# A Review Study on Total Ankle Replacement
### Contribution  
This paper provides a comprehensive review of the history, evolution and the current state of TAR. The five main categories and specifications that describe the various types of implants and the methodologies used for further improvement of TAR prostheses are discussed in detail. These categories and areas where work is still in progress includes (1) optimal patient selection, (2) number of prosthesis components, (3) bearing type, (4) revision rates and causes and (5) comprehensive finite element models (FEM) of the ankle. These five areas have been the focus of research on improvement of TAR operative techniques, post-operative recovery and prosthesis longevity. While TAR has certainly progressed from its early implementation, there are still essential areas of research that will improve the function and long term outcomes. Proper patient selection is essential for the success and survival of any type of joint arthroplasty, including TAR. Indications for TAR, however, are still being defined. Traditional indications have been patients that are >50 years old with end-stage ankle arthritis, minimal deformity at the ankle joint, non-obese, with lower mobility demands. However, these indications are continuing to evolve, as younger patients with inflammatory or post-traumatic arthritis may also be good candidates for this procedure [13; 14; 15; 16]. First generation TAR implants were composed only of two-component constrained or unconstrained designs with a polyethylene component without a separate bearing. Second and third generation designs progressed to include three-component systems allowing  
Figure 2: Difference between fixed-bearing prosthesis and a mobile-bearing prosthesis, where the polyethylene component is either fixed to the tibial component or free to move [11]. mobile-bearing with independent mobility of the spacer as it articulates with the tibial and talar components. Fixed bearing systems utilize highly congruent components with one articulation between the tibial and talar components where the polyethylene insert is attached to one of the components. This requires greater constraint in order to provide stability. Mobile bearings involve two articulating surfaces, where a moving polyethylene bearing acts as a meniscus between the tibial and talar components. The goal of this construct is to allow for greater stress distribution and theoretically decrease the risk of having points of high concentrated stress between components. Currently, Stryker's Scandinavian Total Ankle Replacement (STAR) implant is the first and only three component mobile bearing ankle implant with FDA approval in the United States. The remaining TAR implants in use in the United States are fixed bearing. Bearing type-either fixed or mobile-has not been shown to affect clinical outcome [17]. Figure 3 below shows one design of a TAR. Wear in the polyethylene component is one of the main causes of revision in TAR. Debris from polyethylene wear can cause osteolysis at the interface between the bone and the implant, leading to aseptic loosening. Further, high contact pressures and subsurface stresses also contribute to polyethylene liner wear over time. Currently, TAR has almost double the number of revisions when compared to hip and knee replacements. This is partly due to the relatively high stresses experienced in the ankle, 64% higher than that of the knee and 45% more than that of the hip. Thus, TAR prostheses must be designed to withstand much higher stresses than other joints [18]. Understanding the stress-strain distribution, plantar pressures and wear on the polyethylene liner are important parameters for improving TAR prostheses. Comprehensive analysis of the biomechanical nature of the ankle is needed before prostheses can be improved, which is when FEM becomes essential. This will help create ankle replacement prostheses that will better mimic the motion of normal ankle function and ideally prolong implant longevity.