Forms and Mechanisms of Wear at Explanted Metal-Metal Hip Implants - Relation to Life Cycle

Metal on metal bearings for artificial hip prosthesis are considered as one alternative to metal on polyethylene bearings, because large amounts of polyethylene wear debris can cause osteolysis and subsequent loosening of the joint. This study includes the inspection of 42 McKee-Farrar-Prosthesis using light and scanning electron microscopy. The goal of the study was to describe the wear appearances and identify the related wear mechanisms. Eleven out of 42 prosthesis showed remarkable wear on the top of the head and in the cup. However, their difference in life-time was not significant compared to the rest. A total of 29 prostheses had a recess in the centre of the cup. Nine of those showed excessive wear at the rim of the recess. Again, there was no significant difference in life-time between these two groups. Potential tribochemical layers were identified on nearly every bearing surface. Inside the layers, scratches with agglomerated wear particles of the prosthetic material were found. The presence of wear particles within the scratched out layers is suggesting that layers were generated as a result of tribochemiocal reaction rather than chemical apposition. It is further speculated, that these layers prevent direct metal on metal contact and thus adhesion. Further they may act as a solid lubricant, but the effectiveness is unknown at the present time and goal of future analyses.