Validation of a new 2-D technique for radiographic wear measurement of cemented, highly cross-linked polyethylene acetabular cups.

A new 2-D radiographic wear measurement system has been developed which enables the low wear of highly cross-linked polyethylene acetabular cups to be accurately and precisely measured from standard, pelvis radiographs. The software was validated using radiographic images of a measurement jig which could vary the cup orientation and simulate the effect of pelvic tilt/rotation. Wear was simulated using accurately measured plastic shims to vary the position of the femoral head relative to the cup. The effects of varying "wear" penetration, "wear" direction, cup orientation and X-ray focus position were assessed. Further direct comparison tests were also carried out using radiostereometric analysis. Inter/intra-observer repeatability of the new system was assessed using clinical radiographs. The mean (SD) "wear" penetration error was -0.002mm (0.028mm). The "wear" penetration precision was 0.055mm. Changing the position of the X-ray focus point made no difference to the measurement error. Inter/intra-observer repeatability and limits of agreement had similar low values. Comparison tests with RSA showed the accuracy was similar.