Abstract: A knee joint prosthesis has a femur part and a tibia part which are pivotable relative to one another about a joint-flexing axis. One of the parts is rotatable about an axis perpendicular to the flexing axis. An eccentric member, the angular position of which is determined by the degree of joint flex, engages a pair of abutments on the rotatable part so that rotation is only possible when the joint is flexed with the angle of rotation being determined by the size of the gap between the abutments and eccentric member when the joint is in its flexed position.