An evaluation of the tribological performance of zirconia and CoCrMo femoral heads

Abstract

Five new zirconia, five new CoCrMo and three explanted CoCrMo femoral heads were wear-tested in bovine serum for five million cycles using the Durham Hip Joint Wear Simulator. Wear was measured gravimetrically and surface topography with a 3D non-contacting profilometer. This allowed an evaluation of the different head types on UHMWPE acetabular cup wear rates and the effect of roughening of the femoral head on acetabular cup wear. The mean acetabular cup wear rate against the five CoCrMo femoral heads was 40.8 mm3/106 cycles which was significantly higher (p = 0.03) than against zirconia (33.3 mm3/106 cycles). The initial surface roughness of the CoCrMo femoral heads (Ra = 4.6 nm) was statistically significantly higher than for the zirconia heads (Ra = 3.1 nm). Over the wear test the CoCrMo heads got statistically significantly rougher (Ra = 10.5 nm) whilst the zirconia heads showed no statistically signficant change. The three explanted CoCrMo femoral heads had initial mean surface roughness, Ra, values of 19, 24 and 55 nm with corresponding cup wear rates of 97.6, 131.2 and 148.4 mm3/106 cycles respectively. The very high wear rates against the explanted heads highlight the need for scratch resistant femoral head surfaces.