The developers of a successful prosthetic implant for ankle replacement must consider the biomechanical properties that are unique to this complex joint. The prosthesis needs to provide structural support while allowing for motion in the sagittal, transverse, and coronal planes. Although the design must conform to and function within the soft tissue constraints of the ankle, it is only a component of the overall success. Paying attention to leg alignment and meticulous soft tissue balancing is essential to a satisfactory outcome.