Cortical bone islands

These are small, benign masses of cortical bone which may be found in the medullary cavity of long bones.

• an enostosis or bone island represents a focus of mature compact (cortical) bone within the cancellous bone (spongiosa)
  
  
• this benign lesion is probably congenital or developmental in origin and reflects failure of resorption during endochondral ossification
  • a bone island can be virtually diagnosed based on its characteristic clinical and radiologic features
    
    
• typically asymptomatic, the lesion is usually an incidental finding, with a preference for the pelvis, femur, and other long bones, although it may be found anywhere in the skeleton, including the spine
  
  
• radiological studies:
  • plain radiography reveals
    • a homogeneously dense, delineated, sclerotic focus in the cancellous bone with distinctive radiating bony streaks ('thorny radiation') that blend with the trabeculae of the host bone, creating a feathered or brush-like border
    • on a plain radiograph they are usually less than 1 cm in diameter.
      
  • CT scan - a bone island appears as a low-attenuation focus
    
    
  • MRI a bone island shows low signal intensity like cortical bone
    
    
  • a distinguishing feature of bone islands is that they are usually "cold" on skeletal scintigraphy
    • thus, bone scan has been and continues to be the means of differentiating bone islands from the more aggressive entities
    • however, reports of histologically confirmed bone islands that were scintigraphically active have raised a note of caution about relying on this modality in the differential consideration of lesions otherwise characteristic of bone islands (1)
      • guides to the correct diagnosis should be looked for in the individual clinical situation and in the morphological features of the lesion on plain radiography, CT, and MRI, without regard to the lesions activity on bone scan (1)

They are of no clinical importance except for their striking appearance on radiographs.

Reference:

• Greenspan A. Bone island (enostosis): current concept--a review.Skeletal Radiol. 1995 Feb;24(2):111-5