Osteoma

An osteoma (Latin: osteoma) is a benign, slow-growing tumor composed of well-differentiated mature bone. It does not show invasive growth and has no metastatic potential. On histology, an osteoma may be virtually indistinguishable from normal compact (cortical) or cancellous (trabecular) bone.

Most osteomas occur in the skull and facial skeleton, particularly in the walls of the paranasal sinuses. By contrast, osteomas of long tubular bones are exceptionally rare. When osteomas are multiple, Gardner syndrome should be considered, and appropriate oncologic vigilance is warranted.

Aetiology and Pathophysiology

The exact cause of osteomas remains uncertain. They have been linked to prior trauma and to chronic inflammation (for example, chronic sinusitis). Some authors also consider osteomas to represent a developmental abnormality (a hamartoma) rather than a true neoplasm.

Histologically, osteomas are classified into three types:

• Compact (ivory) osteoma: composed of dense cortical bone with very few vascular canals, giving it an ivory-like density.
• Cancellous (spongy) osteoma: composed of trabecular bone with marrow spaces.
• Mixed osteoma: combines compact and cancellous features.

Growth is typically extremely slow and occurs over many years through appositional growth, with new bone laid down by osteoblasts.

Clinical significance

In most cases, osteomas are small and asymptomatic, and they are detected incidentally on imaging performed for unrelated reasons. Symptoms usually appear only after the lesion enlarges enough to exert pressure on adjacent anatomical structures.

Clinical manifestations depend on location:

• Paranasal sinuses: Osteomas may obstruct the sinus outflow pathway, leading to headaches, recurrent sinusitis, and mucocele formation.
• Orbit: they may displace the globe (proptosis), causing diplopia and visual impairment.
• Jaws: An osteoma may cause cosmetic facial deformity and malocclusion.
• External auditory canal: It may narrow the canal and result in conductive hearing loss.

Diagnosis is established by radiography and, more definitively, computed tomography (CT), where an osteoma appears as a well-circumscribed, high-density lesion. Asymptomatic osteomas generally require no treatment and are managed with observation. Surgical excision is indicated if symptoms develop or if there is a pronounced cosmetic deformity.

Differential Diagnosis

Osteoma should be differentiated from other bone-forming tumors and related lesions. Unlike osteoid osteoma, osteoma is typically painless. In contrast to osteosarcoma, it has smooth, well-defined borders, shows no cortical destruction, lacks a soft-tissue component, and grows extremely slowly. Finally, the presence of multiple osteomas should prompt evaluation for Gardner syndrome, which is associated with a markedly increased risk of colorectal cancer.