ARTICULAR SURFACE MATERIAL EFFECT ON METAL-BACKED PATELLAR COMPONENTS - A MICROSCOPIC EVALUATION

This study evaluated patellar wear with three commonly used metallic surfaces against a polyethylene surface. Wear characteristics of metal-backed patellar components were tested against cobalt chromium, titanium alloy, and ion-implanted titanium alloy (IITi) articular surfaces. Adult bovine serum was used as a lubricant. The specimens were cycled at 2.9 Hz for 1 X 10(6) cycles. The axial load was 343.2 N at 0-degrees flexion and increased to 2255.6 N at 110-degrees flexion. After testing, the polyethylene and metal surfaces were examined for wear using scanning electron microscopy. Patellae tested against the titanium-alloy surfaces had severe abrasion, surface cold flow, and pitting. The patellae tested against IITi were similar to the titanium-alloy group at the edges of the components, and delamination of the surface layers occurred in the high stress areas. The patellae tested against cobalt-chromium alloy had the least wear. Among the three metal surfaces, the titanium surface had the most severe surface abrasion and the cobalt-chromium surface had the least. At higher power magnification (x500), the cobalt-chromium surface had minimal surface abrasion even in the high stress areas. The IITi and titanium surfaces had severe scratching and surface abrasion, but the titanium surfaces were clearly more severely affected. The ion-implanted surface appeared to have been worn away in the high stress areas. Ion implantation does not produce a major improvement in the effect the titanium-alloy surface has on the polyethylene but does improve the wear on the metal surface itself. The cobalt-chromium surface was less damaged than the titanium and IITi and also caused less damage to the polyethylene. This experiment suggests that titanium and ion-implanted titanium are inferior materials to cobalt chromium when used as an articular surface in total knee replacements.