Towards the understanding of hip squeak in total hip arthroplasty using analytical contact models with uncertainty

Osteoarthritis in hip joints affects patients' quality of life such that often only costly orthopaedic surgeries i.e. total hip arthroplasty (THA) provide relief. Common implant materials are based on alumina ceramics and biocompatible metal alloys, steel or titanium-based, and plastics such as ultra-high molecular weight polyethylene. Hard-on-hard (HoH) implant pairs, i.e. ceramic-on-ceramic, or hard-on-soft implant combinations are formed. HoH implants have been known to suffer from squeaking, a phenomenon commonly encountered in friction-induced self-excited vibrations. However, the frictional contact mechanics, its dynamics related to impingement, the effect of socket position, the stem' configuration, in combination with different materials used are poorly understood. This study gives an overview of the state of the art research related to squeak and biomechanics in THA, with a focus on the effects of friction, stability, related wear and lubrication. An analytical model is proposed to study the onset of friction-induced vibrations in a simplified hemispherical hip stem rubbing in its bearing by varying the contact area. Preliminary results of the complex eigenvalue analysis and stick-slip motion analysis indicate that an increased contact fosters the development of instabilities, even at very small values of the friction coefficient owing to large local contact pressures.