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.

The use of prostheses with porous surfaces in cementless total hip arthroplasty now predominates. Beads are popular for use as a porous coating, but their mechanical strength may be insufficient because of displacement of some of the beads from the coating. In this study, we propose a new porous surface, created by making direct holes in the metal surface using a YAG laser. A titanium-alloy (Ti-6Al-4V) rod was used. A Bead-type prosthesis was made by diffusion bonding pure titanium beads to the rod; it was 5 mm in diameter and 35% in porosity. A Laser type was made by directly creating holes in the same rod surface using a YAG laser; it was 5 mm in diameter and 33.7% in porosity. Both implants were evaluated in vivo using the hemitranscortical cylindrical model in two beagle dogs. Four prostheses were implanted into each femur through the lateral cortex, for a total of eight of each type, and remained in place for 12 weeks. Except for the proximal implant, push-out tests were performed to measure the shear strength of fixation of the implants to the cortical bone. For observations of the implant-bone interface, decalcified specimens of the proximal femur were stained with toluidine blue and observed with an optical microscope. The mean push-out strength of the Laser type was approximately 10.2 MPa and that of the Beads type was approximately 10.7 MPa. There was no significant difference in interface push-out strength between the groups. Bone ingrowth into both types was sufficient, however, some specimens of the Beads type demonstrated displacement of some of the beads from the rod surface. This study indicates that a porous surface created with a YAG laser might be useful not only for its shear strength, but also for the strength of the surface itself.