Femoroacetabular 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.
- essentially an over-coverage of the femoral head by acetabulum
- aspherical shape of the femoral head due to a bony protrusion, mostly located at the anterosuperior aspect of the femoral head-neck junction just lateral to the physeal scar 2
- 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: a mixture of the two
The pistol grip deformity first described by Stulberg et al. in 1975 2, is considered a typical 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 the pincer type, the anterior acetabular rim projecting laterally to the posterior rim which is called "crossover sign". The lateral centre angle or extrusion index may be measured to confirm acetabular over coverage.
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. An osseous bump lateral to the physeal closure is indicative of the cam type.
The alpha angle is the most frequently cited parameter to confirm a cam lesion 2. It is measured on axial slices 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 consider an angle >60º for reduced false positive diagnoses 16. This with cam-type FAI and an alpha angle of >65° are considered at increased risk of substantial cartilage damage 19.
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.
In addition to evaluating for the presence of a cam lesion or pincer morphology, degenerative changes of the lateral acetabular margin and the hip joint may be seen. A corresponding labral tear may also be visualised on MRI or CT arthrography.
Direct MR arthrography is the most accurate imaging study to diagnose cartilage damage 8 as well as labral tears, which have a high association with cam FAI 9. Both 3D gradient echo and spin echo sequences have been described with good diagnostic accuracy 11. However, evidence suggests that isometric 3T MR acquisition without direct arthrography has comparable sensitivity to MR arthrography 17.
Treatment and prognosis
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 dependant on the severity of the pathology. Cam lesions can be resected arthroscopically with concomitant repair of labral tears.
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