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A bone cyst is a fluid-filled hole that develops inside a bone. They can occur at any age, but most often affect children and young adults.
Bone cysts don't usually cause any symptoms. They are not cancerous and they do not usually pose a serious threat to health.
However, large cysts can cause a bone to weaken, making it more likely to fracture (break). This can cause problems such as pain, swelling, or not being able to move or put weight on a body part.
Read more about the symptoms of bone cysts.
There are two main types of bone cysts.
Unicameral bone cysts are the most common type of bone cyst. They can develop anywhere in the body, although most cases involve either the upper arm or thigh.
The condition tends to affect younger children between 5 and 15 years of age. Boys are about twice as likely to have unicameral bone cysts as girls.
The exact cause of unicameral bone cysts is unknown, but a leading theory suggests they may occur due to problems with the drainage of interstitial fluid from growing sections of bone.
Aneurysmal bone cysts are a rarer type of bone cyst where the bone contains a pocket of blood. They can also develop anywhere in the body, but most often affect the legs, upper arms, pelvis or spine.
Most cases affect children and young people aged between 10 and 20. It's thought that aneurysmal bone cysts may be slightly more common in females than males.
The exact cause of aneurysmal bone cysts is not clear. They may occur because of an abnormality in the blood vessels inside affected bones, which develop as a result of a previous injury or a non-cancerous growth inside the bone.
Bone cysts can usually be diagnosed by looking at an X-ray of the affected bone. This will highlight any hollow cavities or fractures in the bone.
In many cases, a bone cyst will only be discovered by chance when X-rays are used to diagnose an unrelated condition, or after an affected bone has fractured.
A computerised tomography (CT) scan, a magnetic resonance imaging (MRI) scan, and/or a biopsy (where a sample of fluid is removed from the bone with a needle and tested) may sometimes be carried out to confirm the diagnosis.
Many bone cysts will eventually heal without the need for treatment and won't cause long-term problems – particularly unicameral bone cysts in children, which will usually disappear by the time they stop growing.
If a bone cyst does not get better, or if treatment is recommended to help reduce the risk of a fracture, several options may be available.
For example, steroid medication can be injected into the bone to encourage the cyst to heal. If this doesn't help, surgery may be needed to drain the fluid and fill the hole with chips of bone.
There is a significant chance of the cyst recurring after treatment, so you may need to have regular X-rays for a few years afterwards to look for signs of the condition returning.
Read more about treating bone cysts.