Bone Cyst

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.

There are two main types of bone cysts.

Unicameral 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:
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.

Signs & Symptoms

Bone cysts often have no symptoms. They may not be discovered the bone fractures (breaks), or until a coincidental X-ray is performed.

Unicameral Bone Cysts:
Most unicameral bone cysts do not cause any symptoms unless the affected bone becomes severely weakened, causing it to fracture.

Signs and symptoms of a fracture can include:

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

In most cases, the bones of the upper arms or legs are affected.

Occasionally, unicameral bone cysts may cause pain without any obvious signs of a fracture, and they may disrupt the growth of the affected bone.

Aneurysmal Bone Cysts:
Signs and symptoms of an aneurysmal bone cyst can include:

• persistent pain and swelling
• a noticeable lump or deformity in the bone
• decreased range of movement, weakness or stiffness in the affected body part
• the skin in the affected area being warm to the touch

Most aneurysmal bone cysts affect bones in the legs, upper arms, pelvis or spine.

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 arms or legs (sciatica)
• numbness or a tingling sensation in the arms and legs
• problems with bladder or bowel control

Although it happens less often than in unicameral bone cysts, aneurysmal bone cysts can also sometimes cause fractures in affected bones.

Diagnosis

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.

Treatment

Many bone cysts will eventually heal without treatment and won’t cause any permanent problems.

This is particularly true of unicameral bone cysts in children, which will usually disappear around the time they finish growing.

However, treatment is sometimes necessary if the cyst is painful or is at risk of causing a fracture.

If a cyst has already caused a fracture, this will need to be dealt with before the cyst is treated – usually with painkillers, rest and a plaster cast.

Unicameral Bone Cysts:
If the cyst is small and the affected bone is strong, no immediate treatment is administered, but will be given regular check-ups to make sure the cyst is not getting bigger.

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 injections – where fluid is drained out of the cyst before steroid medication is injected into it; several injections may be required a few months apart to help ensure the cyst fully heals

• Bone marrow injections – similar to steroid injections, but the surgeon injects bone marrow that has been removed from another part of the body to encourage the cyst to heal

• Curettage and bone grafting – where the surgeon cuts into the bone, drains the cyst, and scrapes out the lining of the cyst; the resulting hole is then usually filled with chips of bone from another part of the body or a donor

All three techniques are carried out under general anaesthesia. Most people can go home later the same day, although may need to take things easy for a few months while recovering.

In some cases, unicameral bone cysts can return after treatment, usually within a year or two. Therefore, so may need to have regular X-rays for a few years afterwards to assess the condition of the affected bone and check for signs of the cyst recurring.

Aneurysmal Bone Cysts:
Aneurysmal bone cysts usually need to be treated because they don’t tend to get better on their own and they can get bigger rapidly. In many cases, simple treatment such as carrying out a biopsy of the cyst will start the healing process.

If this does not work, they are treated using curettage and sometimes with bone grafting, as described above.

In some cases, additional treatments such as a procedure to block the blood supply to the blood vessels near the cyst (embolisation) or to freeze and damage the tissue of the cyst using liquid nitrogen may also be carried out.

Like unicameral bone cysts, aneurysmal bone cysts may sometimes come back after treatment. This usually happens within 18 months, if it’s going to.