Bone Island or Enostosis

A bone island or enostosis  is a common benign bone lesion. It is asymptomatic and do not require treatment. It is usually seen as an incidental finding on x-ray, computed tomogram and MRI.

Bone island, is a focus of mature cortical bone within the cancellous bone.  Pelvis, femur, and other long bones are 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 bone island is larger than 2 cm, they are classified as giant bone island.

Enostosis is considered a hamartoma. Hamartoma is a is a benign  tumor like mass composed of of an overgrowth of mature cells and tissues normally present in the affected part, but with disorganization and often with one element predominating.

Other hamartomas in the bone are of following type

Bone-forming

Cartilage-forming

  • Osteochondroma
  • Multiple osteochondroma
  • Epiphyseal osteochondroma
  • Enchondromatosis

Fiber-forming

  • Nonossifying fibroma
  • Fibrous dysplasia

Benign non–matrix-forming

  • Hemangioma of bone
  • Skeletal hemangiomatosis

Presentation of Bone Island

Mostly, the bone island is asymptomatic and the lesion is usually an incidental finding.

Bone islands typically appear as sclerotic, round-to-ovoid intramedullary focus. The long axis of the bone island is aligned parallel to the long axis of the bone. Though bone island 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.

Osteopoikilosis  is a skeletal dysplasia that manifests as multiple bone islands  situated in a periarticular distribution in the epiphyses and e metaphyses of long bones, pelvis and scapulae. The distribution is symmetrical.

Imaging

On xray, the typical feature of bone island is homogeneous, sclerotic focus in the cancellous bone with distinctive radiating bony streaks that blend with the trabeculae of the host bone.. The long axis of a bone island typically parallels the long axis of the involved bone.

bone island xray

Image credit: Radiopedia

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.

T1- and T2-weighted magnetic resonance imaging  scans demonstrate low signal intensity like cortical bone.

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.

But 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 more likely to have increased activity.

Generally speaking, findings on radiographs are considered diagnostic. In cases of large or symptomatic lesions, scintigraphy followed by biopsy may be warranted.

Treatment

No treatment is required for bony island. Lesion at the best , needs to be observed.

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