Highly cross-linked ultra high molecular weight polyethylenes (UHMWPEs) were developed to reduce UHMWPE wear in arthroplasty. These UHMWPEs have manifested a significant improvement in the in-vitro adhesive — abrasive wear resistance of total joint prostheses.
Examination of the first clinically retrieved liners revealed surface features not usually observed on previous retrievals. A flattening of the machining marks is evident, together with the presence of ripples with micro-fissures. To evaluate these features, examinations were performed on a cup-on-ball device, an AMTI hip simulator, and a Stanmore knee simulator, as well as on retrieved components with a maximum follow-up of 15 months. The surface features of the components were investigated with light microscopy, by scanning electron microscopy (SEM), and also by transmission electron microscopy (TEM).
Ripples with micro-fissures were observed on the loaded areas perpendicular to the principal directions of motion. The transverse examinations of all sections of these specimens, which were made with optical microscopy and by TEM, showed that the ripples with micro-fissures may be described as folds. With all the investigated samples, the depth of the micro-cracks at the tip of the folds extended to a maximum of 5 μm below the surface. The folds experienced with UHMWPE were already described in the late 1970s.
Due to the extreme wear resistance of highly cross-linked UHMWPEs, these folds accumulate on the surface of components manufactured in this new generation of UHMWPEs. In view of their depth stability with the number of cycles, we believe that these folds have no influence on the fatigue behavior of such components.