The presence of intraosseous gas is not regularly expected in any human bone,
except in the paranasal sinuses and in the middle ear.
It is thus considered either a variant of normal or a pathological finding,
and it should raise concern for the presence of emphysematous osteomyelitis,
in the adequate clinical setting.
Emphysematous osteomyelitis is a rare and potentially fatal pathological entity in which the presence of intraosseous gas is due to bone infection,
with only a few cases having so far been reported.
It is usually caused by anaerobic bacteria,
seeded from haematogenous spread and it is associated with many comorbidities such as malignancy,
diabetes mellitus,
alcohol abuse,
Crohn’s disease and immunosuppression.
The infection can be either mono or polymicrobial and the responsible organisms are generally anaerobes or members of Enterobacteriaceae family.
Very few cases have been reported in the literature so far.
The most common locations are the pelvis,
femur,
tibia,
fibula,
thoracic and lumbar vertebrae.
There is no gender predilection for its occurrence and the median age of diagnosis is 51 years.
Other entities are known causes of intraosseous air,
such as recent bone surgery,
bone biopsy,
penetrating wounds / compound fractures,
lympangiomiomatosis of bone,
osteonecrosis / Kummel's disease and pneumatocysts.
Very few cases of gas within bone tumors and solitary bone cyst have also been described.
Kummel disease and pneumatocyst are increasingly being recognized,
particularly in CT scans and deserve particular attention because they involve the spine,
a common location for emphysematous osteomyelitis:
- Kummel's disease is a process of delayed post-traumatic osteonecrosis of the vertebrae.
It's an entity which affects vertebra in middle-aged to elderly patients and although rarely reported,
it likely occurs with higher frequency than recognized.
- Pneumatocysts are gas-cointaining cystic structures seen inside the bone,
usually of small size,
that do not communicate with joints.
Their pathogenesis is not well understood,
although some authors believe they are an extension of intradiscal gas / vacuum phenomenon.