Osteochondroma: Understanding Your Pathology Report

Section Editor: Bibianna Purgina MD FRCPC
October 6, 2026

An osteochondroma is a noncancerous bony outgrowth that sticks out from the surface of a bone. It is covered by a cap of cartilage, the firm, smooth tissue found in joints. It is the most common noncancerous bone tumor.

The name comes from three parts: osteo, meaning bone, chondro, meaning cartilage, and oma, meaning a growth. Some reports use the older name exostosis, which means an outgrowth from bone.

Osteochondromas develop and grow during childhood and adolescence, while the skeleton is still growing. They usually stop growing when a person reaches their adult height. Most are found in children, teenagers, and young adults.

This article explains how an osteochondroma is diagnosed and what the findings in your pathology report mean.

Where do osteochondromas develop?

Osteochondromas develop near the ends of long bones, close to the growth plates. The growth plates are layers of cartilage that allow bones to grow longer during childhood.

The most common location is around the knee, at the lower end of the thigh bone or the upper end of the shin bone. Other common locations include the upper arm bone near the shoulder, the pelvis, and the shoulder blade. Osteochondromas usually point away from the nearby joint.

What causes an osteochondroma?

Osteochondromas result from changes in a gene called EXT1 or, less often, EXT2. These genes help cartilage cells at the growth plate grow in an orderly way. When they stop working, a small group of growth plate cells grows sideways out of the bone instead of in its normal direction.

Most people have a single osteochondroma. In these people, the gene change develops in the growth cells during childhood and is not inherited.

A smaller number of people have many osteochondromas, caused by an inherited condition called multiple osteochondromas. It is also known as hereditary multiple exostoses. It is caused by an inherited change in the EXT1 or EXT2 gene, and each child of an affected parent has a 50% chance of inheriting it. People with this condition may develop dozens of outgrowths, and some have shorter height or bowing of the arms or legs.

What are the symptoms of an osteochondroma?

Many osteochondromas cause no symptoms. Some people notice a hard, painless lump near a joint.

When symptoms occur, they usually depend on the size and location of the outgrowth:

Pain or discomfort. The outgrowth can rub against muscles or tendons, especially with activity.
Limited movement. A large outgrowth near a joint can make it harder to bend or straighten the joint.
Nerve or blood vessel symptoms. Pressure on a nerve can cause numbness or tingling. Pressure on a blood vessel can cause swelling or a change in the limb’s color.
Fracture. The narrow base of the outgrowth can break after an injury, causing sudden pain.

How is the diagnosis made?

An osteochondroma has a characteristic appearance on X-ray. The inside of the outgrowth is continuous with the inside of the bone it grows from, which is the key identifying feature. Many osteochondromas are diagnosed from imaging alone, without tissue being removed.

A pathology report is produced when the osteochondroma is removed, usually because it is causing symptoms. Occasionally, a biopsy is done first, although this is uncommon.

Under the microscope, an osteochondroma looks like a small, slightly disorganized growth plate. It has three layers:

A fibrous covering. A thin layer of dense tissue covers the outside.
A cartilage cap. Beneath the covering is a layer of cartilage made of cells that look like normal cartilage cells. In an adult, this cap is usually less than 1 cm thick.
A bony stalk. The deepest part of the cartilage turns into bone, forming the stalk, which contains bone marrow like the rest of the bone.

What is the cartilage cap, and why is it measured?

The cartilage cap is the most important part of the osteochondroma to examine. The radiologist often measures its thickness on MRI and again by the pathologist after the outgrowth is removed.

In children, the cap is naturally thicker because the outgrowth is still growing. In adults, the cap should be thin. A cap thicker than about 2 cm in an adult raises concern that the osteochondroma may have changed into a cartilage cancer, and the pathologist will examine it closely.

Your report may also comment on the cartilage cells. Cells that look atypical, meaning more abnormal than expected, can be another sign of this change.

Can an osteochondroma turn into cancer?

Very rarely. A single osteochondroma changes into a cancer in fewer than 1 in 100 people. The risk is higher in people with multiple osteochondromas, although it is still small, estimated at a few percent over a lifetime.

When this change happens, the cancer is a chondrosarcoma, usually low grade, growing from the cartilage cap. This is called a secondary peripheral chondrosarcoma.

These warning signs are a reason to have the outgrowth checked:

Growth in adulthood. The osteochondroma keeps growing after a person has reached their adult height.
New pain. Pain develops without a clear cause such as an injury.
A thick cartilage cap. The cap is thicker than about 2 cm in an adult.

Margins

When a surgeon removes an osteochondroma, they aim to take the whole outgrowth, including the cartilage cap and its fibrous covering. If the surgeon leaves part of the cap behind, the outgrowth can grow back.

Your report may state whether the surgeon removed the cartilage cap completely. For a noncancerous osteochondroma, this matters only for the chance of the outgrowth coming back.

What is the prognosis?

The outlook is excellent. An osteochondroma is noncancerous, and removal is curative when the whole outgrowth is taken. It rarely comes back after complete removal.

People with multiple osteochondromas may need more than one operation over their lifetime, and an orthopedic surgeon usually follows them.

What happens after the diagnosis?

Observation. An osteochondroma that causes no symptoms does not need treatment. Your doctor may recommend checking it periodically.
Surgery. Removal is recommended when the outgrowth causes pain, limits movement, presses on a nerve or blood vessel, or breaks. It is also recommended when there are warning signs of change.
Multiple osteochondromas. People with this condition are usually followed regularly, sometimes with MRI, and may need surgery to correct bone deformities. Genetic counseling is often offered to the family.
Adults. Any new growth or new pain in an existing osteochondroma after reaching adult height should be checked.

Questions to ask your doctor

Was the whole osteochondroma removed, including the cartilage cap?
How thick was the cartilage cap?
Were any atypical features seen?
Is there a chance it will grow back?
Do I have more than one osteochondroma?
Should my family be checked for multiple osteochondromas?
What changes should prompt me to come back?