THE SIGNIFICANCE OF NONLINEAR MOTION IN THE WEAR SCREENING OF ORTHOPEDIC IMPLANT MATERIALS

Linear reciprocating pin-on-plate-type wear testing has been a standar d technique for the screening of orthopaedic implant materials since t he early 1980s. This investigation compares a wear screening technique based on linear motion with a modem hip joint simulator based on mult i-axial motion. Two groups of differently sterilized UHMWPE samples we re tested. The first group of samples was sterilized by ethylene oxide (EtO) gas that caused no structural changes in the UHMWPE. The second group of samples was sterilized in nitrogen by gamma-irradiation and then subjected to a stabilization treatment that resulted in a signifi cant level of crosslinking in the UHMWPE. When tested on the linear re ciprocating wear machine, the EtO sterilized specimens (non-crosslinke d linear polyethylene) showed an approximately 30% lower wear rate tha n the gamma-irradiated and stabilized specimens (crosslinked polyethyl ene). When tested on the hip simulator, the EtO sterilized specimens e xhibited two to three times higher wear rates than the gamma irradiate d and stabilized specimens. The ranking of wear resistance obtained wi th the hip simulator was strikingly different than that obtained with the linear reciprocating wear machine. This study indicates that scree ning wear-machines based on linear motion do not correlate with multi- axial joint simulators and may produce misleading results in the predi ction of clinical wear performance of UHMWPE bearing materials.