Examination of surface and material properties of explanted zirconia femoral heads

The use of alternative bearing surfaces in total hip arthroplasty (THA) has been advocated as a way to prolong the life of implants by decreasing wear between components. Ceramics, such as Tetragonal Zirconia Polycrystal (TZP) and Alumina have been used as materials for femoral heads in THA because of their desirable mechanical properties (high fracture toughness, surface hardness and low coefficient of friction). TZP femoral heads were introduced in 1985 as an alternative to alumina, because of zirconia’s superior hardness and resistance to fracture; however; there have been recent reports of significant polyethylene wear with TZP ceramic femoral heads. TZP ceramic exists in three different phases of crystalline structure (monoclinic, tetragonal and cubic). TZP, as it is manufactured, is in a metastable tetragonal phase that has the advantage of possessing higher resistance to crack formation than the monoclinic phase, which is the stable phase at room temperature. TZP in a tetragonal phase can undergo transformation into the monoclinic phase under stress or with aging. The phase transformation from tetragonal to monoclinic involves a volume expansion of 3–4%. Because of this volume expansion, crack propagation is halted in the tetragonal phase as the crack is sealed. Once the material undergoes extensive transformation into a monoclinic phase, this advantage is lost, and the implant may be more susceptible to surface damage and to increasing surface roughness. It has been postulated that in an in-vivo environment, TZP ceramic femoral heads undergo aging with increasing conversion of their surface layers into monoclinic phase. Over time, this phase transformation could lead to increasing crack formation and decreased surface hardness, which in turn would leave the surface vulnerable to wear from increasing surface roughness. There have been two recent studies that have examined a limited number (2 in one case, 3 in the other) of explanted TZP femoral head samples for signs of increased wear, but no study has examined a large number of samples for signs of surface deterioration. The purpose of our study was to examine the surface and bulk mechanical properties of explanted TZP femoral heads compared to factory-sealed controls. The hypothesis is that degradation of surface and mechanical properties occurs with increasing time of implantation because of the surface material phase transformation from tetragonal to monoclinic phase. The aim is to compare the control specimens to explanted samples that have been subjected to the rigors of implantation and determine the extent of degradation, if any, which occurs over time in the human body.