The appearances of wear on polyethylene--a comparison of in vivo and in vitro wear surfaces.

An examination has been made of the wear surfaces of 21 ultra-high-molecular-weight polyethylene components of hip and knee joint prostheses removed from patients. Both light optics and scanning electron microscope systems of examination have been used. The appearances are compared with specimens subjected to two different in vitro wear tests. Surface appearances related to the wear experience have been classified into seven modes. At least four of these are evidences of what may be four different wear mechanisms. Entrapment of bone-cement particles between articulating surfaces is fundamentally extraneous but is seen to generate the most severe wear. In the absence of bone-cement particles, in vivo, wear seems to be mild. Wear in vitro produces similar surface modes but with different relative emphasis.