Gas bubbles in the hip joint on CT: an indication of recent dislocation

OBJECTIVE: The purpose of this study was to determine if gas bubbles in the hip joint seen on CT scans after trauma are reliable indicators of recent (< 48 hr) hip dislocation. We believe that the gas seen in the hip joint represents intracapsular nitrogen bubbles that result from the vacuum created by forcible distraction associated with traumatic dislocation. MATERIALS AND METHODS: CT scans of 79 consecutive patients with pelvic injury were reviewed retrospectively. We noted the number and position of intracapsular gas bubbles, presence of joint effusion, soft-tissue injury, and associated fractures or dislocations. Intracapsular gas bubbles were defined as round areas of low attenuation, in an intracapsular position, that on visual inspection were equivalent to air. Seventy-three of 79 patients were imaged within 48 hr of injury. Most patients had been involved in a motor vehicle collision or were pedestrians struck by a motor vehicle. Fifteen patients had hip dislocation, including one patient with bilateral dislocation. Fifty-five patients had pelvic fractures without hip dislocation, and nine patients had soft-tissue injury without fracture or dislocation. Fourteen of 16 dislocations had been reduced at the time of scanning. RESULTS: Gas bubbles were seen in the hip joint on CT scans in 13 (81%) of the 16 dislocated hips, including 12 (92%) of 13 dislocations in patients scanned within 4 hr of admission. Bubbles were present in 11 (79%) of 14 hip joints that had dislocations reduced at the time of scanning and in both hip joints that remained dislocated. Most bubbles were located anterior to the femoral neck; however, bubbles were also seen posteriorly. Bubble size (1-3 mm) and number (1-7) varied. Only two (3%) of 64 patients without dislocation had intracapsular gas bubbles; one had been shot and the other had extensive soft-tissue emphysema. CONCLUSION: In the absence of penetrating trauma, intracapsular gas bubbles on CT are reliable indicators of recent hip dislocation and may be the only objective finding of this injury.