Osteoma
Overview
- Definition: A benign, slow-growing tumour composed of mature lamellar bone.
- Epidemiology:
- Most common benign tumour of the paranasal sinuses.
- Age: Middle-aged adults (30–50 years).
- Gender: M > F (2:1).
- Pathology subtypes:
- Ivory osteoma: Dense, solid bone (no marrow).
- Cancellous osteoma: Trabecular bone with marrow elements.
- Syndromic association with Gardner syndrome.
Anatomical distribution
- Paranasal sinuses (Most common):
- Frontal sinus (80%).
- Ethmoid air cells (15%).
- Maxillary/Sphenoid (Rare).
- Skull vault: Arises from the outer table (exophytic).
- Mandible: Often associated with syndromic cases.
Imaging features
CT (Modality of choice)
- Appearance: Well-circumscribed, homogenously hyperdense (bone density) mass.
- Shape: Round or lobulated ("cauliflower-like").
- Marrow:
- Ivory: Uniformly dense.
- Cancellous: Central trabeculation/marrow density.
- Contrast: No enhancement.
MRI
- T1/T2: Markedly hypointense (signal void) due to dense cortical bone.
- Note: Cancellous osteomas may have intermediate signal corresponding to marrow.
- Utility: Useful to assess for secondary mucocele or intracranial extension.
Complications (The "check areas")
- Mucocele formation: If the osteoma obstructs the frontal recess or ostium.
- Proptosis: Due to intra-orbital extension.
- Pneumocephalus/CSF leak: If the osteoma erodes the posterior wall of the frontal sinus/fovea ethmoidalis.
Differential diagnosis
| Lesion | Key distinguishing features |
|---|---|
| Exostosis | Reactive/developmental outgrowth; usually broad-based (sessile); classic in external auditory canal ("surfer's ear"). |
| Meningioma | Hyperostosis associated with meningioma arises from the inner table and involves the dura/meninges (en plaque). |
| Fibrous dysplasia | "Ground-glass" matrix; expands the bone rather than growing on it; ill-defined margins. |
| Osteoblastoma | > 2 cm; painful; rare in skull vault; lytic/mixed rather than purely sclerotic. |
| Bone island (Enostosis) | Located within the medullary space (intramedullary), not exophytic on the cortex; "thorny" radiating margins. |
High-yield exam pearls
Inner vs Outer table
- Osteoma: Arises from the outer table of the skull (grows outward).
- Meningioma / Hyperostosis frontalis: Affects the inner table (grows inward or thickens inward).
The "blocked sinus"
In the viva, if shown a frontal sinus osteoma, do not stop at the diagnosis.
- Look for: Opacification of the sinus behind it (Mucocele/Sinusitis).
- Mention: "I would check the posterior table for breach to exclude intracranial communication."
