Audible Vibrations of total Hip Replacements
A. Hothan, G. Huber, C. Weiß, K. Sellenschloh, N. Hoffmann, M.M. Morlock
Recently, squeaking of total hip replacements with ceramic on ceramic bearing is a frequently discussed phenomenon. Although numerous publications have discussed clinical factors which are potentially essential for its occurrence, the responsible mechanisms are not yet well understood.
Squeaking occurs when structural vibrations are excited in a manner that the vibration amplitudes allow the emission of audible sounds. This is typically possible when the frequency of the excitation matches the natural frequency of a part of the system or the system’s entity. The prosthesis stems are the main vibrating components detuned by the surrounding bone stock and the induced load.
In an artificial hip joint system the friction between the ceramic bearing surfaces due to relative movement during gait cycles induce energy into the vibrating system – the lower the friction the lower the application of energy. An increased friction coefficient allows the excitation of vibration amplitudes which are high enough to emit sounds which are clearly audible from outside the patient’s body.
To study this phenomenon a hip simulator was built to reproduce squeaking in-vitro and to analyze the dynamic events under realistic conditions. Lubrication, bearing clearance, stem and cup design, loading, bearing surface roughness, component orientation, bone quality are just examples for various factors influencing the susceptibility and characteristics of squeaking. Modal analyses, frequency analysis of acoustic noise, analysis of transfer paths utilizing laser vibrometry, high precise microphones and accelerometers as well as explicit and implicit FE analyses are performed to understand the dynamic behavior of prosthesis-tissue systems.
The results from this study will help to understand the responsible mechanisms and influencing factors for squeaking and to develop a remedy.
This study is finically supported by Ceramtec. Components were donated by Aesculap, Biomet, DePuy, Eska, Mathys, Plus and Smith&Nephew.