A vacuum phenomenon is a radiolucent collection of gas that is present at the sites of negative pressure. When traction is applied to a joint or when a joint dislocates, blood gases composed primarily of nitrogen leak out of the solution due to a reduced intra-articular pressure.1

A man in his 20s fell off scaffolding, landed on his everted foot and came to the emergency department with a deformed ankle. Computed tomographic (CT) scans revealed subtalar and calcaneocuboid fracture dislocation with presence of gas, although there was no break in the skin. We believe this gas was due to a vacuum phenomenon (fig 1).

• Download figure
• Open in new tab
• Download powerpoint

Figure 1 Computed tomographic scan showing presence of vacuum phenomenon.

Because of the low attenuation values of gas, the vacuum phenomenon is readily identifiable on CT scans. In the absence of penetrating trauma, the intra-articular gas seen on CT is a reliable indicator of recent dislocation and sometimes may be the only objective finding of the injury. The emergency medicine staff should be aware of this finding.