ABSTRACT
BACKGROUND: It has been shown that cam deformities are located at a more anterosuperior location than was previously described. PURPOSE: 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). STUDY DESIGN: Cross-sectional study; Level of evidence, 3. METHODS: 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. RESULTS: 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). CONCLUSION: 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.