Last Updated on January 30, 2025
A bone island or enostosis is a common benign sclerotic bone lesion. It is asymptomatic and does not require treatment. It is usually seen as an incidental finding on x-ray, computed tomogram and MRI.
The pelvis, femur, and other long bones are the most common sites. These are also found in ribs, the carpal and tarsal bones, and the thoracolumbar vertebral bodies though it may be found anywhere in the skeleton.
Bone islands usually are 1 mm to 2 cm in diameter.
When the bone island is larger than 2 cm, they are classified as the giant bone island.
Bone islands earlier also known as enostoses, are common benign sclerotic bone lesions. When they occur in the skull they are called osteomas. However, the latest recommendations of WHO have advised that enostosis is not a recommended term. The lesions are more common in males.
Osteopoikilosis is a condition, a skeletal dysplasia, characterized by the presence of multiple bone islands.
Cause
The exact cause of this lesion is not known. However, factors such as genetics, trauma, or mechanical stress on the bone have been suggested to be contributor to the development of the lesions.\
Sites of Occurrence
These often occur in the medullary cavity when it merges with merging with trabecular bone. These may be may be solitary or multiple. Though these can occur anywhere, there is a predilection for the pelvis, long bones, spine, and ribs.
Presentation of Bone Island
Mostly, the lesion is asymptomatic and usually an incidental finding on x-ray. CT or MRI done for other purposes. Rarely joint stiffness or occasional pain (very rare) may be complained in selected locations.
Imaging
X-ray
The lesion appears as small round or oval foci of homogeneously dense bone within the medullary space. They do not cause cortical destruction or periosteal reaction.The long axis of the bone island is often parallel to the long axis of the bone.
Xrays are considered diagnostic in these lesions.
CT
On CT scan, their appearance correlates with their plain film appearance. They are sclerotic and hyperdense foci with “thorny” radiations that blend with surrounding trabeculae.
MRI
These show low signal intensity on all sequences just like compact bone . There is no edema surrounding the edema nor there is any enhancement on contrast.
Nuclear medicine
Bone islands do not usually demonstrate increased radiotracer activity on bone scans (see the image below) in contrast to more aggressive lesions, such as metastases or primary bone tumors.
However, there are reports of bone islands that have shown increased activity which has been suggested either due to increased metabolic activity or to osteoblastic bone remodeling associated with the growth of bone islands. Giant bone islands are reported to be are more likely to show increased scintigraphic activity. They appear kind of warm rather than hot on scintigraphy.
Differential Diagnoses
Generally speaking, findings on radiographs are considered diagnostic. In cases of large or symptomatic lesions, scintigraphy followed by biopsy may be warranted.
Most of the cases have characteristic appearance but some of the lesions, especially the larger lesions may need to be differentiated from other lesions.
The following lesions may be considered for image differentials as these lesions may mimic something sinister.
- Sclerotic bone metastasis
- Osteoid osteoma
- Osteosarcoma [low grade]
- Osteoblastoma
Treatment
No treatment is required for the bony island. The lesion at best needs to be observed. If observation is contemplated, the patient should be asked to follow up at regular intervals.
Though the lesion is completely harmless on its own, it sometime may mimic a more, the bone island may mimic a more threatening process like osteoblastic metastasis and need to be differentiated as outlined above.
References
- Pattamapaspong N, Peh WC. Benign incidental do-not-touch bone lesions. Br J Radiol. 2022 May 31. [Link].
- Sala F, Dapoto A, Morzenti C, et al. Bone islands incidentally detected on computed tomography: frequency of enostosis and differentiation from untreated osteoblastic metastases based on CT attenuation value. Br J Radiol. 2019 Nov. 92 (1103).
