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Bone cyst


A bone cyst is a benign (non cancerous), fluid-filled cavity in the bone, which weakens the bone and makes it more likely to fracture (break). It occurs  mostly in  children and young adults.

It is not known what causes bone cysts.


Most bone cysts heal within six months to a year without the need for treatment.

If a bone cyst does not get better, or if treatment is recommended to help reduce the risk of a bone fracture, several effective treatment options are available. For example, steroids may be injected into the bone to encourage the cyst to heal. See Bone cyst - treatment for more information.

However, there is a one in three chance that a bone cyst will reoccur in the future, usually within two years.

Types of bone cysts

There are two main types of bone cysts. They are:

  • unicameral bone cysts, which are thought to be relatively common and often do not require treatment
  • aneurysmal bone cysts, which are thought to be very rare and can quickly spread through the bone, meaning treatment is necessary

These are described in more detail below.

Unicameral bone cyst

They normally occur in the long bones (legs and arms) of growing children, and are twice as likely to affect boys than girls. Most cases affect younger children who are between five and fifteen years of age, with the average age at diagnosis being nine years.

Aneurysmal bone cyst

As with unicameral bone cysts, aneurysmal bone cysts can develop anywhere in the body. However, most cysts develop inside:

  • the bone of the lower leg
  • the spine
  • flat bones, such as the vertebrae (the horseshoe shaped bones that make up the spine) or the bones of the pelvis (hipbone)

Aneurysmal bone cysts are thought to be very rare,affecting about one person in a million. They are not cancerous, but they can grow quickly and disrupt the normal workings of the affected bone.

The large majority (75%) of aneurismal bone cysts occur in 10-20 yearolds, and they are more common in women than men.

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Unicameral bone cyst

Most unicameral bone cysts do not cause any symptoms unless the bone has been excessively weakened by the cyst. If this occurs, your child may experience painful symptoms when placing weight onto the affected bone.

If the cyst causes the bone to fracture, it is likely that your child will experience additional symptoms, such as:

  • pain and swelling
  • bruising or discoloured skin around the bone or joint
  • angulation, which is where the limb or affected body part is bent at an unusual angle
  • inability to move or put weight on the injured limb or body part

Aneurysmal bone cyst

The most common symptoms of an aneurysmal bone cyst are:

  • pain (usually a persistent dull ache that gets worse over time)
  • swelling
  • a noticeable lump in the bone
  • the affected body part looks deformed
  • a decreased range of movement, weakness or stiffness in the affected body part
  • the skin in the affected area may be warm to the touch

If an aneurysmal bone cyst develops inside the spine, it can disrupt the normal working of the nervous system and cause additional symptoms, such as:

  • muscle weakness
  • a shooting pain in the legs or arms
  • persistent headaches
  • numbness or a tingling sensation in the arms and legs
  • loss of the ability to move one or more of the body's muscles

When to seek medical advice

You should always contact your GP if you or your child experiences persistent bone pain and/or any of the neurological symptoms, such as muscle weakness or paralysis, described above.


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The cause or causes of both unicameral and aneurysmal bone cysts are unclear, although several theories have been suggested. These are described below.

Unicameral bone cyst

A leading theory is that unicameral bone cysts are caused by a build-up of the fluid that surrounds the cells. It is thought that this happens when the rapid bone growth that occurs during childhood occasionally disrupts the workings of the cells.

There is some limited evidence to suggest that there may be certain
genetic mutations that can make a person more likely to develop a unicameral bone cyst.

A genetic mutation occurs when the normal instructions that are carried in certain genes (single units of genetic material) become scrambled. This means that some of the functions and processes of the body will not work in the normal way.

Aneurysmal bone cyst

Blood is the fluid that forms inside an aneurysmal bone cyst. This suggests that some sort of underlying abnormality with the blood vessels inside an affected bone is responsible. However, it is unclear exactly what type of abnormality is involved or what causes it.

One theory is that a previous injury to the bone may damage the blood vessels, leading to a build-up of blood inside the bone. Another theory is that other non-cancerous growths (benign tumours) inside the bone may disrupt the blood supply, resulting in the formation of an aneurysmal bone cyst.

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Bone cysts can usually be diagnosed by looking at an X-ray of your child’s 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. Alternatively, a diagnosis may be confirmed after the affected bone has been fractured.

Further testing

Further testing is usually only required if:

  • The cyst has developed on the end of a long bone that is still growing (an area of the bone that is known as the growth plate) and there is concern that it may affect normal physical development.
  • There are additional symptoms, such as unexplained weight loss, that mean a diagnosis of bone cancer needs to be ruled out. It should be stressed that bone cancer is very rare, with only 500 new cases occurring in the UK each year.
  • The cyst is so large that the affected bone is at risk of fracturing (breaking).

In such circumstances, two tests that may be used are:

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Unicameral bone cyst

If the cyst is small and the affected bone is strong, a policy of watchful waiting may be recommended. This means that your child will not receive any immediate treatment, but they will be given regular check-ups to make sure that the cyst is not getting larger. About 1 in 4 unicameral bone cysts heal by themselves without the need for treatment.

Surgical treatment may be recommended if the cyst does not show any signs of healing or if the bone is thought to have a high risk of fracture. There are three main types of surgical treatment:

  • steroid injection, which is where steroids are injected into the bone to encourage it to heal
  • bone marrow injection, which is where bone marrow is injected into the cyst to encourage it to heal
  • curettage and bone grafting, which is where the cyst is scraped out and replaced with donated bone tissue or chips of bone that are taken from another part of the body 

All three techniques are carried out under general anaesthetic, which means that your child will be asleep during the surgery and will not feel any pain.

In some cases, a combination of two, or all three, techniques may be used. Each technique is discussed in more detail below.

Steroid injection

A type of steroid called methylprednisolone acetate is the preferred steroid for this type of treatment. It is thought that methylprednisolone acetate encourages the stimulation of a chemical called prostaglandin, which helps to prevent the cyst from growing any larger.

The surgeon will drain the fluid out of the cyst before injecting the steroids into it. Repeated injections may be required every few months over the course of a year before the cyst fully heals.

As steroid injection is a relatively straightforward type of treatment, it is often the first treatment to be used. Additional treatment options will only be used if the cyst shows no sign of healing.

Bone marrow injection

Bone marrow injection is similar to a steroid injection. The surgeon injects the cyst with bone marrow that has been removed from another part of your child’s body, usually their pelvis.

Bone marrow contains specialised cells which should encourage the cyst to heal. In most cases, a single injection of bone marrow is required.

Curettage and bone grafting

During this procedure a surgeon cuts into the bone to gain access to the cyst.

The fluid inside the cyst is drained and the lining of the cyst scraped out using a tool called a curette. The resulting cavity inside the bone is filled with chips of bone, either from other parts of your child’s body or from donated bone tissue.


It is relatively common for bone cysts to reoccur during the first two years following treatment. For example, one study estimated that a third of all symptomatic unicameral bone cysts will reoccur. Therefore, it is likely that your child will have regular X-rays to assess the condition of the previously affected bone.

It is impossible to predict the reoccurrence rates of bone cysts that do not cause any symptoms because most go undiagnosed.

Aneurysmal bone cyst

Aneurysmal bone cysts are typically treated using curettage and bone grafting, as described above.

In some cases, additional treatments, such as liquid nitrogen (a very cold substance) may be used to damage the tissue of the cyst.

Some aneurysmal bone cysts will heal spontaneously following a simple biopsy. About one in five aneurysmal bone cysts will reoccur following treatment, almost always within the first 18 months after treatment.

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