WHAT IS FAI?
Femoral Acetabular Impingement (FAI) is caused by increased friction between the head of the femur bone and the acetabulum. This abnormal rubbing leads to damage of the hip joint, possibly including the articular and labral cartilages. FAI is associated with cartilage damage, labral tears, early hip arthritis, hyperlaxity, sports hernias, subspinous AIIS impingement, ischiofemoral impingement, and low back pain.
Types of FAI
There are three types of FAI: pincer, cam, and combined cam & pincer impingement.
Pincer Impingement occurs due to excess bone extending out over normal rim of the acetabulum. The labrum can be impinged between the prominent rim of the acetabulum and the neck of the femur.
Cam Impingement occurs when the femoral head is not round and cannot rotate smoothly inside the acetabulum. The abnormal "cam bump" causes increased tension force on the labrum, eventually tearing the labrum off the bone of the acetabulum.
Combined The most common form of FAI (86% of cases) is a combination of cam and pincer. http://www.nirschl.com/hip-arthrosocpy-FAI.asp