Abstract: A joint prosthesis is provided for replacing a joint between two bones in which one bone is normally permitted a substantial degree of rotary circumduction with respect to a second bone. A first member of the prosthesis is provided with a coupling portion enabling it to be attached to the elongated bone and includes a head having a generally saddle-shaped articulating surface facing away from the stem. The second member similarly has a connector adapted to connect the second member to the second bone, and has a head provided with a saddle-shaped articulating surface configured and sized to articulate with the saddle-shaped surface of the first member to enable one of the long bones to move in rotary circumduction with respect to the other. The saddle-shaped surfaces preferably are substantially congruent in shape.