Dissection of gas into an intraspinal synovial cyst from contiguous vacuum facet

Intraspinal synovial cysts probably originate from herniation of synovium through a weakened facet joint capsule. The computed tomographic (CT) findings of such lesions were recently reported. An additional characteristic is gas within the cyst as demonstrated by CT. We postulate the source to be an adjacent diseased facet joint with a vacuum cleft with consequent dissection of gas into the cyst.