rds),6turns retained type(6rds).To compare the pullout strength and bending strength among different models of CFRP pedicle screws,three-dimensional finite element models were developed to compare and improve the mechanical reliability of these screws.Results:1.In the first part of the experiment,the three-point bending showed that the maximum bending load of pure CFRP material screws was 190±60N,which was significantly lower than that of TI screws of the same size(3957±249N)(P<0.05)and the finite element simulation results suggested that its bending strength was only 33.87% of the same size of titanium alloy screws.At the same time,the result of the finite element simulation indicates that the tail of the screw is the stress concentration part of the screw.2.Based on the experimental results of the first part,we designed different models of CFRP screws to guarantee enough stability and notably decrease the stiffness of the implantation simultaneously.According to the results of the finite element analysis simulation,it was found that the conical screw design of pure CFRP material can meet the mechanica strength:the v0.2 and v0.3 conical screw reached 98.39% and 87.9% of the bending strength of cylindrical titanium alloy screws of the same size respectively and the anti-pull-out experimental test suggested that under the same axial pull-out strength,the deformation of V0.2 screw is relatively small,which is close to the pull-out resistance of titanium alloy.The bending strength of M6.5 cylindrical screws of pure CFRP material can only reach 45.16% of M6.0 titanium alloy screws of the same size.For TI-CFRP composite screws,the 4rds、5rds、6rds can reach the same bending strength of TI pedicle screw,while the the 4rds has best elastic deformation at the same safety factor and achieves 97% of the pull-out strength of titanium alloys of the same size.The design of TI-CFRP composite screw with four titanium alloy threads at the end of the screw was finally determined by us,for it can effectively reduce the screw stiffness as well as guarantee the screw strength,and does not affect the intraoperative operation convenience.Conclusion:Screws of CFRP materials alone fail to meet the mechanical strength of clinical demands.Nevertheless,the results of finite element analysis and the biomechanical study show that the “TI-CFRP” composite screw can reduce the screw stiffness and guarantee the screw strength simultaneously compared with the Ti6l4 V screw.In addition,the“TI-CFRP” composite screw with four titanium alloy threads at the end of the screw is the best design. |
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