Femoroacetabular Impingement - Knee & Sports

Introduction

A common cause of
- early onset hip dysfunction
- secondary osteoarthritis
Epidemiology
- Cam impingement
  - refers to femoral based disorder (femoral acetabular impingement - "FAI") usually in young athletic males and includes
    - decreased head-to-neck ratio
    - aspherical femoral head
    - decreased femoral offset
    - femoral neck retroversion
      - can be due to previous SCFE deformity
- Pincer impingement
  - refers to acetabular based disorder usually in active middle-aged women and includes
    - anterosuperior acetabular rim overhang
    - acetabular retroversion
    - acetabular protrusio
    - coxa profunda
- combined Cam/Pincer impingement
  - can include both patient populations
    - refers to combinations of above (up to 80%)
Mechanism
- result of impingement of the femoral neck against anterior edge of acetabulum
Pathoanatomy
- proximal femur abuts acetabulum with range of motion, especially in flexion
  - occurs if femoral head/neck bone is too broad in Cam impingement
  - occurs if acetabular bone/labrum overhang is too broad in Pincer impingement
Associated injuries
- labral degeneration and tears
- cartilage damage and flap tears
- secondary hip osteoarthritis

Presentation

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

Imaging

Radiographs
- radiographic views
  - false profile view
    - to assess anterior coverage of the femoral head
    - standing position at an angle of 65° between the pelvis and the film
- characteristic 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
- 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
      - values of >42° are suggestive of a head-neck offset 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
      - If the ratio is <0.17, a cam deformity is likely present
CT
- can be used as adjunct to assess for structural abnormalities
MRI
- best modality to evaluate for articular cartilage, and labral degeneration and tears
- can assess anatomy of femoral head/neck junction abnormalities

Treatment

Nonoperative
- observation
  - indications
    - minimally symptomatic patient
    - no mechanical symptoms
Operative
- arthroscopic hip surgery
  - indications
    - symptomatic patient
    - mechanical symptoms
  - outcomes
    - recent literature supports arthroscopy shows equivalent results to open hip surgery
- open surgical hip dislocation
  - indications
    - gold standard for management of FAI for patients with clinical signs and structural evidence of impingement and
    - preserved articular cartilage, correctable deformity, reasonable expectations
  - contraindications
    - age >55, morbid obesity, advanced joint disease
- periacetabular osteotomy
  - indications
    - structural deformity of acetabulum with poor coverage of femoral head
  - technique
    - osteotomy and fixation
- total hip arthroplasty
  - indications
    - age >60 years and end-stage hip degeneration

Techniques

Arthroscopic hip surgery
- approach
  - arthroscopic approach to the hip
- technique
  - trim femoral head/neck in Cam impingement
  - acetabular rim labral debridement vs repair
    - isolated labral debridement will not solve problem without treatment of underlying pathology
Open surgical hip dislocation
- approach
  - anterior (Smith-Peterson) approach
    - best for isolated femoral head/neck pathology due to limited exposure, although it is possible that acetabular side could be treated
    - acetabular treatment involves take down of rectus femoris reflected head
    - femoral osteotomy and fixation
- technique
  - uses a "trochanteric flip" for safe access to proximal femur and acetabulum
    - provides best visualization for hip surgery
    - preserves all external rotators and blood supply to femoral head (medial circumflex femoral artery)
      - no increase in AVN risk
    - provides wide exposure of femoral head and acetabulum
- outcomes
  - median expected time to return to sports is 7 months in adolescent athletes

Complications

Femoral neck fracture
- at risk during open or arthroscopic debridement of Cam lesions
- risk is minimized by limiting depth of femoral head-neck osteochondroplasty to <30% of femoral neck diameter
Heterotopic ossification
Persistent Cam and Pincer lesions following arthroscopic treatment