Introduction: This study evaluated the sphericity of bearing surfaces in total hip arthroplasty.

Methods: All the prosthetic metal femoral heads and the UHMWPE liners evaluated in this study were obtained straight from manufacturers (DePuy Johnson and Johnson, Howmedica Osteonics, Kyocera, Smith and Nephew, Zimmer). Out-of-roundness was assessed as an indicator representing sphericity. A total of 50 femoral heads and 22 UHMWPE liners were evaluated in 1995. Out-of-roundness of ball bearings were measured for the control study. A total of 43 femoral heads and 40 UHMWPE liners were evaluated in 1999 and 2000.

Results: The out-of-roundness of the femoral heads and the UHMWPE liners were significantly inferior to those of ball bearings. The out-of-roundness of the UHMWPE liners was significantly inferior to that of the femoral heads. The out-of-roundness of the femoral head on the sagittal plane was significantly inferior to that on the transverse plane. Several significant differences were found among different manufacturers. Overall, the out-of-roundness of the femoral head on the sagittal plane and UHMWPE liners had improved significantly in 1999/2000 compared to that in 1995.

Discussion: We previously reported that UHMWPE wear in poor out-of-roundness coupling (femoral head: 9.5 μm, socket: 36 μm) was 148% greater compared to those in good out-of-roundness coupling (femoral head: 0.5 μm, socket: 0.6 μm) at 1 million cycle experiments (J Arthroplasty 15:332, 2000). Some prosthetic femoral heads indicated more than 9.5 μm out-of-roundness in the present study, and these femoral heads with poor sphericity might be unfavorable to wear if implanted. We consider that sphericity of UHMWPE liners should be also improved to reduce initial UHMWPE wear. The sphericity of bearing surfaces can be improved by appropriate changes in manufacturing technique. Further improvement is desirable, since this is expected to prolong the functional performance of the prosthesis after total hip arthroplasty.