Cam (a bump at the head–neck junction), pincer (acetabular overcoverage) or both: the radiograph measures the angles, MR arthrography shows the labral tear and the cartilage delamination that decide arthroscopy versus arthroplasty.
Orient first
- Cam: alpha angle raised (> 55–60° — verify); pincer: lateral centre-edge angle > 40°, crossover sign, posterior wall sign.
- Dysplasia (LCEA < 20–25°) must not be missed — it changes the operation.
- Osteoarthritis (Tönnis ≥ 2) predicts poor arthroscopy outcome.
Acquire the study
- Standardised AP pelvis (coccyx 1–3 cm above the symphysis) and a Dunn 45° view.
The manoeuvre
- AP radiograph: lateral centre-edge angle in degrees; Tönnis angle.
- Crossover sign and ischial spine sign on the AP radiograph (retroversion).
- Dunn view: alpha angle in degrees; head–neck offset.
- Joint space width in mm; Tönnis grade.
What confirms it
- Cam or pincer morphology with a labral tear and a concordant clinical impingement test.
What licenses you to exclude it
- Normal angles without a labral tear make FAI an unlikely cause of pain.
The classic misread
- Calling a crossover sign on a tilted pelvis radiograph.