The Effect of Variations in Component Positioning and Swing Phase Load on the Occurrence and Severity of Edge Loading and Wear in Hip Joint Replacements

Edge loading is multifactorial and has been identified as a risk leading to the revision of hip joint replacements. Pre-clinical testing requirements currently do not incorporate edge loading. Previous testing of edge loading has been carried out as controlled dynamic microseparation of 0.5 mm via a spring, where the femoral head contacts the rim section of the acetabular cup. The input conditions of these tests did not allow the individual evaluation of other variables due to the constraints of the methodology. The aim of these studies was to evaluate the effect on the occurrence and severity of edge loading, and wear, under edge loading due to different parameters in a hip joint simulator. A translational mismatch of 1, 2, 3 and 4 mm were applied between the centres of rotation of the head and the cup in order to evaluate the effect on the dynamic separation and severity of edge loading. This allowed differentiating the effect of the 45°, 55° and 65° cup inclination angle studied in the test matrix. Three studies were developed to determine the effect of the cup inclination angle (45°, 55° and 65°), the effect of the swing phase load (50-500 N) and the effect of the spring constant (50, 100 and 200 N/mm), where short biomechanical studies determined the effect and aided to provide an informed decision on the specific test conditions selected for wear testing. The results indicated how the decrease in cup inclination angle and increase in swing phase load decreased the dynamic separation and severity of edge loading and wear. These studies have elaborated on the potential conditions leading to edge loading and their outcome. Overall the severity of edge loading had a positive linear correlation (R2=0.90) with the wear rate for all the conditions tested. This project has emphasised the criteria required to individually evaluate different parameters when considering edge loading and presented a format to develop an ISO standard to consider the risks associated with edge loading.