Discussion of Femoroacetabular impingement
- Femoroacetabular impingement (FAI) is a relatively newly described etiology for hip pain and early onset osteoarthrosis.
- Patients are predisposed to developing FAI due to their intrinsic (congenital or acquired) anatomy.
- The impingement in FAI is caused by abnormal anatomic features of the proximal femur or the acetabulum.
- Impingement can lead to labral tears, cartilage lesions, and eventually premature osteoarthrosis.
 Predisposing factors
- Developmental dysplasia of the hip
- Slipped capital femoral epiphysis
- Legg-Calve-Perthes disease
- Avascular necrosis of the femoral head
- Malunited femoral-neck fractures
- Acetabular protrusio
- Elliptical femoral head
- Prominence of the femoral head–neck junction
- Retroverted acetabulum
- Hip pain most significant with internal rotation.
- The symptoms of impingement are usually unilateral.
- Worse after prolonged periods of sitting or when significant stress is placed on the hip.
 Physical examination
- Loss of internal rotation out of proportion to the loss of range of movement at other positional extremes.
- Osteoarthosis will usually produce a universal limited range of motion.
- A grinding and popping sensation can be felt when the femur is externally rotated when the hip is maximally abducted.
 Pincer FAI
- “Acetabular” FAI
- Abnormal acetabulum contacting a normal femur.
- Usually presents in elderly women.
- This can be due to increased acetabular anteversion or coxa profunda.
- Acquired causes can be acetabular protrusio or postsurgical prominent anterosuperior acetabulum.
- Acetabular retroversion: may be seen as a result of trauma or as part of acetabular dysplasia (either in isolation or part of a complex).
- The acetabular labrum is the first structure to be effected.
- The repetitive impact results in degeneration of the labrum with intrasubstance ganglion formation or ossification of the acetabular rim.
- Ossification can lead to further deepening of the acetabulum and therefore more overcoverage of the femoral head by the acetabulum.
- When cartilage lesions do occur with pincer impingement they are typically focal and involve small areas of the acetabular rim.
- Reduction of anterior acetabular coverage of the femoral head by an excision of the osseous prominence at the acetabular rim.
 Cam FAI
- "Femoral" FAI
- Abnormal morphology of the anterior femoral head–neck junction.
- Prominence of the femoral head-neck junction in the anterior/anterosuperior portion of the proximal femur is known as the “pistol grip deformity"
- Younger males.
- Prominence femoral head-neck junctions produces intermittent and consistent stress (the cam effect) on the associated articular cartilage
- The shear forces cause damage to both the acetabular labrum and the articular cartilage of the femoral head and acetabulum.
- Excision osteoplasty of the femoral neck.
 Imaging Findings for Femoroacetabular impingement
 Pincer type
 Plain films
- Coxa profunda
- Acetabular protrusio
- Acetabular rim ossification
- Acetabular retroversion
- Crossover sign: The anterior acetabular rim is projected laterally relative to the same point of the posterior rim in the superolateral aspect of the acetabulum.
- Same as plain films
 Femoral type
 Plain film
- Anteroposterior radiographs may show the “pistol grip deformity.”
- The lateral view may also be helpful to show the decreased femoral head-neck ratio.
- α angle : objective representation of the prominence of the anterior femoral head-neck junction.
- A section through the narrowest portion of the femoral neck is selected
- A best-fit circle is drawn around the femoral head to determine the head-neck junction
- A line down the center of the femoral neck and the head-neck junction.
- A second line that extends from the intersection of the first line and the center of the femoral head to the point where the osseous anterior femoral head intersects the circle.
- An angle greater than 55 degrees has been shown to be closely associated with symptomatic cam-type femoroacetabular impingement.
- The larger the α angle, the more the predisposition for impingement.
 See Also
 External Links
 References for Femoroacetabular impingement
- Beall DP, Sweet CF, Martin HD, Lastine CL, Grayson DE, Ly JQ, Fish JR. Imaging findings of femoroacetabular impingement syndrome. Skeletal Radiol. 2005 Nov;34(11):691-701.
- Kassarjian A, Yoon LS, Belzile E, Connolly SA, Millis MB, Palmer WE. Triad of MR arthrographic findings in patients with cam-type femoroacetabular impingement. Radiology. 2005 Aug;236(2):588-92.
- Blankenbaker DG, Tuite MJ. The painful hip: new concepts. Skeletal Radiol. 2006 Jun;35(6):352-70.