Bone cysts

A bone cyst is a cavity inside a bone filled with fluid or with blood. It is not cancer: it does not seed into other organs. But the bone around the cavity grows thin and breaks under a load that a healthy bone would have taken without any consequences. Cysts are found mostly in children and teenagers, while the skeleton is still growing, and very often by accident, on an X-ray taken for something else entirely. Some disappear on their own, others need treatment, and the symptoms alone will not tell you in advance which is which.

Where the hollow in the bone comes from

The honest answer is that nobody knows for certain. Two main kinds of cavity are recognised, and they come about in different ways.

• Simple cyst. A single smooth chamber holding clear fluid, usually in the upper third of the upper arm bone or the thigh bone, next to the growth plate. The problem is thought to lie in the drainage of fluid from the growing part of the bone. Once growth finishes, this kind of cavity often closes by itself.
• Aneurysmal cyst. Several blood-filled chambers separated by thin walls. It grows noticeably faster, pushes the bone apart from the inside and shows a preference for the vertebrae and the bones of the pelvis. Sometimes it does not appear on its own but alongside another bone lesion, and then the treatment has to be aimed at whatever caused it.

Neither kind is catching, neither is passed from parent to child, and neither has anything to do with how much dairy or calcium someone eats. A knock or a fall does not create the cyst, but it is usually what reveals it.

How it makes itself known

Most of the time it does not. While the cavity is small the bone carries the load and the person feels nothing at all. When there are signs, they tend to be these:

• a dull ache in the shoulder, thigh or shin that builds up towards the end of the day and with activity;
• swelling, or a firm lump that can be felt directly on the bone;
• a limp, or the habit of favouring one arm for no obvious reason;
• numbness, pins and needles, weak legs or a stiff back, if the cavity sits inside a vertebra;
• a fracture from something trivial: an awkward landing, throwing a ball, a shove during a game.

A break with no adequate cause is the commonest way a cyst announces itself. The bone there is as thin as an eggshell and snaps where a healthy one would only have been bruised. Sometimes the fracture is even useful: it sets healing in motion, and a proportion of simple cysts close up afterwards.

What the doctor has to rule out first

This is the most important part of the whole conversation. On an X-ray a hole in the bone looks much the same in situations of wildly different seriousness, and the calm verdict of "just a cyst" is not reached on first impressions.

There is an uncommon type of malignant bone tumour that on imaging is almost indistinguishable from an aneurysmal cyst: the same multi-chambered, blood-filled cavity. That is precisely why a fast-growing lesion is not scraped out blindly; a piece of tissue is taken for examination first.

The second point is age. In a child, a cavity in the bone is almost always harmless. In an adult, particularly after forty, the same picture more often turns out to be something else: a deposit from a tumour that started in another organ, a disease of the bone marrow, or the effect of an excess of parathyroid hormone leaching calcium out of the skeleton. That is why adults always have their blood calcium checked and a primary source looked for.

Pain that wakes you at night, pain at complete rest, swelling that grows quickly, fever without a cold and weight loss should all raise the level of attention. None of these belongs to an ordinary cyst.

When to see a doctor

• pain in the same spot on a bone lasts longer than two or three weeks;
• a lump or swelling has appeared over a bone and is not settling;
• a child has started limping or protecting an arm without any injury;
• a fracture happened under a very small load;
• pain or a lump has come back after treatment, because the cavity can form again.

Do not wait until morning if a limb is deformed and unusable after a fall; if back pain has suddenly worsened and is joined by weak legs, numbness around the groin or trouble passing urine; or if the skin over the bone is red and hot and there is a high fever. Pressure on the spinal cord and a collection of pus both need help the same day.

How it is investigated

The starting point is a plain X-ray in two views, which shows a simple cyst quite convincingly. Magnetic resonance imaging is needed when the picture is not straightforward: it shows what fills the cavity, the internal walls and the characteristic fluid levels, and at the same time reveals whether the lesion has broken out of the bone. Computed tomography gives a better view of how thin the wall is and of the state of a vertebra.

Blood tests do not make the diagnosis, but they help to rule out other causes. If anything on the images falls outside the reassuring picture, a biopsy is taken. It is worth agreeing to without inner resistance: it answers the one question that genuinely matters, whether the cavity is harmless or not.

What is done about it

A small cavity that neither hurts nor threatens a fracture is usually just watched: X-rays are repeated every few months to see how it behaves over time. In children and teenagers this most often ends in the cyst closing by itself.

If the cavity is large, growing, or sitting in a bone that carries body weight, there is a choice of approaches:

• needle puncture: the fluid is let out and something is injected in that prompts the cavity to fill in, usually a steroid anti-inflammatory or the patient's own bone marrow. The procedure often has to be repeated;
• scraping the cavity out and packing the space with bone chips from the patient, donor bone or a special cement;
• strengthening the bone from inside with a thin rod, if a fracture is likely or has already happened;
• blocking the feeding vessels through a catheter, an option for aneurysmal cysts in awkward places such as the pelvis;
• in stubborn cases, drugs that suppress bone breakdown are used; that decision belongs to the specialist.

These procedures are done under general anaesthetic and usually do not require an overnight stay. A fracture next to a cyst is treated as a fracture first, and the cavity itself is dealt with later, once the bone has united.

While the bone fills in

Healing is slow, taking months, and how you feel is no guide to it: the pain goes long before the cavity closes. For that period contact sport, jumping and loading the treated bone are off the table; the exact timescale comes from the doctor and the follow-up X-rays.

Cysts come back fairly often, especially in the first couple of years and in younger children whose bone is still growing hard. It does not mean the treatment was done badly, it is simply how these cavities behave. That is why follow-up X-rays continue for several years even when nothing is troubling you. The overall outlook is good: a treated cyst usually leaves growth, the shape of the limb and later life unaffected.

Online consultation

In an online consultation the doctor will go through how long the pain has been there and what brings it on, look at any X-rays and reports you already have and explain what they actually say, advise whether magnetic resonance imaging and an orthopaedic opinion are needed, and help you work out what limits on activity make sense right now.

This material is for information only and does not replace medical advice.