Computed tomographic demonstration of the vacuum phenomenon in the subtalar and tibiotalar joints.

Gas has been identified within diarthrodial joints for more than eight decades. From a radiologic standpoint, the so-called "vacuum phenomenon" is the most common demonstration of gas. We retrospectively reviewed computed tomographic scans of 495 feet and ankles performed over a 24-month period. We found 12 patients (2.4%) with clearly demonstrable gas within the tibiotalar or subtalar joints. This gas was not associated with the presence of fixation hardware or infection. Each patient with gas in the subtalar joint had plain radiographic changes consistent with subtalar arthritis. We conclude that gas in the subtalar joint can be effectively demonstrated by high-resolution CT techniques in the subtalar and tibiotalar joints in patients with advanced degenerative disease.