Osteoma & Bone Island (Enostosis / Ivory Exostosis)

A dense intramedullary sclerotic focus is most likely an enostosis, or bone island, which is a benign focus of compact, cortical-type bone within the medulla, but I must distinguish it from an osteoblastic metastasis. I would do this on the characteristic benign features. First, it should be asymptomatic, with no pain or tenderness at the site. Second, on radiographs and CT it has a dense radio-opaque centre with peripheral radiating spicules - the rose-thorn or brush-border appearance - that blend with the surrounding trabeculae. Third, the CT attenuation is very high, with a mean above about 885 Hounsfield units, which is higher than a typical metastasis. Fourth, a bone scan shows no increased uptake, whereas an osteoblastic metastasis usually does. And fifth, it is stable over time on serial imaging. Taken together, these reliably confirm a benign bone island. If instead the lesion is painful, shows increased bone-scan uptake, has aberrant or lower attenuation, or enlarges over time, it loses these reassurances and I would investigate further to exclude an osteoblastic metastasis or a low-grade central osteosarcoma, with serial imaging and occasionally biopsy. I would also remember the associations: multiple bone islands represent osteopoikilosis, a benign sclerosing dysplasia not to be biopsied as metastases, and if I were dealing with multiple osteomas of the skull or jaw I would consider Gardner syndrome and arrange colonic screening, because the associated colonic polyposis is premalignant. A typical bone island needs only reassurance.