Zirconia ceramic femoral head has better mechanical properties than those alumina head has. However, it is concerning whether the wear of UHMWPE against zirconia head is as low as that of alumina head. We compared polyethylene wears against 22.225mm alumina and zirconia heads in total hip arthroplasties (THA) occupied in our hospital.

Kobelco hip prostheses (Kobelco, Kobe, Japan) were used. Titanium alloy stem was cement-fixed and all-polyethylene acetabular components were articulated with alumina or zirconia femoral heads. In the cases from February 1996 to December 1998, 96 primary cemented total hip arthroplasties (THA) in 87 patients of osteoarthritis were observed with a minimum follow-up of five years. The clinical results were evaluated using JOA hip score (100 in normal hip). There were 46 hips with alumina heads and mean follow-up of 6.2 years, and 51 hips with zirconiaheads and mean follow-up of 5.2 years. Mean ages at operation were 58 years. For each patient, the initial postoperative and the latest anterior-posterior radiographs of the pelvis were selected. With the custom software Image-Pro Plus version 4.0, linear wear and volumetric wear were measured.

Clinical results of both heads showed no difference. Linear wear rate of alumina is 0.080±0.047mm/year, while it is 0.147±0.069mm/year for zirconia, significantly higher than alumina (p?0.0001). Volumetric wear rate of zirconia (42.499±20.233mm3/year) is also significantly higher than alumina (27.049±17.216mm3/year) (p?0.0001).

The low thermal conductivity of zirconia is thought to cause local high temperature, leading to the phase transformation and lower-temperature aging degeneration (LTAD) of zirconia head surface. In order to solve this problem, alumina-adding zirconia compound ceramics with lower phase transformation were developed. Hip simulator test revealed much lower wear rate of improved zirconia. This will be discussed.