US perspective on hip simulator wear testing of BIOLOX® delta in’ severe’ test modes
A ceramic on ceramic (COC) simulator wear study was run with 36mm Biolox® implants under’ severe’ microseparation test mode. The implants included the zirconia-toughened alumina (ZTA: Biolox® delta, ‘d’) with alumina used as the historical control (Biolox® forte, ‘f’). The four combinations (ball-cup: dd, df, fd and ff) were run simultaneously in a hip simulator to 5 million cycles duration (5Mc). The femoral balls were 36mm diameter and four material pairings were run with preset ball/cup diametral clearances. All combinations demonstrated stripe wear phenomenon on both balls and cups. The stripes began immediately in the runin phase (100,000 cycles) as well-defined, narrow scars that gradually expanded over the duration of the study. For the cups, there was a narrow superior stripe region along the rim that expanded circumferentially with test duration. The ff combination showed the greatest stripe expansion that also expanded rapidly into the main wear zone. The appearance of such narrow stripes in run-in phase was comparable to prior microseparation simulator studies and short-term ceramic retrieval studies. The gradual expansion of our simulator stripes compared well to our long-term ceramic retrievals with 15 to 22 year use in-vivo.
Wear rates for all 36mm COC combinations were very low even under our severe microseparation test conditions. Our ff pairings demonstrated ‘Average’ wear rate of 1.5 mm3/Mc. Clinical wear rates for contemporary alumina THR retrievals ranged from 0.1 to 3.6 mm3/year, so clearly our simulator wear rates were in mid-clinical range for alumina implants. The general trend showed that ff pairs had the highest THR wear rates and dd pairs had the lowest. Overall wear rates were ranked as follows: ff ≫ (df ≥ fd) > dd. Under our microseparation test condition, the ‘Average’ wear rates for our fd and df hybrid pairings were > 3-fold lower than with the historical control (ff). The ‘Average’ wear for the dd pairing was 6-fold lower than control and 2-fold lower than the hybrids (fd, df). It was interesting that the delta ceramic wore preferentially over the forte ceramic in the hybrid pairings, possibly due to the slightly higher hardness of the pure alumina. Comparing this study with 36mm diameter ceramic balls to prior microseparation COC studies with 28mm diameters, there was little discernable difference in wear rates. Thus the effects of the 36mm versus 28mm ball size, if any, will likely be of small significance clinically.
At the two sites analyzed (pole and stripe), monoclinic transformations were detected in all delta implants. The monoclinic phase for delta balls increased with test duration to 20% and 27% at the pole and stripe, respectively. The delta cups revealed only 15% monoclinic but likely this lower value was due to difficulties of laser access to inside of the intact cups. The wear debris for all combinations was similar in sizing, aspect ratio and circular shape factor at 0.5 and 1.1Mc durations. Particle size appeared to be just slightly larger for the hybrid delta combinations at 1.1Mc duration. Raman spectroscopy, surface roughness and debris studies are continuing.
Overall the ZTA ceramic combinations in microseparation simulator studies consistently showed lower wear than the alumina used as historical control. This was in contradistinction to ytrria-stabilized zirconia balls that showed increased wear compared to the control. Thus the wear performance of ZTA implants in the laboratory was quite different to zirconia implants and appeared superior to the alumina as the historical control. The superior strength and wear resistance of ZTA ceramics may become advantageous in sub-optimal clinical cases that may have had the risk of implant fracture or abnormally high wear.
KeywordsWear Rate Stripe Region Retrieval Study Simulator Wear Equivalent Circle Diameter
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