What is Hereditary Multiple Exostoses (HME)?
The information in this section is based on a talk given at the first meeting of the HME Support Group on 13 September 1997 by Mr Hamish Simpson, Consultant Orthopaedic Surgeon. These pages were edited by Dr D G Small, BM, BCh, MRCP(UK) and were checked by the Speaker. They have been reviewed from time to time by Mr D E Porter, Senior Lecturer & Hon. Consultant in Orthopaedic Surgery, University of Edinburgh, Edinburgh, UK, most recently on 21 May 2000.
Normal Bone Growth
When you look at the X-ray of a child's bone, such as the bone of the upper arm (humerus), you can see a darker band at either end. This is the growth-plate which consists of cartilage. Also, looking along the length of the bone, you will usually find that its width varies, being narrower in the middle and widening towards the ends. This is process called modelling.
In childhood the two growth-plates lay down new bone, causing the bone to become longer. Eventually growth stops, the growth-plates change from cartilage to bone and the darker areas disappear from the X-ray.
Growth at the two ends of the bone is not necessarily equal. The arm bones grow faster at the wrist and shoulder (the ends away from the elbow), while in the leg, growth is faster at the knee. Apart from the main growth at the growth-plates, there is a small amount over the whole surface, causing the width of the bone to increase as it becomes longer.
The bone retains its modelling ability throughout life and is able to repair itself after a break (fracture).
What is an Exostosis?
One of the most common places to develop an exostosis is around the knee, an area of maximum growth. They are also common at the shoulder, the wrist and on the ribs. You don't usually get them on your head which is a different type of bone.
Lumps usually stop growing at the end of childhood.
Multiple Exostoses
A single exostosis is quite common and in approximately 1 in 200 children (ref 1) grows large enough to require surgery. In contrast, a few people develop many exostoses, perhaps even hundreds. This is a condition called Hereditary Multiple Exostoses (HME), first described by a famous surgeon, John Hunter, in 1787. Other names for HME include Multiple Osteochondromata, Osteochondromatosis, Diaphyseal Aclasis and Familial Bony Spurs. These all refer to the same condition.
Although you usually inherit HME from one of your parents, it can sometimes develop unexpectedly. There is a 50% chance of it being passed on, so on average half the children in an affected family will develop exostoses. Boys and girls have an equal chance of being affected but there can be quite big variations within a family, some of the affected children developing many lumps and others only a few.
How common is HME?
A recent study (ref 2) in the state of Washington, USA estimated that about 1 person in 50,000 was likely to have the condition. In the families they studied, 90% had inherited it and about 10% had no family history. If present, HME was discovered by the age of 4 years.
Applying these estimates to the United Kingdom (population approximately 59 million), there are likely to be at least 1,200 people in the UK with HME, of whom 120 will have no family history.