Alpha angle (Notzli)

The contour of the femoral head-neck junction as a predictor for the risk of anterior impingement.

Nötzli HP, Wyss TF, Stoecklin CH, et al. The Journal of bone and joint surgery. British volume 2002 · PMID 12043778 · doi:10.1302/0301-620x.84b4.12014

Question

Can a measurable angle on MRI quantify loss of femoral head-neck offset in patients with anterior impingement?

Design

Comparative imaging study using oblique axial MRI.

Patients

Patients with hip pain and clinical anterior impingement compared with asymptomatic controls.

Intervention

Measurement of the alpha angle on MRI

Comparison

Asymptomatic control hips

Primary outcome

Alpha angle

Key result

Symptomatic patients had significantly larger alpha angles than controls, establishing the alpha angle as a measure of cam morphology.

Limitations

Small numbers and single centre; thresholds for an abnormal alpha angle vary between later studies and imaging planes.

Impact on practice

The alpha angle is the standard measure of cam morphology on radiographs and MRI and is routinely used in assessing young adult hip pain.