# A Review Study on Total Ankle Replacement  
[PERSON]  
1 Department of Orthopedic Surgery, University of California-Irvine, Orange, CA 92868, USA 1, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29,  
understanding of the physiological mechanics of the ankle complex. Geometrical studies of human joints aim to show how ligaments and shapes of the articular surfaces can guide the movements of the bones upon each other. Mechanical studies show how ligaments can act together with the muscles and articular surfaces to transmit load from one bone to the other. Understanding the role of all structures in the ankle joint is important for a successful design of joint replacement prostheses. In end stage ankle arthritis, total ankle replacement is considered as a means to reduce pain and restore joint stability and mobility. Compared to the population without ankle arthritis, patients have prolonged stance time, shortened stride length, increased cadence and a slower walking speed. Ankle arthrodesis has been shown to improve these metrics from preoperatively to post operatively [8]. However, after TAR, there is less change to the natural motion at the ankle joint complex when compared to arthrodesis [9]. This makes TAR an attractive option when compared to ankle arthrodesis.  
From the first total ankle replacement in the 1970s TAR has had a steep evolutionary progress with improved reproducible results. The first generation of the implant had two-main component cemented designs that were either unconstrained or constrained. Both types of design eventually fell out of favor because of poor survivorship due to significant bone loss required for component placement, implant incongruency leading to stress deformation and component loosening leading to failure [10].  
As technology progressed, second and third-generation prostheses used ultra-high molecular weight polyethylene (UHMWPE) as the spacer. The most current designs can be considered fourth generation. These subsequent generations of prostheses can be categorized by component number (two or three) and device type (fixed bearing or mobile bearing). Figure 2 below shows the difference between fixed-bearing prosthesis and mobile-bearing prosthesis.  
Figure 1: Simple schematic of the ankle joint.  
While each generation of ankle prostheses has advanced outcomes, a difficulty of this procedure is that the ankle joint is extremely difficult to model due to its unique articular characteristics. Compared to total hip and knee replacements, TAR is still a procedure that lacks the same long-term success rates, though part of this is due to its more recent advent leading to inherent limitations in longer term outcome data. When loaded, the ankle joint has a smaller area of contact between the articulating surfaces when compared to the hip and knee. At the same load of 500 N, the contact area for the ankle is 350 mm\({}^{2}\), compared to 1120 mm\({}^{2}\) for the knee and 1100 mm\({}^{2}\) for the hip. Thus, the smaller contact area signifies a higher normal peak stress in the ankle [12]. Further, the articular cartilage in the ankle has different tensile and thickness properties than that of the knee and hip. These complexities in the ankle joint make it a more challenging joint to model and to improve upon.