Femoroacetabular Impingement

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Written by: Dr Jeremy Steinberg – created: 15 April 2022; last modified: 17 April 2022

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Femoral acetabular impingement FAI.svg
Normal, CAM, and pincer morphologies.
Femoroacetabular Impingement
Definition Pathological contact between the femoral head-neck junction and the acetabular rim during functional hip movement
Classification CAM, Pincer, or mixed
Risk Factors Dose-response relationship between intense sporting activity during adolescence and CAM morphology.
Clinical Features Insidious mild positional or movement related groin pain.
Diagnosis Presence of cam and/or pincer morphology on imaging in the appropriate clinical picture.
Tests Plain films and MRI
Taxonomy Called femoroacetabular impingement syndrome in the presence of symptoms.
Treatment Exercise, physiotherapy, surgery.
Prognosis Risk factor for hip osteoarthritis

Femoroacetabular impingement (FAI) refers to abnormally shaped femoral head and neck junction and/or acetabulum. This creates a space conflict with hip flexion. It is a risk factor for early onset hip osteoarthritis, but is not a sufficient cause.


There are two patterns, cam and pincer, as well as a combination of both types. In cam FAI there is a thickened, aspherical femoral head-neck junction, which abuts against the anterosuperior labrum with hip flexion, and results in compression of the labrum and acetabular cartilage. This leads to separation of the labrum from the acetabular cartilage and delamination of the acetabular cartilage from the subchondral bone.[1]

In pincer FAI, there is an overcoverage of the acetabulum with a deepened acetabular on the femoral head. The femoral neck abuts against the acetabular labrum and compresses it, damaging both the labrum and underlying cartilage. Lesions usually occur anterosuperiorly from hip flexion like with cam lesions, but also posteroinferiorly from posterior subluxation due to continued flexion causing a contre-coup lesion.

Clinical Features

The pain is often insidious with mild groin pain that occurs over many years and then suddenly worsens. The patient often uses a C grip with their index finger and thumb proximal to the greater trochanter to indicate the painful region. The pain is most noticeable when sitting with the hips flexed to 90 degrees for long periods and reduces with standing. There may be additional pain in the back or thigh. The patient may describe mechanical symptoms such as catching, stiffness, or restricted motion. In the presence of an associated labral tear there may be sharp pain with catching sensations.

There may be limited flexion or internal rotation of the hip and provocation of groin pain with FADDIR test. The flexion internal rotation test may also be positive. However both of these tests are sensitive but not specific.

Differential Diagnosis

Differential Diagnosis


Obtain AP and lateral plain films. See Hip Radiograph for measurements. MRI is the definitive scan.


The CHECK study assessed 1002 patients aged 45–65 years with early symptoms of hip OA but without definite radiographic evidence of the same over 5 years. At baseline, 76% of the included hips had no radiographic signs of osteoarthritis and 24% doubtful osteoarthritis. Cam deformities and acetabular dysplasia were shown to be associated with an increased odds ratio for progression to early onset osteoarthritis.[2] While pincer changes were actually associated with a reduced risk.[3]

There is also risk for development of labral tears.



  1. 1.0 1.1 1.2 Murphy, Nicholas J.; Eyles, Jillian P.; Hunter, David J. (2016-11). "Hip Osteoarthritis: Etiopathogenesis and Implications for Management". Advances in Therapy. 33 (11): 1921–1946. doi:10.1007/s12325-016-0409-3. ISSN 1865-8652. PMC 5083776. PMID 27671326. Check date values in: |date= (help)
  2. Agricola, Rintje; Heijboer, Marinus P.; Bierma-Zeinstra, Sita M. A.; Verhaar, Jan A. N.; Weinans, Harrie; Waarsing, Jan H. (2013-06). "Cam impingement causes osteoarthritis of the hip: a nationwide prospective cohort study (CHECK)". Annals of the Rheumatic Diseases. 72 (6): 918–923. doi:10.1136/annrheumdis-2012-201643. ISSN 1468-2060. PMID 22730371. Check date values in: |date= (help)
  3. Agricola, R.; Heijboer, M. P.; Roze, R. H.; Reijman, M.; Bierma-Zeinstra, S. M. A.; Verhaar, J. a. N.; Weinans, H.; Waarsing, J. H. (2013-10). "Pincer deformity does not lead to osteoarthritis of the hip whereas acetabular dysplasia does: acetabular coverage and development of osteoarthritis in a nationwide prospective cohort study (CHECK)". Osteoarthritis and Cartilage. 21 (10): 1514–1521. doi:10.1016/j.joca.2013.07.004. ISSN 1522-9653. PMID 23850552. Check date values in: |date= (help)

Literature Review