Hip-simulator wear studies of an alumina-matrix composite (AMC) ceramic compared to retrieval studies of AMC balls with 1–7 years follow-up

Wear in large diameter ceramic-on-ceramic bearings were compared in both simulator and patient-retrieval studies. Four combinations of 36 mm diameter alumina matrix composite bearings (AMC: alumina–zirconia) were run in a hip simulator using alumina (ALX) bearings as the historical control. The hip simulator was run in microseparation test mode to 5-million cycles (5Mc) duration. Run-in and steady-state wear, mapping of wear-stripes, monoclinic transformations, surface roughness and wear debris morphology were analyzed sequentially. The balls and cups were inspected at every event and surface changes recorded. For AMC bearings, the zirconia transformation was studied by confocal Raman microprobe spectroscopy (RMS). Bearing surfaces were analyzed by SEM imaging and surface roughness was measured by white-light interferometry. The same techniques were used on the retrieved AMC balls with the exception that zirconia phase transformation was measured by X-ray diffraction (XRD). All balls and cups revealed stripe wear within 0.1Mc duration. Stripes expanded over 5Mc duration to span a 45–90° arc on the balls and covered an area of 150–350 mm 2. The narrow wear scar on cup rims expanded circumferentially and radially with duration. The AMC/AMC performance represented a 5–12-fold wear reduction compared to controls; hybrid ALX/AMC combinations showed a 3-fold reduction. Tetragonal to monoclinic transformation was detected in all AMC bearings and peaked at less than 30% by end of study. In non-wear zones, monoclinic content averaged less than 20%. SEM and roughness mapping at 5Mc showed that AMC surfaces had superior resistance to stripe wear. The stripe wear and monoclinic transformations in the two retrieved AMC balls with alumina liners (3–7 years use) appeared similar to the laboratory study. Thus the monoclinic transformation was anticipated by the laboratory performance. It is therefore hypothesized that once the concentration of monoclinic reaches a certain level, further zirconia transformation is suppressed until the compressive alumina constraint can be overcome by additional wear. This appears to be a very different process compared to that seen with the yittria-stabilized zirconia balls used from year 1985.