A bone island, also known as an enostosis, is a focus of compact bone located in cancellous bone. This is a benign entity that is usually found incidentally on imaging studies; however, the bone island may mimic a more sinister process, such as an osteoblastic metastasis (eg, from prostate cancer).
Pathophysiology: Although the exact etiology of bone islands is not clear, they are almost certainly developmental in nature, likely representing cortical bone that has failed to undergo medullary resorption during the process of endochondral ossification. Histologically, bone islands are intramedullary foci of normal compact bone with haversian canals and "thorny" radiations that merge with the trabeculae of surrounding normal cancellous bone.