Objective - To assess the usefulness of standard radiographic projections for determining acetabular cup position and to compare 2 radiographic methods for calculation of the angle of lateral opening of the acetabular cup. Study Design - In vitro radiographic study. Methods - Acetabular components were mounted on a custom-built inclinometer. The effect of varying the angles of lateral opening, inclination, version, and centering of the x-ray beam on the radiographic appearance of the cup was investigated. The angle of lateral opening was calculated trigonometrically after direct measurement and by means of computer-aided image analysis. Results - Using a ventrodorsal radiograph, the computer-aided measurement of angle of lateral opening was accurate to within 5° when the true angle of lateral opening was ≥40°. In the case of direct measurement, the calculated angle was accurate to within 5° only when the true angle was ≥60°. An increasing angle of inclination was associated with reduced accuracy of calculation of the angles of lateral opening and version. Standard radiographic assessment of acetabular version and inclination was not found to be clinically useful. The displacement of the x-ray beam, which would cause significant error in the determination of lateral opening, is unlikely to be exceeded in practice. Cup size had no significant effect on the accuracy of the results. Conclusions - Computer-aided image analysis can be used to calculate angles of lateral opening to within 10° throughout the range of acetabular positions seen in practice and is appropriate for postoperative documentation of cup position. The angles of inclination and version should be interpreted with caution.
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