An endoprosthetic ankle joint device is provided including a tibial component in the form of a trough defining a concave surface and a talar component having a generally complementary convex surface which is received in the trough in mutual bearing engagement for articulation about the trough long axis. The concave and convex surfaces can have a cross-sectional profile which is wholly formed by revolution of said long axis, or partly so in the centre with rectilinear end parts, but in either case the included angle of this profile is larger for the concave surface. Also, these surfaces can be cylindrical with the profile curves of the same radius, or the convex surface can be convexly longitudinally curved to allow additional articulation orthogonal to the first articulation. In any case the trough ends provide stability in the longitudinal direction. The components can be secured to their respective bones with gap-filling cement by providing grooved or equivalent formations remote from the bearing surfaces.