Rare earth stearates for wear determination of UHMWPE bearings

The wear of ultra-high molecular weight polyethylene (UHMWPE) components has limited the clinical lifespan of total joint replacements (TJR). Current wear quantification methods through gravimetric measurement only allow estimation of total component loss. Wear originating from different bearing surfaces cannot be distinguished. The ability to determine the volume of loss experienced by specific bearing surfaces would be a leap forward in assessing and evaluating prosthesis designs and aid in the development of wear prevention. The purpose of this study was to assess the tracing ability of europium (Eu)- and gadolinium (Gd)-stearates, and to investigate the wear impact of the tracer additions on the base UHMWPE material. Using a pin-on-disc (POD) apparatus, eighteen pins (9.53 mm diameter × 19.1 mm length) were tested in three separate experiments. Each experiment contained n = 2 Eu-stearate doped, n = 2 Gd-stearate doped UHMWPE pins and n = 2 UHMWPE control pins (CP, without tracers). Lubricant samples for chemical quantification of tracer amount and gravimetric pin measurements were obtained every 0.25 million cycles. The overall wear rates of each pin were then calculated. The wear rates of each material grouping were not significantly different from one another (P > 0.5), indicating that tracer addition did not adversely affect the wear performance of the conventional UHMWPE material. Both Eu and Gd yielded highly significant correlations (R2 ≥ 0.95) between the concentration of tracer quantified in the lubricant samples and the corresponding gravimetric measurements. However, in both cases, the slope was different from 1 (approximated 2). This systematic offset was most likely related to component manufacturing and will be addressed in future studies. In summary, the presented tracer-determined wear quantification method looks promising to accurately track polyethylene wear in lubricant samples.