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

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 degrees and the 95% confidence interval was 146.7 degrees-147.4 degrees. Differences of the mNSA between sexes, age groups, and sides were nonsignificant. The absolute difference between NSA and mNSA was 16.3 degrees (range 3-31 degrees; SD 4.4 degrees); 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.