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Background
Although uncommon, emphysematous osteomyelitis (EO) carries a significant level of morbidity, which is amplified by the necessity of amputating the affected bone. The condition is not well-described due to the limited number of cases documented, with fewer than 50 cases in existing literature. Our objective is to further elaborate on the relevant radiological observations and their distinction from common differentials related to this condition.

Educational Goals / Teaching Points
There are less than 10 case reports in the existing literature on EO of the foot and ankle and less than 30 studies about the entity. The objective of this presentation is to acquaint radiologists with this uncommon yet significant diagnosis and its imaging findings. Although EO can potentially affect any bone, it typically manifests in the pelvis, femur, tibia, fibula, and spine. Nevertheless, the occurrence of this condition in the foot is considered atypical. CT excels at detecting early indicators of infection, including the presence of gas, while MRI offers the advantage of enhanced visualization of marrow signal changes and soft tissue alterations. CT pumice stone sign, characterized by clusters of irregularly sized intramedullary gas foci between 2–5 mm resembling pumice stone, is observed in 96% of cases. Most cases reported in the literature do not exhibit cortical bone destruction, distinguishing them from traditional osteomyelitis. Intraosseous gas can be seen in various benign and aggressive conditions, such as postsurgical/postbiopsy changes, trauma, intraosseous and/or soft tissue abscess, subchondral cyst, intraosseous pneumatocyst, osteonecrosis, osteoblastic metastases, or benign bone cysts, to name a few. In addition to the patient's clinical history, distinctive imaging features like the uneven and bubbly distribution of gas, or extension into nearby soft tissues, can be valuable for distinguishing between these infectious conditions.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
The study was conducted as a retrospective review of 10 cases with a review of the literature. Most of the demographic is comprised of diabetic Black men in their 50–60s. Wound/blood cultures grew mostly polymicrobial organisms. Additionally, a qualitative review of existing literature was done by searching the PubMed database using the keywords “emphysematous osteomyelitis,” yielding 30 full-access articles. In our center, radiography was done in eight cases, contrast-enhanced CT in six cases, and MRI in four cases. Plain radiographs show soft tissue gas in 60% of cases. Both CT and MRI showed the presence of intramedullary gas. Some cases also had extramedullary gas in adjacent locations with emphysematous septic arthritis, myositis, and tenosynovitis.

Conclusion
This is the largest single-institution study on EO of the foot and ankle. There are many differentials for intraosseous gas; however, the presence of clusters of irregularly sized intramedullary gas within the appendicular skeleton serves as a pathognomonic indicator of EO.