Effect of absorbed fatty acids on physical properties of ultra-high molecular weight polyethylene

Studies have demonstrated fatty acids diffuse into ultra-high molecular weight polyethylene (UHMWPE) prosthetic components (Costa, Biomaterials, 2001), but their effect on the physical properties remains poorly understood. One of the main fatty acids present in synovial fluid is octadecadienoic acid. We compared the properties of UHMWPE tensile test samples doped with <i>cis</i>-9,<i>cis</i>-12-Octadecadienoic acid (Sigma Aldrich) for 24 h at 100 ^oC to control samples heated to 100 ^oC for 24 h in air. Both cross-sectional area and weight increased after doping (area increase: 1.3% &plusmn 0.2, weight increase: 3% &plusmn 0.28). Infrared spectroscopy (Perkin-Elmer Frontier with ATR, 32 scans, from 4000 to 600 cm^-1) confirmed the presence of octadecadienoic acid (peak at 309 cm^-1). Differential scanning calorimetry results showed doping significantly decreased the crystallinity (p=0.015, n=3) and the melting temperature (p=0.001, n=3). Tensile tests (n=5) were carried out in accordance with ISO527 using an electromechanical testing machine (Instron 5965) and a contact extensometer, at a rate of 50 mm/min. The doped samples had significantly lower yield stress (p&lt0.0001) and elongation at failure (p=0.03), but no change was found in modulus or ultimate stress. The results demonstrate the absorption of octadecadienoic acid, which happens over time <i>in vivo</i>, alters UHMWPE dimensions, reduces crystallinity, melting temperature, yield stress, and elongation at failure. Consequently, it is important that the effect of fatty acid absorption is taken into account when performing <i>in vitro</i> tests of UHMWPE components, such as wear testing. The dimensional change also has implications for close fitting component designs.