Hip contact forces can be directed outside of a well-oriented cup during common activities; implications for implant testing

Background Hip replacements are evaluated under a range of standard tests, including simulations of edge loading of the acetabular cup. The study aim was to provide data for the improvement of preclinical testing methods through patient-specific musculoskeletal simulation of daily activities that quantify the direction of hip joint loading relative to the orientation of the cup. Methods Musculoskeletal modelling data for five total hip patients were analysed for walking, sit-to-stand, and step down. Simulated implant alignment translational and rotational variations were included (cup implantation orientations: inclination 30–50°, version 12–32°). Dynamic cup orientations were determined based on pelvic rotation. The angles between the load vector and cup pole, force while past the rim, and locations on the cup rim where the vector left and returned into the cup were calculated. Findings No forces external to the cup occurred under sit-to-stand or step down. All external forces were directed anteriorly during swing phase, loading with the largest direction changes typically started anteroinferiorly and swept superiorly. The maximum angle between the load vector and cup pole (127°) and maximum external force (245 N) were similar between the worst cases from this dataset and current ISO standard. The largest sweep around the rim was 18°, compared to 0° in the ISO loading, however, and the ISO loading is directed to the superior rim edge. Interpretation The current ISO profile may underestimate the potential for liner fixation feature damage or squeaking from the load vector passing around the rim.