Exostosis is excess bone growth. With exostosis, bone grows outward from the original bone, causing it to thicken. In some cases, there's a palpable lump or area of thickness. Exostosis can be symptomatic or asymptomatic and can show up at any age or stage in life.

What is Exostosis?

An exostosis is a benign or non-cancerous growth of bone on top of other bone. An example of exostosis is a bone spur, also known as an osteophyte. Exostosis is among the most common non-cancerous tumors that affect the bone — 3% of people have one or more. The most frequent location is in the lower extremities, particularly the hips and feet, but they can also occur in the jaw, sinuses, and ear canal. Sometimes they develop later in life from joint damage due to arthritis.

xray of a foot with a bone spur on the heel


What Causes Exostosis?

An exostosis can form at a site on a bone that was previously injured, or it can arise from chronic irritation to a bone. There is also a hereditary form of exostosis — called hereditary multiple exostoses — passed down from parent to child. With this type, the bony growths usually appear during childhood and there may be many of them.

digital illustration diagnosis concept; hereditary multiple exostoses


Risk Factors for Exostosis

People who have had an injury to a bone or have joint damage from arthritis are at higher risk of developing one or more exostoses. Inheriting the gene for hereditary multiple exostoses greatly increases the risk of developing multiple exostoses elsewhere on the body.

Since some joint degeneration occurs with age, aging is also a risk factor for exostosis. For exostoses in the ear canal, swimming in cold water without ear protection is a risk factor.

man holding his knee, knee pain or arthritis concept


Symptoms of Exostosis

Common symptoms of exostosis include intermittent pain, stiffness, or limitation of movement in the area of the growth. If the exostosis pushes on a nerve, it may cause numbness or tingling. Some people can feel the bony overgrowth as a lump or thickening when they press on the area. The symptoms of exostosis may come and go and get better with rest.

woman holding her knee, knee pain concept



Doctors can sometimes diagnose exostosis based on physical examination. They may recommend imaging studies, such as an x-ray. More advanced imaging, such as CT scans or MRI, can provide a detailed look at the bone. In some cases, exostosis shows up on a routine x-ray in a person with no symptoms.

doctor examining an x-ray of a patient's knee with arthritis


Treatment of Exostosis

Unless exostosis is causing symptoms, many people don't require any treatment for their growths. For painful exostoses, a doctor may prescribe non-steroidal anti-inflammatory medications to reduce inflammation and tenderness. For exostoses on the feet, rest and avoiding high-impact exercise may help.

For pain that persists, a doctor may recommend cortisone injections to reduce inflammation. Exostoses in certain areas, such as the ear canal, may need surgical treatment if they are severe enough that the symptoms don't resolve with more conservative therapy.

close up of prescription medication on a prescription pad


Home Therapy

Applying ice to areas that are tender or swollen may decrease pain and tenderness when symptoms flare up. Non-steroidal anti-inflammatory medications are also effective for reducing pain and inflammation, but patients should always speak with their doctor before taking new medications. If exostosis involves the feet, wearing shoes that fit well and aren't too tight will keep the area from getting irritated. Wearing special orthotics may also be beneficial, especially if exostosis affects the heels or ball of the foot.

medical practitioner fitting patient for orthotics


Prognosis for Exostosis

In people with hereditary multiple exostoses, there is a small risk, about 1 to 2%, of the growths becoming cancerous.  That's why ongoing close monitoring is important for people with this condition.

For people who don't have the hereditary form, the prognosis is good. Many people experience few or no symptoms or have only occasional flairs of pain or stiffness in the area where the exostosis is.

doctor examining patients back or shoulder



Beyond the small potential for malignant transformation in people with the hereditary form, serious complications of exostosis are uncommon. People who have exostoses in the ear canal can develop hearing loss and repeated infections if the growth becomes large and goes untreated. Surgery may be necessary to avoid this complication.

doctor looking into young patient's ear


Prevention of Exostosis

It may not be possible to completely prevent exostosis, especially the hereditary form, but reducing irritation and damage to bone and joints helps. One way to do this is to wear shoes that fit well and are not too tight. Also, maintain a healthy posture to reduce stress on the joints in the neck and back. Maintaining a healthy weight also lessens stress and force on the bones and joints.

It is important not to overdo high-impact exercise, like running and jumping, since the impact is hard on the bones and joints. To lower the risk of exostoses in the ear canal, wear earplugs when swimming in cold water.

cropped image of a person in running shoes walking


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