Introduction / Definition

Femoroacetabular impingement (FAI) syndrome is defined by the 2016 Warwick Agreement international consensus as a motion-dependent clinical entity characterised by a triad of:

1. Symptoms (predominantly motion- or position-related anterior groin and hip pain),
2. Clinical signs (notably reduced internal rotation in flexion and positive impingement tests),
3. Relevant diagnostic imaging (cam, pincer, or mixed morphology on plain radiographs, cross-sectional imaging, or magnetic resonance arthrography).

Asymptomatic osseous variants are common in the general population; morphological variance on imaging alone does not constitute FAI syndrome in the absence of concordance with history and examination.

Epidemiology

• Demographics: Cam morphology shows a strong male predominance (frequently presenting in athletic males aged 20–40 years), whereas isolated pincer lesions are more prevalent in active middle-aged females (aged 30–50 years). Mixed deformity accounts for the majority (up to 70–80%) of symptomatic cases.
• Risk Factors: Repetitive high-impact loading during skeletal development (e.g., competitive football, ice hockey, basketball) promotes stress-induced physeal alterations at the anterolateral femoral head-neck junction. Secondary causes include subclinical slipped capital femoral epiphysis (SCFE/SUFE), Perthes disease, avascular necrosis, and femoral neck malunion.

Relevant Anatomy

• Femoroacetabular Articulation: A constrained ball-and-socket synovial joint. The fibrocartilaginous acetabular labrum runs circumferentially along the osseous acetabular rim, deepening the socket, distributing contact stresses, maintaining negative intra-articular hydrostatic fluid pressure (the labral suction seal), and providing passive joint stability.
• Vascular Considerations: The main vascular supply to the femoral head arises from the medial femoral circumflex artery (MFCA). Its deep branch passes anterior to the obturator externus and posterior to the obturator internus/gemelli complex before piercing the joint capsule to form lateral retinacular branches. Preserving the MFCA is mandatory during surgical dislocation (requiring preservation of the obturator externus and a trochanteric flip osteotomy) and during excessive femoral neck osteochondroplasty (which should be confined anteriorly and superiorly, avoiding the posterosuperior retinacular vessels).

Classification & Pathoanatomy

FAI morphology is classified into three distinct subtypes:

1. Cam Impingement

• Mechanism: Aspherical femoral head-neck junction (loss of native concavity or 'pistol-grip deformity'). During flexion, adduction, and internal rotation, the nonspherical prominence is driven into the acetabular rim.
• Pathology: Produces shear and delamination of the acetabular articular cartilage from the subchondral bone at the chondrolabral transition zone ('outside-in' carpet delamination), secondary labral detachment, and eventual full-thickness cartilage loss.

2. Pincer Impingement

• Mechanism: Structural overcoverage of the femoral head by the acetabulum. This can be focal (e.g., anterior cranial overcoverage due to acetabular retroversion) or global (e.g., coxa profunda, protrusio acetabuli).
• Pathology: Repetitive linear abutment crushing the labrum directly between the acetabular rim and femoral neck, leading to intrasubstance labral tearing, cystic degeneration, rim ossification, and a reciprocal posteroinferior 'contre-coup' chondral lesion due to posterior subluxation.

3. Mixed / Combined Impingement

• Involves features of both cam and pincer morphologies; identified in up to 70–86% of patients presenting for surgical intervention.

Clinical Presentation & Examination

Clinical Presentation

• Pain Pattern: Gradual, insidious onset of deep, sharp, or aching anterior groin pain exacerbated by hip flexion, sitting for prolonged periods, driving, or twisting sports. Lateral trochanteric, buttock, or thigh pain can occur.
• Pathognomonic Sign: The 'C-sign'—patients cup their thumb and index finger over the anterior and lateral aspect of the greater trochanter and superior iliofemoral triangle.
• Mechanical Symptoms: Catching, clicking, locking, or 'giving way' indicative of chondral flap tears or unstable labral pathology.

Physical Examination

• Gait & Alignment: Evaluate for Trendelenburg gait, out-toeing (compensatory external rotation), abductor weakness, and sagittal pelvic tilt.
• Range of Motion: Reduced internal rotation in 90° flexion (<15° compared to contralateral side) and limited flexion (<100°).
• Provocative Tests:
  • FADIR Test (Flexion, Adduction, Internal Rotation): Highly sensitive (~90–95%) for anterior/anterosuperior femoroacetabular impingement and labral tear.
  • FABER Test (Flexion, Abduction, External Rotation / Patrick's Test): Stretches the anterior capsule and identifies intra-articular or sacroiliac joint pathology; distance from the lateral knee to examination couch is measured.
  • Posterior Impingement Test: Hyperextension and external rotation produces deep posterior/groin pain in pincer or retroverted variants.

Investigations

Radiographs

1. Standard Standing AP Pelvis:
   • Strict control of pelvic tilt and rotation (coccyx pointing to symphysis pubis, 1–3 cm above it).
   • Crossover sign: Anterior acetabular rim crosses lateral to posterior rim in its upper third, indicative of cranial retroversion.
   • Posterior wall sign: Posterior rim passes medial to the centre of the femoral head (posterior deficiency).
   • Ischial spine sign: Prominent projection of the ischial spine into the pelvic cavity, indicating retroversion.
   • Lateral Centre-Edge Angle (LCEA) of Wiberg: Normal 25°–39°. <20° indicates dysplasia; >40° indicates global pincer overcoverage (coxa profunda/protrusio).
   • Tönnis Angle (Acetabular Roof Angle): Normal 0°–10°. Values <0° indicate overcoverage.
2. 45° or 90° Dunn Lateral / Cross-Table Lateral:
   • Demonstrates anterosuperior head-neck asphericity.
   • Alpha Angle of Nötzli: Angle between a line drawn through the centre of the femoral neck axis and a line from the centre of the femoral head to the point where the contour departs from sphericity. Normal <55°; >55°–60° confirms cam morphology.

Magnetic Resonance Imaging / MR Arthrography (MRA)

• High-field (3.0 T) MRI or 1.5 T MR Arthrography (gadolinium) represents the investigation of choice for chondrolabral assessment.
• Identifies labral detachments, paralabral cysts, chondral delamination, subchondral edema, and fibrocystic reaction (herniation pits) at the femoral neck.

Computed Tomography (CT) with 3D Reconstruction

• Critical for quantifying three-dimensional femoral torsion, acetabular version, mapping complex rim deformities, and preoperative computer-assisted planning.

Diagnostic Intra-articular Infiltration

• Fluoroscopic- or ultrasound-guided injection of local anaesthetic (e.g., ropivacaine). ≥80% acute reduction of Concordant mechanical pain confirms intra-articular origin, differentiating it from athletic pubalgia, adductor tendinopathy, snapping psoas, or lumbar radiculopathy.

Management

                      Suspected FAI Syndrome
       (Warwick Criteria: Symptoms + Signs + Imaging)
                             │
                             ▼
                 Conservative Management
       (Physiotherapy, Activity Modification, NSAIDs, Injections)
                             │
              ┌──────────────┴──────────────┐
         Successful                     Unsuccessful
              │                         (≥ 3-6 Months)
          Discharge /                        │
         Maintenance                         ▼
                               Consider Chondral Status
                       (Radiographs: Tönnis Grade, Joint Space)
                                             │
                   ┌─────────────────────────┴─────────────────────────┐
                   ▼                                                   ▼
       Tönnis ≥ 2 or Space < 2 mm                             Preserved Joint Space
                   │                                          (Tönnis 0-1, > 2 mm)
                   ▼                                                   │
         Total Hip Arthroplasty                                        ▼
                                                          Correct Deformity & Labrum
                                                                       │
                                                    ┌──────────────────┴──────────────────┐
                                                    ▼                                     ▼
                                             Hip Arthroscopy                   Open Surgical Hip Dislocation
                                        (Cam/Pincer Osteochondroplasty,      (Global Retroversion, Large Posterior
                                         Labral Repair, Capsular Closure)     Cam, Associated Dysplasia/PAO)

Non-Operative Management

First-line therapy for all patients for at least 3–6 months:

• Activity modification (avoiding terminal flexion and high-pivot sports).
• Structured, hip-specific physical therapy (strengthening lumbopelvic core, gluteal musculature, deep hip rotators; avoiding aggressive stretching into impingement positions).
• Non-steroidal anti-inflammatory drugs (NSAIDs).
• Fluoroscopically guided intra-articular corticosteroid or hyaluronic acid injection for symptomatic relief and confirmatory diagnosis.

Operative Management

Indicated in patients with persistent disabling symptoms despite optimised non-operative care, clear morphological concordance, and preserved joint space (Tönnis grade 0 or 1, joint space >2 mm).

1. Hip Arthroscopy (Gold Standard in Contemporary Practice)

• Portals: Anterolateral (AL), Mid-Anterior (MAP), and Modified Anterior/Distal Anterolateral Accessory (DALA) portals, established under fluoroscopic traction.
• Procedures:
  • Cam decompression (Femoral osteochondroplasty): Resection of the aspheric bone at the anterolateral head-neck junction using a high-speed burr until dynamic clearance through 120° of flexion and 40° internal rotation is confirmed. Resection should not exceed 30% of the neck diameter to prevent fracture.
  • Acetabular rim trimming (Pincer decompression): Conservative resection of the retroverted/overcovering rim.
  • Labral Management: Labral refixation with suture anchors is superior to labral debridement/resection in preserving the fluid seal and preventing arthritic progression. In severe degeneration or hypoplastic labra, labral reconstruction with autograft (iliotibial band, gracilis) or allograft is performed.
  • Capsular Management: Complete interportal or T-capsulotomy closure is recommended to prevent iatrogenic micro-instability.

2. Open Surgical Hip Dislocation (Ganz Technique)

• Preserves the deep branch of the MFCA through a lateral approach with a digastric trochanteric flip osteotomy.
• The capsule is incised with a 'Z' capsulotomy, allowing 360° direct visualization of the acetabulum and femoral head-neck junction without risking avascular necrosis.
• Indicated in complex multi-planar deformities, severe SCFE malunion, large posterosuperior cam lesions unreachable arthroscopically, or where concurrent periacetabular osteotomy (PAO) or femoral osteotomy is required.

3. Periacetabular Osteotomy (PAO / Reverse Bernese PAO)

• Indicated for true structural global acetabular retroversion (where rim trimming alone would critically reduce weight-bearing lunate surface).

Complications

• Iatrogenic Micro-instability: Secondary to unclosed capsulotomies or over-resection of the acetabular rim/labrum.
• Femoral Neck Fracture: Caused by aggressive cam resection (>30% of femoral neck diameter resected).
• Avascular Necrosis (AVN) of Femoral Head: Disruption of lateral retinacular branches of the MFCA via improper retractor placement or posterolateral neck over-resection.
• Nerve Injuries: Traction neurapraxia (pudendal nerve from perineal post, lateral femoral cutaneous nerve injury during portal placement, or sciatic nerve neuropraxia).
• Heterotopic Ossification: Prophylaxis with postoperative NSAIDs (e.g., indomethacin or naproxen for 2–3 weeks) is routine.
• Persistent Pain / Revision: Residual unaddressed cam deformity is the most common cause of revision surgery.
• Conversion to Total Hip Arthroplasty (THA): High failure rates occur if arthroscopy is undertaken in hips with pre-existing osteoarthritis (joint space ≤2 mm or Tönnis grade ≥2).

Landmark Evidence

• Ganz et al. (2003): Seminal paper in CORR describing femoroacetabular impingement as an etiologic mechanism for early osteoarthritis of the hip and describing surgical hip dislocation.
• The Warwick Agreement (Griffin et al., BJSM 2016): International consensus defining FAI syndrome as the triad of clinical symptoms, physical signs, and diagnostic imaging.
• The UK FASHIoN Trial (Griffin et al., Lancet 2018): Multicentre randomised controlled trial comparing hip arthroscopy with personalised hip therapy (PHT). Hip arthroscopy demonstrated superior clinical outcomes and quality of life at 12 months (iHOT-33 improvement: 6.8 points above PHT, reaching statistical significance).
• The FAIT Trial (Palmer et al., BMJ 2019): Multi-centre pragmatic RCT confirming hip arthroscopy provided superior symptomatic and quality of life improvement compared to structured physiotherapy at 8 and 24 months.

Exam Pearls for Fellowship Vivas

• Defining FAI: Always state the Warwick criteria first: Symptoms + Signs + Imaging findings. Asymptomatic cam or pincer morphologies alone must never be operated on.
• Pathoanatomy Rule: Cam lesions cause cartilage delamination at the chondrolabral junction with secondary labral avulsion; Pincer lesions cause labral crush/calcification with a reciprocal posterior 'contre-coup' chondral lesion.
• The Safe Resection Limit: Resection of the femoral neck prominence should never breach 30% of the anteroposterior femoral neck diameter; going beyond this dramatically elevates risk of subcapital fracture.
• Blood Supply Viva Trap: Identify the deep branch of the MFCA passing anterior to obturator externus and posterior to the obturator internus tendon. The trochanteric flip osteotomy leaves obturator externus attached to the stable proximal femur, safeguarding the MFCA.
• Red Flags for Arthroscopic Failure: Tönnis grade ≥2, joint space narrowing <2 mm, marked chondral wear on Outerbridge scale, or underlying undetected hip dysplasia (LCEA <20°). Dysplasia must not receive aggressive rim trimming.