Hereditary Multiple Osteochondromas (HMO): Understanding Benign Bone Tumors in Children
Updated: 3 days ago
Medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: October 2026
Quick Answer: What are Hereditary Multiple Osteochondromas?
Hereditary multiple osteochondromas (HMO) is a lifelong genetic disorder in which many benign, cartilage-capped bone tumors develop, mainly during a child's growing years. The tumors often stop growing once the skeleton matures but can cause pain, uneven limb length and joint stiffness. Management aims to relieve symptoms, correct bone growth problems and surgically remove troublesome tumors.
What the research says
Diagnosis is mostly clinical and radiological. A 2025 educational review reports an incidence of 1 in 50,000 to 1 in 100,000 and notes that the WHO criteria require at least 2 radiological osteochondromas near the growth plates of long bones, with genetic testing reserved for unclear cases. It estimates a 10% risk of malignant transformation to secondary chondrosarcoma, compared with about 1% in people with a single osteochondroma, and recommends follow-up and MRI of the cartilage cap if new pain appears (Rueda-de-Eusebio et al., Insights into Imaging, 2025) [2].
Severity varies with the gene involved. In a study of 529 patients, EXT1 mutations and involvement of more than twenty skeletal sites were linked to a severe phenotype, while EXT2 mutations and fewer than five affected sites were linked to a mild one. Malignant transformation was observed in 5% of patients, with no association found with the specific mutation, sex or number of lesions (Pedrini et al., Journal of Bone and Joint Surgery, 2011) [3].
Key Takeaways
Feature | Hereditary Multiple Osteochondromas (HMO) Details |
|---|---|
Primary Cause | Changes in the EXT1 or EXT2 genes (about 90% of cases) [1]. |
Prevalence | About 1 in 50,000 to 100,000 people [1]. |
Common Symptoms | Bony lumps, pain, restricted joint motion, and short stature [1]. |
Diagnostic Tools | Physical exam, X-rays, and CT or MRI when more detail is needed [1]. |
Management | Watchful waiting, pain management, physical therapy, and surgery [1]. |
Cancer Risk | About 2% to 5% of people develop a cancer such as chondrosarcoma or osteosarcoma [1]. Some studies report higher figures. |
The Nature of Osteochondromas
An osteochondroma is a benign tumor made of bone and covered by a cartilage cap [1]. These growths typically form on the ends of long bones such as the femur (thigh bone) or humerus (upper arm bone), but they can also occur on flat bones such as the pelvis or shoulder blade [1].
The condition is developmental. Tumors generally appear in the first decade of life and keep growing until the growth plates close at skeletal maturity [1]. They do not spread like cancer, but they can interfere with the normal development of nearby tissues and cause secondary skeletal problems.
Symptoms and Physical Impact
Symptoms vary widely. Some people have only a few small, painless bumps, while others develop dozens of tumors that cause significant physical challenges.
Common symptoms and complications include:
Visible bumps: hard, stationary lumps near joints that may be tender to pressure [1].
Growth abnormalities: changes in bone alignment, especially in the forearms, and differences in limb length [1].
Mobility limits: reduced range of motion in the ankles, knees or elbows depending on where tumors sit [1].
Pain and discomfort: acute or chronic pain when tumors press on nerves, tendons or blood vessels [1].
Secondary conditions: early osteoarthritis and pinched nerves from skeletal misalignment [1].
Genetic Basis and Inheritance
HMO is inherited in an autosomal dominant pattern, so one changed copy of the gene from one parent is enough to cause it. A person with HMO has a 50% chance of passing the gene change to each child [1].
About 90% of people with HMO have a change in the EXT1 or EXT2 gene, which are involved in making proteins essential for bone development. The cause in the remaining 10% is unknown [1]. HMO can also appear in families with no prior history, because the gene change can be new in that person [1].
Diagnostic Procedures
Diagnosis combines a physical assessment with imaging. Because the tumors are bony, they are often easy to identify.
Management and Treatment Strategies
There is no medical cure for HMO, so treatment aims to manage symptoms and prevent long-term complications. The plan is personalized to the severity of the condition.
Watchful waiting: regular exams and imaging to make sure tumors are not causing problems [1].
Pain management: medicines and therapies to ease discomfort [1].
Physical therapy: exercises to keep joints flexible and muscles strong [1].
Surgery: to remove tumors that cause pain or nerve compression, correct bone deformities or lengthen limbs [1].
Prognosis and Long-Term Outlook
Most people with HMO lead full, active lives with a normal life expectancy [1]. The main long-term concern is the small chance that a benign osteochondroma turns into cancer. Cleveland Clinic cites 2% to 5%, and some studies report higher rates [1]. Regular check-ups are essential, especially if new pain develops, new tumors appear after puberty, or a tumor's cartilage cap is thicker than 2 centimeters [1].
Frequently Asked Questions
Is HMO the same as bone cancer?
No. HMO causes benign (noncancerous) tumors. However, a small percentage can become cancerous over time, so regular follow-up matters.
When are children usually diagnosed with HMO?
Usually in early childhood or during the adolescent growth spurt, when the tumors become more visible.
Can HMO tumors be prevented?
No. Because the condition is genetic, there is no way to prevent the tumors from forming.
Do all HMO tumors need to be removed?
No. Many tumors are painless and do not interfere with growth, so they are simply monitored.
What is a cartilage cap?
A layer of flexible tissue that covers the bony part of an osteochondroma.
Does HMO affect life expectancy?
Most people with HMO have a normal life expectancy.
What is the risk of passing HMO to a child?
A person with HMO has a 50% chance of passing the gene change to each child.
Can HMO cause short stature?
Yes. Effects on the growth plates can sometimes lead to shorter-than-average height.
What are the signs that a tumor might be cancerous?
Warning signs include new tumors appearing after puberty, a tumor that becomes painful, or a cartilage cap thicker than 2 centimeters. See a doctor promptly if you notice these.
Is HMO contagious?
No. It is a genetic condition and cannot spread from person to person.
What are the EXT1 and EXT2 genes?
Genes that provide instructions for proteins involved in the development of cartilage and bone.
Can HMO be diagnosed before birth?
If the family's gene change is known, prenatal genetic testing is possible.
What is watchful waiting?
A strategy in which doctors monitor the condition closely and act only if symptoms develop or change.
Does HMO cause joint pain?
It can. Tumors near joints may restrict movement and cause pain, and misalignment can contribute to early arthritis.
What is a limb length discrepancy?
A difference in length between one leg or arm and the other because bone growth is affected.
Can new tumors appear in adulthood?
Tumor growth usually stops once the skeleton is mature. New growths or pain after puberty should be checked by a doctor.
Is physical therapy helpful for HMO?
Yes. It helps maintain mobility and manage pain related to skeletal changes.
What kind of surgery is used for HMO?
Surgery can remove tumors that cause pain or nerve compression and can correct bone deformities or lengthen limbs.
Can HMO be outgrown?
No. It is lifelong, although tumor growth usually slows or stops at skeletal maturity.
What is chondrosarcoma?
A type of cancerous bone tumor that can occasionally develop from a benign osteochondroma.
Are the bumps from HMO always painful?
No. Many people have painless bumps that cause no functional problems.
What is diaphyseal aclasis?
An older medical term for hereditary multiple osteochondromas.
Can HMO cause pinched nerves?
Yes. A tumor growing near a nerve can cause numbness, tingling or pain.
What is a growth plate?
The area of developing tissue near the ends of long bones in children and adolescents.
How common is HMO?
About 1 in 50,000 to 100,000 people are affected.
Can genetic testing confirm HMO?
Yes. A change in the EXT1 or EXT2 gene is found in about 90% of people with HMO.
What happens if a tumor grows back after surgery?
Doctors reassess the need for further treatment based on symptoms and imaging.
Are there support groups for HMO?
Yes. Patient organizations provide information and community for families.
Can HMO affect the spine?
Less commonly, tumors can grow on the vertebrae and potentially affect the spinal cord.
What is a de novo mutation?
A new genetic change that appears for the first time in a family. Some people with HMO have no affected parent.
What is the primary goal of HMO treatment?
To reduce pain, maintain function and correct significant skeletal deformities.
Is HMO the same as a single osteochondroma?
No. HMO involves multiple tumors, whereas a solitary osteochondroma is a separate, non-hereditary condition.
Can HMO cause forearm deformities?
Yes. It frequently affects the forearm bones, which can lead to bowing or shortening.
How often should someone with HMO see a doctor?
Regular follow-up is usually recommended, often once a year or more often during growth spurts. Your specialist will set the schedule.
Can people with HMO play sports?
Many can, but those with significant joint involvement may need to avoid certain high-impact activities. Ask your orthopedist.
What is an MRI used for in HMO?
MRI helps show the cartilage cap and nearby nerves and blood vessels, especially when malignant change is suspected.
Does HMO cause early arthritis?
It can. Joint misalignment caused by tumors may lead to premature cartilage wear.
Can HMO affect the ribs?
Yes. Tumors can grow on the ribs and are often painless there.
Is surgery for HMO risky?
All surgery carries risks such as infection or scarring, and operations near nerves and vessels need an experienced surgeon.
What is the EXT gene family?
A group of genes involved in making heparan sulfate, which is important for bone signaling.
What does MHE stand for?
Multiple hereditary exostoses, the former name for HMO.
Can HMO cause blood vessel problems?
Occasionally a tumor can press on a blood vessel and affect circulation in that limb.
Are there dietary restrictions for HMO?
No specific diet is required, though a healthy weight reduces stress on the joints.
Where do HMO tumors commonly appear?
Common sites include the ends of long bones near the knee, ankle, shoulder and wrist, as well as the pelvis and shoulder blade.
Does HMO affect boys and girls equally?
The condition affects both sexes, although severity can differ between individuals.
Disclaimer: The information provided in this article is for educational purposes only and should not be considered medical advice. Always consult with a pediatric orthopedist or genetic specialist regarding skeletal conditions.
References
Cleveland Clinic. Multiple Hereditary Exostoses (Hereditary Multiple Osteochondromas): Symptoms, Diagnosis & Treatment. Cleveland Clinic. Last updated July 5, 2022. https://my.clevelandclinic.org/health/diseases/23433-hereditary-multiple-osteochondromas
Rueda-de-Eusebio A, Gomez-Pena S, Moreno-Casado MJ, et al. Hereditary multiple exostoses: an educational review. Insights into Imaging. 2025;16(1):46. PMID 39982564. https://doi.org/10.1186/s13244-025-01899-6
Pedrini E, Jennes I, Tremosini M, et al. Genotype-phenotype correlation study in 529 patients with multiple hereditary exostoses: identification of protective and risk factors. Journal of Bone and Joint Surgery (American volume). 2011;93(24):2294-2302. PMID 22258776. https://doi.org/10.2106/JBJS.J.00949
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