Background. An increasing number of hip prostheses are inserted without bone cement. Experimental research has shown that hydroxyapatite (HA) coated implants are strongly fixated in the bone, which is believed to reduce the likelihood of prosthetic loosening. However, in recent years, there has been much debate about the role of HA particles in third-body polyethylene (PE) wear and formerly we have shown the revision rate to be high among older-design HA coated cups. Purpose. We hypothesized increased PE wear-rate using HA coated acetabular components in comparison with non-HA coated components (control group). Materials and Methods. We performed a retrospective comparative clinical study based on two patient populations identified in the Danish Hip Arthroplasty Registry (October 2006). All patients had primary total hip arthroplasty (THA) between 1997 and 2001 with cementless Mallory-Head acetabular components. One group received HA coated acetabular components (75 patients, 77 hips). The other group received identical components without HA (70 patients, 73 hips). In all cases the liner was similar and 28 mm metal femoral heads were used. All patients were invited for a radiographic follow-up in 2007. The AP radiographs were analysed for two-dimensional (2D) polyethylene wear using the semi-automated PolyWare software. All cases of non-responders, stem revisions, hip dislocations and patients with less than 5 years of follow-up were excluded from the analysis. Findings/Results. The 2D linear PE wear-rate of 0.18 mm/year (SD 0.09) was higher (P<0.001) in the group with HA coated cups (n = 54) compared with 0.12 mm/year (SD 0.07) in the group of non-HA coated cups (n = 35). The Effect size of the difference in linear PE wear-rate, established as Cohen's d, was large (0.9). The time of follow-up was similar (p = 0.11) in the HA group (7.2 years) versus the non-HA group (7.6 years). There was no case-mix concerning distribution of gender and operated side in the groups; however, the mean age was lower (P = 0.001) in the HA group (57 years) compared with the non-HA group (63 years). Conclusions. We found a significantly increased PE wear rate in HA coated acetabular components at
