Alpha Angle Measurements in Healthy Adult Volunteers Vary Depending on the MRI Plane Acquisition Used.

Abstract

It has been shown that cam deformities are located at a more anterosuperior location than was previously described. To establish, in a large group of asymptomatic participants, the normative range of the alpha angle in the anterosuperior location in both the oblique axial and radial views of magnetic resonance imaging (MRI). Cross-sectional study; Level of evidence, 3. In 197 asymptomatic participants (394 asymptomatic hips) with a mean age of 29.4 years (range, 21.4-50.6 years), T1-weighted MRI scans were studied. The anterosuperior alpha angle measurement was performed by 2 observers using a previously described methodology and also using the radial view. The intraclass correlation coefficient (ICC) was determined for interobserver and intraobserver reliability. Descriptive statistics, the Student t test, correlation studies, and the Bland-Altman technique were used for data analysis. The ICC for interobserver and intraobserver reproducibility was 0.74 (good agreement) and 0.84 (very good agreement), respectively. Anterosuperiorly, the mean (±SD) alpha angles in the oblique axial and radial views were 45.11° ± 8.52° and 50.30° ± 7.91°, respectively ( P < .0001). The upper limits of the 95% reference interval for the oblique axial and radial views were 63° and 66°, respectively. In the oblique axial view, the mean (±SD) alpha angle for male participants was 48.3° ± 7.5° compared with 42.6° ± 6.2° for female participants ( P < .0001), and in the radial view, it was 53.0° ± 7.1° compared with 48.1° ± 5.6°, respectively ( P < .0001). Linear regression analysis demonstrated an insignificant relationship between age and alpha angle, regardless of the imaging plane ( r² = 0.06). We suggest using a higher threshold of 63° (in the oblique axial view) and 66° (in the radial view) at the 1:30 clockface position for the diagnosis of a cam-type deformity. This is significantly higher than 50° to 55° at the 3-o'clock position traditionally used based on the oblique axial view that has been initially described.