Design,Manufacture And Bio-mechanical Research Of A Novel Tailless Cap Self-locking Pedicle Screw

Background Posterior spinal fixation is the core technique for spinal stabilization and orthopedics and has been widely used to treat spinal deformities,spinal fractures,and spinal degenerative diseases.At present,the commonly used pedicle screw locking design includes top vertical locking or side locking,and both need to use a screw plug to rotate and press the fixing rod.Although the design can effectively lock the fixed rod and complete the stable structure of the nail rod,there are some defects,such as:1.The operation steps in the operation are complicated,and various steps such as pre-tightening,locking,and breaking the cap are required,and the operation time is long;2.There are phenomena such as staggering wire and sliding wire during the installation of the screw plug,which may cause the fixing rod to loosen and fail to fix;3.The tail cap must be high enough to lock the fixing rod,so the cutting marks at the end of the screw are increased,occupying more soft tissue area,especially for children.The postoperative foreign body sensation is obvious,so patients often feel pain in certain body part that.In view of the above defects of tail cap locking design,this study designed a set of new pedicle screws with zero notch and fast locking,so as to eliminate the defects of traditional screws,simplify the intraoperative operation process and improve the efficiency of clinicians.ObjectivesDesigne and manufacture a zero-notch,self-locking pedicle screw.Analyze and verify the safety and effectiveness of the product by finite element analysis and bio-mechanical experiments.Research Methods1.Design and Manufacture:The design of a new type of screwless self-locking pedicle screw.Specifically,the integrated sliding lock and the nail rod joint portion consist of two parts.1)One-piece sliding lock design: It includes two structures: the fastening seat and the screw seat.The sliding lock and unlocking of the fastening seat are realized by the difference between the diameter of the screw seat and the moderate elastic deformation of the metal.2)Nail rod joint: The inner wall of the screw seat is combined with the rod by slot,so as to achieve no relative movement after the nail rod is locked.Cooperate with Shandong Weigao Orthopedic Materials Co.,Ltd.to manufacture samples according to design requirements.2.Finite Element Analysis:The finite element model of L3-S1 was established,and the material properties of each part of the model were given.Computer simulation of lumbar 4/5 single segmental resection,simulation of posterior fusion internal fixation surgery.After 3D calculation,the bio-mechanical data of the lumbar spine under four states of forward flexion,lateral flexion,backward extension and rotation were calculated.Then the stress of each part of the zero-notch headless screw under various pressure states and the stress concentration of the screw in the test are obtained by analyzing the pressure cloud diagram.3.Bio-mechanical Test:Using the special clamp and ultra-high molecular weight polyethylene experimental module as the carrier,the biomechanical axial pull-out experiment,static compression bending yield test and periodic anti-flexion test of the new non-spiral pedicle screw and common screw were compared.The axial pull-out force of the screw,the static compression bending yield load,the bending stiffness,the cyclic maximum load and the displacement are comprehensively evaluated to evaluate the stability of the new screwless pedicle screw.Results1.A tailless cap self-locking pedicle screw is designed and manufactured,which is characterized by 1)zero top notch,2)all-in-one lifting sliding locking,and 3)the screw rod is combined with embedded slot joint.After the screw rod is combined with the metal elastic force,there is no relative motion between the two.Obtained a national invention patent,Patent number: CN107157564 A.Cooperate with Shandong Weigao Orthopaedic Materials Co.,LTD.,to manufacture screw samples that meet the design requirements for testing.2.Finite element analysis results: under the four test conditions of anterior flexion,lateral flexion,posterior extension and rotation,the maximum stress values of tailless self-locking pedicle screw structure and interbody fusion cage were as follows,respectively.Anterior flexion:29.15±0.992×106N/m2;Posterior extension: 29.87±1.054×106N/m2;Lateral flexion: 31.73±2.290×106N/m2;Rotation: 28.80±1.862×106N/m2;the maximum stress values of traditional pedicle screw structure and interbody fusion cage were as follows,respectively.Anterior flexion: 29.92±1.537×106N/m2;Posterior extension: 29.03±1.282×106N/m2;Lateral flexion: 32.53±1.994×106N/m2;Rotation: 29.47±2.431×106 N/m2;During loading,the stress of the internal fixation system was mainly concentrated at the end of the screw and the junction of the screw rod.3.Bio-mechanical test results: the maximum axial pull-out force of the new pedicle screw rod combination without tail cap was 1387.45±96.84 N,while the maximum axial pull-out force of the traditional pedicle screw rod combination was 1477.75±75.28 N.The bending yield load of the new pedicle screw was 1477.75±75.28 N,and the bending stiffness was 73.91±7.454 N/mm.The bending yield load of traditional pedicle screw was 1008.34±40.93 N,and the bending stiffness was 84.58±4.051N/mm.In the cyclic flexion resistance test,the vertical displacement of the new pedicle screw test was 1.593±0.134 mm,and that of the traditional screw group was 1.603±0.126 mm within 800 cycles and 200 N of maximum load.There was no significant difference in the axial pull out test,static compression bending yield test and cyclic bending resistance test.Conclusions1.The new type of tailless cap self-locking pedicle screw has the characteristics of integration,zero notch and quick locking,which avoids the disadvantages of traditional tail cap such as high notch occupation and tail cap slip wire,simplifies the operation steps after screw placement,and provides a new structural basis for shortening the operation time.2.Finite Element Analysis and bio-mechanical tests showed that the use of the new type of tailless cap pedicle screw can provide stability and fixation strength comparable to that of the traditional pedicle screw.