Abstract: A ball and socket joint for implanting in the body is provided which comprises (1) a ball; (2) a cup with a spherical cavity, said cup to be affixed to bone; (3) a projection associated with the cup and extending into the spherical cavity at the geometric pole of the cavity; and (4) a bearing member surrounding a portion of the ball and rotatable within said spherical cavity, said bearing member including a walled aperture for receiving the projection, the rotation of the bearing within the cavity being constrained by engagement of a wall of the aperture with the projection.