Overview & Definition
Femoroacetabular impingement (FAI) is a motion-related clinical disorder of the hip resulting from premature or repetitive contact between the proximal femur and the acetabular rim during terminal ranges of hip motion. It is among the most common causes of hip pain in young active adults and is now recognised as a leading cause of hip osteoarthritis, particularly in the non-dysplastic hip. FAI was formally described and classified by Ganz and colleagues in 2003, transforming understanding of hip mechanics and triggering a revolution in hip preservation surgery. The condition produces characteristic patterns of labral tearing and cartilage damage at the anterosuperior acetabulum — the primary zone of impingement contact — and if untreated leads to progressive chondral loss and secondary osteoarthritis.
- Three morphological types: (1) Cam impingement — an aspherical, `pistol-grip` deformity of the femoral head-neck junction; the cam lesion is a bony prominence on the anterolateral femoral head-neck junction that enters the acetabulum during flexion and internal rotation, generating a shear force that levers against the anterosuperior acetabular cartilage and labrum; predominantly affects young athletic males; (2) Pincer impingement — acetabular over-coverage (too much acetabulum); the acetabular rim is excessively deep or anteverted, causing circumferential rim abutment against the femoral neck during motion; more common in middle-aged active females; (3) Mixed — the most common clinical presentation (~86% of symptomatic FAI cases); elements of both cam and pincer morphology coexist
- Epidemiology: cam morphology is present in approximately 10–25% of the general population but is symptomatic in a minority; prevalence is highest in high-level athletes (up to 55% of elite footballers); the cam deformity is thought to develop during the adolescent growth spurt from repetitive loading of the proximal femoral physis before closure; the condition is therefore considered a developmental deformity rather than a congenital one; pincer morphology is present in approximately 20% of the population
Morphological Classification & Measurement
| Type | Morphology | Key Measurement | Radiological Threshold | Predominant Damage Pattern |
|---|---|---|---|---|
| Cam | Aspherical femoral head-neck junction; `pistol-grip` deformity; the cam bump is typically located on the anterosuperior femoral head-neck junction; during hip flexion, the cam lesion rotates into the acetabulum and generates compressive and shear forces on the anterosuperior acetabular cartilage | Alpha angle — the angle between the femoral neck axis and the line from the femoral head centre to the point where the femoral head-neck junction deviates from a perfect circle (measured on the Dunn lateral or MRI radial sequences at the 1-o`clock position); also: head-neck offset ratio (Eijer); asphericity index | Alpha angle >55° on Dunn lateral view = cam morphology (Notzli 2002); some sources use >60° as the diagnostic threshold; the 1-o`clock position on radial MRI is the most sensitive location | Outside-in cartilage damage at the anterosuperior acetabulum (the cartilage peels away from the subchondral bone in a `carpet-avulsion` pattern); labral tearing at the anterosuperior quadrant |
| Pincer | Excessive acetabular coverage; the acetabular rim protrudes too far and contacts the femoral neck directly; subtypes: (1) global over-coverage — coxa profunda (acetabular fossa touching the ilioischial line on AP pelvis), protrusio acetabuli (femoral head medial to ilioischial line); (2) focal anterior over-coverage — acetabular retroversion (`crossover sign`, `posterior wall sign`, `ischial spine sign`) | Lateral centre-edge angle (LCEA — normal 25–40°; >40° = over-coverage); anterior centre-edge angle (ACEA); acetabular index (Tönnis angle — normal 0–10°; <0° = over-coverage); crossover sign (the anterior acetabular wall crosses medial to the posterior wall on AP pelvis view = acetabular retroversion) | LCEA >40° = global over-coverage; positive crossover sign = focal anterior over-coverage (retroversion); `figure-of-8` configuration on AP pelvis (the anterior and posterior walls of the acetabulum cross and then diverge — the anterosuperior acetabulum is retroverted) | Inside-out labral damage — the labrum is crushed between the rim and the femoral neck; it may undergo degenerative ossification (os acetabuli) and thus worsen the over-coverage; paradoxically, the posterior cartilage may be damaged by counter-coup mechanism as the femoral head levers posteriorly |
| Mixed | Elements of both cam and pincer morphology; this is the most common pattern in clinical practice (~86%); the combined impingement produces more complex patterns of labral and chondral damage | Both alpha angle and LCEA/crossover sign are abnormal | Both thresholds met | Combined patterns of inside-out and outside-in damage |
Tönnis Classification — Hip Osteoarthritis Grading
The Tönnis classification grades the degree of hip osteoarthritis on plain radiography and is critical for determining surgical eligibility for hip preservation (arthroscopy/osteoplasty) vs arthroplasty. Advanced osteoarthritis is a contraindication to hip preservation surgery.
| Grade | Radiological Features | Surgical Implication |
|---|---|---|
| Grade 0 — Normal | No signs of arthritis; normal joint space; no subchondral changes; no osteophytes | Ideal for hip preservation surgery; best outcomes expected |
| Grade 1 — Mild | Increased sclerosis of the femoral head and acetabulum; slight joint space narrowing; small osteophytes; no cystic change | Hip preservation surgery still appropriate; good outcomes with concurrent chondral treatment |
| Grade 2 — Moderate | Small cysts in the femoral head or acetabulum; moderate joint space narrowing; moderate osteophytes; loss of head sphericity | Borderline — outcomes of hip preservation are less predictable; individualise; some centres proceed with arthroscopy; careful patient counselling required |
| Grade 3 — Severe | Large cysts; severe joint space narrowing or obliteration; large osteophytes; deformity of the femoral head; necrosis possible | CONTRAINDICATION to hip preservation surgery; total hip arthroplasty is the appropriate definitive treatment |
Clinical Assessment
- Symptoms: groin pain is the cardinal symptom (reported by ~90% of patients); the pain is typically deep, activity-related, and worsened by prolonged sitting, getting into/out of a car, and high-flexion activities; patients describe `C-sign` — they cup the lateral hip and groin with the thumb and index finger in a C-shape when asked to point to the pain; lateral hip pain (trochanteric bursitis pattern) is less common; locking, clicking, or giving way may indicate labral tear; reduced range of motion — particularly hip flexion, internal rotation, and adduction
- FADIR test (Flexion-Adduction-Internal Rotation) — the most sensitive clinical test for FAI: the hip is passively flexed to 90°, adducted to the midline, and internally rotated; a positive test (reproduction of the patient`s groin pain) has sensitivity ~88%, specificity ~43%; it is the best screening test for intra-articular hip pathology including FAI and labral tears; a negative FADIR essentially rules out significant FAI
- FABER test (Flexion-Abduction-External Rotation — `Patrick`s test`): the heel of the affected leg is placed on the opposite knee in a figure-of-4 position; positive = groin pain or limited range compared to the contralateral side; more specific for posterior hip and sacroiliac joint pathology but also positive in FAI
- Posterior impingement test: the hip is extended and externally rotated (the position of posterior impingement); positive = posterior hip pain; tests for posterior FAI or posterior labral pathology; distinguish from the FADIR (anterior impingement) — different anatomical zones are tested
- Hip range of motion: document flexion (normal 120°), internal rotation in flexion (normal 30–40°; reduced <15° is significant in FAI), external rotation (normal 40–60°), abduction, adduction; compare bilaterally; loss of internal rotation in flexion is the most consistent finding in cam FAI
Investigations
- Plain radiographs: the essential first investigation; obtain: (1) AP pelvis (standing — to assess the pelvis in the functional position; measure LCEA, Tönnis angle, crossover sign, coxa profunda, protrusio, head-neck offset); (2) Dunn lateral 45° or 90° view — the optimal view for alpha angle measurement; the Dunn 45° lateral is now the standard (the 90° Dunn has more inter-observer variability); (3) Cross-table lateral — useful for assessing femoral anteversion and head-neck offset; the AP pelvis and Dunn lateral together provide the complete morphological assessment for FAI classification
- MRI and MR arthrography (MRA): MRI is the investigation of choice for soft tissue assessment (labrum, cartilage, ligamentum teres); standard MRI sequences include coronal T2, axial oblique, and radial sequences through the femoral head-neck junction; MR arthrography (intra-articular gadolinium injection) improves labral tear sensitivity from ~65% to ~90% and is the gold standard for labral assessment; MRI also assesses: (1) the alpha angle on radial sequences (the 1-o`clock position is most sensitive for cam); (2) labral morphology (anterior, superior, posterior labrum); (3) chondral thickness and signal; (4) subchondral bone marrow oedema; (5) associated pathology (ligamentum teres tears, iliopsoas pathology)
- Diagnostic intra-articular injection: an intra-articular LA injection (under ultrasound or fluoroscopic guidance) is an invaluable adjunct — if symptoms are significantly relieved by intra-articular LA, the pain is confirmed to be of intra-articular origin (FAI, labral tear); if no relief → consider extra-articular sources (lumbar spine referral, trochanteric bursitis, iliopsoas tendinopathy, athletic pubalgia); the injection also helps confirm candidacy for surgical intervention
Management
| Option | Indication | Detail |
|---|---|---|
| Conservative management | All patients initially (3–6 months minimum); Tönnis 0–1; mild symptoms; no labral tear on MRI | Activity modification (avoid deep flexion, pivoting, impact); physiotherapy targeting hip abductor and core strengthening, hip flexor stretching, movement pattern retraining; NSAIDs; intra-articular corticosteroid injection (short-term symptom relief); the FORCe RCT (UK, 2022) compared physiotherapy to arthroscopy for FAI and found that at 8 months there was no significant difference — however at 12 months and beyond, the arthroscopy group had superior outcomes, suggesting that conservatively managed FAI frequently fails long-term in symptomatic patients |
| Hip arthroscopy — cam resection (femoral osteoplasty) | Symptomatic cam FAI; failed 3–6 months conservative; Tönnis 0–2; alpha angle >55°; no advanced OA | The cam bump is resected under arthroscopic visualisation using a burr to restore a spherical femoral head-neck junction; the goal is to restore the alpha angle to <55° (ideally <50°); fluoroscopic confirmation of adequate resection; residual cam (under-resection) is the most common cause of failure; over-resection risks femoral neck fracture (risk is very low if <30% of the femoral neck diameter is removed); concurrent labral repair is performed if a labral tear is present |
| Hip arthroscopy — acetabuloplasty (rim trimming) | Symptomatic pincer FAI with focal anterior over-coverage; crossover sign; Tönnis 0–2 | The anterosuperior acetabular rim is trimmed (acetabuloplasty) to reduce over-coverage; care must be taken to avoid over-resection (which causes iatrogenic instability — global pincer coverage should generally not be reduced by rim trimming); labral repair or reconstruction is performed concurrently; for global coverage (coxa profunda, protrusio), periacetabular osteotomy (PAO) is preferred to rim trimming (trimming alone will not adequately address global over-coverage) |
| Labral repair vs reconstruction | Labral tear present (MRI confirmed); repair preferred for intact labral tissue; reconstruction for irreparable tears | Labral repair: the torn labrum is reattached to the acetabular rim using suture anchors (placed through portals); superior outcomes compared to labral debridement (Larson 2012); the labrum provides the seal of the joint, maintains intra-articular fluid pressure, and contributes to hip stability; labral reconstruction: for irreparable or previously debrided labrums, a graft (iliotibial band, ligamentum teres, gracilis tendon, capsular graft) is used to reconstruct the labrum |
| Periacetabular osteotomy (PAO — Ganz/Bernese) | Hip dysplasia (LCEA <20°, Tönnis angle >15°); global acetabular under-coverage; young patient | A different operation from FAI surgery — PAO reorients the acetabulum to improve coverage in dysplasia; NOT a treatment for pure FAI (though FAI can co-exist with dysplasia); the acetabulum is osteotomised in four cuts and rotated to provide better coverage of the femoral head; typically combined with hip arthroscopy (labral repair); requires careful pre-operative planning with LCE angle measurement |
Complications of Hip Arthroscopy for FAI
- Traction-related nerve injuries: the most common complication of hip arthroscopy (up to 10% of cases); the hip is placed in traction during the central compartment phase of arthroscopy (to distract the joint and allow instrumentation); the perineal post compresses the pudendal nerve (perineal numbness, sexual dysfunction) and excessive traction time compresses the sciatic or femoral nerves; minimise traction time (<2 hours); use a well-padded perineal post; use intermittent traction; most traction neuropraxias resolve within weeks
- Iatrogenic chondral or labral damage: during portal placement and instrumentation, the acetabular cartilage and labrum are at risk from the portals and instruments; careful portal placement and use of cannulas minimises this risk; the anterolateral portal is the primary working portal (placed under fluoroscopic guidance at the anterosuperior acetabular rim)
- Femoral neck fracture: a rare but serious complication of cam resection; over-resection of the femoral neck (>30% of the neck width) weakens the femoral neck and predisposes to insufficiency fracture; restrict weight-bearing for 4–6 weeks post-operatively; fluoroscopic monitoring of resection depth during surgery
- Avascular necrosis of the femoral head: extremely rare; risk increases if the lateral epiphyseal vessels (on the anterior femoral head-neck junction) are damaged during cam resection; the capsule and vessels must be protected during arthroscopy
- Iatrogenic instability: over-resection of the acetabular rim (acetabuloplasty) or incomplete capsular closure can produce post-operative hip instability or micro-instability; close the capsulotomy at the end of arthroscopy
Exam Pearls
- FAI types: cam (aspherical head-neck junction, alpha angle >55°, young athletic males, outside-in anterosuperior cartilage damage); pincer (over-coverage, LCEA >40° or crossover sign, middle-aged females, inside-out labral crushing); mixed (most common, ~86%)
- Alpha angle: measured on Dunn 45° lateral (or MRI radial 1-o`clock); >55° = cam morphology; normal <55°; the Dunn lateral is the standard radiographic view for alpha angle
- FADIR test: most sensitive test for FAI (sensitivity ~88%); hip flexion 90° + adduction + internal rotation = reproduction of groin pain; a negative FADIR largely excludes significant FAI
- Tönnis grading: 0 (normal — best outcomes with arthroscopy); 1 (mild — good outcomes); 2 (moderate — borderline); 3 (severe — contraindication to hip preservation, proceed to THA)
- Crossover sign (acetabular retroversion): on AP pelvis X-ray, the anterior wall projects medial to the posterior wall in the superior acetabulum (they `cross`); indicates focal anterior over-coverage; pincer-type FAI; also assess the posterior wall sign (posterior wall is medial to the femoral head centre = insufficient posterior coverage — risk of posterior instability if rim is trimmed) and ischial spine sign (ischial spine projects medially into the pelvis on AP view)
- FORCe trial (UK, 2022): RCT of physiotherapy vs hip arthroscopy for FAI; at 8 months — no difference; at 12 months and beyond — arthroscopy superior; conclusion: physiotherapy first, surgery for failures
- Cam lesion development: develops during adolescent growth spurt from repetitive loading; NOT congenital; high prevalence in elite athletes (up to 55% of elite footballers); the cam deformity is on the ANTEROSUPERIOR femoral head-neck junction — this is why the Dunn lateral at the 1-o`clock MRI position is the most sensitive