Elevated-rim acetabular components. Effect on range of motion and stability in total hip arthroplasty

The effect of "elevated-rim" (ER) acetabular components on prosthetic range of motion (ROM) and stability was studied in a laboratory simulator using three contemporary total hip arthroplasty (THA) systems. Acetabular components were tested in positions simulating both normal alignment and excess abduction. The geometry of the implants differed between systems--two types were identified, and their effect on ROM in comparison with their corresponding plain liners were quantified. The ability of the liners to improve instability appeared to be dependent on the cause of instability, the orientation of the metal shell and ER liner, and the ER liner geometry. The routine use of ER liners in otherwise satisfactorily positioned acetabular components appeared to offer no demonstrable benefit and raised concern over theoretical disadvantages. The primary indication for these implants appeared to be in cases of instability due to acetabular malposition in which the metal shell is already well fixed by cement or bone ingrowth or cannot be readily changed.