Serum Lubricant Absorption by UHMWPE Orthopaedic Bearing Implants

This study investigates fluid absorption behavior of an unirradiated and a heavily crosslinked ultra-high molecular weight polyethylene (UHMWPE) exposed to bovine serum with pressures varying from 0.1 to 10 MPa. The time-response of serum absorption is biphasic, with an initial “soak-in” phase (<1 week) of rapidly increasing mass before settling to a steady-state phase of mass increasing more gradually and linearly with time. Initial soak-in absorption of the unirradiated and crosslinked UHMWPE are very similar, increasing from 0.8 to 4.3 g/m² as serum pressure is increased from 0.1 to 10 MPa. Steady-state absorption rates of the unirradiated UHMWPE increase from 0.8 to 1.5 (mg/m²)/hr as serum pressure is increased from 0.1 to 10 MPa, while absorption rates for the crosslinked UHMWPE are lower, increasing from approximately 0.28 to 0.85 (mg/m²)/hr over the same pressure range. Steady-state rate of absorption is also found to depend on temperature, with values at room temperature being two-fold less than at 37°C. Such fluid absorption characterization is critical to hip and knee simulator testing, in order to separate wear loss and serum absorption gain contributions to total mass change measurements of UHMWPE bearings. In addition to its pertinence in quantification of UHMWPE wear, the extent of fluid film lubrication in total joint replacements as implied by reported serum absorption behavior is also discussed.