Osteoma

Definition

Osteoma is a benign, slow-growing, painless, bone-surface lesion that forms mature, well-differentiated bone. It is commonly seen in the paranasal sinuses and calvaria, and less commonly in juxtacortical locations of the long bones.

Incidence

• The exact incidence is unknown as lesions are often asymptomatic (especially medullary osteomas).
  • Peak incidence: 30 – 50 years
• Female to male ratio of 3:1 (some sources say no sex predilection)
• Associated with Gardener Syndrome
  • Gardner Syndrome is multiple skull, sinus or mandible osteomas associated with colon polyps and soft tissue skin tumour

Aetiology

• Composed of well-differentiated mature bone tissue
• No Haversian canals or fibrous component

Types

• Mature osteomas may consist of a radiolucent nidus surrounded by dense sclerosis (ivory osteoma)
• Trabecular osteomas are composed of cancellous bone surrounded by denser cortex

Clinical Presentation

• Usually asymptomatic; incidental finding on imaging
• Osteomas of the paranasal sinuses can obstruct the ostia, blocking drainag and causing headache

Imaging Signs

General features

• Well-differentiated bone formation without aggressive features
• Location: predominantly in bones formed by membranous ossification:
  • Paranasal sinuses: 75%
    • Named for sinus invaded by osteoma, not sinus of origin
    • Frontal (80%) > ethmoid (20%) > maxillary > sphenoid
    • May extend intraorbitally or intracranially
  • Calvarium: typically outer table
  • Mandible, maxilla
  • Long bones (rare)
    • Femur > humerus > short tubular bones
• Size: Usually 1-4 cm
• Morphology:
  • Round to oval, smooth borders,
  • Sessile or pedunculated

Plain Radiography

• Homogenously sclerotic 
• Rounded sharply marginated lesion arising from the outer table
• Spares the diploe and underlying cortex
• No space between lesion and cortex
• Intramedullary extension or expansion within long bones rare

CT

• Distinguish parosteal osteoma from myositis ossificans
  • Myositis ossificans is characterised by a zonal pattern with a radiolucent central area of immature bone tissue surrounded by a dense ring of mature bone

MRI

• Hypointense on T1-weighted and T2 weight images
• Sometimes perilesional oedema

Nuclear Medicine

• Negative typically
  • If positive, indicates active growth but this is atypical

Treatment

• Not required for asymptomatic lesions
• Surgical excision considered for cosmetic reasons or from obstruction of a sinus producing mucocele formation.

Course & Prognosis

No risk of recurrence following removal

Differential Diagnosis

• Osteochondroma – Lesion is continuous with host bone cortex
• Juxtacortical myositis ossificans – Zonal pattern (peripheral zone/ring of mature bone)
• Parosteal osteosarcoma – Less opaque and homogenous radiographic appearance
• Periosteal osteoblastoma – Round/oval shaped, sessile lesion located on the cortex. Radiopacity varies
• Ossifying parosteal lipoma – Lobulated mass. Contains irregular foci of ossification and radiolucent adipose tissue
• Melorheostosis – Cortical expansion resembling dripping candle wax. Longer extent.
• Meningioma – Dural tail sign on MRI
• Sclerotic metastasis – More rapid growth. Margins may be indistinct. Negative bone and PET scans.