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 fluid, resulting in high pressure and obstruction. This is thought to happen when rapid bone growth during childhood occasionally disrupts the workings of the cells.

There is some limited evidence to suggest 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 carried in certain genes (single units of genetic material) become scrambled. This means 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 an underlying abnormality with the blood vessels inside an affected bone causes the problem. 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.

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 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, which mean a diagnosis of bone cancer needs to be ruled out (it should be stressed that bone cancer is very rare, with only 600 new cases occurring in the UK each year)
• the cyst is so large that the affected bone is at risk of fracturing (breaking)

In these circumstances, two tests may be used:

• a computerised tomography (CT) scan, where a series of X-rays are taken and a computer is used to produce a more detailed three-dimensional image of the bone
• a magnetic resonance imaging (MRI) scan, which uses strong magnetic fields and radio waves to produce a detailed image of the inside of the bone

A bone cyst is a fluid-filled hole that develops inside a bone. They mostly occur in children and young adults.

Bone cysts do not usually cause any symptoms, they are not cancerous and they do not usually pose a serious threat to health.

However, larger cysts can cause a bone to weaken, making it more vulnerable to fracture. This can lead to symptoms 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.

It is not known exactly what causes the build up of fluid that leads to a bone cyst, but several theories have been suggested, including damage to the blood vessels in the bone.

Read more about the causes of bone cysts.

Types of bone cysts

There are two main types of bone cysts.

Unicameral bone cysts can develop anywhere in the body, although 90% of cases involve either the bone of the upper arm or the thigh bone. They often do not need treatment.

Most cases affect younger children between five and 15 years of age, with the average age at diagnosis being nine years. Boys are twice as likely to be affected by a unicameral bone cyst as girls.

Aneurysmal bone cyst

Aneurysmal bone cysts can also develop anywhere in the body. However, most cysts develop inside:

• the bone of the thigh, lower leg or upper arm 
• the vertebral bones (in the spine)

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

Most cases of aneurysmal bone cysts affect young people who are between 10 and 20 years of age. It is thought that aneurysmal bone cysts are slightly more common in females.

Treating 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 fracture, several treatment options are available.

For example, steroids may be injected into the bone to encourage the cyst to heal. If the bone is still not healing, surgery may be needed to treat the cyst.

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

Read more about how bone cysts are treated.

Bone cysts do not usually have any symptoms. They may not be discovered until you fracture (break) the bone, or until you have an X-ray.

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 or any of the neurological symptoms, such as muscle weakness or paralysis, described above.

Many bone cysts heal without treatment. However, treatment is sometimes necessary if the cyst is painful or to prevent the bone fracturing.

Unicameral bone cyst

If the cyst is small and the affected bone is strong, a policy of watchful waiting may be recommended. This means your child will not receive any immediate treatment, but they will be given regular check-ups to make sure the cyst is not getting larger. About one in four 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
• bone marrow injection
• curettage and bone grafting

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 prevent the cyst growing.

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.

Follow-up

In around 10-20% of cases, bone cysts may recur during the first two years after treatment, especially with larger cysts or in young people. Therefore, it is likely your child will have regular X-rays to assess the condition of the previously affected bone.

It is impossible to predict the recurrence 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 recur following treatment, almost always within the first 18 months.