Images in Emergency Medicine

Article Outline

• Diagnosis
  • Lipohemarthrosis
• Acknowledgment
• Copyright

[Ann Emerg Med. 2007;50:120.]

Missed fracture is a major cause of dissatisfaction, litigation, and sometimes morbidity. A knee radiograph that reveals effusion but no fracture usually indicates only soft-tissue injury. Figure 1, Figure 2 show the knee of a 22-year-old woman who fell onto her flexed knee while ice skating. Plain radiographs revealed normal-appearing bones and an effusion. The patient was discharged, but a second radiologist reread the film and found an abnormality. The patient was contacted and asked to return for a magnetic resonance imaging (MRI) study, which revealed a fracture of the patella.

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• Figure 1. 

  Lateral radiograph of the knee revealing an effusion that contains a linear interface between 2 soft tissue densities.

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• Figure 2. 

  Lateral MRI of the knee revealing in greater detail the linear interface between 2 soft tissue densities, suggestive of lipohemarthrosis. Used with permission of Daniel J. Pallin, MD, MPH, Department of Emergency Medicine, Brigham and Women’s Hospital, the Division of Emergency Medicine, Children’s Hospital Boston, and the Division of Emergency Medicine, Harvard Medical School, Boston, MA.

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Diagnosis 

Lipohemarthrosis 

The first image is a lateral plain radiograph, with a linear interface between 2 soft-tissue densities, just superior to the posterior aspect of the patella. This linear interface indicates lipohemarthrosis, resulting from entry of marrow fat into the joint cavity. Lipohemarthrosis can also be detected when fat globules are observed on needle aspiration of a hemarthrosis. The second image is a sagittal MRI that shows the interface in greater detail, with blood products posterior and fat anterior. (The fracture is not shown in these images.) Emergency physicians should be vigilant for lipohemarthrosis on radiographs of traumatized joints, just as they are vigilant for the fat pad sign on radiographs of traumatized elbows. While the fat pad sign indicates displacement of a fat pad by an effusion, lipohemarthrosis indicates release of fat into the joint cavity, ie, an effusion composed of both blood and fat.

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The author is grateful to Aaron Sodickson, MD, for his help.