An exostosis, also called an osteoma, is a benign overgrowth of a pre-existing bone. It forms during the active growth stage. In the majority of cases the bone overgrowth is capped with cartilage, and it is then called an osteochondroma. Exostoses do not show any consistent pattern of occurrence as far as location, size or number is concerned. They are often the result of trauma. In many cases an exostosis is asymptomatic, and found incidentally during a medical checkup.
One common type of exostosis occurs inside the paranasal sinuses. These may be diagnosed after a CT scan of the head taken for some other indication. This type of exostosis occurs more often in males (1.5-2.6:1), and most frequently in middle age. They cause symptoms such as pain, swelling or joint pain because they put pressure on nearby blood vessels, nerves or muscles. They also often block the sinus openings, so that nasal secretions accumulate and cause sinus congestion. They occur most often in the frontal sinuses, followed by the ethmoid sinuses.
Single osteomas can also occur in the skull vault or mandible. An osteoma of the skull can cause a visible bump, or it can erode the meninges (the brain coverings). It can result in the leak of cerebrospinal fluid, the entry of air from the nasal or paranasal cavities into the brain, and infection of the meninges or the brain itself.
Another common site for a solitary exostosis is in the external ear. This is also called ‘surfer’s ear’ and is due to the combined action of wind and water producing extreme cooling of the ear. A third situation is a bony growth under a nail, called a subungual osteoma. This often follows trauma to the nail, and causes pain, swelling and sometimes ulceration of the nail. When it is resected and the underlying unhealthy area is curetted, healing occurs.
Osteomas may or may not have a connecting stalk. Most of the symptoms of an osteoma are due to:
- pressure on neighboring structures
- inflammation of surrounding muscle and tendons
- the formation of an inflammatory bursa between the bony growth and the neighboring soft tissue
- occasionally, fracture of the thin stalk of the osteoma. Less than 1% of solitary exostoses turn malignant
Osteochondromas are the most common benign tumors arising from bone. They are cartilage-capped tumors originating from the cartilage growth plate of bones. They are most common at the ends of the long bones, 30% of them occurring on the femur. They are found slightly less often in the arm bones and spinal bones. They grow as the child grows, and cease to enlarge once maturity is reached. Sometimes these occur after surgery, radiation or certain fracture types, and they form the most common type of benign radiation-induced tumors. Local resection is sufficient for cosmetic reasons, or if there are bothersome symptoms. Recurrence sometimes happens. The risk of a cancer is low, about 1%.
Another type of exostosis is found in the condition called hereditary multiple exostoses (HME). This condition is also called Bessel-Hagen syndrome or diaphyseal aclasia. Of the affected children, 96% have no exostoses at birth, but experience several growths before the age of 12. These continue to grow as long as the child’s bones do, and stop growing when bone growth is complete. They are most commonly seen in the long bones (of the arms and legs) or flat bones (as in the hip joint or the scapula).
Complications of this condition include:
- unequal limb growth because the exostoses disturb the normal progressive increase in bone length on one side of the body
- impingement on various joints making movement painful
- pain even at rest, due to stiff and painful joints
- bow legs or bowed ankles due to disturbed bone growth
- shooting pains, weakness, swelling, or discoloration due to the pressure of the exostosis on nearby blood vessels, nerves, or even the spinal cord, depending on the location of the overgrowth
- risk of a malignancy arising in one of these tumors, ranging from 1 in 20 to 1 in 200, and found most often in the pelvis.
While the overall incidence of HME is only 1 in 50,000, some people groups like the Ojibway Indians of Canada have an astonishing risk of 1 in 77. There is a strong association with two gene mutations, in DNA segments labeled EXT1 and EXT2. These are involved in processing a molecule called heparan sulfate for use by the cell. However, the mechanism connecting the abnormal gene function with the growth of an exostosis is not yet clear. In about 15% of these people, no mutation is found in these genes.
HME is an autosomal dominant condition. Thus a single abnormal copy of a gene will produce symptoms even if a normal copy is also present inside the cell. If either parent has a mutated gene, there is a 25% chance of passing it on to any given child. However, many cases occur in people with normal parents, and are thus due to new mutations.
Treatment is initially medical, and is directed at relieving symptoms due to the pressure of the osteoma on neighboring structures. Resection of the tumor is performed if the symptoms persist, or for cosmetic reasons. Malignant tumors are treated by resecting the tumor with a wide margin around the visible mass.