This 2007 review by Tannast et al. defines the radiographic criteria for diagnosing cam and pincer FAI on standard AP pelvis and cross-table axial radiographs. It establishes key measurement thresholds, describes the pathomechanics linking morphology to cartilage damage, and identifies critical imaging pitfalls from pelvic malpositioning.
A young patient with groin pain, limited flexion/internal rotation, and a "normal" hip X-ray is the classic FAI presentation — because early FAI shows no joint space narrowing, sclerosis, or osteophytes. Standard hip-centered AP films make this worse: centering over the hip rather than the pelvis overcalls acetabular version and can fabricate or hide a cross-over sign.
Get a true AP pelvis (legs 15° IR, beam between ASIS and symphysis, 1.2 m focus distance) and a cross-table axial. On the axial, measure the alpha angle (cam if >50°) and anterior offset (cam if <10 mm). On the AP, trace both acetabular rims for the cross-over sign and measure the lateral center edge angle (overcoverage if >39°).
Before calling a cross-over sign positive, check pelvic tilt: symphysis-to-sacrococcyx should be within 3.2 cm in men and 4.7 cm in women. Increased tilt or ipsilateral rotation generates a false-positive retroversion sign.
If you spot a herniation pit on the anterosuperior femoral neck, do not reassure the patient. It occurs in 33% of FAI patients and marks a joint at risk. Surgical correction is recommended at first symptom onset. Prognosis is significantly better before cartilage damage progresses.
This 2007 review by Tannast et al. defines the radiographic criteria for diagnosing cam and pincer FAI on standard AP pelvis and cross-table axial radiographs. It establishes key measurement thresholds, describes the pathomechanics linking morphology to cartilage damage, and identifies critical imaging pitfalls from pelvic malpositioning.
A young patient with groin pain, limited flexion/internal rotation, and a "normal" hip X-ray is the classic FAI presentation — because early FAI shows no joint space narrowing, sclerosis, or osteophytes. Standard hip-centered AP films make this worse: centering over the hip rather than the pelvis overcalls acetabular version and can fabricate or hide a cross-over sign.
Get a true AP pelvis (legs 15° IR, beam between ASIS and symphysis, 1.2 m focus distance) and a cross-table axial. On the axial, measure the alpha angle (cam if >50°) and anterior offset (cam if <10 mm). On the AP, trace both acetabular rims for the cross-over sign and measure the lateral center edge angle (overcoverage if >39°).
Before calling a cross-over sign positive, check pelvic tilt: symphysis-to-sacrococcyx should be within 3.2 cm in men and 4.7 cm in women. Increased tilt or ipsilateral rotation generates a false-positive retroversion sign.
If you spot a herniation pit on the anterosuperior femoral neck, do not reassure the patient. It occurs in 33% of FAI patients and marks a joint at risk. Surgical correction is recommended at first symptom onset. Prognosis is significantly better before cartilage damage progresses.