Femoral neck torsion angle measurement by computed tomography.

To establish the accuracy and reproducibility of measuring femoral neck torsion angles (FNTA) on CT images, this angle was measured on 12 dry femora directly on the gross specimens, on plain radiographs, and on CT images. The selection of the CT level, both at the femoral neck and the condyles, that may affect the measurement of FNTA was analyzed. The CT measurements of FNTA were found to have a reproducibility of 2.5 and 0 degrees mean offset error when compared with readings on plain radiographs. A new CT method was developed to measure this angle particularly in patients with coxa valga in whom this measurement is difficult if not impossible to accomplish by current methods. For this purpose composite CT images were made of the femoral head and the femoral shaft, immediately below the lesser trochanter. On these images the line connecting the centers of the femoral head and shaft were inferred to represent the cervical plane. In comparison with the standard CT method, this new method had the same reproducibility, although it slightly overestimated this angle by 2 degrees. This degree of error is negligible when compared with the normal variations of this angle, which ranges from -20 to +38 degrees. Furthermore, it compares favorably with the limited accuracy of derotational osteotomy surgery.