# A Review Study on Total Ankle Replacement
## 3 Revision TAR
### Finite Element Modeling  
Comprehensive finite element modeling (FEM) is essential to improve future designs of TAR. It is necessary to understand the biomechanics of the ankle and adjacent joints before the analysis of an implanted ankle prosthesis can begin. FEM allows for realistic implant modeling, as in vivo experiments are invasive, costly, time consuming and clinically impractical [56; 57].  
There have been many iterations of models of the ankle joint used to analyze the midterm and long-term outcomes of primary and revision TAR. These models have been used to validate biomechanical bases of prosthesis components. In one such study, researchers instrumented uncomented mobile-bearing tibial component of a TAR in eight cadaveric tibias in three different positions to validate a FEM of TAR. An axial force was applied to the implants in each of the positions. CT scans were used to evaluate the anatomy of the tibias, to assess implant position and to gauge the environment at the bone-implant interface. The combination of this data was used to calculate bone strains. The finite element model was implemented in Abaqus and analyzed with an implicit solver. Both the predicted and measured strains were highly correlated with a coefficient of determination of 0.91. This model showed great potential for understanding local biomechanics and could assist in optimizing implant design, fixation and positioning [58].  
FEM can also be used to understand the stress and wear in TAR. One of the most common reasons for revision is due to polyethylene liner wear [59; 60; 61; 62]. A model of the axial loading profile was coupled with Hertzian contact mechanics to develop a new model to evaluate the maximum contact pressure in TARs. Then, optimization models were developed to consider the geometric characteristics of the TAR systems that could be altered to minimize stress. FEM and mathematical analysis determined that contact area between the talar and polyethylene components was a significant contributor to total stresses in TAR. This information can be utilized to design new generations of TARs that reduce refitting wear and delamination in the UHMWPE liners. One way to decrease contact and subsurface stresses is through optimization of the implant geometry to decrease wear rate. Optimization models are powerful tools to help design new low-stress implants [43].  
Similarly, another FEM was used to assess the kinematics and contact pressures in a three-component TAR when subjected to forces of a typical gait cycle. The FEM incorporated a previously validated model of the ankle joint. During the simulated stance phase, the models reported reaction forces that remained within physiologic values. The maximum experienced force was below the load at failure. This model was able to allow the necessary range of motion with maintained constrained motion of the prosthetic components. The congruency of the design led to small peak contact pressures that were almost evenly distributed [63].  
The number of segments used in foot modeling has also been shown to affect the calculated gait kinetics. The older one segment foot models overestimate ankle joint range of motion and power generated. A multi-segment foot model is more accurate when assessing the outcome of total ankle replacement [64].  
The use of FEM is therefore essential to understanding ankle biomechanics, which will provide the foundation for improving upon current TAR prostheses. Analyzing specific TAR implants can better elucidate what works well and what needs improvement. The limitations of FEM lie in what assumptions can be made when modeling in terms of material properties, loading directions and articulating surfaces. Further research, such as the utilizing specific patient's information such as a CT based information, to be utilized in finite element modeling, can improve the understanding of how TAR prostheses interact in vivo.