An osteochondroma is a benign bone tumor made of bone and cartilage that grows near the growth plates of bones, most commonly around the knee. It is the most common type of benign bone tumor, and it typically appears during childhood or adolescence when bones are still growing. Most osteochondromas never cause problems and are discovered by accident on an X-ray taken for something else entirely.
How Osteochondromas Form
During normal bone growth, cartilage cells in the growth plate multiply in organized columns, gradually turning into bone as the skeleton lengthens. An osteochondroma forms when a small fragment of this growth plate cartilage pushes outward through the membrane surrounding the bone (the periosteum) and keeps growing on the bone’s surface. The result is a bony stalk or bump capped with cartilage, almost like a small mushroom attached to the side of a bone.
At the genetic level, this process involves a gene called EXT1. Cells in the growth plate that lose both working copies of EXT1 appear to trigger the initial formation of the tumor. Once the osteochondroma starts growing, however, other cellular signals may keep it going even without complete loss of that gene. This is why the tumor behaves more like a developmental quirk than an aggressive cancer: it grows alongside normal bone and typically stops growing once a person reaches skeletal maturity.
Where They Appear and Who Gets Them
Osteochondromas most often develop near the ends of long bones, particularly around the knee (lower femur and upper tibia), the upper arm bone near the shoulder, and the hip. They can also form on the pelvis, shoulder blade, or spine, though these locations are less common. The knee is the single most frequently affected site.
Most solitary osteochondromas are diagnosed in children, teenagers, or young adults. They affect boys slightly more often than girls. Because these tumors grow along with the skeleton, they tend to become noticeable during growth spurts and stop enlarging once bone growth is complete, usually by the late teens or early twenties.
Solitary vs. Multiple Osteochondromas
Most people develop a single osteochondroma with no family history involved. A smaller group of people, however, develops multiple osteochondromas throughout the skeleton, a condition called multiple hereditary exostoses (MHE). MHE is inherited in an autosomal dominant pattern, meaning a child needs only one copy of the mutated gene from one parent to develop the condition.
MHE is caused by mutations in the EXT1 or EXT2 genes. EXT1 mutations account for roughly 44 to 70 percent of cases, while EXT2 mutations explain another 27 to 40 percent. The condition has nearly complete penetrance: virtually 100 percent of males and about 96 percent of females who carry the mutation will develop osteochondromas. About 10 percent of people with MHE have a new (de novo) mutation rather than inheriting it from a parent. People with MHE may develop dozens of bony bumps and face additional complications like limb length differences and joint deformities that solitary osteochondromas rarely cause.
Symptoms and Complications
The majority of osteochondromas cause no symptoms at all. When they do, the most common sign is a hard, painless lump you can feel under the skin near a joint. This bump doesn’t move and feels like bone because it is bone.
Symptoms develop when the tumor presses on surrounding structures. Because osteochondromas near the knee are so common, the popliteal artery (the main artery behind the knee), the common peroneal nerve, and the posterior tibial nerve are the most frequently compressed structures. Nerve compression can cause tingling, numbness, or weakness in the affected limb. Vascular compression can lead to changes in skin color, weakened pulses, blood clots, or, rarely, an aneurysm.
Osteochondromas sitting beneath tendons can cause pain and irritation during movement as the tendon repeatedly slides over the bony bump. The friction can also trigger formation of a fluid-filled sac (bursa) over the tumor, leading to bursitis. In rare cases, the stalk of the osteochondroma can fracture, causing sudden, sharp pain at the site.
How It’s Diagnosed
A standard X-ray is usually enough to identify an osteochondroma. The tumor has a characteristic appearance: a bony projection growing away from the joint, with the inner bone (medullary canal) continuous with the parent bone. This continuity is the hallmark that distinguishes an osteochondroma from other bone surface tumors.
MRI is used when more detail is needed, particularly to measure the cartilage cap on top of the bony stalk. This cap is visible on MRI but not on X-ray, and its thickness matters. In children and adolescents who are still growing, the cap is naturally thicker. In adults whose bones have finished growing, a cartilage cap thicker than about 1.5 to 2 centimeters raises concern for possible malignant transformation and usually prompts further evaluation or removal.
Risk of Becoming Cancerous
The most serious concern with osteochondromas is the small chance of malignant transformation into a type of cancer called chondrosarcoma. For solitary osteochondromas, this risk is estimated at less than 1 percent. For people with MHE, the risk is higher, estimated at 1 to 5 percent over a lifetime. When malignant transformation does occur, chondrosarcoma accounts for 94 percent of those cases.
Warning signs include an osteochondroma that starts growing again after skeletal maturity, new or worsening pain at the site of a previously painless bump, or a rapidly enlarging soft tissue mass around the tumor. Any of these changes in an adult warrants imaging to evaluate the cartilage cap thickness and rule out cancerous change.
When Surgery Is Needed
Most osteochondromas require no treatment. If the tumor isn’t causing pain or compressing anything, the standard approach is simply to monitor it. Surgery becomes an option when the osteochondroma causes persistent pain, presses on nerves or blood vessels, restricts joint movement, or shows signs of possible malignant change such as growth after skeletal maturity.
The procedure involves surgically removing the entire bony stalk along with its cartilage cap. Complete removal of the cartilage cap is critical: it prevents the tumor from recurring and eliminates the future risk of malignant transformation in that cap.
Recovery After Surgery
Recovery time depends on the size and location of the tumor. For small osteochondromas, the incision typically heals and swelling resolves within about two weeks. Larger tumors that require more extensive surgery take four to six weeks to recover from, and crutches may be needed during that period. If the surgery includes correction of a bone deformity, which is more common in MHE patients, recovery can stretch to several months and may involve physical therapy to restore strength and range of motion.
Recurrence after complete removal is rare. The key factor is whether the entire cartilage cap was excised. When it is, the tumor does not come back and the risk of future cancerous change at that site is effectively eliminated.