Femoroacetabular Impingement - Knee & Sports

summary
- Femoroacetabular impingement (FAI) is the abnormal contact between the femur and acetabulum which may lead to labral damage, various degrees of chondral injury and progressive hip pain.
- Diagnosis is made radiographically with hip radiographs showing an aspherical femoral (Cam impingement) or anterosuperior acetabular overhang (Pincer impingement), or a combination of both.
- Treatment may be nonoperative or operative depending on the chronicity of symptoms, patient age, patient activity demands, and development of secondary insult to the hip joint (i.e. labral tear, secondary osteoarthritis).

Epidemiology
- Incidence
  - femoral and acetabular deformity common in general population and often asymptomatic
  - may become more apparent with participation in activities requiring extreme range of motion (ballet, gymnastics, martial arts)

Etiology
- Pathophysiology
  - mechanism of injury
    
    proximal femur abuts acetabulum with range of motion, especially during flexion
  - pathoanatomy
    
    Cam impingement
    
    refers to femoral based disorder usually in young athletic males
    
    occurs if femoral head/neck bone is too broad, mostly on the anterolateral neck
    
    characterized by any of the following
    
    decreased head-to-neck ratio
    
    aspherical femoral head
    
    decreased femoral offset
    
    femoral neck retroversion
    
    this sphericity mismatch causes shearing at the chondro-labral junction, leading to cartilage delamination and labral separation
    
    Pincer impingement
    
    refers to acetabular based disorder usually in active middle-aged women
    
    occurs if acetabular bone/labrum overhang is too broad, mostly at the anterosuperior quadrant
    
    anterosuperior acetabular rim overcoverage
    
    acetabular retroversion
    
    acetabular protrusio
    
    coxa profunda
    
    the femoral neck impinges and crushes the labrum creating intra-substance tearing
    
    this levers the femoral head into the postero-inferior acetabulum leading to a contrecoup cartilaginous injury
    
    combined Cam/Pincer impingement
    
    can include both patient populations
    
    refers to combinations of above (up to 80%)
    
    SCFE deformity causes variable patterns of impingement
- Associated injuries
  - labral degeneration and tears
  - cartilage damage and flap tears
  - secondary hip osteoarthritis

Anatomy
- Osteology
  - highly congruous joint formed by
    
    Acetabulum
    
    formed by confluence of ischium, ilium and pubic bones
    
    Femur
    
    spherical head on neck, anteverted 15 degrees in relation to femoral condyles
- Muscles
  - 5 major muscle groups acting across hip
  - hip flexors, extensors, abductors, adductors, and external rotators
- Capsule and Ligaments
  - 3 ligaments of the form joint capsule
    
    iliofemoral ligament (Y ligament of Bigelow)
    
    ischiofemoral ligament
    
    pubofemoral ligament
  - labrum
    
    horseshoe-shaped fibrocartilaginous tissue extending around periphery of acetabulum
    
    connected by transverse acetabular ligament at inferior acetabulum
    
    increases acetabular volume and provides suction seal
  - ligamentum teres
    
    extends from cotyloid fossa to femoral head
    
    negligible contribution to vascular supply of femoral head in adult

Presentation
- Symptoms
  - common symptoms
    
    activity related groin or hip pain, exacerbated by hip flexion
    
    difficulty sitting
    
    mechanical hip symptoms of clicking or popping
    
    can present with gluteal or trochanteric pain
    
    due to aberrant gait mechanics
- Exam
  - motion
    
    limited hip flexion (<90 degrees), especially with internal rotation (<5 degrees)
    
    anterior impingement test (flexion, adduction, internal rotation) elicits pain
  - inspection
    
    externally rotated extremity
    
    can be due to post-SCFE deformity

Imaging
- Radiographs
  - recommended views
    
    AP with true lateral view (hip placed in 15 degrees of internal rotation)
  - optional views
    
    Dunn or modified Dunn view
    
    false profile view
    
    to assess anterior coverage of the femoral head
    
    standing position at an angle of 65° between the pelvis and the film
  - findings
    
    asphericity and contour of femoral head and neck
    
    pistol grip deformity
    
    indicates Cam impingement
    
    examine for acetabular protrusio, retroversion, and coxa profunda
    
    crossover sign
    
    indicates acetabular retroversion in Pincer impingement
    
    posterior wall sign
  - measurements
    
    alpha angle
    
    method
    
    measured frog-leg lateral radiograph
    
    first line is drawn connecting the center of the femoral head and the center of the femoral neck.
    
    second line is drawn from the center of the femoral head to the point on the anterolateral head-neck junction where prominence begins
    
    the intersection of these two lines forms the alpha angle
    
    normal values
    
    < 42° considered normal
    
    values of >42° are suggestive of a head-neck offset deformity, >50-55° indicates Cam deformity
    
    head-neck offset ratio
    
    method
    
    measured from lateral radiographs
    
    a line #1 is drawn through the center of the long axis of the femoral neck
    
    line #2 is drawn parallel to line 1 through the anteriormost aspect of the femoral neck
    
    line #3 is drawn parallel to line 2 through the anteriormost aspect of the femoral head
    
    the head-neck offset ratio is calculated by measuring the distance between lines 2 and 3, and dividing by the diameter of the femoral head
    
    normal values
    
    > 0.17 considered normal
    
    If the ratio is <0.17, a cam deformity is likely present
    
    lateral center-edge angle (angle of Wiberg)
    
    normal value under 40 degrees
    
    anterior center-edge angle
    
    normal value over 20 degrees
    
    acetabular index or Tonnis roof angle
    
    normal value above 0 degrees
- CT
  - indications
    
    can be used as adjunct to assess for structural abnormalities
  - views
    
    3D reconstructions aid in pre-operative assessment
- MRI and/or MR arthrogram
  - indications
    
    best modality to evaluate for articular cartilage and labral damage
    
    can assess anatomy of femoral head/neck junction abnormalities
  - views
    
    ensure MRI is formatted to be in-line with femoral neck
  - findings
    
    labral fraying or frank tears, chondral damage, subchondral cyst formation

Differential
- Various pathologies will refer pain to the hip region
- Ischiofemoral impingment
- adductor strains and athletic pubalgia
- lumbar radiculopathy
- iliopsoas pathology
- hip instability

Treatment
- Nonoperative
  - activity modification, PT, NSAIDs
    
    indications
    
    minimally symptomatic patient
    
    no mechanical symptoms
    
    modalities
    
    period of rest or activity modification followed by physical therapy to address kinetic chain abnormalities
    
    NSAIDs
- Operative
  - arthroscopic osteoplasty
    
    indications
    
    symptomatic patient with mechanical symptoms
    
    failure of non-operative measures
    
    non-arthritic
    
    outcomes
    
    recent literature supports arthroscopy shows equivalent results to open hip surgery
    
    decreased functional and symptomatic outcomes in patients with evidence of hip osteoarthritis (Tonnis grade 1 or greater)
  - open surgical hip dislocation and osteoplasty
    
    indications
    
    previous gold standard for patients with clinical signs and structural evidence of impingement
    
    preserved articular cartilage, correctable deformity, reasonable expectations
    
    significant femoral deformity (residual SCFE or Perthes)
  - periacetabular osteotomy
    
    indications
    
    structural deformity of acetabulum with significant retroversion
  - hip arthroplasty
    
    indications
    
    arthritic and end-stage hip degeneration
    
    controversial regarding hip resurfacing versus total hip arthroplasty

Techniques
- Arthroscopic osteoplasty
  - approach
    
    arthroscopic approach to the hip
  - soft tissue
    
    capsulotomy required to access peripheral component to address CAM
    
    labral repair/refixation required following acetabuloplasty if labrum is destabilized
  - bony work
    
    trim femoral head/neck in Cam impingement
    
    acetabular rim trimming followed by labral debridement vs repair/reconstruction
    
    isolated labral debridement of labral tears will not provide long-term benefit without treatment of underlying bony pathology
  - outcomes
    
    equivalent success compared to open procedure
  - complications
    
    neurapraxias associated with hip arthroscopy
- Ganz open surgical hip dislocation and osteoplasty
  - approach
    
    Kocher-Langenbeck incision while in lateral decubitus position, gluteus maximus split
    
    digastric "trochanteric flip" performed and fragment mobilized anteriorly
    
    capsulotomy performed, hip dislocated anteriorly, ligamentum teres likely transected
    
    allows safe access to proximal femur and acetabulum
    
    alternatively, a direct anterior approach may be utilized but grants limited visualization to posterior acetabulum
  - bony work
    
    same as arthroscopic osteoplasty
  - soft tissue
    
    labral repair/refixation/reconstruction required following acetabuloplasty if labrum is destabilized
  - outcomes
    
    provides wide exposure of femoral head and acetabulum while preserving all external rotators and blood supply to femoral head (medial circumflex femoral artery)
    
    no increase in AVN risk
    
    median expected time to return to sports is 7 months in adolescent athletes
  - complications
    
    trochanteric hip pain
- Combined arthroscopic and limited open approach
  - combines aspects of both procedures to gain access to entire femur and acetabulum
  - early results promising

Complications
- Femoral neck fracture
  - at risk during femoroplasty
  - risk is minimized by limiting depth of femoral head-neck osteoplasty to <30% of femoral neck diameter, using multiple fluoroscopy views of femoral neck during procedure
- Heterotopic ossification
- Residual deformity following arthroscopic treatment
  - use of multiple fluoroscopy views

Prognosis
- Natural history believed to lead to early onset hip dysfunction and arthritis