Y-TZP zirconia run against highly crosslinked UHMWPE tibial inserts: Knee simulator wear and phase-transformation studies

Background: Zirconia (ZrO₂) ceramics combined with highly cross-linked polyethylene appears to be a promising approach to minimize wear in artificial knee joints. The wear performance of yttria-stabilized zirconia (YZr) femoral condyles on 7-Mrad tibial inserts was compared in a knee simulator to CoCr bearing on 3.5-Mrad inserts. Methods: The knee design was the Bi-Surface™ type with a 9-year clinical history in Japan (JMM, Japan). A displacement-controlled knee simulator was used with kinematics that included 20° flexion, ±5° rotation, and 6 mm anterior/posterior translation. Lubricant was alpha-calf serum, test duration was 10 million cycles (10 Mc), and wear was measured by weight-loss techniques. The wear zones were studied by laser interferometry, scanning electron microscopy, and Raman microprobe spectroscopy. Results: At 10 Mc the wear rates of the CoCr controls averaged 4.5 mm³/Mc. This was within 7% of the prior estimate at 5-Mc duration and comparable to Bi-Surface wear data from another laboratory. The CoCr condyles increased in roughness (R_a) from <50 nm to average R_a = 250 nm due to linear scratching. The ceramic condyles remained pristine throughout the wear study (R_a <7nm). With the YZr/7-Mrad combination, the weight change had a positive slope over at 10 Mc, which meant that the actual polyethylene wear was unmeasurable. Microscopic examinations at 10 Mc showed that the zirconia surfaces were intact and there was no detectable change from tetragonal to monoclinic phase. Interpretation: Our laboratory knee wear simulation appeared very supportive of the 9-year YZr/PE clinical results with Bi-Surface total knee replacements in Japan.