The in vivo linear and volumetric wear of hip resurfacing implants revised for pseudotumor.

BACKGROUND: Metal-on-metal arthroplasty-related pseudotumors can cause severe local destruction of bone and soft tissues. The cause of pseudotumors is unknown, although some authors have implicated metal wear debris. The aim of this study was to measure the location and magnitude of wear on resurfacing devices that were retrieved during revision procedures for pseudotumor (the pseudotumor group) and for other reasons (the control group). METHODS: We examined thirty-six hip-resurfacing implants, which were divided into two groups: eighteen implants from patients with a diagnosis of pseudotumor and eighteen control implants. Implant orientation and patient demographics were recorded. Three-dimensional, contactless metrology was used to scan the surface of the femoral and acetabular components to a resolution of 20 nm. Linear and volumetric wear were measured, and the components were examined for evidence of edge wear. RESULTS: There was three times more total linear wear and over six times more total volumetric wear of the femoral and acetabular components in the pseudotumor group as compared with that in the control group. The mean linear wear rate and standard deviation of the femoral components in the pseudotumor group (8.4 ± 8.7 µm/yr) were significantly greater than those in the control group (2.9 ± 3.9 µm/yr; p = 0.01). The mean volumetric wear rate of the femoral components was also significantly greater in the pseudotumor group (3.3 ± 5.7 mm3/yr) than it was in the control group (0.8 ± 1.2 mm3/yr; p = 0.009). Seventeen of eighteen subjects in the pseudotumor group had edge wear, compared with six of eighteen in the control group (p < 0.001). CONCLUSIONS: Implants that were retrieved because of pseudotumor had a significantly higher wear rate and prevalence of edge wear than the control implants did. There was a strong association between pseudotumor and the high levels of wear debris that are generated during edge-loading. However, not all patients with high wear developed pseudotumors, and not all pseudotumors had high wear; therefore, other factors are most likely involved in the cause of pseudotumors.