A "bone island" is a small region of compacted bone located within the more porous cancellous bone. The medical term for a bone island is an endostenosis, according to Bone & Spine.
Bone islands appear as small circular or oval regions of slightly denser bone when viewed with an X-ray, CT scan or similar imaging. They are essentially benign growths of high-density bone tissue. Bone islands aren't harmful and don't pose a health risk, states Radiopaedia.org. They are indistinguishable on first inspection from several more serious conditions. For instance, bone tumors such as osteomas and osteosarcomas can appear indistinguishable from a bone island and may require additional imaging or biopsy if the density is ambiguous or the patient presents other symptoms.