A Comparative Study Of Different Internal Fixation Methods For Ankle Arthrodesis Based On Finite Element Method

Objective:In this study,the finite element models of different internal fixation methods of ankle fusion were established to simulate and analyze the common clinical fusion internal fixation methods and the optimized internal fixation methods designed on the basis of the conventional steel plate,and to study the influence of different internal fixation methods on the ankle fusion effect.Methods:A high-resolution CT image of the foot of a healthy adult male was obtained and a finite element model of the ankle including bone,cartilage,and ligaments was constructed using Mimics 21.0,Geomagic 2017,Solid Works 2019,and Workbench 2019 software.The distribution region and peak value of the contact pressure of the ankle joint were obtained by applying neutral position load to verify the validity of the model.The ankle fusion model was established based on different internal fixation methods of conventional fusion surgery.Based on this,the fusion internal fixation was optimized,and all the internal fixation methods were simulated by applying four kinds of loads,including neutral position,dorsiflexion position,internal rotation position and external rotation position.The advantages and disadvantages of different internal fixation methods were evaluated by analyzing the fretting of the fusion surface,the contact pressure of the fusion surface,the change of the maximum stress before and after the tibial fusion,and the stress shielding of the fusion surface by the implant.Results:(1)Under four loading conditions of neutral position,dorsiflexion position,internal rotation position and external rotation position,the fusion surface fretting of each internal fixation method has its advantages and disadvantages.The results of each method under four loading conditions were scored respectively.The optimal stability was obtained by optimizing the method of plate group combined with half-thread lag screw(D2);(2)The peak contact pressure of optimized plate group combined with half threaded lag screw(D2)and conventional plate group combined with half threaded lag screw(C2B)were 28.528 MPa,35.110 MPa,28.591 MPa and 26.724 MPa,22.127 MPa,22.572 MPa,respectively.The contact pressure of optimized plate group combined with half threaded lag screw(D2)was slightly higher;(3)The maximum stress of tibia in the optimized plate group combined with transarticular screw(D1)was 59.2MPa,and that in the optimized plate group combined with half threaded lag screw(D2)was 32.187 MPa,which were higher than those in the conventional plate group combined with transarticular screw(C2a)and the conventional plate group combined with half threaded lag screw(C2b);(4)Both the stress shielding rate of conventional steel plate group combined with half thread tension screw(C2b)and that of optimized steel plate group combined with half thread tension screw(D2)are lower than that of the corresponding cross joint fixation screw(C2a and D1 scheme),and D2 and C2 b have the same stress shielding rate of 0.86.Conclusion:(1)The fusion effect of the three-screw internal fixation method was better than that of the two-screw method,which ensured the fusion stability and increased the anti-rotation ability,and the performance of the front approach of the third screw was better than that of the back approach.(2)In all fusion methods,the optimized steel plate group combined with half thread tension screw has smaller micro motion of fusion surface and more averaged contact pressure distribution of fusion surface,the peak stress of tibia after fusion does not increase significantly,and the stress shielding of implant on fusion surface is more consistent than that of conventional plate group,so it is the best solution of all internal fixation methods.(3)The optimized steel plate ensures the strength of fusion while reducing the volume.Therefore,it is suggested that the anterior plate design of ankle joint fusion should be appropriately thickened and widened at the joint transition,and slightly thinned and narrowed at both ends of the plate,so as to reduce the overall volume of the plate while ensuring the mechanical strength,and obtain a certain amount of elastic fixation,so as to achieve better performance in the fusion stability.