* 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.

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