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FAI generally occurs as two forms: Cam and Pincer. The Cam form describes the femoral head and neck relationship as aspherical or not perfectly round. This loss of roundness contributes to abnormal contact between the head and socket as the hip goes through a range of motion. The Pincer form describes the situation where there is overcoverage of the socket or acetabulum relative to the ball or femoral head. This over-coverage typically exists along the front-top rim of the socket (acetabulum).
The end result is that the labral cartilage gets pinched between the rim of the socket and the front part where the femoral head meets the femoral neck. The Pincer form of the impingement is typically the results of retroversion, where the socket is pointed backwards a bit (rather than the usual situation where it is angled forwards), or where the socket is too deep. Very often, the Cam and Pincer forms exist together. The cause of these bony variations is not known at this time.
FAI is associated with articular cartilage damage and labral tears and may result in hip arthritis at a younger age than usually occurs.
Who is at risk?
FAI is common in high level athletes, but also occurs in active individuals. While either type of impingement can occur in men or women at any age, most frequently the Cam type of impingement tends to affect young (20s) male athletes, while Pincer tends to occur more commonly in women in their 30s and 40s who are athletically active.
Sports associated with FAI include martial arts, ballet, cycling, rowing, golf, tennis, soccer, football, ice hockey, baseball, lacrosse, field hockey, rugby, water polo, and deep squatting activities such as power lifting.