Tribology of artificial hip joints

Trauma and arthritis can severely impair hip joint function resulting in pain and restricted motion. Often, affected hip joints are replaced by artificial joints which are assumed to be one of the best clinical treatment options. Presently, over one million hip replacements are performed annually world-wide. Due to the limited lifespan of implants and unfavourable results of revision total hip arthroplasty, it is critical, especially for the younger population, that the longevity of the hip implant becomes maximised. The present study aims to firstly better understand the dynamic mechanism of the artificial hip joint and secondly to investigate important issues affecting the lifespan and performance of hip implants. To this end, a three-dimensional computational biomechanics model based upon multibody dynamics methodology is developed taking the spatial nature of the physiological loading and motion of the human body, the inertial forces of the hip components and bearing surface tribology into account. The calculation of the intra-joint contact forces developed is based on a continuous contact force approach that accounts for the geometrical and materials properties of the contacting surfaces. In addition, the friction effects due to the contact between hip components are also taken into account using the Stribeck friction model. Nonlinear dynamics and vibration of artificial hip joints, contact pressure distribution, hip moment and hip squeaking are investigated using this multibody model. Moreover, the Archard’s wear law is considered and embedded in the dynamic hip multibody model, which allows for the prediction of the wear developed in the hip joint. With the purpose of having more realistic wear simulation conditions, the geometries of the acetabular cup and femoral head are updated throughout the dynamic analysis. The main results obtained from computational simulations are compared and validated with those available in the best-published literature. Finally, from the study performed in the present work, it can be concluded that the main reason of hip squeaking is friction-induced vibration owing to different phenomena such as stick-slip friction, negative-sloping friction and contact force changes. Friction influences the trajectory of the contact point between the cup and head over the gait cycle. In addition, a potential cause of the high wear rates seen in vivo may be due to friction-induced vibration.