Femoroacetabular Impingement (FAI) series, Part 1: Pathophysiology

Femoroacetabular impingement (FAI) is a common condition among sports where athletes utilize rotational forces or high levels of impact during hip flexion. FAI has been shown to lead to secondary breakdown of the acetabular cartilage within the hip joint overtime. Eventually, the athlete will likely develop osteoarthritis, sooner than their less athletic counterparts. Many times, FAI begins as non-descript groin pain and is treated as such with temporary relief, but as the athlete continues to participate in high risk sports (golf, baseball, hockey etc…) the pain will increase. As athletes continue their careers, increased breakdown occurs which can lead to increased joint damage, ultimately lowering the threshold for deterioration. This causes lower than normal loads to negatively impact the joint.

There are 2 types of impingement: pincer and cam. Pincer impingement is when there is an overgrowth or increased prominence of the anterolateral acetabular rim. The site of the impingement is usually anteriorly and occurs when the hip is brought into a flexed/adducted/internally rotated position. Normally the acetabulum sits and faces slightly anteriorly/laterally with differences between males and females.  It can also be caused by an os acetabulum – a separate piece of bone along the anterolateral rim of the acetabulum. With hip flexion, the rim of the acetabulum begins to “crush” the labrum and with repeated motions, the labrum will start to fail leading to the typical anterior groin pain.

Cam impingements happen when there is a non-spherical femoral head rotating inside the acetabulum. Often times cam impingements are a sequela to Slipped Capital Femoral Epiphysis (SCFE) and can result in significant deficits into hip internal rotation. With hip flexion, the non-spherical shape of the femoral head eventually leads to increased deterioration of the cartilage inside the socket due to the shear forces placed on the surface – resulting in early onset arthritis. One theory on the cause of cam impingements, which has not be proven, is that there is early closing of the growth plate resulting in the non-spherical shape of the femoral head. Intense athletic activity may precipitate the premature closing of the capital physis, but it is unclear whether it causes the impingement.

As with all pathology, there is a chance for having both cam and pincer lesions present.


Differential diagnosis will be touched on in the next post covering assessments of athletes with FAI. However, I would like to mention that there are a number of diagnoses that can cause anterior groin pain in the subjective history including athletic pubalgia (sports hernia) and muscle trigger points in the iliopsoas, QL, rectus femoris, and obliques. As mentioned above, many times FAI is treated locally and symptoms will subside with rest/time, but it will be imperative to treat movement dysfunction that the athlete will present with in order to help them return to sport with decreased risk of re-injury/minimal flare up.

Stay tuned for follow up posts in this series that will cover in-depth assessments in athletes for FAI (including at risk sports) as well as treatment interventions.

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