Lipohemarthrosis

Fig.1&2: Cross-table lateral radiograph (1) and sagittal-reformatted CT image (2) of the knee show a fat-blood interface (arrow) in the suprapatellar bursa of a 57-year-old woman who was a pedestrian hit by a car. Yellow asterisk = fat in the bursa, red asterisk = blood.

Lipohemarthrosis

• Mixture of blood and fat in a joint cavity following trauma
• Fat from the marrow space enters the joint through intra-articular fracture
• Fat is less dense than blood; therefore it layers above blood.
• On x-ray, fat is less dense than blood; therefore it can be seen as different density using radiography and CT
• Horizontal beam of x-ray needs to be parallel to the fat-blood interface, to be able to demonstrate the interface.
  
• Described in intra-articular fractures of the knee, shoulder, hip and elbow. However, it is not seen in all cases of intra-articular fracture. On the other hand, it is helpful for the diagnosis of an occult fracture.

Reference:
Arger PH, Oberkircher PE, Miller WT. Lipohemarthrosis. AJR 1974;121:97-100.