A prosthetic ankle joint device for replacement of articular bearing surfaces of the natural joint comprises: a first component (2) for attachment to a first tibial bone segment (2a) having a first articulating bearing surface (5); a second component (3) for attachment to a second tarsal bone segment (3a) having a second articulating bearing surface (6) opposite to the first articulating bearing surface (5). The first articulating bearing surface (5) and the second articulating bearing surface (6) are substantially different in shape to the corresponding shapes of the articular bearing surfaces of the natural joint. The first surface (5) being concave, or flat, or convex. Each articulating bearing surface (5, 6) presents a respective shape designed on the basis of the shape of the opposite articulating bearing surface (5, 6) according to the common normal to their point of mutual contact at every position over the range of passive plantar and dorsi-flexion. The common normal theorem being applicable because of a four-bar linkage mechanism identified at this joint in the sagittal plane, formed by the tibio-calcaneal calcaneo-fibular ligaments.