Where is the neck? Alpha angle measurement revisited

Abstract

Background and purpose: The alpha angle is the most used measurement to classify concavity of the femoral head-neck junction. It is not only used for treatment decisions for hip impingement, but also in cohort studies relating hip morphology and osteoarthritis. Alpha angle measurement requires identification of the femoral neck axis, the definition of which may vary between studies. The original "3-point method" uses 1 single point to construct the femoral neck axis, whereas the "anatomic method" uses multiple points and attempts to define the true anatomic neck axis. Depending on the method used, the alpha angle may or may not account for other morphological characteristics such as head-neck offset.

Methods: We compared 2 methods of alpha angle measurement (termed "anatomic" and "3-point") in 59 cadaver femora and 83 cross-table lateral radiographs of asymptomatic subjects. Results were compared using Bland-Altman plots.

Results: Discrepancies of up to 13 degrees were seen between the methods. The 3-point method had an "equalizing effect" by disregarding femoral head position relative to the neck: in femora with high alpha angle, it resulted in lower values than anatomic measurement, and vice versa in femora with low alpha angles. Using the anatomic method, we derived a reference interval for the alpha angle in normal hips in the general population of 30-66 degrees.

Interpretation: We recommend the anatomic method because it also reflects the position of the femoral head on the neck. Consensus and standardization of technique of alpha angle measurement is warranted, not only for planar measurements but also for CT or MRI-based measurements.

Figure 1.

3-point and anatomic method compared in high alpha angle (A) and low alpha angle (B). 3-point method (A.1 and B.1) uses the midpoint of the femoral neck at its narrowest point. The anatomic method (B.2 and B.2) defines the femoral neck axis by connecting the centers of 3 circles projected over the neck contour. The axis is translated to the center of the femoral head if necessary, to measure the alpha angle. In this example, alpha angle A.1 = 64˚, A.2 = 73˚. Angle B.1 and B.2 are both 30˚, while the femoral head is positioned central on the femoral neck.

Figure 2.

Bland-Altman plot of the measurements of cadaver photographs. The x-axis depicts the average of 2 measurements for each cadaver femur. The y-axis depicts their difference (anatomic minus 3-point).

Figure 3.

Bland-Altman plot of the measurements of radiographs. The x-axis depicts the average of 2 measurements for each cadaver femur. The y-axis depicts their difference (anatomic minus 3-point).