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Femoro Acetabular Impingement (FAI): Overview

What is femoro acetabular impingement?

If your child is suffering from hip pain it may be the result of a mechanical disorder of the hip joint called femoro acetabular impingement (FAI).  A normally developed hip joint is formed by the end of the thigh bone (technically called the femoral head) that normally is shaped as a ball or sphere and the  socket (also referred to as the acetabulum) on either side of the pelvic bone.  FAI involves abnormal contact between the end of the thigh bone and neck against the frontal part of the hip socket

FAI may occur from deformities of the femur, which is called cam impingement, or from deformities of the socket which is called pincer impingement. It can also occur in children who suffer from deformities of both the thigh bone and the hip socket simultaneously.

What causes femoro acetabular impingement?

Cam impingement (deformity of the thigh bone) occurs when the abnormally shaped femoral head (ball) and head-neck junction jam, or run into, the hip socket during certain activities like bending over, sitting for long periods of time, or riding a bike.

Pincer impingement is caused by direct contact between the femoral head-neck junction and the acetabular rim in conditions where the coverage of the femoral head in the anterior region is excessive (acetabular retroversion, protrusion acetabulum etc.).  Impingement of a child’s hip can also be caused due to pediatric hip disorders such as Legg-Calvé-Perthes disease, slipped capital femoral epiphysis (SCFE), or post-traumatic deformities.  

Activities such as running, hockey, dancing, and gymnastics are not causes of FAI, but because of the increased demand these activities place on the hips, the symptoms of FAI may occur earlier than in less active children.

Who gets femoro acetabular impingement?

Risk factors for developing femoro acetabular impingement:
  • Consequence of Legg-Calvé-Perthes altering the shape of the ball and socket of the hip joint
  • Consequence of Slipped Capital Femoral Epiphysis altering the shape of the femoral head and acetabulum
  • Activities involving repetitive motion of the hip (running, weight lifting, ice skating)
  • Activities involving extreme flexion of the hip (dance, gymnastics)
  • Trauma to the hip

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