Phase transformation in explanted highly crystalline UHMWPE acetabular cups and debris after in vivo wear

Retrieved Hylamer™ acetabular cups were examined by differential scanning calorimetry (DSC) and Raman micro-spectroscopy and compared with conventional ultra-high molecular weight polyethylene (UHMWPE) cups to evaluate their crystallinity and crystal structure. At the same time, wear debris were recovered and analyzed by Raman micro-spectroscopy. DSC results showed that Hylamer™ manufacturing conditions enhanced crystallinity, lamellar thickness, and lamellar thickness distribution. Also Raman coupled to partial least squares (PLS) analysis was utilized to estimate the crystallinity of the samples; the intensity of the orthorhombic Raman band at 1415 cm^-1 was used to calculate the orthorhombic content. The crystallinity values obtained by DSC and Raman measurements (both PLS and orthorhombic percentages) agreed in the case of conventional cups while for Hylamer™ samples important discrepancies were observed. This finding suggests a partial transformation from orthorhombic into monoclinic phase and/or ill-defined orthorhombic one during the manufacturing of Hylamer™. This transformation is much more evident in the debris. This is the first case of observed phase transformation in Hylamer™ acetabular cups after in vivo wear and in PE debris. The phase transformation could be responsible for the poor clinical performances of Hylamer™ acetabular cups.