| Abstract: | The validity of wear measurements from total-hip radiographs was assessed using a wear simulator which consisted of a total hip prosthesis mounted on a Plexiglas orientation jig. Wear was mesured by a micrometer device and uni- and duoradiographic methods of wear measurement were used with a variety of acetabular cup orientations. In the initial calibration studies the duoradiographic technique proved to be more accurate. The source of the inaccuracies of the uniradiographic method was determined analytically by means of a geometrical model. Forty-inch or 72' source-to-object film-distances provided similar accuracy and the errors in wear measurement varied from --0.1 +/- 0.4 mm (2 S.D.) to 0 +/- 0.9 mm for wear magnitudes of 1 and 5 mm respectively, regardless of cup position. In subsequent calibration studies the previous radiographs were compiled to represent seven hypothetical patients at various stages of follow-up whose prostheses exhibited between 1 and 5 mm wear. Clinicians assessed the degree of wear, using both the uni- and duoradiographic techniques, under supervision but unaware of the true position and orientation of the wear track. The resulting errors were of the same order as the wear magnitudes being measured. The location of the measurements on the radiographs was then restricted to approximately the true wear axis but again the errors were so large that it was still impossible to make any valid wear assessments. The calibration runs were therefore terminated at this point and it was concluded that wear measurements could not be made from clinical radiographs. |
