The quest for optimal femoral torsion angle measurements: a comparative advanced 3D study defining the femoral neck axis

Abstract

Purpose There is high variability in femoral torsion, measured on two-dimensional (2D) computed tomography (CT) scans. The aim of this study was to find a reliable three-dimensional (3D) femoral torsion measurement method, assess the influence of CAM deformity on femoral torsion measurement, and to promote awareness for the used measurement method. Methods 3D models of 102 dry femur specimens were divided into a CAM and non-CAM group. Femoral torsion was measured by one 2D-CT method described by Murphy et al. (method 0) and five 3D methods. The 3D methods differed in strategies to define the femoral neck axis. Method 1 is based on an elliptical least-square fit at the middle of the femoral neck. Methods 2 and 3 defined the centre of mass of the entire femoral neck and of the most cylindrical part, respectively. Methods 4 and 5 were based on the intersection of the femoral neck with a 25% and 40% enlarged best fit sphere of the femoral head. Results 3D methods resulted in higher femoral torsion measures than the 2D method; the mean torsion for method 0 was 8.12° ± 7.30°, compared to 9.93° ± 8.24° (p < 0.001), 13.21° ± 8.60° (p < 0.001), 8.21° ± 7.64° (p = 1.00), 9.53° ± 7.87° (p < 0.001) and 10.46° ± 7.83° (p < 0.001) for methods 1 to 5 respectively. In the presence of a CAM, torsion measured with method 4 is consistently smaller than measured with method 5. Conclusion 2D measurement might underestimate true femoral torsion and there is a difference up to 5°. There is a tendency for a higher mean torsion in hips with a CAM deformity. Methods 4 and 5 are the most robust techniques. However, method 4 might underestimate femoral torsion if a CAM deformity is present. Since method 5 is independent of a CAM deformity, it is the preferred technique to define expected values of torsion.

A surface registration‐based approach for assessment of 3D angles in guided growth interventions in the growing femur

“…In the literature, the assessment of femoral torsion using 3D femur models often relies on landmark registration. In such approaches, proximal and distal femoral 3D models are realigned to the mechanical axis, and femoral torsion is evaluated as the intersection between the femoral neck axis and the retrocondylar plane [ 4 , 5 , 8 , 11 ]. However, it has been demonstrated that 3D measurements can be sensitive to how the femoral neck axis is defined [ 8 ].…”

“…In such approaches, proximal and distal femoral 3D models are realigned to the mechanical axis, and femoral torsion is evaluated as the intersection between the femoral neck axis and the retrocondylar plane [ 4 , 5 , 8 , 11 ]. However, it has been demonstrated that 3D measurements can be sensitive to how the femoral neck axis is defined [ 8 ]. In this recent study, van Fraeyenhove et al conducted a study examining femoral torsion through five different 3D analyses based on the selection of the femoral neck axis.…”

“…This approach differs from the evaluation strategies used in studies advocating for 3D femoral analysis. In such studies, the effectiveness of a novel 3D femoral rotation assessor is juxtaposed with conventional 2D analysis [ 4 , 5 , 8 , 12 , 25 ]. For example, in Iwasaka–Neder et al [ 12 ], the reliability of the assessment of femoral torsion through a 3D femur model is compared to Murphy's 2D axial technique [ 22 ].…”

“…In these studies, a landmark‐based method is commonly applied, which involves measuring the femoral torsion as the angle between the femoral neck axis and the retrocondylar plane. Nevertheless, consistent reliability in identifying these landmarks may remain a challenge [ 8 ], also relying solely on a single set of landmark points may not adequately account for rotational changes in both the coronal and sagittal planes as side effects of the intervention [ 7 , 15 , 18 , 24 ].…”