Enostosis (also known as bone islands) are common and benign bone lesions that are usually seen as an incidental findings. They constitute a small focus of compact bone within cancellous bone.
Enostoses are likely congenital or developmental, and are thought to represent either hamartomatous lesions or failure of osteoclastic activity during bone remodelling 1.
Bone islands may occur anywhere in the skeleton, although there is some predilection for pelvis, long bones, spine and ribs.
- osteopoikilosis - multiple bone islands
Plain film / CT
They are commonly seen as small round or oval foci of dense bone within the medullary space. The appearance of radiating spicules at the margins that blend with the surrounding trabeculae is pathognomonic.
In a diaphysis, the long axis of a bone island typically parallels the long axis of the involved bone. In the metaphyses and other regions where trabeculation is more random, the bone islands are typically more spherical.
The size of a bone island is typically less than 1cm, although large ones may occur particularly in the pelvis, and are termed "giant bone islands" 3. Sometimes a slow increase in size is seen.
Low signal intensity on all sequences.
There is usually no tracer uptake, and a normal bone scan can exclude an osteoblastic metastasis or osteosarcoma. However, low grade scintigraphic activity has sometimes been reported in histologically proven enostoses 2.
Enostosis is one of the skeletal “don’t touch” lesions.
In the vast majority of cases, bone islands have a pathognomonic appearance. Larger lesions may sometimes pose a diagnostic dillema, particularly in the setting of known malignancy.
Imaging differential considerations include: