Design A Novel Integrated Screw For Minimally Invasive Atlantoaxial Anterior Transarticular Screw Fixation:A Finite Element Analysis

Objective: To design a new type of screw for minimally invasive atlantoaxial anterior transarticular screw(ATS)fixation with a diameter that is significantly thicker than that of traditional screws,threaded structures at both ends,and a porous metal structure in the middle.The use of a porous metal structure can effectively promote bone fusion and compensate for the disadvantages of traditional ATSs in terms of insufficient fixation strength and difficulty of bone fusion.The biomechanical stability of this screw was verified through finite element analysis.This instrument may provide a new surgical option for the treatment of atlantoaxial disorders.Methods: According to the surgical procedure,the new type of ATS was placed in a three-dimensional atlantoaxial model to determine the setting of relevant parameters such as the diameter,length,and thread to porous metal ratio of the structure.A finite element model of the upper cervical vertebrae was established,and the validity of the model was verified.Then,finite element-based biomechanical analysis was performed using three models,i.e.,atlantoaxial posterior pedicle screw fixation,traditional atlantoaxial ATS fixation,and atlantoaxial ATS fixation with the new type of screw,and the biomechanical effectiveness of the novel ATS was verified.Results: By measuring the atlantoaxial parameters,the design parameters of the 新型 screw were determined as follows: diameter,6-7 mm;length of the head thread structure,8-10 mm;length of the middle porous metal structure,8 mm(a middle porous structure containing an annular cylinder with an outer diameter of 3 mm and an inner diameter of 1 mm,which facilitated the placement of the guide wire);length of the tail thread structure,6-8 mm;and total length,23-26 mm.When applying a torque of 1.5 Nm to stimulate physiological load to the atlantoaxial complex along different directions in the established finite element models of the three types of atlantoaxial fusion modes,under the same load conditions,the atlantoaxial joint underwent angular changes in the directions of axial rotation, flexion,extension,and lateral bending,and the construction stiffness of the models was calculated.The construction stiffness of the atlantoaxial posterior fusion was recorded as A,the construction stiffness of the traditional atlantoaxial anterior fusion was recorded as B,and the construction stiffness of the new ATS was recorded as C.A comparison of the data when the same load conditions were applied showed the following: in terms of axial rotation,C > A > B;in terms of flexion,A > C > B;in terms of extension,A > C > B;and in terms of lateral bending,C > B > A.Due to the larger diameter of the new ATS,the maximum local stress on the screw head,the maximum local stress on the first cervical vertebra(C1)and the second cervical vertebra(C2)in the atlantoaxial anterior surgery were less than those of traditional atlantoaxial anterior surgery.Conclusion: By measuring relevant atlantoaxial data,we found that screws with a larger diameter can be used in ATS surgery,and the new ATS can make full use of the atlantoaxial lateral mass space and increase the stability of fixation.The finite element analysis and verification revealed that the biomechanical stability of the new ATS was superior to the ATS used in traditional atlantoaxial ATS fixation.The porous metal structure of the new ATS may promote fusion between atlantoaxial joints and allow more effective bone fusion in the minimally invasive anterior approach surgery.