Abstract
Introduction and Aims: The extremely low wear rates of third generation alumina-alumina bearings in traditional hip simulators are not reflected in vivo. Separation of the bearing during swing phase and edge loading with heel strike is reported to account for this discrepancy.
Method: We have had the opportunity to visually inspect 21 bearings at re-operation from a group of 1588 hip arthroplasties with third generation alumina ceramic-ceramic bearings. Re-operations were for heterotopic ossification (one), loosening (three), femoral fracture (six), psoas tendonitis (six), sepsis (three) and dislocation (two). There were no re-operations for bearing failure. Sixteen of these 21 bearings (16 heads and 12 inserts) were retrieved and analysed. We mapped the location and we measured the volume of the wear and we performed microscopy and measured roughness of worn and unworn areas.
Results: Eleven bearings had visual evidence of edge loading wear, making an incidence of 52% in the 21 patients having re-operations. These 11 bearings and five visually undamaged bearings were analysed. The wear on the insert was always located at the rim indicating edge loading. The location and orientation of the stripe on the head was not consistent with subluxation during normal gait but was consistent with subluxation and edge loading with the hip flexed at 90 degrees. The average wear volume was 0.7mm3 per year (heads plus liners). Longer service bearings had signs under SEM of repolishing of the wear area suggesting that the process of edge loading wear will be self-limiting. The heads without a wear scar showed very little damage: under SEM, a slight relief polishing of individual grains and minor pitting was noted.
Conclusion: The subluxation causing the stripe wear in these patients did not occur during normal walking gait. It probably occurred with rising from a chair. Simulator testing of third generation alumina-alumina components must include edge loading if it is to give a realistic indication of in vivo performance. Alumina-alumina bearings remain an excellent option for total hip arthroplasty, however more work is required to understand the clinical consequences.
These abstracts were prepared by Editorial Secretary, George Sikorski. Correspondence should be addressed to Australian Orthopaedic Association, Ground Floor, The William Bland Centre, 229 Macquarie Street, Sydney, NSW 2000, Australia.
At least one of the authors is receiving or has received material benefits or support from a commercial source.
