WEAR OF NOVEL CERAMIC-ON-CERAMIC HIP REPLACEMENT BEARINGS UNDER ADVERSE EDGE LOADING CONDITIONS DUE TO TRANSLATIONAL MAL-POSITIONING

Introduction

Ceramic composites have been developed to further improve the mechanical properties, reduce risk of fracture, and increase the survivorship of ceramic-on-ceramic bearings in total hip replacement¹.

The aim of this study was to evaluate the wear of two novel ceramic composite materials under edge loading conditions due to translational mal-positioning when used in both like-on-like and mixed pairing configurations; and to compare their performance to earlier generation ceramic-on-ceramic bearings.

Materials and Methods

The head-on-cup configurations of three ceramic materials (see Figure 1), were ATZ-on-ATZ, ZTA-on-ZTA, Al₂O₃-on-Al₂O₃, ATZ-on-ZTA, ZTA-on-ATZ, Al₂O₃-on-ATZ, ATZ-on-Al₂O₃and Al₂O₃-on-ZTA. All combinations were size 28mm and were supplied by Mathys Orthopädie GmbH (Morsdorf, Germany). They were tested for four million cycles on the Leeds II hip simulator under microseparation^2,3,4 conditions representing translational mal-positioning. The gait cycle comprised extension/flexion (−15º/+30º), internal external rotation (+/−10º) and a twin peak load with a maximum of 3kN. Microseparation was achieved by applying a 0.5mm dynamic medial/lateral displacement using a spring load resulting in edge loading at heel strike. New-born calf serum (25%) was used as a lubricant. Wear was assessed gravimetrically every million cycles. Statistical analysis was performed using one-way ANOVA (significance taken at p<0.05).

Results

The wear of ATZ-on-ZTA, ATZ-on-Al₂O₃ and Al₂O₃-on-Al₂O₃ was biphasic with a bedding in wear rate between zero and one million cycles under translational malpositioning conditions and a lower steady state wear rate between one and four million cycles. The bedding in and steady state wear rates of ATZ-on-ZTA (1.16mm³/million cycles bedding in and 0.18mm³/million steady state) and ATZ-on-Al₂O₃ (0.66mm³/million cycles bedding in and 0.20mm³/million steady state) were lower than that of Al₂O₃-on-Al₂O₃ (1.54mm³/million cycles bedding in and 0.55mm³/million steady state) bearing combination (see Figure 2). However, there was statistically no significant difference (p=0.35) between the wear rates.

The wear rates of the other bearing combinations under these adverse microseparation conditions, ZTA-on-ZTA and ATZ-on-ATZ, Al₂O₃-on-ATZ, Al₂O₃-on-ZTA, and ZTA-on-ATZ were very low with no clear bedding in and steady state phases (see Figure 1). The wear rates of these combinations, over the four million cycles of test under adverse microseparation conditions, were all lower than 0.14mm³/million cycles.

Discussion and Conclusion

The mixed material combinations (ATZ-on-ZTA, ATZ-on-Al₂O₃, Al₂O₃-on-ATZ, Al₂O₃-on-ZTA and ZTA-on-ATZ) tested in this study have shown slightly higher wear rates when compared to ATZ in like-on-like configuration, but superior wear resistance when compared to Alumina BIONIT® (Mathys) and BIOLOX® forte (CeramTec) Al₂O₃-on-Al2O₃ bearings tested under the same adverse microseparation conditions.