Femoro-Acetabular Impingement-MRI

There is evidence of altered signal intensity in the left femoral head in its superolateral aspect along with altered signal and cystic changes in the acetabulum. Also noted is bump in the femoral head just at the level of physeal scar. These findings are indicative of cam type femoro-acetabular impingement with associated degenerative changes in labrum/subchondral bone.

Cam impingement

Cam impingement results from an abnormality of the femoral head. On MRI or radiography, a prominent lateral extension of the femoral head at the expected step-off to the adjacent femoral neck can be seen  as in this case. This femoral “bump” is typically located lateral to the physeal scar. 

Pincer impingement

In pincer FAI, the acetabulum, rather than the proximal femur, has a morphologic abnormality that results in abnormal contact between the femur and acetabulum. Radiographic findings of acetabular retroversion include the crossover or “figure-8” sign and the posterior wall sign. The crossover sign is caused by the anterior wall of the superior aspect of the acetabulum projecting lateral to the posterior wall of the superior aspect of the acetabulum. The line that demarcates the posterior wall passes medial to the center point of the femoral head, rather than following the normal passage at or lateral to the center of the femoral head.