For parents, first
Your child has a hard lump on a bone, and someone has used the word tumour. For most families that word lands before any of the others do. So here is the whole page in three sentences. An osteochondroma is the commonest bone tumour in children, and it is benign. It grows while your child grows and stops when they stop. Most are left alone, some are removed because they get in the way, and the one thing to watch for is a lump that grows again after growth has finished.
The essentials
- It is benign. An osteochondroma does not spread and does not shorten life. It is a growth, not a cancer.
- It is common. This is the most frequent benign bone tumour, and 85% are a single lump in a child with no other problems.
- It grows with the child and stops with the child. Most stop by the late teens.
- Most are left alone. Removal is for lumps that hurt, rub, press on a nerve or vessel, or bend a growing bone.
- Surgery usually waits for the end of growth, because the lump sits beside the growth plate. A lump that causes real trouble is removed sooner, carefully.
- The inherited form, with many lumps, is a different situation: more deformity, more operations, organised follow-up and a higher lifetime risk that one lump changes.
- The single rule: a lump that grows after growth has finished, or becomes newly painful, should be looked at.
What an osteochondroma is
Bones grow at their ends, from a layer of cartilage called the growth plate. Sometimes a small piece of that growth cartilage ends up on the outside of the bone instead of inside it. It keeps doing what growth cartilage does: it makes bone. The result is a bony outgrowth with a cap of cartilage on top, pointing away from the joint. That is an osteochondroma. The older name, exostosis, means the same thing.
It is usually found near the end of a long bone, most often around the knee, the upper arm near the shoulder, or the hip. It can be broad-based or sit on a stalk. Because it is made of normal bone and cartilage doing a normal job in the wrong place, it behaves like the rest of the skeleton: it grows during childhood and stops at skeletal maturity, usually between about 14 and 18 depending on the child.
Most are found by chance. A parent notices a hard bump, or a knock to the area draws attention to it, or it turns up on an X-ray taken for something else. An X-ray usually settles the diagnosis, because the lump’s bone runs continuously into the bone it grows from, which is the feature that tells it apart from anything worrying. An MRI is used when the lump is in an awkward place, when the cartilage cap needs measuring, or when symptoms suggest something is being pressed on. A biopsy is almost never needed.
When it is left alone
Most of the time. A lump that does not hurt, does not rub on anything and is not distorting a bone needs no treatment and no operation. Children live their whole lives with these.
Whether it needs regular scans depends on the situation. A single, typical osteochondroma that has stopped growing often needs nothing scheduled, provided the family knows the one rule below. Where the lump is near a nerve or vessel, is unusually large, or is one of many, review is arranged.
When it is removed, and why surgery usually waits
Removal is offered for a reason, not for the lump’s existence. The usual reasons are:
- Pain from the lump itself.
- Rubbing on a tendon or muscle, often with a fluid sac (a bursa) over the lump that keeps getting inflamed.
- Pressure on a nerve or a blood vessel, which can cause pins and needles, weakness or swelling.
- A bone that is bending or growing unevenly because of the lump. This is mostly a problem in the inherited form.
- A stalk that has fractured after a knock.
- A lump that genuinely bothers a teenager because of where it sits. This is a reasonable conversation rather than an automatic operation.
The timing matters in a child. The lump sits next to the growth plate, and operating near a growth plate in a child who is still growing carries a small risk of disturbing it. There is also a practical point: a lump removed while growth cartilage is still active has a slightly higher chance of growing back. So where a lump is not causing real trouble, surgeons generally prefer to wait until growth has finished. Where it is causing trouble now, it is removed now, with the operation planned around the growth plate.
The operation
The lump is removed at its base together with its cartilage cap, which is the part that could otherwise regrow. For most lumps this is a day case or a one-night stay, with a small scar and a return to school within a week or two. Lumps in awkward places, such as the inner side of the knee near the main vessels or behind the shoulder blade, need more planning.
The numbers from the published series are reassuring and worth having:
- In a series of 92 symptomatic lumps removed in 86 patients, 93% of the symptoms the operation was done for went away. Major complications occurred in 5%, mainly nerve irritation that recovered.
- In a children’s series of 80 patients who had 285 lumps removed, complications occurred in about one in eight, most often a temporary nerve problem at the outer knee. The authors’ conclusion is the one parents should hear: these lumps should not be removed routinely, only when there is a proper reason.
- Recurrence is uncommon, about 2%, when the whole cap is removed.
The removed lump is usually looked at by a pathologist, as a matter of routine, to confirm what everyone expects it to be.
The small long-term risk, and the one rule
Very rarely, the cartilage cap of an osteochondroma changes, over years, into a slow-growing cancer called a secondary chondrosarcoma. For a single lump the lifetime risk is about 1%. It is a risk of adult life, not of childhood; the average age at which it happens in the inherited form is the late twenties.
That is the reason for the one rule every family is given. A lump that grows after your child has finished growing, or that becomes newly painful without an injury to explain it, should be looked at. Osteochondromas stop growing when the child does; one that does not is asking a question.
On a scan, the feature that is measured is the thickness of the cartilage cap. The thresholds that raise concern are different for children and adults: a cap of more than 2 cm in an adult, but more than 3 cm in a child, because a growing child’s cap is naturally thicker. The same picture is read differently at twelve and at twenty-five, which is one more reason these decisions belong with people who see these lumps often.
The inherited form: hereditary multiple exostoses
About 15% of children with osteochondromas have many of them, and that is a different condition with its own name: hereditary multiple exostoses, also called multiple osteochondromas. It is rare, affecting roughly one child in 50,000 to 100,000.
How it is inherited. It is passed on in a dominant pattern: a child of an affected parent has a one in two chance of inheriting it, and nearly everyone who inherits the gene change develops lumps. About one in ten children with the condition have no family history at all, because the gene change arose new in them. It is caused by a change in one of two genes, EXT1 or EXT2, found in close to nine out of ten families. The diagnosis is usually made from the X-rays and the examination, often by age three; genetic testing is kept for families where the picture is unclear, and it can tell a family which gene is involved.
What it does. More lumps means more chance that one of them causes a problem, and the lumps around the forearm, knee and ankle can bend a bone or make one bone grow shorter than its partner. In a long-term study of affected families, about four in ten had a visible forearm deformity, one in ten a difference in leg length, and about one in twelve a knee that angled. Children with the condition had, on average, about two operations over their growing years. Boys, children with an EXT1 change and children with more than twenty affected sites tend to have the more severe form.
The long-term risk is higher. Older estimates quoted up to 10% for a lump changing to a chondrosarcoma over a lifetime; the larger modern studies put it lower, at about 3 to 5%, and most likely towards the lower end because the people who join studies tend to be those more severely affected. It does not shorten life expectancy. It does mean that these families keep regular reviews into adult life, know the one rule well, and have any removed lump examined. When a change does occur it is most often in the pelvis or the shoulder blade, where a lump is hard to feel, which is why those are the places reviewed most carefully.
What this means for you
Your child has a benign bony lump that will grow with them and then stop. If it is not hurting, rubbing or bending a bone, the right treatment is usually no treatment, and the right follow-up is knowing the one rule. If it is causing trouble, it can be removed, with good results and a small risk of complications that you should hear about before deciding. If there are many lumps, your child has a condition that needs an organised plan through the growing years and a watchful habit afterwards, and the rest of the family may want to be looked at too. The benign cartilage tumours page covers the other cartilage lesions a scan can turn up, and the referrers page lists the pathways your GP can use.