The ankle is a complex joint of the human body which is most often affected by trauma. Osteoarthritis is a joint disease that can require surgical intervention and when it affects the ankle there are two options to achieve pain relief. For years ankle fusion has been the most common technique but it entails a huge reduction of the ankle joint. This last decades the trending to total ankle replacement (TAR) to deal with that disease has increased. However, the longterm results are far away from the success of other joint replacements like the knee or the hip. The complexity of the ankle joint makes difficult to predict and to know what causes the failure of the replacement. Therefore, a numerical model that includes mainly the bones of the ankle joint and the prosthetic elements is created. Then two different kind of prosthesis are compared using the same model. Finally, in order to test how the ankle acts for a daily life movement, another model performing a phase of the ascent of stairs cycle is created. This last model guides the movement by the contraction of the triceps surae muscle. Results show that the bone strain levels differ a lot depending on the geometry of the prosthesis tested being more recommendable the tibial implant that is designed with an horizontal keel because it leads to a more uniform distribution of the load. For all the simulations the tibia is the most affected bone and the strain results surpass the limit established in some cases. For the climbing stairs simulation none of the elements surpass the strain limits. Even so, the failure of the TAR may not be only related to the deformation of the bone as there are other non-prosthetic issues that can affect the success of it. This is the first time that a musculoskeletal model of the ankle has been developed in order to study a prosthetic joint. Even though improvements are needed to make the model more complete and reliable it has been shown that it is useful to obtain the behaviour of the joint and to assess different implant designs.

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