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 strongly influenced by the presence of abrasive particulate entrapped between the articulating surfaces. The aim of the present study is to evaluate the dependence of such damage on the geometry of the particles entrapped in the joint, with reference to the UHMWPE/chrome-cobalt coupling. Five chrome-cobalt femoral heads and their coupled UHMWPE acetabular cups, retrieved at revision surgery after a short period of in situ functioning, have been investigated 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 derived from profilometry evaluations, ranged from 0.3-1.3 μm. The observed damage has been divided into four classes, related to the particle motion while being embedded into the polymer. Two kinds of particle morphology have been studied, spherical and prismatic, with size ranging from 5-50 μm. In order to provide an estimation of the damage induced by such particles, a finite element model of the third-body interaction was set up. The peak-to-valley 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 μm for spherical particles of size ranging from 10-40 μm. In the case of prismatic particles, the peak-to-valley height can reach 1.3 μm and depends both on the size and width of the particle's free corner, indenting the chromecobalt. As an example, a sharp-edged particle of size 30 μm can induce on the chrome-cobalt an impression with peak-to-valley height of 0.75 μm, when embedded into the polyethylene with a free edge of 5 μm facing the metallic surface. Negligible damage is induced, if a free edge of 7.5 μm is indenting the counterface. Our findings offer new support to the hypothesis that microscopic third-body particles are capable of causing increased roughening of the femoral head and provide a quantitative evaluation of the phenomenon.