Effect of Lubrication Conditions on the Wear of UHMWPE with Noncyclic Motion and Load

Abstract

In orthopaedic tribology, one of the most debated issues is the optimal lubricant. Many different types, concentrations, additives and temperatures of serum-based lubricants are in use. With the multidirectional RandomPOD wear test device, several different lubrication conditions were studied for conventional, gamma-sterilized ultrahigh molecular weight polyethylene (UHMWPE) against polished CoCr. The conditions included dry, deionized water, phosphate buffered saline (PBS) and Alpha calf serum. Only with serum, wear was similar to that known to occur clinically. It was highly linear and of substantial magnitude. Polyethylene surface was burnished and no macroscopic wear debris was produced. The absence of polyethylene transfer, however, was not limited to serum lubrication only. With PBS, no transfer occurred and the same held true occasionally with dry sliding. An increased temperature, 37 °C, as opposed to 20 °C, of 1:1 diluted serum was found to increase the standard deviation of wear factor 2.7 to 4.4-fold, depending on whether an antimicrobial additive, NaN3, was absent or present (0.2%). In the absense of NaN3, the mean wear factor at 20 °C was 2.9-fold higher than at 37 °C. In the presence of NaN3, the corresponding difference was 1.8-fold. A one-day vs. a 6-day serum change interval resulted in wear factors not statistically different from each other. The same held true for the wear factors with undiluted serum vs. serum diluted 1:1. The lubrication conditions appear to have significant effects in wear studies of orthopaedic implant materials and so they need to be carefully chosen.