Ganglion cysts are fluid-filled lumps that typically develop in the soft tissue along tendons and joints [6, 7]. Intraosseous ganglia are rare and commonly located in the tibia, fibula, humerus, ulna, radius, carpal and tarsal bones, acetabulum, and scapular bone [1, 8,9,10,11]. The pathophysiology of intraosseous ganglia remains unclear, but it has been suggested that mechanical stress or trauma, synovial herniation, mucoid degeneration, and intramedullary metaplasia of mesenchymal cells may be the main causes [1, 12, 13].
Gas within the bone can be seen under various conditions, such as emphysematous osteomyelitis, pneumatocyst, osteonecrosis, and postoperative emphysema . The gas results from gas-forming pathogens, exogenous air, and nitrogen gas from soft tissue due to distraction induced negative pressure [14, 15]. In negative-pressure related cases, some lesions have been reported in near completely normal joints without evidence of degenerative changes or a clear connection to the joint space . Maldague et al.  believed that the presence of gas in a fracture was due to a lack of tissue, fluid and blood. Likewise, gas within bone tumors suggests a cellular paucity, indicating that reabsorption of gas to surrounding tissues does not occur easily or well liberating environment of nitrogen gas [1, 17]. In our case, a lack of tissue with mucoid content in the intraosseous ganglia may have contributed to the development of negative pressure, and the juxtaarticular location was also affected by negative pressure due to joint movement and maintenance. That negative pressure can result in a decrease in gas solubility. There was adjacent subcortical sclerosis and articular surface irregularities, but no clear connection was observed in CT and MRI. Gas collection was only identified on CT.
According to previous literature, gas in the vertebra or intervertebral discs, and formation of gas due to infection can be excluded if there is no evidence of concomitant infection, such as osteomyelitis or paravertebral soft tissue lesions . To the best of our knowledge, no malignant bone tumors with air in the lesion have been reported in the English literature. It is believed that the high-density tissue of malignant tumors contributes to gas reabsorption. In our case, the reasons for a diagnosis of chondrosarcoma based on MRI were as follows: greater than two-thirds endosteal scalloping of the normal cortical thickness, a peripheral lobular enhancing pattern, cortical breach, and an enhancing solid extraosseous component. These findings strongly suggest chondrosarcoma . Even on retrospective MRI review, air was not clearly defined. CT is optimal diagnostic method for intraosseous ganglia, because it can accurately illustrate underlying mineralization and even small amounts of gas [14, 18].
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