A COMPARISON OF ALTERNATIVE METHODS OF MEASURING FEMORAL ANTEVERSION

Purpose: Although CT scans are widely believed to provide the most acc urate measurements of femoral anteversion, any estimate of the antever sion of the femur depends on the accuracy of the calculated axis of th e femoral neck. We devised a method to measure the anteversion of the femur precisely using a 3D femoral computer model reconstructed from d igitized 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 classi c 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 superimpo sed 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 a ccuracy of the single slice method was also examined using slices take n at four different neck slice levels within the proximal femur. CT sc ans of 30 femora were obtained using a helical CT scanner and reconstr ucted using custom software. Results: Based on the 3D model, the true anteversion of the femora averaged 19.8 +/- 9.3 degrees (SD), Using th e method of Weiner et al,, the anteversion of the femora was underesti mated by an average of 6.4 degrees (predicted value 13.4 +/- 10.4 degr ees). Conversely, Murphy et al.'s method overestimated anteversion by an average of 6.3 degrees with an average value of 26.0 +/- 9.1 degree s. The difference between the true anteversion and the values predicte d by both of these methods was statistically significant (p < 0.001). The average anteversion measured according to the method of Reikeras e t al. was 17.8 +/- 8.9 degrees, 2.0 degrees less than the true antever sion of the sample (p < 0.05). Anteversion angles predicted from a sli ce just below the inferior edge of the head averaged 18.3 +/- 9.5 degr ees, 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 accu racy 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 diff iculty is encountered in positioning the patient, 3D reconstruction ap pears essential for accurate measurement of anteversion.