Hereditary Multiple Exostoses

Also known as: Diaphyseal Aclasis; Multiple Osteochondromatosis


Hereditary multiple exostoses (HME), also known as multiple osteochondromatosis (MO) is a condition in which people develop multiple benign (noncancerous) exostoses (osteochondromas). These growths are bony in nature and include a cap or cartridge. Exostoses appear at the end of many bones, especially the long bones of the arms and legs.


Medical text written May 1998 by Professor DE Porter. Last updated May 2016 by Professor DE Porter, Tsinghua University Medical Centre, Beijing, China.

Although great care has been taken in the compilation and preparation of all entries to ensure accuracy, we cannot accept responsibility for any errors or omissions. Any medical information is provided is for education/information purposes and is not designed to replace medical advice by a qualified medical professional.

What are the symptoms?

Exostoses grow throughout childhood, but usually become fully bony and stop growing at the end of teenage years. When they grow large, they are frequently painful and can disturb growth resulting in variable short height and bone deformity which might need to be corrected.

Less commonly symptoms occur when exostoses press on nearby nerves and blood vessels. Forearm pain, hip, knee and walking problems may occur and flat foot may also exist. Extremely rarely, exostoses may become cancerous. Rapidly-growing and painful exostoses in late adolescence or adulthood with a cartilage-cap thickness in excess of 10mm are usually those with this potential.

What are the causes?

The gene responsible for 50% of affected families is EXT1 located on chromosome 8 and causes a slightly more severe form of HME. The EXT2 gene is located on chromosome 11 and is responsible for about 30% of HME. Other genes may be responsible for the remainder of families.

How is it diagnosed?

Usually children will be born to families in which HME is already known. Less commonly a child is born with no parents affected. The average age at which HME is diagnosed in childhood is four years, usually by the presence of one or more new bony lumps that are identified on an X-ray. A single exostosis is usually not due to HME.

How is it treated?

Surgery to remove problematic exostoses is often required during childhood. In childhood, regular review by an orthopaedic surgeon is recommended (perhaps annually or every second year). A specialist exostosis clinic which takes national referrals occurs at the Royal Hospital for Sick Children in Edinburgh every three months. No other specialist exostosis clinic is known to exist in the United Kingdom. In adulthood, an affected person may still require occasional monitoring by an orthopaedic specialist.

Inheritance patterns and prenatal diagnosis

Inheritance patterns
Autosomal dominant. Up to one third of affected individuals represent new gene mutations in whom neither parent has the condition.

Prenatal diagnosis
A prenatal ultrasound will not identify babies at risk. Prenatal or family gene testing is occasionally available through consultation with a consultant clinical geneticist.

Is there support?

HME Support Group


The Group is a Registered Charity in England and Wales No. 1091069. It provides information and support to anyone affected by Hereditary Multiple Exostoses, and offers linking with other families where possible. 

Group details last updated May 2016.

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