When there is an intraarticular fracture of the tibial plateau or of one of the femoral condyles, bone marrow (mostly fat) migrates from the marrow space into the joint space. With fracture, blood also escapes, and a lipohemarthrosis is produced. Fat is less dense than blood, and it layers superior to it, producing the change in radiographic density referred to as a fat-blood interface (FBI sign).
CASE 1: These four radiographs are from a 50 year old male who fell 1 day prior to presentation. There is a tibial plateau fracture and a lipohemarthrosis. Click on each image for a larger view and notice the location of the fat-fluid level.
CASE 2: This radiograph is from a 52 year old male who was hit by a forklift. It is a cross table lateral view acquired in the emergency room. Click to enlarge.
The FBI Sign:
CT, MRI, and Radiographic Appearance of Lipohemarthrosis