A re-appraisal of wear features of acetabular sockets using atomic force microscopy

The morphological and material features of 10 worn ultra-high molecular weight polyethylene (UHMWPE) acetabular sockets were assessed using atomic force microscopy (AFM) imaging of topography, stiffness and adhesion. Six of the sockets were simulator worn, four with a physiological gait cycle and two under linear motion. The remaining four sockets were explanted cemented joints. Polishing of the UHMWPE was shown in the pole of all cups. The presence of rippling or fatigue cracking within the surface of the UHMWPE at the pole was found to require motion approaching unidirectionality. Fibril formation within the polished region was highly associated with explanted cups and scratching was seen at the pole was found in simulator tested cups. The periphery of the contact region showed polishing of the UHMWPE in the simulator testing but the explanted cups showed highly fibrillar wear areas. Third bodies were positively identified in the periphery of one cup. Evidence in support of current wear models was not forthcoming. An alternative explanation of the formation of fibrils within the polished regions has been proposed; partial particle release is followed by the teasing of a fibril through a rolling action between the two articular surfaces.