Why are yttria stabilized zirconia and alumina used to fabricate the femoral heads of hip prostheses?

Yttria stabilized zirconia-alumina is used to fabricate the femoral heads of hip replacement prostheses because of its hardness, low coefficient of friction, resistance to wear, and its bio-compatibility. While this ceramic compound does wear after several years of use it is still one of the best materials available today for this application.