Even after worldwide implantation of more than 400,000 zirconia hip joint heads versus UHMWPE cups, the ageing behaviour of zirconia is still sometimes questioned. This study has for objective to explain the mechanisms involved in the tetragonal to monoclinic phase transformation and its consequences on the intrinsic properties of zirconia hip joint heads. The positive effect of process improvements like Hot Isostatic Pressing (HIP) and slight alumina doping in the 3Y-TZP structure is described, Next generation zirconia heads will exhibit a 4 to 5 times higher ageing resistance. The monoclinic content at the surface of a PROZYR (R) head was also shown to have no influence neither on the mechanical burst strength nor on the wear behaviour of the heads.
