The Modified Femoral Neck-Shaft Angle: Age- and Sex-Dependent Reference Values and Reliability Analysis

Christoph Kolja Boese, Michael Frink, Janine Jostmeier, Stefan Haneder, Jens Dargel, Peer Eysel, Philipp Lechler, Christoph Kolja Boese, Michael Frink, Janine Jostmeier, Stefan Haneder, Jens Dargel, Peer Eysel, Philipp Lechler

Abstract

Background. The femoral neck-shaft angle (NSA) is of high importance for the diagnostics and treatment of various conditions of the hip. However, rotational effects limit its precision and applicability using plain radiographs. This study introduces a novel method to measure the femoral NSA: the modified NSA (mNSA), possibly being less susceptible against rotational effects compared to the conventional NSA. Patients and Methods. The method of measurement is described and its applicability was tested in 400 pelvis computed tomography scans (800 hips). Age- and gender-dependent reference values are given and intra- and interrater reliability are analyzed. Results. The mean age of all 400 patients (800 hips) was 54.32 years (18-100, SD 22.05 years). The mean mNSA was 147.0° and the 95% confidence interval was 146.7°-147.4°. Differences of the mNSA between sexes, age groups, and sides were nonsignificant. The absolute difference between NSA and mNSA was 16.3° (range 3-31°; SD 4.4°); the correlation was high (0.738; p < 0.001). Overall, the intra- and interrater reliability were excellent for the mNSA. Interpretation. We introduced a novel concept for the analysis of the neck-shaft angle. The high reliability of the measurement has been proven and its robustness to hip rotation was demonstrated.

Conflict of interest statement

The authors declare that they have no conflict of interests.

Figures

Figure 1
Figure 1
Effect of hip rotation on the projected neck-shaft angle (NSA). The axial view of the femur in a CT reconstruction of a right hip is shown (a). Three coronal reconstructions are projected. Internal hip rotation of 0°, 30°, and 60° is demonstrated as shown in the axial view. The NSA is included and the overestimation of the projected NSA due to the rotation is visualized.
Figure 2
Figure 2
Schematic drawing of the measurement of the NSA (a) and mNSA (b). The NSA requires the definition of the centre of rotation and the waist of the femoral neck to define the femoral neck axis (FNA). Two circles in the femur are drawn to define the long femoral axis (FLA). The NSA is the angle between the FLA and FNA (a). The mNSA requires no FNA. A perpendicular to the FLA is drawn, cutting the apex of the minor trochanter. The cutting point of both axes is connected to the centre of rotation; the angle between this axis and the FLA is the mNSA.
Figure 3
Figure 3
Box-Plot of the distribution of mNSA in 800 hips measured in three different planes: the scout-view, the anterior pelvic plane (APP), and the femoral neck plane (FNP). Asterisk (∗) is an extreme outlier. Circles are outliers of 1.5 times the IQR (interquartile range) and extreme outliers are beyond 3 times IQR.
Figure 4
Figure 4
Histogram of the distribution of the mNSA in 800 hips in the femoral neck plane (FNP).
Figure 5
Figure 5
Scatterplot of the differences between the NSA and mNSA in 800 hips in the femoral neck plane (FNP).
Figure 6
Figure 6
Scatterplot of the NSA and mNSA in 800 hips in the femoral neck plane (FNP).

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