Abstract: The patented prosthesis device of U.S. Pat. No. 3,813,699 is effectively limited in its drifting movement deeper into its cooperating acetabular opening, which could cause rupture of the acetabular floor, by a laterally oriented lip, even though the size of the lip, in and of itself, is not physically capable of this performance. This is achieved by providing an initial clearance position to the lip and thus allowing a settling movement in the device before the lip is permitted to contact bone structure. During said settling movement, favorable secondary tissue changes occur in the acetabular opening which impede movement; thus, said tissue changes together with the lip making contact with bone structure hold the device in position, whereas premature contact of the lip would have merely indented or otherwise adversely modified the bone structure and interfered with achieving this result.