Adult fat cams

The normal hip above has enough space at the femoral waist to accommodate the acetabular rim throughout the normal range of motion.

The hip below has localized osseous protuberance (asterisk) and an aspherical humeral head (blue arrow) that impinge on the superolateral portion of the acetabulum during flexion and internal rotation, injuring the hyaline cartilage and adjacent labrum, resulting in labral detachment (red arrow).

Most cases of FAI are a combination of both cam and pincer-type mechanisms, with cam-type usually predominating.

In order for the hip to flex normally without impingement, the femoral head needs to be round and there must be a suitable space at the femoral head-neck junction (aka the femoral waist).To a lesser degree there may be damage to a thin adjacent strip of acetabular cartilage.Later in this process, when dystrophic ossification has developed in the injured labrum, the prominent acetabular rim acts as fulcrum, leveraging the femoral head posteroinferiorly and causing more significant chondral injury at that site.Femoroacetabular impingement (FAI) is not a disease, but a common pathomechanical process which occurs when the proximal femur comes into contact with the acetabular rim.This can occur in normal hips in extreme flexion and internal rotation or may be from morphologic changes that cause premature contact between the structures.Figure 2: Laterally prominent femoral head margins (yellow arrowheads) create femoral head asphericity bilaterally.

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