Hip contact force pathways in total hip replacement differ between patients and activities of daily living

One of the main causes of implant failure and revision surgery in total hip replacement (THR) is aseptic loosening often caused by the accumulation of wear debris arising between the contact surfaces of the acetabular cup and femoral head during activities of daily living (ADL's). However, limited information is available regarding the contact force pathways between these two surfaces during specific ADL's. In this study, through musculoskeletal modelling, we aimed to estimate the orientation of the hip contact force pathway on the acetabular cup. One hundred and thirty-two THR patients underwent motion capture analysis whilst undertaking locomotor and non-locomotor ADL's. Musculoskeletal simulations were performed to calculate contact force pathways using inverse dynamics analysis. We then qualitatively compared differences in the contact force pathways between patients and between ADL's. Walking resulted in a typical figure-of-eight pattern, with the peak contact forces occurring in the superior-anterior area of the cup. The non-locomotive activities such as stand up, sit down and squat had a more linear shape, spanning across the superior-posterior quarter of the cup. Our results showed a large inter-patient variability in the shape and location of the contact force pathway. There is a distinct difference in the location and shape of the pathway between locomotor and non-locomotor activities and this could result in different wear accumulations. These results could enhance our understanding why revision rates vary across the population and could inform the development of personalised implant design.