DISCREPANCY BETWEEN PENETRATION DEPTHS DERIVED FROM RADIOGRAPHIC AND DIRECT MEASUREMENT OF ACETABULAR COMPONENTS

The most common technique for assessing penetration due to wear in ace tabular components is with the aid of the most recent serial radiograp h. This approach, which is often termed the uni-radiographic method, h as been shown to underestimate the more reliable value of the penetrat ion depth deduced from direct measurement of explanted sockets. In thi s article the causes of the discrepancies between the two data sets ar e explored. Ninety-six sockets were available from revision surgery fo r which both the penetration depth and angle could be measured using t he shadowgraphic technique in both the coronal and wear planes. Furthe r, the penetration depth for each of the sockets was also assessed fro m pre-revision X-rays. A significant discrepancy was observed between the penetration depths measured in the wear plane of the replica Delta P-w and that measured from the radiograph, Delta PX-ray. The discrepa ncy was greatest for loose sockets as opposed to those that were still fixed at revision surgery. Using the corresponding data from the shad owgraph measurements, it was possible to deduce that the errors have a risen from the radiographic measurement of wear in the coronal plane a nd the formula used in calculating Delta PX-ray. If these errors (whic h cannot be calculated from the X-ray data alone) were taken into cons ideration, then the systematic bias between radiographic and shadowgra phic measurement was greatly reduced. The largest portion of the discr epancy was accounted for by wear occurring out of the plane of the rad iograph, and this, in general, coincides with the coronal plane. Overa ll, these results indicate that the accurate measurement of wear from serial radiographs is not possible and that improved performance in te rms of accuracy can only be achieved when a three-dimensional system i s used. An alternative method for deducing the radiographic penetratio n depth is proposed which, theoretically, negates the error arising fr om the inaccuracy of the formula.