Abstract
Introduction
Variations in component position can lead to dynamic separation and edge loading conditions. In vitro methods have been developed to simulate edge loading conditions and replicate stripe wear, increased wear rate, and bimodal wear debris size distribution, as observed clinically [1, 2]. The aim of this study was to determine the effects of translational and rotational positioning on the occurrence of dynamic separation and severity of edge loading, and then investigate the wear rates under the most severe separation and edge loading conditions on an electromechanical hip joint simulator.
Materials and Methods
A 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. Three axes of rotation conditions (ISO 14242-1 [3]) was applied to the femoral head. This study was in two parts. I) A biomechanical test was carried out at 45° (n=3) and 65° (n=3) cup inclination angles with 1, 2, 3 and 4 (mm) medial-lateral translational mismatch between the centres of the head and cup. The amount of dynamic separation displacement between the head and cup was measured using a position sensor. The severity of edge loading was determined from the area under the axial force and medial-lateral force outputs during the time of separation [4]. II) A wear test was carried out at 45° (n=6) and 65° (n=6) cup inclination angles for three million cycles with translational mismatch of 4mm between the head and cup. The lubricant used was diluted new-born calf serum (25% v/v). Volumetric wear measurements were undertaken at one million cycle intervals and mean wear rates were calculated with 95% confidence limits. Statistical analysis was carried out using ANOVA and a t-test with significance levels taken at p<0.05.
Results
Dynamic separation increased significantly with 3mm (p<0.01) and 4mm (p<0.01) translational mismatch at a 45° cup inclination angle (Figure 1). At 65° the separation increased significantly as the translational mismatch increased from 1mm to 4mm (p<0.01). The most severe edge loading conditions occurred at a 65° cup inclination angle with 4mm of translational mismatch (p<0.01, Figure 2). Mean wear rates were greater at a 65° cup inclination angle compared with a 45° cup inclination angle (p<0.01, Figure 3).
Conclusion
Different levels of rotational and translational mismatch affected the separation between the head and cup during gait. Higher levels of translational mismatch and a steeper cup inclination angle may lead to more severe edge loading conditions and increased wear of ceramic-on-ceramic bearings in vivo. A new preclinical testing approach was developed to study the effects of edge loading due to variations in rotational and translational surgical positioning under ISO loading and angular displacement conditions. The first stage comprised of biomechanical tests to determine the occurrence and severity of edge loading in a range of component positions. The second stage investigated the tribological performance of the bearing surface under the worst case edge loading conditions.
