Bony sequestrum
The radiological definition of a bony sequestrum refers to an image of calcification within a lucent lesion, completely separated from the surrounding bone and without referring to the histological nature and vascular status of the calcified tissue .
The pathological definition of a sequestrum is defined as a piece of devitalised bone that has been separated from its surrounding bone during the process of necrosis. The pathological definition usually refers to a complication of osteomyelitis; and represents devascularisation of a portion of bone with necrosis and resorption of surrounding bone leaving a 'floating' piece. The sequestrum acts as a reservoir for infection and as it is avascular is not penetrated by antibiotics. It usually requires excision if cure is to be achieved. In some instances the sequestrum becomes encased in a thick sheath of periosteal new bone, known as an involucrum.
When the sequestrum is small and surrounded by lucent rim it is known as a button sequestrum. These are usually seen in the calvarium.
The main conditions that may present with a bony sequestrum are osteomyelitis and skeletal tuberculosis. Eosinophilic granuloma, lymphoma and malignant fibrous histiocytoma (which include fibrosarcoma and desmoids) can also present with a bony sequestrum. Some primary bone tumors such as osteoid osteomas can mimic a bony sequestrum .
As a bony sequestrum can have several differential diagnoses; a careful analysis of the sequestrum together with the clinical and radiological findings should help to narrow down the differential.
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