Measurement of wear in retrieved acetabular sockets.

Twenty-eight standard Charnley sockets were retrieved at revision surgery. The penetration angle with respect to the cup coordinate system, beta, and penetration depth, d, of the sockets were measured using both the traditional shadowgraph technique and by analysing data obtained from a coordinate measuring machine (CMM). In addition, d was deduced radiographically from pre-revision X-rays. Limits of agreement between the three methods of measuring d were of the order of +/- 0.5 mm. Using the data obtained from the CMM it was possible to deduce the wear volume Vmeas directly. It was found that, in general, values of the wear volume calculated from d and beta using equations cited elsewhere (1, 2) were both imprecise and inaccurate. The direct measurement of the wear volume using the CMM depends on the location of reference points external to the wear surface. If such surfaces were damaged, then it was concluded that the shadowgraph technique provided the most suitable method for measuring the dimensional changes in the retrieved socket, due to its relative ease of use.