Nötzli and colleagues developed an MRI angle to quantify concavity at the anterior femoral head‑neck junction and tested whether it separates hips with clinical impingement from normal hips.
Case‑control MRI comparison of 39 patients with clinical anterior impingement and 35 asymptomatic controls, University of Zürich.
Methods
Design
Cross‑sectional case‑control MRI study
Data source
Single center, Zürich
Patients
39 symptomatic patients with groin pain, reduced internal rotation and a positive impingement test
35 asymptomatic controls with normal hip examination
Exposure
Clinical anterior impingement vs normal hips
Outcomes
Alpha angle on tilted axial MRI through the head center
Scaled head‑neck width at two sites
Intra- and interobserver reproducibility across four observers
Key Findings
The alpha angle averaged 74.0° in impingement hips vs 42.0° in controls (p < 0.001), with ranges of 55° to 95° vs 33° to 48° that did not overlap.
Scaled anterior neck width was greater in patients at both sites, 0.73 vs 0.61 for T1a/r (p < 0.001) and 0.93 vs 0.85 for T2a/r.
Acetabular anteversion and center‑edge angle on MRI did not differ significantly between patients and controls.
MRI showed acetabular cartilage damage in 85% of symptomatic patients, mostly anterolateral and less often at the posterior rim.
Alpha angle measurement varied by ±3% within observers and ±7% between the four observers.
Within each group, internal rotation correlated only relatively poorly with the alpha angle (r = 0.66 in controls, r = 0.51 in patients).
Board pearl
The alpha angle is read on a tilted axial MRI cut through the femoral head center, and a higher value reflects lost anterior head‑neck concavity.
Authors' conclusion
Using standardised MRI, the symptomatic hips of patients who have impingement have significantly less concavity at the femoral head-neck junction than do normal hips. This test may be of value in patients with loss of internal rotation for which a cause is not found.
Nötzli and colleagues developed an MRI angle to quantify concavity at the anterior femoral head‑neck junction and tested whether it separates hips with clinical impingement from normal hips.
Case‑control MRI comparison of 39 patients with clinical anterior impingement and 35 asymptomatic controls, University of Zürich.
Methods
Design
Cross‑sectional case‑control MRI study
Data source
Single center, Zürich
Patients
39 symptomatic patients with groin pain, reduced internal rotation and a positive impingement test
35 asymptomatic controls with normal hip examination
Exposure
Clinical anterior impingement vs normal hips
Outcomes
Alpha angle on tilted axial MRI through the head center
Scaled head‑neck width at two sites
Intra- and interobserver reproducibility across four observers
Key Findings
The alpha angle averaged 74.0° in impingement hips vs 42.0° in controls (p < 0.001), with ranges of 55° to 95° vs 33° to 48° that did not overlap.
Scaled anterior neck width was greater in patients at both sites, 0.73 vs 0.61 for T1a/r (p < 0.001) and 0.93 vs 0.85 for T2a/r.
Acetabular anteversion and center‑edge angle on MRI did not differ significantly between patients and controls.
MRI showed acetabular cartilage damage in 85% of symptomatic patients, mostly anterolateral and less often at the posterior rim.
Alpha angle measurement varied by ±3% within observers and ±7% between the four observers.
Within each group, internal rotation correlated only relatively poorly with the alpha angle (r = 0.66 in controls, r = 0.51 in patients).
Board pearl
The alpha angle is read on a tilted axial MRI cut through the femoral head center, and a higher value reflects lost anterior head‑neck concavity.
Authors' conclusion
Using standardised MRI, the symptomatic hips of patients who have impingement have significantly less concavity at the femoral head-neck junction than do normal hips. This test may be of value in patients with loss of internal rotation for which a cause is not found.