Femoroacetabular Impingement
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Direct answer
A young athletic man's groin pain that deepens with sitting, pivoting and deep squats — often localised with the C-sign, a hand cupped over the hip because patients point to the side, not the groin — characterises femoroacetabular impingement, abnormal contact between the femoral head-neck junction and the acetabular rim during terminal hip motion. Two morphologies produce it: cam impingement, a non-spherical femoral head with an aspherical anterosuperior neck bump (alpha angle above roughly 55 degrees) that shears the chondrolabral junction, typical of young athletic males; and pincer impingement, acetabular over-coverage that crushes the labrum directly, more typical of women — with combined cam-pincer the commonest real pattern. The anterior impingement test (flexion-adduction-internal rotation) provokes it, MRI arthrography shows the labral tear, and arthroscopic femoroplasty with rim trimming and labral repair addresses symptomatic pathology after activity modification and physiotherapy fail.
What you must remember
- Cam morphology: aspherical head-neck junction with diminished offset, quantified by alpha angle above about 55-60 degrees; produces shear delamination of cartilage at the chondrolabral junction in young men.
- Pincer morphology: focal or global acetabular over-coverage — centre-edge angle above about 40 degrees, crossover sign, protrusio — pressing the labrum, which fails first; more common in women.
- Clinical tests: anterior impingement test — flexion, adduction and internal rotation reproduces groin pain; FABER distance supplements; the sitting and deep-squat aggravation history is nearly as diagnostic.
- Imaging pathway: standing pelvic radiograph for crossover sign, centre-edge and Tonnis angles; MRI arthrogram for labral tear and cartilage delamination; CT with 3D reconstruction for offset mapping before femoroplasty.
- Treatment logic: morphology alone is not disease — asymptomatic cam is common; first-line is activity modification and hip-focused physiotherapy; persistent symptomatic impingement with confirmed labral pathology goes to arthroscopy for femoroplasty, rim trimming with labral refixation, and capsular closure.
- Secondary causes to exclude: slipped capital femoral epiphysis sequelae, Perthes deformity, coxa vara and post-traumatic deformity — treat the mechanics, not just the bump.
- Natural history caution: impingement morphology is a risk factor for early osteoarthritis, but prophylactic surgery on asymptomatic hips is not supported — a favourite viva boundary.
A typical case from bike seat to arthroscope
A 26-year-old state-level hockey player has eight months of right groin pain — worst after matches and long drives, improved sitting with the leg dropped open — and he grips his hip with a cupped hand when asked to localise it. Internal rotation at 90 degrees flexion is painful, the anterior impingement test is positive, and FABER shows 6 cm of knee-to-table distance. Radiographs reveal a pistol-grip deformity with a crossover sign; the alpha angle measures 64 degrees, and MRI arthrogram demonstrates an anterosuperior labral tear with a paralabral cyst.
Management proceeds in two phases. He first undergoes a structured 12-week programme — avoiding terminal-range flexion and rotation, strengthening gluteal and deep rotator groups, modifying training — because a proportion of impingement pain is soft-tissue overload rather than structural necessity. Symptoms persist through the season, so he proceeds to hip arthroscopy: femoral osteochondroplasty resecting the cam bump, rim trimming with labral refixation using suture anchors, chondroplasty of the delaminated flap, and capsular plication — followed by staged rehabilitation over 3-4 months before return to pivot sport. Counselling includes realistic expectations: pain relief is the goal, arthritis prevention plausible but unproven.
Where students slip
Two confusions dominate. First, labral tear versus impingement as the diagnosis: a labral tear is the injury, impingement the mechanism, and treating the tear without correcting the bony morphology recurs — the answer that pairs refixation with femoroplasty scores over the one that stops at labral debridement. Second, number-free morphology talk: the exam expects alpha angle thresholds and centre-edge angles as the quantitative backbone, and candidates who cannot attach numbers to the two shapes lose the image-interpretation marks. The C-sign earns its place as the bedside giveaway that pain is truly intra-articular. A final boundary: finding cam morphology on an asymptomatic hip is not an indication for surgery, and differentiating FAI groin pain from the adductor, iliopsoas and sports-hernia cluster is the diagnostic discipline the exam probes.
Frequently asked questions
How do cam and pincer impingement differ mechanically?
Cam is an aspherical head-neck junction that shears the chondrolabral junction in flexion (young men, alpha angle over 55-60 degrees); pincer is acetabular over-coverage that directly crushes the labrum (centre-edge angle over 40 degrees, women) — combined lesions commonest.
What is the anterior impingement test?
Passively flexing the hip to about 90 degrees, adducting and internally rotating it; reproduction of the familiar groin pain indicates anterior impingement, often with labral pathology.
What does the C-sign indicate?
The patient cups a hand over the hip, thumb posterior and fingers anterior — localising true intra-articular hip pain better than pointing, a bedside discriminator from lumbar referred pain.
Which imaging confirms labral pathology?
MRI arthrography — contrast distends the capsule and shows labral tears, paralabral cysts and cartilage delamination; radiographs and CT define the bony alpha angle and coverage first.
Why is prophylactic surgery not done for asymptomatic cam morphology?
Bony morphology is common in asymptomatic hips and osteoarthritis prevention from surgery is unproven; intervention is reserved for symptomatic impingement after conservative failure.