<aside>

Enostosis, also known as a bone island, is a benign focus of compact (cortical-like) bone located within the medullary cavity of cancellous (trabecular) bone.

</aside>

It is asymptomatic, non-expansile, and typically found incidentally on radiographs or CT scans.

Pathogenesis

Represents a hamartomatous proliferation of mature bone.
Composed of dense lamellar bone with normal Haversian systems.
Thought to arise during bone development, persisting into adulthood.

Epidemiology

Feature	Detail
Age	Seen in all age groups; more common in adults
Sex	Equal male and female prevalence
Prevalence	~14% on pelvic radiographs and >30% on CT scans
Sites	Pelvis, proximal femur, sacrum, ribs, humerus, spine

Clinical Features

Feature	Description
Symptoms	Asymptomatic in nearly all cases
Discovery	Incidental finding during imaging for other causes
No malignant potential	Rare variants exist (e.g., osteopoikilosis, multiple bone islands)

Radiology

Modality	Imaging features
XR	• Appearance: Small, round or oval, sclerotic lesion
• Margins: Feathered or brush-like margins, blending with trabeculae
• Size: Usually <2 cm, but can rarely exceed 2 cm ("giant bone island")
• No periosteal reaction, no cortical destruction
CT	• Hyperdense lesion with sharp but non-aggressive borders
• Helps distinguish from osteoblastic metastases
MR
• T1: Very low signal (dark) due to dense bone
• T2: Low to very low signal
• No associated bone marrow edema or soft tissue mass
Bone scan	No or minimal uptake, which helps differentiate from metastatic lesions

Lesions >2 cm raise concern for other entities, especially in patients with known malignancy.
No change over time on serial imaging supports diagnosis of enostosis.

Differentials