The sphericity of the bearing surface in total hip arthroplasty

This study evaluated the sphericity of bearing surfaces in total hip arthroplasty. The out-of-roundness of metal femoral heads, the inner surface of polyethylene liners, and commercially available ball bearings was measured. The hip prostheses were obtained directly from the manufacturers. The sphericity of the bearing surfaces was significantly inferior to that of the ball bearings. The sphericity of the femoral head on the sagittal plane was inferior to that on the transverse plane. Several significant differences were found among different manufacturers. The sphericity of the femoral head on the sagittal plane and that of polyethylene significantly improved in 1999 and 2000 compared with those in 1995. Further improvement is desirable, however, because good sphericity is expected to prolong the functional performance of the prosthesis after total hip arthroplasty.