Osteochondromas occur as solitary osteochondroma (osteocartilaginous exostosis) or as multiple osteochondromatosis. These are discussed separately.

Solitary osteochondroma is a developmental abnormality of bone. It occurs when part of the growth plate forms an outgrowth on the surface of the bone. This bone outgrowth may or may not have a stalk. When a stalk is present, the structure is called pedunculated. When no stalk is present, it is called sessile.

An osteochondroma may grow in a child or adolescent. Its growth usually stops at maturity.

Solitary osteochondromas are thought to be the most common noncancerous (benign) bone tumor. They account for 35 percent to 40 percent of all benign bone tumors.

Solitary osteochondroma is diagnosed in patients aged 10 to 30. It occurs equally in males and females. It does not result from injury. It is thought to arise during skeletal growth when bone grows away from the growth plate instead of in line with it. Because the cause of solitary osteochondroma is unknown, doctors have not been able to find a way to prevent it.

The most common symptom of an osteochondroma is a painless bump near the joints. The knee and shoulder are more commonly involved.

Solitary osteochondroma can be found at the ends of any long bone and along the pelvis and bones that make up the shoulder. If the stalk of a pedunculated osteochondroma breaks, pain and swelling may start immediately.

An osteochondroma can be located under a tendon. When it is, snapping of the tissue over the tumor may cause activity-related pain.

An osteochondroma can be located near a nerve or blood vessel, such as behind the knee. When it is, there may be numbness and tingling in that extremity. A tumor that presses on a blood vessel may cause periodic changes in blood flow. This can cause loss of pulse or changes in color of the limb. Changes in blood flow resulting from an osteochondroma are rare.

A plain X-ray will show the bony growth. A magnetic resonance imaging (MRI) scan may be used to look for cartilage on the surface of the bony growth. Such cartilage in an adult patient should be checked for cancer if it is larger than two centimeters in size, or if there is pain. A computed tomography (CT) scan may also be used.

Osteochondromas are thought to be associated with a gene called EXT 1. This is currently poorly understood. Researchers are investigating it.

Multiple osteochondromatosis is also called multiple osteocartilaginous exostosis, multiple hereditary exostosis (MHE), familial osteochondromatosis, multiple hereditary osteochondromatosis, or diaphyseal aclasia.

About 70 percent of the time, it is inherited. About 30 percent of the time, it occurs randomly.

Multiple osteochondromatosis affects males more often than females. It usually happens within the first three decades of life. Alterations in genes called EXT genes are thought to be the cause of this disease. It can be passed along in families. There is no other known risk factor or cause. There is increased risk of benign tumors changing to cancer (malignant transformation).

Symptoms are the same as solitary osteochondroma. The number and location of osteochondromas varies. A patient may also have deformities of the forearms, a short stature, and knock-knees and ankles. This is caused by abnormal growth in the bones affected by the osteochondromas.

Symptoms and signs of a tumor becoming cancerous are:

• Growth of an osteochondroma after puberty
• Pain at the site of an osteochondroma
• A cartilage cap larger than two centimeters

It is diagnosed when two or more osteochondromas are seen on plain X-rays. Multiple osteochondromatosis has been associated with EXT 1 and EXT 2 genes. These genes are now being studied. The way they act is not well understood.

X-ray findings are the same as with solitary osteochondromas.

If cancer is suspected, you may need a thorough evaluation. This includes magnetic resonance imaging and computed tomography scans of the tumor.

Pain at the site of a mass or growth after puberty should be evaluated by a doctor who specializes in treatment of bone tumors. The most common malignancy found in this transformation is called chondrosarcoma.

A computed tomography scan of the chest helps look for any disease that may have traveled through the bloodstream to the lungs.

Sometimes the doctor may remove a piece of tissue from the tumor (biopsy). This is used to look at cells under a microscope.