Normal Hip Alpha Angle Size on MRI: Reference Guide
The alpha angle is a quantitative measure of the roundness of the femoral head-neck junction, assessed on MRI (or radiographs) in the oblique axial plane. It reflects the degree to which the femoral head extends beyond a perfect sphere at the anterosuperior neck. Accurate measurement is clinically important because an elevated alpha angle is the hallmark of cam-type femoroacetabular impingement (FAI), a leading cause of hip pain and early osteoarthritis in active adults.
Normal Reference Values
| Measurement |
|---|
| 55¡ |
Clinical Significance
An alpha angle of 55° or greater is considered the threshold for cam morphology associated with a significantly increased risk of anterior femoroacetabular impingement. As the alpha angle rises, the aspherical portion of the femoral head abuts the acetabular rim during hip flexion and internal rotation, causing labral tears, chondral damage, and progressive joint degeneration.
It is important to recognize that mild elevation (55–60°) may be an incidental finding in asymptomatic individuals, particularly athletic males. Clinical correlation with symptoms, physical examination (positive FADIR test), and labral pathology on MRI is essential before attributing pain solely to an elevated alpha angle. Bilateral assessment is recommended, as cam morphology is frequently bilateral.
- Cam-type FAI — primary pathology when alpha angle ≥55°
- Slipped capital femoral epiphysis (SCFE) sequelae — results in pronounced cam deformity
- Legg-Calvé-Perthes disease sequelae — irregular head-neck junction
- Post-traumatic deformity — femoral neck fracture malunion
- Normal athletic variant — borderline values in high-level athletes without symptoms
Reference: Ntzli HP, Wyss TF, Stoecklin CH et al. The contour of the femoral head-neck junction as a predictor for the risk of anterior impingement. J Bone Joint Surg Br. 2002;84(4):556-60.
Imaging Notes
On MRI, the alpha angle is measured on the oblique axial (radial) sequence centered along the femoral neck axis, with the image plane passing through the center of the femoral head and the midpoint of the femoral neck. A circle is fitted to the femoral head; one line is drawn from the center of the femoral head to the point where the head-neck contour exits the circle (the cam lesion), and a second line runs along the femoral neck axis. The angle between these two lines is the alpha angle.
Radial MRI sequences, which acquire images at multiple clock-face positions around the femoral neck, improve detection of cam morphology compared to a single plane, as the deformity is most pronounced anterosuperiorly (12–1 o'clock position). Adequate field-of-view, high-resolution sequences (≤3 mm slice thickness), and a consistent measurement technique are essential to minimize inter-reader variability.