Spontaneous vacuum pneumarthrography revisited: the significance of the vacuum phenomenon in the lateral compartment of the knee.

We report seven cases of spontaneous vacuum phenomenon of the lateral compartment of the knee. Previous reports of the vacuum phenomenon suggest that it is related to traction on a joint or the absence of an effusion. The presence of this finding on plain radiographs, or artifacts associated with it on magnetic resonance imaging, have been said to create a false-positive indication for meniscal tears, especially in the medial compartment. We describe plain radiographic, magnetic resonance, and arthroscopic findings in seven consecutive patients with vacuum phenomenon of the lateral compartment of the knee and conclude that this finding may be a true-positive indication for degenerative meniscal tears and chondrosis. The etiology of this finding and its long-term significance remains unclear.