BIREFRINGENT STUDIES OF POLYETHYLENE WEAR SPECIMENS AND ACETABULAR CUPS

Birefringent studies have been performed on ultra-high molecular weight polyethylene (UHMWPE) components, from pin-on-disc wear tests, from hip-joint simulator tests and on an explanted hip prosthesis. Pins from tri-pin-on-disc machines showed incremental changes in the wear rate associated with changes in the transverse surface profile of the pin, and removal of material from longitudinal peaks oriented in the direction of sliding. Birefringent studies of thin sections of the pin, cut perpendicular to the wear surface, showed residual strains in the polymer, up to 40-mu-m deep below the surface. The strains were greatest underneath the longitudinal peaks. Similar birefringent studies of an explanted acetabular cup and a cup from a hip-joint simulator test also showed residual strains in the sub-surface layer with evidence of crack formation. Sub-surface deformations and residual strains, along with the discontinuous nature of the wear process, indicate that fatigue can play an important role in the wear of UHMWPE sliding on smooth surfaces.