Bone cysts are fluid-filled holes that form in bones. They mainly affect children and teenagers. They're not usually serious, but they sometimes need to be treated with surgery.

Symptoms of a bone cyst can include:

• pain, swelling or a lump, usually in your arm, leg or spine
• numbness or tingling, if the cyst affects your spine
• a break (fracture), sometimes from a relatively minor injury

Bone cysts do not always cause symptoms. Sometimes they're found after they cause a fracture, or if you have an X-ray for another reason.

A bone cyst may not need treatment if it's small and not causing any problems.

They often get better on their own, especially in children and teenagers.

You may have regular X-rays for a few years to check your bone is healing and the cyst is not getting bigger.

If the bone cyst is large or causing problems

A bone cyst might need to be treated if it's:

• large or getting bigger – this could make the bone weaker and more likely to break (fracture) if you injure it
• causing problems like pain, swelling or a lump

The main treatments are:

• draining the fluid with a needle and injecting medicine into the bone to help it heal – this may need to be done several times over a few months
• cutting or scraping out the cyst – the hole may be filled with small pieces of bone taken from another part of your body or from a donor, or with a bone cement mixture

Treatment is done under general anaesthetic. You will not usually need to stay in hospital overnight.

If you have treatment for a bone cyst it normally takes at least a few months for the bone to heal.

You may need to avoid activities or sports that could damage the bone until it has healed.

You'll have regular X-rays for a few years to check it's getting better.

It's quite common for bone cysts to come back, especially in the first couple of years after treatment.

See a GP if:

The exact cause of bone cysts is unknown. They're not cancer and do not spread to other parts of the body.

The main types of cyst are thought to have different causes:

• unicameral bone cysts – fluid-filled holes that may form if fluid does not drain properly from a bone as it's growing
• aneurysmal bone cysts – blood-filled holes that may be caused by a problem with the blood vessels in a bone (possibly due to an injury or a non-cancerous growth)