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THREE-DIMENSIONAL MODELING OF THE HIP KINEMATICS UNDER MICRO-SEPARATION REGIME AFTER TOTAL HIP REPLACEMENT WITH PARTICULAR REFERENCE TO SQUEAKING

Institute of Medical and Biological Engineering, Leeds University, Leeds, UK

The use of hard-on-hard hip prostheses has highlighted specific problems like the "stripe-wear" and the squeaking. Many authors have related these phenomena to a micro-separation between the cup and the head. The goal of the study was to model the hip kinematics under micro-separation regime in order to develop a computational simulator for total hip prosthesis including a joint laxity, and to use it to perform a sound analysis.

A three-dimensional model of the Leeds II hip simulator was developed on ADAMS® software. A spring was used to introduce a controlled micro-separation (less than 500 microns) during the swing phase of the walking cycle. The increase of the load during the stance phase induced a relocation of the head in the cup. Values of the medial-lateral separation predicted from the model were compared to experimental data measured using a LVDT of less than 5 microns precision. Theoretical wear path predicted from the model was compared to the literature data. The frequencies of the vibratory phenomena were determined, using the Fourier transformation.

There was an excellent correlation between the theoretical prediction and the experimental measurement of the medial-lateral separation during the walking cycle (0.92). Edge-loading contact occurred during 57% of the cycle according to the model and 47% according to the experimental data. Velocity and acceleration were increased during the relocation phase in a chaotic manner, leading to vibration. The contact force according to the model had also a chaotic variation during the micro-separation phase, suggesting a chattering movement. Fourier transformation showed many frequencies in the audible area.

A three-dimensional computational model of the kinematics of the hip after total replacement was developed and validated with an excellent precision under micro separation. It highlighted possible explanations for the squeaking that may occur during either relocation phase or edge loading.

Correspondence should be addressed to Mr K Deep, General Secretary CAOS UK, Dept of Orthopaedics, Golden Jubilee National Hospital, Glasgow G81 4HX, Scotland. Email: caosuk{at}gmail.com