Synonyms
- Acetabular rim syndrome
- Acetabular retroversion
- Pistol grip proximal femur deformity
- Bony spurring of the hip
- Cervicoacetabular impingement
Introduction
Femoral acetabular impingement (FAI) is a major cause of early osteoarthritis of the hip, especially in the young and active patients. It is characterized by an early pathologic contact causing high frictional forces during hip joint motion between skeletal prominences of the acetabulum and the femur that limits the physiologic hip range of motion, typically flexion and internal rotation. FAI is very common in high level athletes, but also occurs in active individuals. FAI is important due to its association or development into cartilage damage, acetabular labral tears, early hip osteoarthritis, hyperlaxity and low back pain.
NORMAL HIP JOINT
Classification of FAI
Based on clinic-radiological findings the following types of FAIs are described:
1. CAM Type. (Cam meaning cog) describes the femoral head and neck relationship as aspherical or not perfectly round.
CAM TYPE IMPINGEMENT
Cam impingement is more common in young men, occurring at an average age of 32 years. Cam impingement is the femoral cause of femoral acetabular impingement and is caused by an aspherical shape of the femoral head where the nonspherical portion is jammed into the acetabulum either idiopathic or secondary to other causes. The recurrent irritation leads to an abrasion of the acetabular cartilage or its avulsion from the subchondral bone. The cartilage area involved in cam impingement is much larger than pure pincer impingement and may be associated with large areas of cartilage softening, delamination or fissuring. Cam impingement can be caused by an bony elevation on the femoral head-neck junction or by a retroverted femoral neck or head. Bony bumps are typically located in the lateral quadrant (so-called pistol grip, seen on an anteroposterior pelvic radiograph or on an axial cross-table view of the proximal femur. A pistol-grip deformity is characterized on radiographs by flattening of the usually concave surface of the lateral aspect of the femoral head due to an abnormal extension of the more horizontally oriented femoral epiphysis.
Cam impingement is usually considered to be caused by a growth abnormality of the capital femoral epiphysis, but it can also be the result of a subclinical slipped capital femoral epiphysis or Legg-Calv
