Effect of Water and Serum Absorption on Wear of Unirradiated and Crosslinked UHMWPE Orthopaedic Bearing Materials

Abstract

In addition to confounding mass-based wear measurements in serum-lubricated hip simulator experiments, fluid absorption by the acetabular cups may simultaneously modify the wear resistance of the ultra-high molecular weight polyethylene (UHMWPE) from which they are composed. To decouple the fluid absorption and wear processes enabling clearer investigation of this effect, absorption was first imposed during an initial stage where UHMWPE was exposed to pressurized (10MPa) fluid. This was followed by a second stage, where resultant wear behavior was assessed by a multidirectional pin-on-flat technique that, though still providing a serum-lubricating environment, does not promote the simultaneous fluid absorption occurring in hip simulator testing. Both unirradiated and highly-crosslinked UHMWPE were investigated, each with both bovine calf serum and water soaking exposures of duration to 129 days. The pressurized soaking of a highly-crosslinked UHMWPE decreased its wear resistance, causing an increase in wear rate by approximately 50% during subsequent serum-lubricated multidirectional pin-on-flat sliding tests as compared to non-soaked material. The magnitude of this effect did not appear to depend on whether the soaking fluid was water or serum, nor did it appear to depend on soak time provided it was at least of a 14-day duration during which more rapid transient fluid absorption occurs. Such soaking did not produce as pronounced an effect on unirradiated UHMWPE, at its lack of wear resistance likely causes the absorption-affected surface region to the completed removed within the earliest stages of sliding contact.

Comparative wear tests of ultra-high molecular weight polyethylene and cross-linked polyethylene

“…There are few in vitro reports of the comparative wear performance of XLPE and UHMWPE using a multidirectional motion device in the presence of a dilute bovine serum lubricant. 39,40 The present in vitro results are compared with these and are summarized in Table 4. Although test conditions have varied in terms of load, contact stress, wear track path and so on, it is interesting to note the relatively consistent reduction in wear of XLPE compared to UHMWPE of between 78% and 81%.…”

“…37,38 There are only a few reports of comparative in vitro wear testing of UHMWPE and XLPE using a wear testing device that has applied multidirectional motion in the presence of a dilute bovine serum lubricant. 7,39,40 Therefore, the objective of this work was to determine the wear of UHMWPE and XLPE in a clinically validated, high-capacity wear test device and compare the test results with clinically reported data for the two polyethylenes. A second objective was to verify the test results with ASTM F732-00 (standard test method for wear testing of polymeric materials for use in total joint prostheses).…”