Bony sequestrum: a radiologic review

According to a pathological definition, a bony sequestrum is defined as a piece of devitalized bone that has been separated from the surrounding bone during the process of necrosis. However, the radiological definition of a sequestrum is different and refers to an image of calcification visible within a lucent lesion, completely separated from the surrounding bone, without referring to the vascular status and histological nature of the calcified tissue. The term "button sequestrum" has been used in calvarial lesions. The prototype conditions that may present with a bony sequestrum are osteomyelitis and skeletal tuberculosis. Other conditions such as radiation necrosis, eosinophilic granuloma, metastatic carcinoma, primary lymphoma of bone, aggressive fibrous tumors may also manifest as osteolytic lesions containing a sequestrum. In addition, some primary bone tumors produce a matrix that may mineralize and sometimes simulate a bone sequestrum. These include osteoid tumors (osteoid osteoma, osteoblastoma), cartilaginous tumors (chondroma and chondroblastoma), lipomatous tumors (lipoma), and benign fibrous tumors (fibromyxoma, myxoma, and desmoplastic fibroma). Therefore, various conditions may present at imaging as a small area of osteolysis containing central calcifications. However, a careful analysis of the sequestrum as well as the associated clinical and radiological findings often enables to point toward a limited number of conditions.