Abstract
Introduction
Edge loading of hip replacements may result in plastic deformation, creep and wear at the rim of the cup and potentially fatigue failure. Variations in component positioning can lead to dynamic separation and edge loading [1]. The aim of this study was firstly to investigate the effects of translational and rotational positioning on the dynamic separation and severity of edge loading, and secondly to determine the wear rates of metal-on-polyethylene bearings under the more severe separation and edge loading conditions.
Materials and Methods
A hip joint simulator (ProSim EM13, Simulation Solutions, UK) was set up with 36mm diameter metal-on-polyethylene hip replacements (Marathon™, DePuy Synthes Joint Reconstruction, Leeds, UK). 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 head and cup centres. The severity of edge loading was calculated from the area under the axial force and medial-lateral force outputs during the time of separation when the load was acting on the edge of the cup [2]. II) For two conditions (two million cycles), the head and cup were concentric for cups inclined equivalent clinically to 45° (n=3) and 65° (n=3). For two further conditions (three million cycles), 4mm medial-lateral translational mismatch between centres was applied for cups inclined equivalent clinically to 45° (n=6) and 65° (n=6). Volumetric wear measurements were undertaken at one million cycle intervals. The lubricant was diluted new-born calf serum (25% v/v). Plastic deformation and wear were determined using a coordinate measurement machine. Mean values 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
The largest dynamic separation measured was at a 65° cup inclination angle with 4mm translational mismatch (Figure 1). At 1mm and 2mm of translational mismatch there was insignificant or no edge loading due to dynamic separation. The most severe edge loading conditions occurred at 4mm of translational mismatch at 45° and 65° inclination angles (p<0.01, Figure 2). The wear rates under standard concentric conditions were 12.9±3.8 and 15.4±5.0 mm3/million cycles for cups inclined at 45° and 65° respectively. Higher wear rates were observed under 4mm of translational mismatch at 45° (21.5±5.5mm3/million cycles, p<0.01) and 65° (23.0±5.7mm3/million cycles, p<0.01) cup inclination angles. The mean maximum penetration depth at the edge of the cup at three million cycles was 0.10±0.05mm and 0.28±0.04mm at 45° and 65° cup inclination angles respectively (p<0.01), indicating substantial plastic deformation due to edge loading (Figure 3).
Conclusion
Surgical positioning is important for long term clinical success of hip joint implants. A method has been developed to study the effects of rotational and translational positioning of metal-on-polyethylene bearings. Severe edge loading increased the wear and deformation of polyethylene liners at the rim. Minimising the occurrence and severity of edge loading and reducing the dynamic separation in vivo may reduce revision rates associated with polyethylene bearings.
