Quantitative evaluation of the prosthetic head damage induced by microscopic third-body particles in total hip replacement

The increase of the femoral head roughness in artificial hip joints is stro ngly influenced by the presence of abrasive particulate entrapped between t he articulating surfaces. The aim of the present study is to evaluate the d ependence of such damage on the geometry of the particles entrapped in the joint, with reference to the UHMWPE/chrome-cobalt coupling. Five chrome-cob alt femoral heads and their coupled UHMWPE acetabular cups, retrieved at re vision surgery after a short period of in situ functioning, have been inves tigated for the occurrence of third-body damage, This was found on all the prosthetic heads, where the peak-to-valley height of the scratches, as deri ved from profilometry evaluations, ranged from 0.3-1.3 mum The observed dam age has been divided into four classes, related to the particle motion whil e being embedded into the polymer. Two kinds of particle morphology have be en studied, spherical and prismatic, with size ranging from 5-50 mum, In or der to provide an estimation of the damage induced by such particles, a fin ite element model of the third-body interaction was set up. The peak-to-val ley height of the impression due to the particle indentation on the chrome- cobalt surface is assumed as an index of the induced damage. The calculated values range from 0.1-0.5 mum for spherical particles of size ranging from 10-40 mum In the case of prismatic particles, the peak-to-valley height ca n reach 1.3 mum and depends both on the size and width of the particle's fr ee corner, indenting the chrome-cobalt. As an example, a sharp-edged partic le of size 30 mum can induce on the chrome-cobalt an impression with peak-t o-valley height of 0.75 mum, when embedded into the polyethylene with a fre e edge of 5 mum facing the metallic surface. Negligible damage is induced, if a free edge of 7.5 mum is indenting the counterface, Our findings offer new support to the hypothesis that microscopic third-body particles are cap able of causing increased roughening of the femoral head and provide a quan titative evaluation of the phenomenon. (C) 2001 John Wiley & Sons. Inc.