Delamination wear on two retrieved polyethylene inserts after gamma sterilization in nitrogen

Two self-aligning mobile bearing knee replacements (SAL-1) with gamma-in-nitrogen sterilized polyethylene inserts were revised due to instability after 6.3 years and after 14.2 years in vivo in two patients. The predominant damage features were burnishing, cracking, and delamination and were observed on the proximal bearing surface of the retrieved polyethylene inserts. This suggested an association with sub-surface fatigue, perhaps initiated by in vivo oxidative degradation which was confirmed by developing a sub-surface white band in one insert. The damage features observed on the distal bearing surface of the polyethylene inserts suggested both an adhesive wear mechanism and an abrasive wear mechanism. The titanium-nitrite coated, titanium-alloy tibial tray was severely worn in one case and possibly contributed to third-body abrasive wear at the distal surface interface. We suggest to carefully follow-up patients who received this type of mobile bearing knee system.