Causes / typical injury mechanism: Femoroacetabular impingement (FAI) describes abnormal contact between the femur and acetabulum due to a group of morphological changes that lead to damage of the labrum and cartilage, ultimately resulting in hip pain. FAI can result from morphological alterations at the femoral head-acetabular joint (cam type), the acetabular side (pincer type), or a combination of both (combined type). Overall, cam-type impingement is more common than pincer-type impingement, but both rarely occur in isolation. Over 70% of patients with hip impingement symptoms have a combined-type FAI with evidence of both cam- and pincer-type FAI.

Classic history and presentation: All types present as hip pain with extreme ranges of motion (ROM), especially flexion and internal rotation, and can gradually worsen to pain with normal ROM. Untreated FAI can cause gradual and serious damage to all aspects of the hip joint and can lead to osteoarthritis, cartilage lesions, labral tears, and chronic pain.

Prevalence: The exact incidence of FAI is unknown, but up to 90% of asymptomatic adolescents have at least 1 radiographic finding suggesting FAI (50% have 2 findings), highlighting that these morphologic alterations do not always cause symptoms.

• Age –
  • Cam-type: 2nd and 3rd decades of life
  • Pincer-type: 4th and 5th decades of life (middle age)
• Sex / gender –
  • Cam-type: 3:1 male to female ratio
  • Pincer-type: 1:1 male to female ratio

Risk factors: General risk factors for FAI include untreated pediatric hip / pelvis deformities, a family history of FAI, Northern European descent, and participation in activities with extreme ROMs (hockey goalie, gymnastics, ballet, martial arts, etc).

Pathophysiology:

• In cam-type FAI, an abnormal femoral head-neck junction results in a nonspherical femoral head (most commonly on the anterolateral surface) and decreased head-neck offset. In addition, the superior aspect of the femoral neck is often convex in shape. During hip ROM, especially flexion and internal rotation, the aspherical head creates a shear force across the acetabular cartilage and causes delamination. Over time, the labrum sustains secondary damage as well. The exact cause of cam-type FAI is unknown. It is also unknown if cam-type FAI results from a primary or a secondary process. Proposed mechanisms include congenital femoral pistol grip deformity, untreated slipped capital femoral epiphysis (SCFE), and abnormal physeal closure, among many others.
• Pincer-type FAI is caused by an excessive prominence of acetabular bone or labrum at the anterolateral rim of the acetabulum. During hip ROM (especially during flexion), the femoral neck impinges upon this prominence, trapping and crushing the labrum inside, leading to breakdown and tearing over time. Furthermore, the head levers on this prominence and causes a contrecoup injury to the articular cartilage of the posteroinferior acetabulum and/or the posteromedial femoral head. The exact cause of pincer-type FAI is unknown and is likely multifactorial. Causes may include simple overgrowth of the anterior edge of the acetabulum, coxa profunda (general acetabular over-coverage), acetabular protrusio, os acetabulum, or acetabular retroversion.