FBI sign

Fat-Blood Interface Sign. A semi-lunar post-traumatic soft tissue effusion, or fat fluid interface seen on lateral radiographs of the knee due to lipohaemarthrosis, a finding highly suggestive of an underlying fracture extending into the joint

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FBI sign

Fat-blood interface Orthopedics A semi-lunar soft tissue effusion, seen in post-traumatic lipohemarthrosis, using a 'horizontal' beam X-ray; the FS is most commonly seen in the knee and in the shoulder, because of ↑ marrow fat and blood in the joint space.

McGraw-Hill Concise Dictionary of Modern Medicine. © 2002 by The McGraw-Hill Companies, Inc.