= repetitive microtrauma due to an anatomic conflict between proximal femur + acetabular rim at extreme range of motion, especially hip flexion and internal rotation

Mean age: 20–45 yrs; in patients with increased physical activity; M÷F = 3÷2 to 9÷1

Prevalence: 10–15%

Cause: anatomic variations; developmental dysplasia of the hip; slipped capital femoral epiphysis; Legg-Calvé-Perthes; posttraumatic deformity

Pathophysiology:

• labrum caught between femoral head and acetabulum (impingement) → labral tear typically anterosuperiorly → chondral damage → secondary osteoarthritis (other causes of labral tears: trauma; capsular laxity; dysplasia; degeneration)

Types:

1. Cam FAI (56% of all labral tears from sports injury)
   = nonspherical shape of femoral head with reduced depth of femoral waist (= femoral waist deficiency) leads to abutment of femoral head-neck junction against acetabular rim
   Age: athletic male 20–30 years
   • aspheric (= osseous bump) femoral head / head-neck junction (50%)
   • large areas of labral avulsion
   • broad areas of cartilage lesions often >1 cm in width
   • carpet phenomenon = focal cartilage delamination
   • α-angle of >55° (measured at the anterosuperior position on radial images rotated around center line of femoral neck)
     N.B.: substantial overlap in α-angle measurements between volunteers and patients
2. Pincer FAI (12% of all labral tears from sports injury)
   = acetabular overcoverage limits range of motion
   Cause: protrusio acetabuli, acetabular retroversion
   Age: athletic woman 30–40 years
   Associated with: increased femoral antetorsion
   • deep acetabulum (head center >5 mm below rim)
   • acetabular retroversion = anterior acetabular rim overlaps posterior rim on AP
   • osseous bump deforming femoral head-neck junction (33%)
   • anterosuperior labral avulsion (pincer FAI)
   • thin rim of adjacent cartilage lesion often <5 mm in width
3. Mixed pattern (frequent)

• groin pain (83%) owing to initially activity overparticipation + later osteoarthritis; impaired ability to squat
• decreased range of motion (ROM)
• positive impingement test in supine position:
  1. anterior impingement (= groin pain during passive hip flexion + adduction + internal rotation)
  2. posterior impingement (= groin pain during hip extension + external rotation)
• cystic / bony proliferative changes at femoral head-neck junction
• ossicle along acetabular rim / os acetabuli

MR:

• requires use of multiplanar capability to obtain radial sections perpendicular to surfaces of hip joint → true cross section of cartilage + labrum without partial-volume effects

Cx: premature osteoarthritis initially with cartilage damage + labral tears

DDx: healthy young adult (common!), adult hip dysplasia, pseudoacetabular overcoverage in diffuse idiopathic skeletal hyperostosis (abnormal thoracolumbar spine) / ankylosing spondylitis (abnormal sacroiliac joints)

Rx: surgery in patients without osteoarthritis consisting of reshaping of femoral waist / trimming of acetabular rim / periacetabular osteotomy