Unstable mode coupling at the origin of hip endoprosthesis squeaking noise

Recently hip endoprosthesis design has focused on improvements of bearing surfaces in order to decrease wear rates and osteolysis. In this sense, the ceramic-on-ceramic (CoC) bearing shows excellent qualities that are at the origin of the spread of their application and development. However, enthusiasm for CoC bearing has been tempered recently by reports of disturbing squeaking noises. Clinical reports have shown worrisome rates of squeaking occurrences, which has been increasingly reported as a complication of modern CoC bearings, becoming a real social problem for patients. This paper focuses on the investigation of hip endoprosthesis squeaking noise that is here considered as the result of a mode coupling instability of the system, excited by the frictional forces at the ceramic contact pair. Parametrical pre-stressed complex eigenvalue analyses have been developed by a finite element model of a real prosthesis in-vitro conditions. The frequencies of the unstable modes predicted by the numerical analysis have been compared with the squeaking noises obtained on a dedicated set-up. The same analysis has been performed to a model including the prosthesis, the bones and the soft tissue in order to explain the squeaking frequencies recovered by in-vivo measurements in the literature. The presented approach has been used here for comparing the squeaking propensity of two different commercial prosthesis designs, showing a good agreement with the occurrence of squeaking events reported in the clinical literature. Effect of design modifications on the squeaking propensity has been addressed as well. The complex eigenvalue analysis is proposed as a useful tool for the design of "squeaking-free" prostheses by understanding and avoiding the critical dynamic configurations. While this approach has shown to not be applicable for brake squeal prevention, because of the high modal density and complex dynamics of a brake system, the relative simple dynamics of a hip endoprosthesis and its lower modal density allow for a simpler control of the mode coupling configurations.