What is an intrinsic bone island anterior?

What is a Bone Island (Enostosis)?

A bone island, also called an enostosis, is a benign focus of compact cortical bone within the cancellous (spongy) bone that represents a developmental abnormality from failure of resorption during endochondral ossification—it is typically an incidental, asymptomatic finding requiring no treatment. 1

Definition and Pathophysiology

• A bone island is a focus of mature compact (cortical) bone embedded within the cancellous bone (spongiosa), likely congenital or developmental in origin 1
• It represents a failure of normal bone resorption during endochondral ossification rather than a true neoplasm 1
• These lesions are classified among benign developmental abnormalities affecting bone 2

Clinical Characteristics

• Bone islands are typically completely asymptomatic and discovered incidentally on imaging studies 1, 3
• They are common benign sclerotic bone lesions considered "do not touch" lesions in skeletal radiology 3
• Most measure 0.1 to 2.0 cm in diameter, though giant bone islands (>2 cm) have been rarely reported 4
• The true incidence is unknown because most benign bone tumors and developmental abnormalities are asymptomatic 2

Anatomic Distribution

• Bone islands show a preference for the pelvis, femur, and other long bones 1, 5
• They can occur anywhere in the skeleton, including the spine 1
• The diaphyseal and metaphyseal regions of long bones are common locations 4

Radiographic Features (Diagnostic)

The diagnosis can be made virtually on imaging alone based on characteristic features:

• Plain radiographs show a homogeneously dense, sclerotic focus within cancellous bone with distinctive radiating bony streaks ("thorny radiation" or "pseudopodia") that blend with surrounding trabeculae, creating a feathered or brush-like border 1, 6
• The lesion appears ovoid, round, or oblong in shape 5, 4
• On CT, bone islands appear as low-attenuation foci 1
• On MRI, they demonstrate low signal intensity on all sequences, similar to cortical bone 1

Nuclear Medicine Findings

• Bone islands are classically "cold" (no uptake) on skeletal scintigraphy, which historically distinguished them from aggressive lesions 1
• However, some histologically confirmed bone islands can show increased radiotracer uptake on bone scan, so scintigraphic activity alone should NOT exclude the diagnosis 1, 6
• When bone islands show scintigraphic activity, histopathology reveals increased osteoblastic activity with a mixture of compact and trabecular bone 6

Diagnostic Algorithm

The correct diagnosis should be based on morphologic features, not scintigraphic activity:

1. Evaluate plain radiographs first for characteristic features (feathered borders, thorny radiation, homogeneous sclerosis) 6
2. If radiographic features are typical, no further workup is needed regardless of bone scan activity 6
3. CT or MRI may be obtained if radiographic features are atypical or the lesion occurs in anatomically complex areas 2
4. An asymptomatic, isolated sclerotic bone lesion with feathered borders is most likely an enostosis regardless of size or scintigraphic activity 6

Clinical Management

• No treatment is required for typical bone islands 1, 3
• These are stable, nonprogressive lesions that do not require follow-up imaging 6
• Biopsy is unnecessary when radiographic features are characteristic 1, 6

Important Caveats

• While bone islands are typically asymptomatic, rare case reports describe painful bone islands (even small ones <2 cm) that required surgical resection for symptom resolution 5
• The differential diagnosis for sclerotic bone lesions includes osteoblastic metastasis, chronic osteomyelitis, and fibrous dysplasia, which must be excluded based on clinical context and imaging features 7
• Do not let a "hot" bone scan dissuade you from the diagnosis if the radiographic morphology is classic for bone island 1, 6