Abstract
Introduction
Recent clinical studies found no apparent reduction in wear using yttria-stabilized zirconia (Y-TZP) instead of cobalt chromium alloy femoral heads bearing against cross-linked UHMWPE. The purpose of this study was to compare the surface topography of retrieved Y-TZP and magnesia-stabilized zirconia (Mg-PSZ) femoral heads and evaluate the influence of time in vivo. The increase in average roughness (Ra, Sa) of Y-TZP due to phase transformation in vivo is well documented, while Mg-PSZ does not roughen or undergo phase transformation in vivo. However, the effects of phase transformation on the polarity (skewness, Ssk) of the surface of retrieved ZrO2 heads has not been reported. We hypothesized that phase transformation associated with the increased roughness of Y-TZP would influence skewness and thus the wear potential of the heads.
Materials and Methods
Y-TZP (n = 18) and Mg-PSZ (n = 17) femoral heads were retrieved from revision THA. Heads were cleaned and scanned by optical profilometry (magnification = 10x) at three locations per specimen. After subtracting the curvature of the heads, roughness statistics (Sa, Ssk) were calculated and averaged for each specimen and then correlated to age in vivo, with p < 0.05 for significance.
Results and Discussion
As expected, the roughness Sa of Y-TZP heads increased exponentially with age in vivo (p < 0.001, r2 = 0.766), while Mg-PSZ heads did not roughen with age (r2 = 0.007; Figure 1). The skewness data of Y-TZP retrievals were noisy with a weak positive correlation to age in vivo (r2 = 0.016), but were consistently positive (average Ssk of all Y-TZP specimens = 0.770), indicating its roughness was caused by positive features. Positive features such as raised edges have been reported to adversely affect wear rates in the lab, and combined with the increased average roughness suggests an accelerated wear potential with age in vivo. In contrast, the skewness data for the Mg-PSZ retrievals were negative (average Ssk = -1.00), indicating its roughness was caused by small negative features, with no relationship to age in vivo (r2 < 0.001). A surface best described by negative features would tend to entrap lubricant for better wear characteristics. Recent hip wear simulator tests reported Mg-PSZ femoral heads to exhibit significantly less wear compared with CoCr femoral heads. These in vitro data suggest that the low average roughness and negative polarity of Mg-PSZ femoral heads should provide superior wear characteristics in vivo.
Conclusions
This study illustrated that Mg-PSZ zirconia ceramics remained stable and did not roughen or develop raised edges in vivo, in contrast to Y-TZP ceramic femoral heads. We believe that clinical studies with Mg-PSZ femoral heads would illustrate a significant reduction in wear of cross-linked UHMWPE liners. Future work will expand this study with additional specimens, and attempt to reduce the noise of the skewness data to more clearly discern any trends between Ssk and age in vivo.
