Abstract
Introduction This study was performed to evaluate the wear rates of Zirconia ceramic heads manufactured prior to the introduction of hot isostatic pressing, on a modular cementless polyethylene cup, and to compare this with the wear rate of a cobalt chrome head coupled with the same cup.
Methods Radiological analysis of 281 primary uncemented ABG total hip arthroplasties performed between 1991 and 1994 was carried out. Patients were divided into three groups: 203 patients with 28 mm zirconia head/polyethylene bearings, 47 patients with 32 mm zirconia ceramic head/polyethylene bearings, 31 patients with 28 mm cobalt chrome head/polyethylene bearings. Wear analysis was performed using the Martel software after digitising radiographs on a flatbed scanner. Initial radiographs were at a minimum of one year to exclude bedding in, and follow-up radiographs a minimum of two years later. Mean follow-up was 6.5 years.
Results The mean linear wear rate of the groups following age and gender matching: for 28 mm Zirconia heads 0.22 mm/yr, for 32 mm Zirconia heads 0.20 mm/yr and for 28 mm metal heads 0.14 mm/yr (p< 0.05). The patients with Zirconia heads had higher rates of osteolysis and revision than those with cobalt chrome heads.
Conclusions The reason for this increased wear rate of Zirconia on polyethylene may well be the transformation of the tetragonal stable phase of the Zirconia to the monolithic unstable phase on the surface of the material. This transformation at grain boundaries on the surface results in ceramic grain “pull-out” by the sliding action of the head against the polyethylene cup. This phenomenon may well occur as a result of the Zirconia processing; the ceramic studied was produced prior to hot isostatic pressing, which produces an inherently more stable ceramic i.e. a higher percentage of the tetragonal phase.
In relation to the conduct of this study, one or more of the authors is in receipt of a research grant from a non-commercial source.
The abstracts were prepared by Mr Jerzy Sikorski. Correspondence should be addressed to him at the Australian Orthopaedic Association, Ground Floor, William Bland Centre, 229 Macquarie Street, Sydney NSW 2000, Australia.
