An osteoma is a benign bone tumor made of well-organized, mature bone tissue. It grows slowly, stays in one place, and does not spread to other parts of the body. Osteomas most commonly develop on the skull or in the sinuses, and the vast majority cause no symptoms at all. They’re often discovered by accident on imaging done for an unrelated reason.
Where Osteomas Grow
Osteomas have a strong preference for the head and face. The most common location is the frontal sinus (the air-filled space behind your forehead), followed by the ethmoid sinuses (between your eyes), the maxillary sinuses (behind your cheeks), and the sphenoid sinuses (deeper in the skull). They also appear on the outer surface of the skull and along the jawbone, where they’re sometimes called “button osteomas” because of their small, round shape.
Less frequently, osteomas can develop on long bones in the arms or legs, though this is uncommon enough that a bone growth in those areas is more likely to be something else entirely.
What Causes Them
The exact cause of osteomas is unknown. Researchers haven’t definitively established whether they are true tumors (new, abnormal growth) or dysplastic lesions (normal bone tissue that developed incorrectly). Either way, they grow very slowly and have limited growth potential, meaning they don’t keep enlarging indefinitely the way aggressive tumors do.
One important genetic connection exists. When a person develops multiple osteomas, particularly in the jaw and skull, it can be a sign of Gardner syndrome. This is a variant of familial adenomatous polyposis, an inherited condition that causes hundreds of polyps to form in the colon and rectum, typically during the teenage years. Gardner syndrome affects roughly 1 in 4,000 to 1 in 40,000 people depending on the population. It also involves dental abnormalities and soft-tissue growths called fibromas. A single osteoma is almost never a cause for concern about Gardner syndrome, but multiple osteomas, especially with a family history of colon polyps, warrant further evaluation.
Histological Types
Under a microscope, osteomas fall into three categories. Compact (or “ivory”) osteomas are made of very dense, tightly packed bone without the normal internal channels that healthy bone uses for blood supply. Cancellous (or “mature”) osteomas look more like regular bone, with a spongy internal framework. Mixed osteomas have features of both. Despite the different structures, all three types behave the same way clinically. None is more likely to grow faster, cause symptoms, or recur after removal.
Symptoms Based on Location
Most osteomas produce no symptoms whatsoever. When they do cause problems, it’s almost always because of where they’re growing rather than what they are. A small osteoma tucked into a corner of a large sinus might never bother you. The same size growth near the eye socket or blocking a sinus drainage pathway can cause real trouble.
The most common symptom is headache, particularly with frontal sinus osteomas. Other symptoms of sinus osteomas include nasal congestion, runny nose, postnasal drip, and a reduced sense of smell. These overlap heavily with chronic sinusitis, which is why some people are treated for sinus infections before the osteoma is identified on imaging.
Osteomas near the eye socket can produce more distinctive symptoms: watery eyes from blocked tear ducts, a bulging eye (proptosis), double vision, or visual field loss. In rare cases, a large ethmoid sinus osteoma can lead to an orbital abscess that threatens vision. When an osteoma presses on the optic nerve, it becomes an emergency requiring prompt surgical treatment.
How Osteomas Are Diagnosed
CT scans are the standard tool for identifying and evaluating osteomas. On a CT image, an osteoma appears as a sharply defined, uniformly dense mass of bone. The dense, bright appearance on imaging is distinctive enough that osteomas are rarely confused with other types of tumors. CT is especially useful for pinpointing the exact size and position of the growth relative to surrounding structures like the brain, eyes, and sinus drainage pathways.
Many osteomas are found incidentally. Studies estimate that sinus osteomas show up on about 1% to 3% of CT scans done for other reasons, making them far more common than most people realize. MRI is sometimes used alongside CT to assess soft tissue involvement, particularly if the osteoma is near the brain or orbit, but CT alone is sufficient for most diagnoses. A biopsy is rarely needed because the imaging appearance is so characteristic.
When Treatment Is Needed
The majority of osteomas never need treatment. If an osteoma is found incidentally and isn’t causing symptoms, the standard approach is simply to monitor it with occasional imaging to confirm it isn’t growing in a direction that could cause future problems.
Surgery becomes necessary in specific situations: when the osteoma causes persistent headaches, chronic sinus obstruction, or any eye-related symptoms like double vision, tearing, or visual changes. It’s also recommended when the growth invades the base of the skull or when complications like infection develop. For sinus osteomas, an endoscopic approach (operating through the nostrils with a camera) is often possible, avoiding external incisions. Larger or more complex osteomas may require an open surgical approach.
Osteomas on the outer surface of the skull that are purely cosmetic concerns can also be removed if the bump is bothersome. This is a straightforward procedure since the growth sits right on the bone surface.
Osteoma vs. Osteoid Osteoma
Despite the similar names, an osteoma and an osteoid osteoma are different tumors. Osteoid osteomas are smaller (typically under 1.5 cm), occur mainly in the long bones of the legs and spine rather than the skull, and are known for causing intense pain, especially at night. They disproportionately affect young men under 30, with a male-to-female ratio of roughly 2 to 3:1. Osteoid osteomas account for about 10% to 14% of all benign bone tumors, making them the third most common type.
The hallmark of osteoid osteoma is a small central core called a nidus, visible on CT as a tiny round area of low density surrounded by a ring of dense reactive bone. Pain from osteoid osteomas characteristically improves with over-the-counter anti-inflammatory medications, which can actually be a useful diagnostic clue. Treatment options include minimally invasive heat-based ablation or surgical removal, though recurrence rates can be significant. One study of pediatric patients found that 50% experienced recurrence, with a median time to return of just under one year.
A conventional osteoma, by contrast, is painless in most cases, recurrence after complete removal is uncommon, and it requires no treatment at all unless it’s pressing on something important.