On plain radiography, pneumatocysts appear as lucent areas with sclerotic margins projecting over an osseous structure. Although occasionally they are visible, plain radiography has its limitations. The small size and location of the lesion are variables that affect the sensitivity of plain radiography.
CT is the most helpful modality in making this diagnosis. CT has greater contrast resolution and high sensitivity for gas collections, even when they are small.6 The CT scan demonstrates the gas density within the medullary cavity of the bone and its borders and documents precisely the size and location of the lesion in the bone and its proximity to the joint.
On MRI, intraosseous pneumatocysts show low signal on T1- and T2-weighted images with no enhancement on contrast-enhanced T1-weighted pulse sequences.9 Bone scintigraphy performed in 2 cases did not identify these lesions.2,6
Intraosseous pneumatocyst is a rare, asymptomatic, benign entity that can occur in association with degenerative joint disease and vacuum phenomenon or as an isolated idiopathic finding. The lesion resides in subchondral bone, and the sine qua non is gas attenuation in bone. Other entities that can present with intraosseous gas include postsurgical and posttraumatic states, anaerobic osteomyelitis, ischemic osteonecrosis, osteoarthritis, osteochondrosis, neoplasm, and microtrauma in patients with osteoporosis.
Recognizing intraosseous pneumatocysts and distinguishing them from other gas-containing intraosseous lesions seen in various clinical settings is essential. The presence of an incidental intraosseous gas collection surrounded by a sclerotic rim or demonstrating well-circumscribed borders is pathognomonic for intraosseous pneumatocyst, which requires no follow-up, biopsy, or surgical intervention.
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