Evaluation of Femoral Neck Anteversion based on Clinical Epicondylar Axis using Three-dimensional Preoperative Planning Software “Athena”

Abstract

Objective: Three-dimensional preoperative planning software has become popular over the past several years, especially for total hip arthroplasty (THA). Although femoral neck anteversion (FNA) has been evaluated based on the posterior condylar axis (PCA), it cannot be used in cases undergoing total knee arthroplasty (TKA) or those with marked degenerative changes in the posterior condyles. As a result, the determination of FNA is difficult using software. In such cases, the clinical epicondylar axis (CEA) can be used. In this study, we measured FNA using the CEA.
Methods: Computed tomography (CT) scans were obtained from 79 patients with osteoarthritis (64 women; 120 hips without degenerative changes) who were candidates for TKA. CT data of the femur was used to measure FNA by the three-dimensional preoperative software called Athena® (Soft Cube, Osaka, Japan). The mean condylar twist angle (CTA) was 5.8±1.7°. The longitudinal dimension was defined as the medial and lateral epicondylar prominence and the centroid of the transverse slices taken just below the lesser trochanter. The proximal femoral shaft axis was defined as the line connecting the centroids of the transverse slices taken just below the lesser trochanter to the proximal third of the femur. The position of the femoral head center was defined as the center of the sphere of best fit to the head surface. The femoral neck axis was defined as the line connecting the femoral head center and the point at which the axis of the proximal femoral shaft passed over the transverse slice, just one centimeter above the lesser trochanter. The neck axis, PCA and CEA were projected onto a field perpendicular to the proximal femoral axis, and FNA was measured as the angle of the neck axis, and the PCA and CEA as the pFNA and cFNA, respectively.
Results: The pFNA and cFNA values were 27.3±9.0° (8.9-60.6°) and 21.4±8.8° (1.9-52.4°), respectively. The difference between pFNA and cFNA, which was approximately 6°, was believed to be due to the CTA. pFNA was highly correlated with cFNA (r = 0.979, p < 0.01).
Conclusions: The FNA value, based on the CEA, measured by three-dimensional preoperative software was approximately 21°. CEA is useful for measuring FNA, and cFNA is helpful in determining the stem anteversion in THA.