Femoral Neck Anteversion Measured Using a 3D CT Scan Perpendicular to the Mechanical Axis of the Femur

Abstract

Purpose: We wanted to measure the femoral neck anteversion (FNA) angles using a 3D CT scan that perpendicularly cut the mechanical axis of the femur and to assess the accuracy and reproducibility of different measuring methods. Materials and Methods: We obtained 95 cases of 3D CT images of the cross-section perpendicular to the mechanical axis of the femur. The methods used to measure the FNA angles included a method using the CT image of the area where the femoral neck is confluent to the greater trochanter (method 1), a method using the CT image taken from the neck base immediately prior to the beginning of the area of the lesser trochanter (method 2) and a method by which measurements are made after putting 3D bone models on a horizontal plane in virtual space (method 3). The reference axes of the distal femur we used were the anatomical transepicondylar axis, the surgical transepicondylar axis and the real posterior condylar axis. Results: The FNA angles measured by method 1 were 4.79±6.41。to the anatomical transepicondylar axis (ATEA), 6.09±6.58。to the surgical transepicondylar axis (STEA) and 7.96±6.81。to the real posterior condylar axis (rPCA). The FNA angles measured by method 2 were 16.01±8.31。to the ATEA, 19.52±8.38。to the STEA and 21.79± 8.52。to the rPCA. The FNA angles measured by method 3 were 20.15±12.89。to the rPCA. Conclusion: The measurement of the FNA angle using a 3D CT scan perpendicular to the mechanical axis is reproducible. The measurement method on the neck base level is more reliable than the one on the proximal neck confluence, and more similar to the measurement method by classic definition.