On the material and the tribology of alumina-alumina couplings for hip joint prostheses.

The yearly sphericity deviation ranged from 0.03 mu to 3.7 mm in 48 retrieved implants with alumina--alumina bearing balls and cups. Excessive amounts of wear are predominantly design dependent or caused by malalignment. But the abrasion phenomena occurring in the cup centers and in the corresponding ball areas are related to unfavorable function zones. The influence of the lubricating gap geometry as studied in laboratory tests shows the form of sphericity deviations to be of decisive importance for wear and friction, if extended over small angles. Then, the contact stresses attributable to the actual effective curvatures of the bearing exceed the resistance to abrasion of the material under boundary lubrication conditions. There are remarkable differences between the early and actual material quality. Based on the state of knowledge of material aspects of wear criteria for the optimization of material and design, promising tolerable wear rates may be achieved.