Abstract
INTRODUCTION
Ceramic-on-ceramic hip replacements have generated great interest in recent years due to substantial improvements in manufacturing techniques and material properties1. Microseparation conditions that could occur due to several clinical factors such as head offset deficiency, medialised cup combined with laxity of soft tissue resulting in a translation malalignment, have been shown to cause edge loading, replicate clinically relevant wear mechanisms2,3 and increase the wear of ceramic-on-ceramic bearings3,4. The aim of this study was to investigate the influence of increasing the femoral head size on the wear of ceramic-on-ceramic bearings under several clinically relevant simulator conditions.
MATERIALS AND METHODS
The wear of size 28mm and 36mm ceramic-on-ceramic bearings (BIOLOX® Delta, CeramTec, Germany) was determined under different in vitro conditions using the Leeds II hip simulator. For each size bearing, two clinical cup inclination angles were considered, 55° (n=3) and 65° (n=3) for the 28mm bearing and 45° (n=3) and 65° (n=3) for the 36mm bearing. The first two (28mm study) or three (36mm study) million cycles ran under standard gait conditions and a subsequent three million cycles ran under microseparation conditions. A standard gait cycle included a twin peak load (300N–3000N), extension/flexion (−15°/+30°) and internal/external rotation (±10°). Microseparation3 was achieved by applying a 0.4–0.5mm medial displacement to the cup relative to the head during the swing phase of the standard gait cycle resulting in edge loading at heel strike. The lubricant was 25% (v/v) new-born calf serum which was changed approximately every 333,000 cycles. The wear volume was ascertained through gravimetric analysis every million cycles. One way ANOVA was performed (significance: p<0.05), and 95% confidence limits were calculated.
RESULTS AND DISCUSSION
The mean wear rate under standard gait conditions was 0.05mm3/ million cycles for the 28mm bearings and significantly lower (p=0.003) for the 36mm bearings (Figure 1) which could be due to improved lubrication regime. The wear of ceramic-on-ceramic bearings was not influenced by the increase in cup inclination angle for either bearing size (Figure 1). The introduction of microseparation into the gait cycle resulted in stripe wear on the femoral head with a corresponding wear area at the rim of the acetabular cup and significantly higher wear rates of the ceramic-on-ceramic bearings (Figure 2). The wear rate of BIOLOX® Delta bearings under microseparation conditions was still low (<0.25mm3/million cycles) compared to the third generation alumina ceramic-on-ceramic bearings (1.84mm3/million cycles)4 under the same adverse conditions. Under microseparation conditions, the wear rate of size 36mm bearings was significantly higher (p=0.004) than that for size 28mm bearings. This was thought to be due to the larger contact area for the larger bearings and deprived lubrication under edge loading conditions. For both bearing sizes, the combination of both steep cup inclination angles and microseparation conditions did not increase the wear rates any further compared to microseparation conditions alone (Figure 3).
This study shows the importance of surgical positioning of the femoral head and acetabular cup and the importance of testing new bearing materials and designs using these adverse simulator methods.
ACKNOWLEDGEMENT
This study was supported by the Furlong Research Charitable Foundation (FRCF) and the National Institute of Health Research (NIHR) as part of a collaboration with the Leeds Musculoskeletal Biomedical Research Unit (LMBRU).
