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Femoroacetabular Impingement (FAI) - Complete Orthopaedic PG Notes

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1. DEFINITION

• First described and popularized by Ganz et al.
• Consensus on FAIS (FAI Syndrome) established in 2016 (Warwick Agreement)
• FAIS is a triad: symptoms + clinical signs + bony deformity (all three required)

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2. CLASSIFICATION / TYPES

A. CAM Impingement (Femoral-sided)

• Abnormal bony prominence at the anterosuperior femoral head-neck junction (reduced offset / non-spherical head)
• During flexion + internal rotation, the non-spherical portion rotates into the acetabulum
• Results in labral avulsion at the labro-chondral junction and articular cartilage delamination (carpet lesion) progressing from the acetabular rim inward
• The labrum is relatively spared in cam impingement; cartilage damage predominates
• More common in young athletic males
• Etiology: possibly a developmental abnormality of the lateral femoral physis related to high-intensity sports during adolescence
• Pistol-grip deformity = non-spherical femoral head on AP radiograph

B. PINCER Impingement (Acetabular-sided)

• Overcoverage of the femoral head by the acetabulum causes the femoral neck to abut the acetabular rim during motion
• Causes:
  • Acetabular retroversion (crossover sign)
  • Coxa profunda (acetabular fossa medial to ilioischial line)
  • Protrusio acetabuli (femoral head crosses ilioischial line)
  • Global overcoverage / deep acetabulum
• Results in intrasubstance labral tears (typically anterosuperior quadrant); labrum is primarily injured
• Contrecoup injury = posteroinferior acetabular cartilage damage from levering of femoral neck on rim
• More common in middle-aged women
• Worsens with time due to reactive bone growth / labral calcification increasing overcoverage

C. Combined CAM + PINCER (Most Common)

• Both morphologies coexist
• Damages both acetabular rim cartilage and labrum to varying degrees
• Worst clinical prognosis of all three types

D. Femoral Retrotorsion

• Contributes to anterior hip impingement as an additional mechanism

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3. EPIDEMIOLOGY & RISK FACTORS

• Young to middle-aged adults (teens to 50s)
• Male sex (cam type); Female sex (pincer type)
• Athletes in high-intensity sports: hockey, soccer, football, tennis, ballet, cycling
• Caucasian ethnicity
• Familial FAI morphology
• High-impact activities during adolescent skeletal development
• Cam morphology prevalence in elite athletes approaches 60-70%

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4. PATHOANATOMY

• Labral tear site: anterosuperior quadrant is most frequently affected in both types
• Carpet lesion: acetabular cartilage delamination from subchondral bone - specific to cam FAI
• FAI is a leading cause of secondary hip osteoarthritis in young adults

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5. CLINICAL FEATURES

Symptoms

• Groin pain - the most common presentation; typically slow onset, persistent
• Pain worsens with: deep hip flexion, prolonged sitting, twisting, sporting activity
• C-sign: Patient cups the hand over the lateral hip in a "C" shape to describe pain location
• Hip stiffness and limited range of motion

Clinical Signs

• Limited internal rotation in flexion (most consistent finding)
• Patients have more passive external rotation than internal rotation
• Positive anterior impingement test (FADIR test): Flexion + ADduction + Internal Rotation reproduces groin pain
  • Most sensitive test for FAI (~95% sensitive but low specificity)
• FABER test (Flexion + ABduction + External Rotation): assesses posterior impingement and labral tears; pain = positive
• Posterior impingement test: Extension + external rotation reproduces posterior hip pain
• Log roll test: patient supine, passive internal/external rotation - pain suggests intraarticular pathology
• Stinchfield test: resisted hip flexion at 30° reproduces pain

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6. IMAGING

A. Plain Radiographs

• AP pelvis (gold standard initial view)
• 45° Dunn lateral view (preferred by most hip arthroscopists - best for measuring alpha angle; identifies subtle cam deformities not visible on AP)
• Cross-table lateral
• False-profile view: evaluates anterior coverage, subspine deformities, posterior joint space narrowing

B. CT Scan / 3D-CT

• Best for assessing bony morphology and version of femur and acetabulum
• 3D reconstruction guides surgical planning for amount of bony resection
• Delineates extent of cam and pincer deformity
• Measures femoral version and acetabular version accurately

C. MRI / MR Arthrography (MRA)

• MRA (intraarticular gadolinium) is the gold standard for labral pathology
  • Sensitivity increases from ~60% (plain MRI) to >90% with contrast
• Reveals labral tears, cartilage delamination, paralabral cysts
• Axial oblique sequence: alpha angle measured on MRI (see diagram below)
• Contrast tracking beneath articular cartilage suggests delamination
• Higher-resolution 3T MRI may eventually replace MRA

D. Intraarticular Injection

• Diagnostic/therapeutic: Fluoroscopy-guided intraarticular LA ± steroid
• If injection relieves pain, confirms intraarticular source of pain

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7. KEY MEASUREMENTS (Exam High-Yield)

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8. CLASSIFICATION OF ARTICULAR CARTILAGE DAMAGE

• Grade 0: No signs of OA
• Grade 1: Slight narrowing, sclerosis
• Grade 2: Small cysts, moderate narrowing, loss of sphericity
• Grade 3: Large cysts, severe narrowing, avascular necrosis signs

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9. DIFFERENTIAL DIAGNOSIS

• Hip dysplasia (DDH) - LCEA <20°, acetabular index >10°
• Labral tear without FAI
• Avascular necrosis (AVN) of femoral head
• Iliopsoas tendinitis / internal snapping hip
• Trochanteric bursitis
• Hip OA
• Stress fracture of femoral neck
• Lumbar spine pathology
• Sports hernia / athletic pubalgia
• Piriformis syndrome

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10. TREATMENT

A. Conservative (Non-Operative) - First Line

• Activity modification: avoid deep flexion, high-impact sports
• Physiotherapy: core strengthening, hip rotator strengthening, ROM exercises
• NSAIDs / analgesics
• Intraarticular corticosteroid injection (diagnostic + therapeutic)
• Conservative management for 3-6 months before surgery

B. Operative Treatment

• Failed conservative treatment (3-6 months)
• Symptomatic FAI with confirmed labral/chondral pathology on MRI
• Good/adequate joint space (Tönnis grade 0-1; joint space >2 mm)
• No severe osteoarthritis

• Tönnis grade 2+ OA (relative contraindication)
• Joint space <2 mm (predicts high THA conversion rate)
• Advanced age with significant OA → proceed to THA

B1. Hip Arthroscopy (Most Common Surgical Approach)

• Preferred method for most FAI cases
• Performed through 2-3 portals (anterolateral, anterior, mid-anterior)
• Traction applied to distract the joint
• Procedures performed:
  • Cam resection (femoral osteochondroplasty): reshaping the femoral head-neck junction
  • Pincer resection (rim trimming / acetabuloplasty): removing excess acetabular rim
  • Labral repair: preferred over debridement (superior outcomes); labral refixation using suture anchors
  • Labral reconstruction: if labrum is irreparable (using IT band or ligamentum teres graft)
  • Chondral treatment: microfracture, fibrin glue, cartilage repair as needed
  • Capsular plication or repair (especially in borderline dysplasia)
• Return to sport: median 6 months
• Outcomes: Excellent in properly selected patients; 80-90% patient satisfaction

B2. Surgical Hip Dislocation (Ganz Trochanteric Flip Osteotomy)

• Allows excellent 360° exposure of the femoral head and acetabulum
• Indicated for: severe deformities not amenable to arthroscopy, complex combined pathology
• Technique (Ganz):
  • Trochanteric flip osteotomy
  • Z-shaped capsulotomy (anterior Z-capsulotomy)
  • Safe surgical dislocation preserving posterior blood supply (medial femoral circumflex artery)
  • Preserves femoral head blood supply - very important
  • Allows complete labral repair and chondral flap treatment
• Complications (<5%): trochanteric nonunion, heterotopic ossification
• Anterior Z-capsulotomy preserves posterior vessels to femoral neck, minimizing AVN risk

B3. Periacetabular Osteotomy (PAO) / Bernese PAO

• For acetabular overcoverage with significant retroversion
• Posterior column is preserved - maintains pelvic stability
• Allows correction of acetabular tilt and version
• Less commonly used for FAI but helpful for retroverted acetabula

B4. Reverse PAO (Anti-protrusio / Derotation)

• Reduces anterior overcoverage in retroverted acetabulum
• Less commonly performed

B5. Femoral Osteotomy

• For femoral retrotorsion or other femoral-sided deformities
• Derotation osteotomy of the proximal femur

B6. Total Hip Arthroplasty (THA)

• Reserved for patients with significant arthritic changes (Tönnis grade 2-3)
• Joint space <2 mm = high conversion to THA risk after arthroscopy

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11. PROGNOSIS / PROGNOSTIC FACTORS

• Younger age
• Tönnis grade 0-1 OA
• Joint space >2 mm
• Isolated cam impingement (better than pincer)
• Labral repair > labral debridement (superior 10-year survivorship)
• Normal acetabular coverage (LCEA 25-40°)

• Tönnis grade ≥ 2 OA
• Joint space < 2 mm → risk of conversion to THA
• Borderline dysplasia (LCEA 18-25°) - capsular plication important
• Combined FAI morphology
• Advanced age
• Significant chondral damage at time of surgery

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12. COMPLICATIONS OF HIP ARTHROSCOPY

• Traction neuropraxia (lateral femoral cutaneous nerve, pudendal nerve)
• Instrument breakage
• Avascular necrosis (rare)
• Heterotopic ossification
• Capsular instability (if over-release without repair)
• Incomplete resection → recurrence of symptoms
• Conversion to THA

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13. SUBSPINE IMPINGEMENT (AIIS Impingement)

• Impingement between the anterior inferior iliac spine (AIIS) and the femoral head-neck junction
• A form of extra-articular impingement
• Common in patients with prior AIIS avulsion injuries or prominent AIIS
• Addressed arthroscopically with AIIS decompression

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14. SPECIAL POINTS FOR PG EXAM

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15. SUMMARY OF RADIOGRAPHIC ANGLES

Alpha angle:
  - Measured on: 45° Dunn lateral / cross-table lateral / axial oblique MRI
  - Method: Circle of best fit around femoral head → angle between 
    (1) femoral neck axis and 
    (2) line from head center to point where head protrudes from the circle
  - Normal: <50-55° | Abnormal (cam): >55°

Lateral Centre-Edge Angle (LCEA / Wiberg angle):
  - Measured on: AP pelvis
  - Method: Angle between vertical line from femoral head center 
    and line from femoral head center to lateral acetabular edge
  - Normal: 25-40° | Overcoverage (pincer): >40° | Dysplasia: <20°

Head-Neck Offset Ratio:
  - Normal: >0.17 | Reduced: <0.17 = cam morphology

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