A bone cyst is a fluid-filled lesion that develops inside a bone. They are not cancerous and usually only affect children and young adults.
Someone with bone cysts won't usually notice any symptoms, but if a cyst gets bigger, it can weaken a bone, making a fracture more likely.
Unicameral bone cysts are the most common type of bone cyst. These affect boys more than girls, are most common between 5-15 years old, and mostly occur in the upper arms and thighs.
A rarer type are aneurysmal bone cysts, where there's a pocket of blood inside the bone. These cysts may cause symptoms including pain, lumps, swelling or reduced movement.
It isn't clear what causes bone cysts to develop. Unicameral bone cysts may be caused by fluid drainage problems in growing bones. Aneurysmal bone cysts may be due to problems with blood vessels in a bone from a previous injury or a non-cancerous growth. Another theory is that genetic abnormalities cause the cysts to form.
Bone cysts are usually picked up during tests such as X-rays, CT scans or MRI scans for other conditions.
Bone cysts often get better on their own as a child gets older without needing any treatment. This is probably why bone cysts are rare in adults.
If treatment is needed to help prevent bone fractures, steroid injections may be given into the bone, or an operation carried out to drain the fluid and fill the hole with bone chips.
Doctors may be concerned if a cyst is affecting normal bone growth, or if there are other symptoms, which in rare cases may suggest bone cancer.
Doctors will also be concerned if an aneurysmal bone cyst affects the spine, where it may disrupt the body's nervous system, causing muscle weakness, numbness or headaches.