Mechanical properties of PSZs are compared with those of new alumina ceramic with grain size of less than 2 μm in Table 3.2. PSZs have significantly higher bending strength, compressive strength, fracture toughness, and impact strength, but have a lower Vickers hardness and elastic modulus than the alumina ceramic, although they are slightly different depending on the grain size and kind of chemical stabilizers used. Among them, yttrium oxide PSZ (Y-PSZ) has the highest bending strength and fracture toughness followed by cerium oxide PSZ (Ce-PSZ). Breaking tests for Y-PSZ and alumina femoral heads, 22 mm in outer diameter, were performed by static loading over a polyethylene liner which was set against the ceramic head. The alumina heads were broken by loads of 2,400–3,400 kg (average 2,800 kg), while Y-PSZ heads were broken by loads of 2,770–4,480 kg (average 3,700 kg). Thus, Y-PSZ head was significantly stronger than the alumina head against breakage [7]. Fatigue test was performed on eight Y-PSZ femoral heads on a hip simulator in physiological saline at 37 °C, by applying 10⁷ cycles of repeated loading with 450 kg. This loading is considered to correspond approximately to 20 years of a person walking. After the test, no breakage was observed in all the eight Y-PSZ heads.

Wear tests for the polyethylene liner against the Y-PSZ, alumina and stainless steel head, all 22 mm in outer diameter, were performed using a hip simulator in physiological saline at 37 °C by applying a load of 450 kg at 1 Hz. After 5 × 10⁵ cycles of loading, the polyethylene liner against the stainless steel head showed significant wear, while those against the Y-PSZ head and alumina head did not show any measurable wear, even after 2 × 10⁶ loading [7].

Thus, alumina is chemically more stable than PSZ in vivo, while PSZ is mechanically stronger than alumina, and both of them exhibit much better wear-resistant character comparing the stainless steel or Co-Cr alloy as assessed in a form of bearing components of hip prosthesis. For these reasons, alumina is used to fabricate a ceramic-on-ceramic hip prosthesis where head size is not a key issue, while PSZ is used to fabricate a PE-on-ceramic hip prosthesis where the head size must be made reasonably small. One of reasons why the zirconia-on-zirconia or alumina-on-zirconia hip prosthesis is not yet brought to the market is that, even with the PSZ, its crystallographical stability in vivo in a long term has not been confirmed.

On the other hand, recently a combined ceramic (Zirconia 20 % and Alunina 80 %) is brought into clinical use as a XLPE-on-Ceramic hip prosthesis. This combination of Zirconia and Alumina is to aim at covering the weak points each other. It is said, however, when a proportion of Zirconia exceeds 6 %, effect of phase transformation can not be neglected in vivo. Therefore, clinical follow-up longer than 10 years is required to confirm the long term results in this combination as well.

A new technology developed by Smith and Nephew Co. in 1998 using Zirconium-Niobium alloy made it possible to solve the problem of phase transformation of zirconia ceramic in vivo. When a high temperature is applied on Zr-Nb alloy, its surface transforms to a monoclinic zirconia layer in about 5 μm thick. The layer is called Oxinium. Thus made surface monoclinic zirconia layer (Oxinium) is a gradient material made from Zr-Nb alloy. When a femoral head of hip prosthesis is made by the use of the above technique, its surface is not affected by phase transformation in vivo as it is made of monoclinic zirconia, and, in addition, the femoral head is not broken as it is made of Zr-Nb alloy, a metal. For these reasons, Oxinium-on Oxinium hip prosthesis is considered reasonable theoretically.