Third-body damage of prosthetic heads in total hip replacement: combined experimental and computational study

Introduction. In artificial hip joints, the roughening of the femoral head has been associated to higher wear rates of the polymeric cup, eventually leading to osteolysis and implant failure. The increase of the femoral head roughness is strongly influenced by the presence of abrasive particulate entrapped between the articulating surfaces (Bauer et al., 1994 and Raimondi et al., 2000). This damaging mechanism has been simulated in controlled in vitro wear experiments (Que et al. 1999). The mechanical parameters influencing the scratch formation are still to be determined. The aim of the present study is to evaluate the dependence of such damage on the geometry of the hard particles entrapped in the joint, with reference to the commonly implanted UHMWPE/chrome-cobalt coupling. To this purpose, a series of chrome-cobalt femoral heads, retrieved at revision surgery after a period of in situ functioning, has been investigated for the occurrence of third-body damage. The damage observed on the surfaces has been divided into four typologies, on the basis of the defect morphology. A FE model has been built, which simulates the local behaviour of the two coupled bearing surfaces, in the presence of those entrapped hard particles. An index of induced head damage has been defined on the basis of the computational results and discussed in relation to the experimental findings.