FAI Femoroacetabular Impingement | Hip Impingement
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FAI Femoroacetabular Impingement

James McCormack
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What is FAI hip?

FAI, or femoroacetabular impingement, is a condition or syndrome that is a common cause of hip pain. The hip is a ball and socket joint, the femoral head and the acetabulum. Impingement of the hip joint can occur when the shape of the ball or socket (or both) causes a pinch in certain positions.

There is normal variation between our skeletons, and different shapes (morphologies) of the femoroacetabular joint are common. If there is additional bone on the femoral head, it can be less round and more oval or can have a wider neck, this can be called a Cam morphology.

If the acetabulum has extra bone, making it too deep or the opening too narrow, this is called a Pincer morphology. Femoroacetabular impingement syndrome can be caused by a Cam or Pincer morphology or a combination of both. 

How common is femoroacetabular impingement?

The painful condition of FAI affects 10-15% of the population, with most symptoms being experienced by patients between 20-40 years old. The sports most associated with this condition are ice hockey, soccer, ballet, dance and gymnastics, rowing and golf (Casta, 2015). Martial arts also have a high prevalence (Lee et al, 2016).

Picture of a person holding a painful hip


One of the main symptoms of FAI (femoroacetabular impingement) is C-sign hip pain. This is the description of pain where the patient places the thumb and index finger in a C shape around the side of the hip. The C sign is a recognised diagnostic sign.

Other symptoms include worsening pain with activity or prolonged sitting, a limp when walking or running, and stiffness of the joint. There is a common pattern of pain felt at the end of the range of hip movement into flexion, adduction and internal rotation.

Occasionally the patient may feel reduced balance or unstable on one leg, and it is fairly common for there to be a palpable and/or audible click or pop.


Femoroacetabular Impingement can be diagnosed clinically by a sports doctor or physical therapist through a series of clinical tests. These clinical tests include moving the hip through a full range of movements and noting when pain is felt.

Pain with the combined movement of flexion, adduction and internal rotation of the hip is common with FAI affecting the front of the joint. This test is called a FADDIR test, “F” for flexion, “ADD” for adduction and “IR” for internal rotation.

For posterior hip impingement, a FABER test can be performed, “F” for flexion, “ABD” for abduction and “ER” for external rotation.

Picture of FAI hip assessment

While these tests provide some information about irritation in the joint, they are not specific to diagnose FAI when used on their own. Clinicians will use a cluster of different tests to improve the accuracy of their diagnosis. Further confirmation can come from diagnostic imaging such as X-ray or MRI to show a particular morphology of the hip that is relevant to their symptoms.

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