Biopolymer wear screening rig validated to ASTM F732-00 and against clinical data

Abstract

The ASTM standard F732-00, 'standard test method for wear testing of polymeric materials used in total joint prostheses' offers several pieces of data against which a test rig for such biopolymers can be validated. These conditions include a linear wear in the range from 0˙07 to 0˙2 mm/million cycles for ultrahigh molecular weight polyethylene (UHMWPE), and a wear rate ranking of poly tetra fluoro ethylene (PTFE)>>polyacetal≥UHMWPE. Using a modified pin on plate wear test rig, these three biopolymers were tested under a 40 N load against polished stainless steel plates while using dilute bovine serum heated to 37°C as the lubricant. When subject to multidirectional motion, mean wear factors of 37×10–6 mm3 Nm–1 for PTFE, 3˙8×10–6 mm3 Nm–1 for polyacetal and 1˙1×10–6 mm3 Nm–1 for UHMWPE were obtained. Therefore the wear ranking recommended by the ASTM standard was achieved. When the corrected mean weight loss of the UHMWPE test pins was converted to a linear wear rate, a value of 0˙12 mm/million cycles was obtained. Therefore the ASTM recommended target for linear wear rate was met. When the wear factors from the test materials were compared with clinical wear factors of 37×10–6 mm3 Nm–1 for PTFE, 3˙5×10–6 mm3 Nm–1 for polyacetal and 0˙95– 1˙45×10– 6 mm3 Nm–1 for UHMWPE, good correspondence between in vivo and in vitro wear factors was obtained, further validating the modified pin on plate wear test rig.