Abstract
Introduction
Translational surgical mismatch in the centres of rotation of the femoral head and acetabular cup in hip joint replacements can lead to dynamic microseparation resulting in edge loading contact [1]. Increased wear in retrieved ceramic-on-ceramic bearings has been associated with edge loading [2]. Hip joint simulators were used to replicate increased wear rate, stripe wear and bimodal wear debris size distribution, as seen clinically [3,4]. Recently developed electromechanical simulators are able to comply with the latest international standards, which include three axes of rotation conditions [5]. Previous simulators had applied two axes of rotation under microseparation conditions [6]. Therefore, the aim of this study was to compare the wear of ceramic-on-ceramic bearings obtained under edge loading due to microseparation conditions during gait using the same electromechanical hip joint simulator with two axes of rotation and three axes of rotation conditions.
Materials and Methods
A six-station electromechanical hip joint simulator (ProSim EM13, Simulation Solutions, UK) was set up with 36mm diameter ceramic-on-ceramic (BIOLOX® delta, PINNACLE®, DePuy Synthes, UK) hip replacements. The wear was determined for two million cycles under standard conditions with two axes of rotation conditions (n=6), two million cycles under microseparation conditions with two axes of rotation conditions (n=6) (Figure 1a), and two million cycles under microseparation conditions with three axes of rotation conditions (n=6) (Figure 1b). The loading profiles [5,7] comprised of 3kN twin peak loads and 300N swing phase load under standard conditions. The swing phase load was reduced to approximately 70N under microseparation conditions. Approximately 0.5mm of dynamic microseparation between the head and the cup was applied in the medial/lateral direction. The components were lubricated with 25% new-born calf serum supplemented with 0.03% sodium azide to minimise bacterial growth. The gravimetric wear rates were compared over two million cycles for each test (XP205, Mettler Toledo, UK). The mean wear rates of the head and cup were calculated with 95% confidence limits and statistical analysis was carried out (t-test) with significance levels taken at p<0.05. A coordinate-measurement machine (Legex 322, Mitutoyo, UK) was used to construct a three-dimensional map of the femoral head surface wear.
Results
Under standard conditions, the mean wear rate of BIOLOX® delta ceramic-on-ceramic bearings was 0.03±0.01 mm3/million cycles. The mean wear rates under microseparation conditions for two axes and three axes of rotation conditions were 0.14±0.01 mm3/million cycles and 0.14±0.03 mm3/million cycles respectively. There was no statistically significant difference between the wear rates using two axes and three axes of rotation conditions under microseparation conditions (p=0.86). Stripe wear was observed and wear depth measured on the femoral heads under microseparation conditions using two axes (Figure 2a) and three axes (Figure 2b) of rotation.
Conclusion
Higher wear rates were observed under microseparation compared with standard conditions, as reported in a previous study [6]. Similar wear rates were obtained under microseparation conditions with two axes and three axes of rotation conditions using the same simulator.
