The biomechanics of the ankle present a unique set of challenges for arthroplasty surgery. Its biomechanics are not simple. Although the ankle joint may seem like a hinge, it is not in a line perpendicular to the tibia. The axis of rotation does not stay constant during range of motion, despite a relative congruency of this joint. Allowing for rotational forces must be accomplished, while maintaining the stability of the joint and its components. Success of the arthroplasty depends on how successful designs can dissipate these rotational forces, while maintaining the stability of the joint. It is not yet clear from the biomechanical analysis of the normal ankle joint that this dissipation of forces has been accomplished successfully in modern implants, although early results in the semiconstrained designs are encouraging. Careful assessment of long-term follow-up will determine how close the present designs are to mimicking the unique requirements of the arthritic foot and ankle. Further work on the biomechanics of these replacements would be beneficial.