What Is Exostosis?

Written by Alyson Powell Key
Medically Reviewed by Poonam Sachdev on June 08, 2025
5 min read

It's a noncancerous bone tumor that doesn't spread to other parts of your body. It forms on top of existing bone tissue, usually in places such as your ankles, feet, toes, ear canals, sinus cavity, fingers, and jaw.

Anyone can get exostoses. But they're more common in children and teens because their bones grow quickly. In many cases, they don't cause any symptoms. But sometimes, they might cause pain.

Exostoses can also develop over time in older adults who have arthritis. "If I'm seeing someone who has hip, knee, or any form of joint pain, the first study we get is an X-ray. And if we're seeing bone spurs, that's a sign of arthritis," says Gbolahan Okubadejo, MD, a board-certified orthopedist and spine specialist at The Institute for Comprehensive Spine Care located in New York and New Jersey.

Exostoses are defined in two ways: sessile or pedunculated.

Sessile exostoses

These types of bumps are permanently attached to your bone.

Pedunculated exostoses

A narrow stem connects these types of bumps to your bone. 

Although exostoses (also known as bone spurs) can occur on any bone, they're more common in some parts of your body, particularly in or around your foot or in the hip. Here are some types:

Heel exostosis

Under the heel. This type of bone spur can be caused by plantar fasciitis. When your plantar ligament (located in the sole of your foot) pulls on your heel, it can cause a buildup of extra tissue.

Hallux rigidus

Big toe. The base of the big toe is the most common place to develop arthritis in the foot. When a bone spur forms on the top of the big toe, it can limit the movement of your big toe when you walk. This is called hallux rigidus or stiff big toe.

Haglund's deformity

Back of the heel. Also called a "pump bump," Haglund's deformity refers to a bone spur on the back of the heel. This type of exostosis can occur when pump-style shoes rub against the back of the heel. Though it's called a pump bump, any type of shoe with a rigid back can cause this irritation.

Hip exostosis

Hip. Osteoarthritis wears away the cartilage in the hip and causes the bones to start rubbing together. This friction leads to the growth of bone spurs.

Other types of exostoses include:

Hereditary multiple osteochondromas (HMO)

Previously known as multiple hereditary exostosis, hereditary multiple osteochondromas (HMO) is a genetic condition usually diagnosed in young children and adolescents. It affects boys and girls equally. It's most often passed down to children from their parents. But it's possible to have this condition even if no one else in your family does. This condition is also sometimes called diaphyseal aclasis.

The gene for HMO affects bone growth and causes multiple exostoses to form. The bumps are noncancerous but can come with other challenges. The exostoses become bigger as children grow but stop growing once they reach adulthood.

The exostoses vary in size, number, and location, but they’re typically found on the:

  • Legs
  • Arms
  • Fingers
  • Toes
  • Shoulders
  • Pelvis

External auditory exostoses

Also called surfer's ear, this type of exostosis is caused by chronic cold water exposure. It's a buildup of bone growth in your external ear canal. There are usually no symptoms, but you may have hearing loss, ear infections, ear drainage, and earwax impaction.

There are many different factors that can cause exostosis. They include: 

Exostoses can also form as a reaction to your body's loss of cartilage. "The body tries to stabilize itself by forming more bone. It's almost a reaction to more stress seen by bones around the joint, and that manifests as bone spurs," Okubadejo says.

You may not have any symptoms. The bone growths themselves don't cause pain, but they can cause problems when they put pressure on your nearby nerves, limit your movement, or cause friction by rubbing against other bones or tissues.

When symptoms do happen, they can include:

  • Pain near the joint
  • Stiffness
  • Limited movement
  • Bumps, especially in the hands or feet
  • Swelling
  • Weakness
  • Numbness
  • One leg or arm being longer than the other

What are the symptoms of HMO?

Your child may have short stature and bowed arms or legs. They might also have stiff elbows, forearms, and hips. One of the main symptoms of this disorder is chronic pain due to the growing bumps. Exostoses can make it hard to walk, run, or extend your arms.

Other symptoms include:

  • Deformities in arms and legs
  • Differences in the length of arms or legs
  • Cancerous growths within the bumps
  • Pinched nerves
  • Premature osteoporosis 
  • Bone and growth plate fractures
  • Problems with ankles and knees

The treatment for exostosis will depend on the cause of the growth and symptoms. There are some things you can do at home to help, such as:

If home remedies don't help ease your symptoms, your doctor may suggest other treatments, including:

How is HMO treated?

Treatment depends on your symptoms. Your doctor might recommend:

  • Pain management
  • Physical therapy
  • Watchful waiting 
  • Surgery to remove any tumors
  • Surgery to lengthen limbs that aren't growing as they should

You can't prevent some causes of exostosis, but you may help reduce your risk by:

  • Staying at a healthy weight to avoid putting too much stress on your joints
  • Practicing good posture
  • Using good ergonomics while working
  • Stretching before exercising
  • Not pushing your body too hard when exercising
  • Eating a healthy diet
  • Staying physically fit
  • Making sure your shoes are supportive and fit well
  • Wearing earplugs when your ears are exposed to cold water

What's the difference between an exostosis and a sarcoma?

An exostosis is a noncancerous bone growth, while a sarcoma is a type of cancer that begins in your bones and soft tissues.

What's the difference between an exostosis and an osteoma?

Like an exostosis, an osteoma is also a noncancerous growth, but it starts in your skull or the bones of your face.

What's the difference between exostosis and Tori?

Tori are extra bone growths that can appear on the inside of your lower jaw near your tongue, or on the outer side of your upper or lower jaw. They're common and usually harmless, so you may not need treatment.

What's the difference between an exostosis and an osteochondroma?

Osteochondromas are the bumps that happen with HMO. These tumors grow as your child gets older and usually stop when they reach puberty.

Where is the most common location for exostosis?

They're most common in your legs, arms, fingers, toes, shoulders, and pelvis.