What causes the intravertebral vacuum cleft on radiography, CT and MRI of the spine?
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Answer:
The intravertebral vacuum cleft is a linear or crescentic collection of gas within a collapsed vertebral body, seen as a lucency on radiographs and CT and as a signal void, or a fluid-filled cleft, on MRI. It indicates avascular necrosis of the vertebral body with a non-united fracture, the condition known as Kümmell disease when it follows a delayed collapse weeks or months after apparently minor trauma. Its diagnostic value is that it is regarded as a sign of a benign, non-neoplastic collapse and argues against malignant infiltration, in which the marrow is replaced by tumour and gas does not form. On MRI the cleft may fill with fluid when the patient has been recumbent, giving a bright T2 cleft rather than a signal void, and this fluid sign carries the same meaning; imaging in extension or a delayed sequence increases its conspicuity. Associated features are dynamic mobility of the collapsed vertebra between flexion and extension or between supine and upright radiographs, which is the hallmark of an unstable non-union, and preserved posterior element signal. The distinction from infective spondylodiscitis, which shows disc destruction, endplate erosion and paravertebral inflammatory change, is essential. Treatment of a painful mobile non-union is usually vertebroplasty or kyphoplasty, and cement fills the cleft directly.
Why is it called so?
The cleft appears as a vacuum, a gas-filled space, within the vertebral body itself rather than in the disc.
Pathophysiology
After an osteoporotic compression fracture, the fractured trabecular bone may fail to heal because of impaired vascular supply to the fragment, continued micromotion or steroid use. The non-united fracture line becomes a cleft lined by fibrous tissue and necrotic bone. As with the vacuum phenomenon in a degenerate disc, negative pressure generated within this cleft during extension or traction draws nitrogen out of solution from the surrounding tissue fluid, so gas accumulates in the space. When the patient lies down and the cleft is compressed, the gas is resorbed and replaced by transudated fluid, which is why the same cleft can appear as a signal void or as fluid depending on positioning and timing.
Alternative names: Kümmell disease; vertebral osteonecrosis cleft
Other associated named signs: Vacuum disc phenomenon, the fluid sign of a benign collapse, and vertebra plana
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