Femoro-acetabular impingement

Femoro-acetabular impingement (FAI) refers to a clinical syndrome of painful, limited hip motion resulting from certain types of underlying morphological abnormalities in the femoral head/neck region and/or surrounding acetabulum. FAI can lead to early degenerative disease.

Pincer impingement is more common in middle-aged women, occurring at an average age of 40 years, and can occur with various disorders 2

Cam impingement is more common in young men, occurring at an average age of 32 years 2.

In osteoarthritis of the hip, primary causes (probably genetically determined cartilage quality) are traditionally differentiated from secondary causes, such as congenital, developmental or post-traumatic deformities of the hip bones.

Recent studies suggest, however, that more subtle developmental abnormalities at the femoral head-neck junction or the acetabulum play a substantial role in cases that formerly would have been classified as primary 1.


Two basic types of joint deformities, either alone or in combination, have been identified as important causes of early degenerative disease. This can lead to three patterns of disease. 

  • pincer type: essentially an over-coverage of the femoral head by the acetabulum
  • cam type:
    • typically becomes symptomatic in physically active young males, and refers to a bony protrusion, mostly located at the anterosuperior aspect of the femoral head-neck junction 2 just lateral to the physeal scar
    • technically, the word "cam" refers to a bump on a rotating shaft that transmits force to another part of an engine
    • in the case of the hip joint, this asphericity leads to unwanted force transmission to the acetabulum during flexion and internal rotation, leading to wear and tear of the labrum and cartilage 3
  • combined: mixture of the two occurring together
Plain radiograph

The pistol grip deformity first described by Stulberg et al in 1975 2, is considered a typical radiographic sign of cam impingement. The shape of the proximal femur in this deformity is reminiscent of a flintlock pistol known from old pirate movies. Since the visual aspect only provides a qualitative assessment of the deformity 4, several attempts at quantification have been made for use with conventional two-plane radiographs.

In pincer type, the anterior acetabular rim projecting laterally to the posterior rim which is called "crossover sign".


Because of the three-dimensional character of the deformity, CT or MR volume imaging with secondary radial (oblique) reformats along the axis of the femoral neck is more reliable to locate and quantify the cam deformity 7. Degenerative changes of the lateral acetabular margin and subchondral cyst may be seen. Osseous bump lateral to the physeal closure is indicative of cam type. Associated labral tear and hip osteoarthritis can be identified.

The most frequently cited parameter is the alpha angle, described by Nötzli et al 5 in 2002. It is described on axial views, as the angle between a line from the centre of the femoral head through the middle of the femoral neck and a line through a point where the contour of the femoral head-neck junction exceeds the radius of the femoral head. An angle >55° is considered indicative of cam impingement 2 but some authors 16 consider an angle >60º for reduced false positive diagnoses. 

A large systematic review by Wright et al 12 in 2015 concluded that increased alpha angle is the only FAI prognostic factor associated with the development of early osteoarthritis and a labral tear. However, inter- and intra-rater reliability with FAI parameters measured on conventional radiographs turned out poor in several studies 6

MR arthrography

MR arthrography is the most accurate imaging study to diagnose cartilage damage 8 as well as labral tears, which are highly associated with cam FAI 9. 3D gradient echo sequences have not been used much in direct MR arthrography so far. However, available results do not show a disadvantage over spin-echo sequences. Maybe, 3D MR arthrography may evolve to a one-stop-shop for measuring FAI parameters such as the alpha angle and detecting early secondary changes in symptomatic athletes, who may benefit from surgical correction of the deformity 11.

The natural history if untreated is for damage to acetabular cartilage leading to labral tears manifesting as anterior hip pain and progression to early osteoarthritis of the hip. Management options can range from nonoperative to operative dependent on the severity of the pathology.

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Article information

rID: 13803
Section: Pathology
Synonyms or Alternate Spellings:
  • Femoroacetabular impingement (FAI)
  • FAI
  • Femoro-acetabular impingement (FAI)

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Cases and figures

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    Fig. 2

Axial rad...
    Case 1: cam type - showing pistol grip deformity
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    Case 2: pincer type
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    Fig. 1

Marked bu...
    Case 3: combined type
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    Case 4: pincer type with protrusio acetabuli
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    Case 5: cam type on CT
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    Case 6: mixed type with osseous bump
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    Case 7: cam type
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    Case 8: cam type
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    Case 9
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    Case 10
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    Case 11: cam type
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