Femoral acetabular Impingement (FAI)

Femoroacetabular impingement (FAI) is a cause of early degenerative changes in the hips of young adults (1)

• FAI reflects abutment of the acetabular rim and the femoral neck and is associated with abnormalities of the proximal femur and of the acetabulum
  • in FAI, the normal congruity of the hip joint has been lost due to various anatomical abnormalities, such as excessive or maloriented acetabular coverage, an insufficient femoral head-neck offset, or asphericity of the femoral head
    • in all of these conditions, the reduced joint clearance results in mechanical impingement leading to acetabular labral tears and progressive damage to the adjacent articular cartilage
• femoroacetabular impingement is classified into two types, cam and pincer, based on the underlying anatomic anomaly
  • pincer impingement results when the femoral head-neck junction makes abnormal contact with the acetabular rim. The repeated microtrauma induces ossification of the acetabular rim and eventual failure of the labrum
    • click here for diagram of pincer impingement
      
      
    • the 'pincer' does not cause an impingement because of an asphericity of the femoral head
      • dominant feature is that of a deep socket, in which the range of movement of the hip is limited by the overcovering acetabular rim. At the limit of movement the femoral neck abuts against the labrum, which acts like a bumper
      • the labrum is compressed between the femoral neck and the underlying bone and the force is further transmitted to the acetabular cartilage. The transmission of force to the cartilage is restricted to a narrow band along the acetabular rim
      • repeated microtrauma induces bone growth at the base of the labrum which subsequently ossifies
        
        
  • cam impingement - the squeezing or jamming of an abnormally shaped femoral head and head-neck junction into the acetabulum during motion characterizes cam impingement. Articular damage occurs before labral damage, and the subsequent damage to the articular cartilage is usually in the anterosuperior region of the acetabulum
    • click here for diagram of cam impingement
      
      
    • principal problem in the hip with a cam impingement is absent anterior-to-anterolateral waisting of the junction of the femoral neck and head. This is equivalent to a cam, which is an eccentric part added to a rotating device
    • during flexion the eccentric part slides into the anterosuperior acetabulum and induces compression and shear stresses at the junction between the labrum and the cartilage and at the subchondral tidemark. The labrum is stretched and pushed outwards, and the cartilage is compressed and pushed centrally, causing a separation between the labrum and cartilage. Therefore, for so-called undersurface tears of the labrum the correct terminology would be 'separation of the acetabular cartilage from the labrum'
      
      
  • it is not uncommon for FAI patients to have a combination of both cam and pincer impingement
    
    
• several imaging modalities have been used to identify specific features of FAI. Diagnoses are currently made with a combination of a conventional radiographs and MRI. Once identified, the morphological abnormalities can be surgically corrected
  • conventional radiography provides only an indirect measurement of cartilage integrity as reflected by joint space narrowing and osseous changes
    
    
• treatment
  • treatment for FAI focuses on improving the clearance for hip motion and alleviation of femoral abutment against the acetabular rim
    • interventions range from arthroscopy to safe surgical dislocation. However, surgical outcome is directly related to the amount of damage to the labrum and cartilage found intraoperatively such that if cartilage delamination is present outcome is often poor

Reference:

• (1) Beck M, Kalhor M, Leunig M, Ganz R. Hip morphology influences the pattern of damage to the acetabular cartilage: femoroacetabular impingement as a cause of early osteoarthritis of the hip. J Bone Joint Surg Br 2005; 87: 1012-1018.