SIMULATOR WEAR STUDY OF ALUMINA-MATRIX-COMPOSITE HIP JOINTS COMPARED TO 1ST RETRIEVAL STUDIES OF 3 CASES FOLLOWED 1 TO 6 YEARS

Abstract

Ceramic-on-ceramic bearings (ALX: pure alumina) have been used for human hip joints for almost 40 years. However an alumina matrix composite with zirconia (AMC) was introduced in year 2000 as a high-strength ceramic with almost double the fatigue resistance (AMC = 80.5%ALZ and 18vol% ZrO2). However we have not found any retrieval studies reported for this new ceramic bearing.

Wear maps were generated on three retrieved AMC femoral heads (28 and 36mm diameters) using x-ray diffraction, roughness and SEM imaging techniques. The wear study ran a physiologically appropriate, micro-separation test on 36mm ceramic balls and liners (AMC/ALZ). Wear rates were determined for the four combinations of balls and cups (ALX:AMC) with mapping of main-wear and stripe-wear zones, surface-roughness and analysis of debris morphology. In addition, the zirconia transformation to monoclinic phase was studied in AMC bearings

The retrieval study showed for the first time the wear phenomena occurring on three retrieved AMC femoral heads (at 1, 3, 6 years). Two had been paired with alumina liners and one with a polyethylene liner. Case-1 featured a 36mm ball in an UHMWPE socket, case-2 was an intact 28mm AMC ball and case-3 had a fractured ball from an IDE study. Laser interferometry and SEM were used to image ceramic wear and x-ray diffraction for analysis of transformation in the zirconia phase. Main-wear zones, stripe-wear zones, metal contamination and sites of implant impingement were also characterized. Surface roughness and in-vivo aging were quantified for both non-worn and worn areas. The SEM studies showed well-preserved articular surfaces, some with faint parallel scratches still evident. The latter likely represented the manufacturer’s original polishing marks. Multiple stripe-wear sites were identified with roughness 25–65nm (Sa) whereas polished main-wear zones averaged very low at 2–3nm. Metal impingements sites stained black with transfer of titanium increased roughness up to 140nm. Mildly worn areas of case-2 AMC ball averaged 10% transformation in the zirconia phase (tetragonal to monoclinic). In the stripe-wear zones, the monoclinic phase increased to 30%. The taper-bore and fracture surfaces in case-3 averaged 30% to 40% monoclinic, respectively. The stripe-wear zones and black metal contamination on these retrieved 28mm balls were correlated to multiple impingement sites on the rim of the alumina liners and titanium shells.

The laboratory model produced stripe wear on the ceramic balls and liners. The AlX/AlX controls produced the highest run-in and steady-state wear rates at 6.3 and 2mm3/Mc respectively). In contrast, the AMC/AMC combination produced the lowest wear rates at 0.5 and 0.1 mm3/Mc, respectively). With hybrid ball:cup combinations (AlX:AMC; AMC:AlX) the wear rates were similar and showed a 3-fold reduction compared to controls. In hybrid pairings, the AMC ceramic wore preferentially more than its AlX counterpart, regardless if present as a ball or cup implant. Thus the AMC ball contributed 66% to AMC/AlX total wear whereas the ALZ ball contributed only 33% of the total AlZ/AMC wear.

This study appears to be the first documentation of wear in retrieved AMC bearing surfaces. In general, the AMC surfaces worn in-vivo corresponded well to our in-vitro wear model. The stripe-wear zones in AMC femoral heads had rougher surfaces and higher monoclinic transformation than the main-wear zone. Overall the AMC ceramic appeared more resistant to stripe-wear effects created by the micro-separation and impingement phenomena.