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PURPOSE Although CT scans are widely believed to provide the most accurate measurements of femoral anteversion, any estimate of the anteversion of the femur depends on the accuracy of the calculated axis of the femoral neck. We devised a method to measure the anteversion of the femur precisely using a 3D femoral computer model reconstructed from digitized femoral contours. Using this method, we compared the accuracy of three popular methods of anteversion measurement based on CT scans. METHOD The three popular CT methods were as follows: (a) the classic method of Weiner et al., based on a single CT image; (b) the method of Reikeras et al., in which the neck axis is defined by two superimposed images of the femoral head and neck; and (c) the method of Murphy et al., utilizing centroids of the head and the medullary canal. The accuracy of the single slice method was also examined using slices taken at four different neck slice levels within the proximal femur. CT scans of 30 femora were obtained using a helical CT scanner and reconstructed using custom software. RESULTS Based on the 3D model, the true anteversion of the femora averaged 19.8 +/- 9.3 degrees (SD). Using the method of Weiner et al., the anteversion of the femora was underestimated by an average of 6.4 degrees (predicted value 13.4 +/- 10.4 degrees). Conversely, Murphy et al.'s method overestimated anteversion by an average of 6.3 degrees with an average value of 26.0 +/- 9.1 degrees. The difference between the true anteversion and the values predicted by both of these methods was statistically significant (p < 0.001). The average anteversion measured according to the method of Reikeras et al. was 17.8 +/- 8.9 degrees, 2.0 degrees less than the true anteversion of the sample (p < 0.005). Anteversion angles predicted from a slice just below the inferior edge of the head averaged 18.3 +/- 9.5 degrees, only 1.5-3.1 degrees less than the true anteversion of the femur (p = 0.14). CONCLUSION The single slice CT method has sufficient accuracy for use, provided the slice is taken just below the femoral head. In cases with a femoral head deformity or a valgus neck or where difficulty is encountered in positioning the patient, 3D reconstruction appears essential for accurate measurement of anteversion.