Abstract
Introduction and Aims
There are many variables that can affect the occurrence and severity of edge loading in hip replacement. A translational mismatch between the centres of rotation of the head and cup may lead to dynamic separation, causing edge loading and increased wear. Combining a steep inclination angle with such translational mismatch in the medial-lateral axis caused a larger magnitude of separation and increased severity of edge loading. Previous studies have shown variation in the hip Swing Phase Load (SPL) during gait between different patients. The aim of this study was to apply a translational mismatch and determine the effect of varying the SPL on the occurrence and severity of edge loading under different cup inclination angles in a hip joint simulator.
Methods
The Leeds II hip joint simulator with a standard gait cycle and 36mm diameter ceramic-on-ceramic bearings (BIOLOX® delta) were used in this study. The study was in two stages; [1] a biomechanical study where the magnitude of dynamic separation, the duration of edge loading and the magnitude of force under edge loading (severity) were assessed under variations in component positioning and SPLs. [2] A wear study to assess edge loading with selected input conditions. For the biomechanical study, a combination of four mismatches, three cup inclination angles, and eight SPLs (Table 1) were investigated. For the wear study, three SPL conditions were selected with one cup angle and one mismatch (Table 1). Three million cycles were completed under each condition. Mean wear rates and 95% confidence limits were determined and statistical analysis (one way ANOVA) completed (significance taken at p<0.05).
Table 1: Study matrix
Results
For any given translational mismatch or cup inclination angle, increasing the SPL from 50N to 450N resulted in a decrease in the magnitude of dynamic separation (Figures 1 and 2). In some scenarios when the mismatch between the centres of rotation was low and the SPL was high, no separation was observed. Under 150N SPL, the severity of edge loading was similar to that determined for the 50N SPL conditions although the magnitude of dynamic separation was lower.
Higher wear rates were found for the 70N and 150N compared to 300N SPL (Figure 3). No significant difference was found between wear rate under the SPLs of 70N and 150N (p=0.05), but significant differences were found between the wear rates under 150N and 300N SPL and between 70N and 300N SPL (p<0.01 and p<0.01 respectively).
Conclusion
The SPL contributed to the resistance of separation between the head and the cup, hence a lower dynamic separation was measured under higher SPL. The wear study demonstrated that edge loading was present even under a higher SPL.
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