Hereditary Multiple Osteochondromas (HMO)

What Is HMO?

HMO, previously called hereditary multiple exostoses (HME), is a genetic condition that causes non-cancerous bone growths called osteochondromas. These growths usually happen near the growth plates of bones, especially in the arms and legs. They grow as a child grows and typically stop once a person reaches their final height.

HMO is rare, affecting about 1 in 100,000 people in the U.S. However, 1–2% of people may develop just a single osteochondroma instead of multiple. The number, size, and location of osteochondromas can vary widely, even among family members.

What Causes HMO? 

HMO happens because of a change (variant) in one of two genes: EXT1 or EXT2. These changes lead to the formation of benign (non-cancerous) bone growths near growth plates.

What Are the Signs & Symptoms of HMO? 

Signs of HMO usually appear between ages 2 and 12. Osteochondromas look like bony bumps under the skin. Once a child reaches their adult height, these bumps typically stop growing, and no new ones form.

Other possible signs and symptoms include:

• Shorter height than expected
• Bowing of the legs
• Bone deformities caused by the growths
• Joint swelling or arthritis
• Limited joint movement or stiffness
• Ongoing pain
• Pressure on nearby nerves or blood vessels
• Hip problems (hip dysplasia)
• Different leg lengths

In rare cases (2–5%), an osteochondroma can become cancerous (osteochondrosarcoma). Warning signs include faster growth, severe pain, or growth that continues after a person stops growing.

Even though HMO can cause a variety of symptoms, most people with HMO live active, healthy lives.

How Is HMO Diagnosed? 

If a healthcare provider thinks your child may have HMO, they may:

• Order x-rays to look for bone growths
• Order blood tests to rule out other conditions
• Recommend genetic testing to check for EXT1 or EXT2 variants

How Is HMO Treated? 

Most of the time, no specific treatment is needed unless there is pain, a bone deformity, or trouble with everyday activities. In those cases, surgery may be done to remove one or more growths.

If there are concerns about height or growth, your provider may check for growth hormone issues.

What to Expect with Our Care Team

If HMO is suspected or already diagnosed, our team may ask for blood work, urine tests, or imaging before your first visit. At the appointment, we’ll review the results and create a personalized care plan based on your child’s needs and symptoms.