What is a bone island?

Bone Island (Enostosis): Characteristics and Clinical Significance

A bone island (enostosis) is a benign, non-progressive focus of mature compact (cortical) bone within cancellous bone, typically asymptomatic and discovered incidentally during radiographic examinations. 1

Definition and Pathophysiology

• Bone islands represent small foci of mature compact bone within the spongiosa (cancellous bone), considered to be either hamartomas or developmental anomalies 1
• They likely result from failure of bone resorption during endochondral ossification, representing a congenital or developmental condition rather than a true neoplasm 1
• Histologically, bone islands consist of normal, mature cortical bone tissue that has formed within the medullary cavity 1

Epidemiology and Distribution

• Bone islands can be found in patients of any age but are most commonly identified in adolescents and young adults 2
• They show a preference for the pelvis, femur, and other long bones, though they may occur anywhere in the skeleton, including the spine 1
• In the maxillofacial region, they are more commonly found in the mandible, particularly in the molar region, with a reported incidence ranging from 2.3% to 9.7% 2

Radiographic Characteristics

• On plain radiographs, bone islands appear as homogeneously dense, sclerotic foci with distinctive radiating bony streaks ("thorny radiation") that blend with surrounding trabeculae 1
• The characteristic feathered or brush-like border helps distinguish bone islands from more aggressive lesions 1
• On CT scans, bone islands appear as low-attenuation foci with density similar to cortical bone 1
• MRI sequences show low signal intensity on all sequences, similar to cortical bone 1
• Bone islands are typically "cold" (non-active) on skeletal scintigraphy, though some histologically confirmed bone islands have shown activity on bone scans 1

Clinical Significance

• Most bone islands are asymptomatic and considered "do not touch" lesions that require no treatment 3
• Bone islands are typically small (2-3 mm), but can occasionally grow larger 4
• Giant bone islands (>2 cm) have been reported to cause symptoms in some cases 5
• Rarely, even small bone islands (<2 cm) may be associated with pain, potentially warranting surgical intervention if other causes of pain have been excluded 5

Differential Diagnosis

• Bone islands must be differentiated from more aggressive bone lesions such as osteoblastic metastases, primary bone sarcomas, or sclerotic bone dysplasias 1
• Key distinguishing features include the characteristic radiating margins, lack of growth over time, and typically negative bone scan findings 1
• In cases of diagnostic uncertainty, correlation with clinical findings and additional imaging modalities may be necessary 1

Management Considerations

• Most bone islands require no treatment and only periodic monitoring if discovered incidentally 1, 3
• In orthodontic patients, large bone islands may potentially impact treatment by creating difficulties with space closure or achieving adequate root tip or torque 4
• Multiple bone islands, particularly when associated with osteomas, may warrant further investigation as they could potentially be associated with adenomatous intestinal polyps 4
• Surgical intervention is rarely indicated and should only be considered when symptoms are clearly attributable to the bone island and all other potential causes have been excluded 5