Abstract
BACKGROUND:
Implant wear continues to be a limitation of total knee replacement (TKR). Wear simulator studies are a valuable screening tool in new implant development. The purpose of this study was to determine the ability of micro-CT to prospectively measure wear in TKR implants during a wear simulator trial.
METHODS:
Three identical cruciate-retaining, fixed bearing cobalt-chromium-molybdenum (CoCrMo) on conventional EtO-sterilized polyethylene TKA implants underwent wear simulator testing up to 3.2 million cycles using gait inputs; loaded-soaks were used to correct for fluid absorption. The implants were weighed and scanned with micro-CT (at 50 micron resolution) before and after testing. The gravimetric mass was converted to volume based on the density of polyethylene. Volume change due to wear was calculated from both the gravimetric and micro-CT methods. The pre- and post-wear test micro-CT geometries were co-registered and the deviations between the two were measured.
RESULTS:
The mean wear volume was 90.8 ± 12.2 mm3 measured gravimetrically and 66.4 ± 13.3 mm3 measured by micro-CT (p = 0.002). While underestimating wear volume compared to gravimetric analysis, micro-CT demonstrated strong correlation (r2 = 0.99, slope difference from zero p < 0.0001). No subsurface changes such as cracks were noted in the micro-CT images. Maximum penetration (from wear and creep) was 0.77 ± 0.20 mm medially and 0.41 ± 0.06 mm laterally. Deformation up to 0.180 mm was noted on each side of the inserts after wear testing.
Discussion:
Micro-CT underestimation of wear volume compared to gravimetric analysis is consistent with previous reports of micro-CT use. The discrepancy could be caused by a scaling error in the micro-CT volume reconstruction, or an error in the fluid uptake correction for gravimetric analysis. Micro-CT had a strong correlation with gravimetric analysis, and also enables analysis of the implant subsurface, and measurements of penetration and surface deformation. The wear volume was consistent with other fixed bearing TKR couples made from CoCrMo and conventional non-irradiated polyethylene.
