Abstract
We studied hips with these two different ceramics during the same period and with a minimum follow-up of 15 years. Because the sizes of the alumina and zir-conia heads were different, hips with 32 mm alumina heads and those with 28 mm zirconia heads were compared with control hips with stainless-steel heads of the same sizes. The same stem and the same PE cup were implanted with cement at the same period. The femoral head was made of alumina with a diameter of 32 mm in 62 cases, and made of yttrium-oxide-partially-stabilized zirconia with a diameter of 28 mm in 40 cases. These ceramic heads were compared with 32 and 28 mm stainless steel heads (40 hips)
There was an increased linear rate of penetration of the femoral heads into the liner between years five and 15 for the zirconia and the stainless-steel groups. This was severe in the zirconia group (0.4 mm/year compared with 0.13 mm/year for the stainless-steel group). During the same 15-year period there was, however, no significant change in the rate of wear in the alumina group (0.07 mm/year). The mean wear at the most recent follow-up was 1740 mm3 for the 28 mm zirconia group, 842 mm3 for the 28 mm stainless-steel group, 825 mm3 for the 32 mm alumina group and 1416 mm3 for the 32 mm stainless-steel group. The three femoral heads retrieved in the zirconia group were analysed using x-ray diffraction. The transformation rate of the tetragonal to the monoclinic crystal was 19 mol%, 25 mol% and 30 mmol% respectively. By comparison, the percentage of monoclinic phase was 4 mol% on a non-implanted femoral head at its surface. Their surface roughness was increased. Scanning electron microscopy of the surface of the retrieved heads showed more craters than on the zirconia heads before implantation. Changes were observed in the volume and sphericity of the retrieved heads. Their volume was measured by fluid displacement and had increased respectively by 0.5%, 0.7% and 1.1%. This variation was greater than that due to manufacturing tolerances . The morphological appearance of the surface of the retrieved cups was inspected. The most surprising change was found on the periphery of one cup with an increase in volume of the polyethylene on the non-articular surface of the liner as if the polyethylene had melted and then cooled. Although experimental studies have shown encouraging results, the long term clinical results of zirconia are not favorable.
Theses abstracts were prepared by Professor Roger Lemaire. Correspondence should be addressed to EFORT Central Office, Freihofstrasse 22, CH-8700 Küsnacht, Switzerland.
