Abstract
Oxidized Zirconium (OxZr), metallic zirconium alloy oxidized to form a ceramic surface, was developed as an alternative bearing material to cobalt-chrome (CoCr) alloy for improvements in roughening resistance, frictional behavior, and biocompatibility without a risk of brittle fracture. Knee simulator testing without intentional addition of abrasives demonstrated that the ultra-high molecular weight polyethylene (UHMWPE) wear rate was 85% less with OxZr than with CoCr femorals. The relative performance of articulating materials can change when tested under abrasive conditions, so test protocols were investigated with abrasives added directly to the simulator test lubricant.
Testing was conducted on a six-station, four-axis, physiological knee simulator. OxZr and CoCr medium-sized, cruciate-retaining, femoral components were tested against UHMWPE tibial inserts sterilized by ethylene oxide. Alumina powder was mixed into 50% bovine serum lubricant at a concentration of 0.2 mg/cm3. Tests were conducted with different powder sizes in the range of 0.3 to 150 μm. Measurements included tibial insert weight and femoral surface roughness.
The lubricant in CoCr tests became opaque with gray debris while the femoral condyles became scratched. In contrast, the lubricant in OxZr tests remained normal (as in tests without abrasives), and femoral condyle scratching was much less severe. Despite these obvious effects, the UHMWPE wear produced by each material did not increase appreciably over that of tests without abrasives, with OxZr maintaining a wear rate about 85% less than for CoCr. It was noted that the scratches were aligned, or became realigned, with the translation motion and had little evidence of the swirls or cross-hatching often observed on retrieved components. Previous testing indicates that UHMWPE wear increases significantly only if scratches are oblique to the sliding direction. Thus, a test technique that produces scratches with more clinically relevant orientations is needed for a performance comparison between femoral materials under abrasive conditions.
The abstracts were prepared by Nico Verdonschot. Correspondence should be addressed to him at Orthopaedic Research Laboratory, University Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands.
