DIFFERENCES IN THE RATES OF PENETRATION DETERMINED FROM RADIOGRAPHIC AND SHADOWGRAPHIC MEASUREMENTS OF ACETABULAR SOCKETS

A number of previous studies have observed a marked difference in the penetration depths recorded between direct and radiographic measuremen t, although no assessment of how these discrepancies affect the rates of penetration has been undertaken. In this report, the penetration de pth of 95 sockets was determined from both prerevision radiographs and casts of the retrieved sockets using the uniradiographic and shadowgr aphic techniques, respectively. It was observed that the mean discrepa ncy between the penetration rates derived from the two measurement met hods was 0.046 (SE 0.017) mm yr(-1), which was significantly different from zero at P = .007. It was concluded that the penetration rates de rived from this method of radiographic assessment seriously underestim ated the true value as measured by a direct method by approximately 20 %. As a consequence, the discrepancy in penetration rates between thos e sockets that are loose at revision surgery (direct measurement) and the general population of acetabular components (radiographic measurem ent) may not be as large as previously thought.