A ball and socket joint for implanting in the body is provided which comprises (1) a ball; (2) a cup to be affixed to bone which includes (a) a spherical cavity and (b) two coaxial pin members which extend into the cavity; and (3) a bearing member surrounding a portion of the ball and rotatable within said spherical cavity about said coaxial pin members, the bearing member having an asymmetric opening therein, the opening having an angular extent of less than 180.degree. in at least one plane. In certain preferred embodiments, the cup includes first and second portions, the first portion to be affixed to bone, the second portion having associated therewith the coaxial pin members and being moveable with respect to the first portion to provide a plurality of possible orientations for the axis of rotation of the bearing member within the spherical cavity.