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Unicameral bone cysts (UBCs) are one of the more common noncancerous bone tumors that affect children. The true number of these tumors is not known because many are never discovered.

Unicameral bone cysts:

• Are found more frequently in boys than in girls.
• Do not metastasize (spread to other parts of the body).
• Most often occur in just one bone, typically at the end of the bone, near a joint and growth plate. Growth plates are areas of developing cartilage tissue near the ends of long bones in children. The growth plate helps determine the length and shape of the adult bone.

As a bone grows, the shape and size of a cyst can change. Usually, the cyst will stop growing when the child is full-grown, and then will gradually fill in with normal bone and disappear.

Doctors classify unicameral bone cysts according to whether the cyst develops next to a growth plate.

• Active. These cysts are in contact with the growth plate. They can grow large enough to weaken bone and cause fracture. An active cyst can expand into and damage the growth plate, resulting in bone deformity or limbs of uneven length.
• Latent. These cysts are not in contact with the growth plate. As the bone grows, the distance between the growth plate and latent cyst becomes greater.

It is not known what causes unicameral bone cysts to form. Likely, as the bone is growing, the body accidentally lays down the wrong tissue or a cyst lining that fills with fluid. 

Most unicameral bone cysts do not cause symptoms and are discovered incidentally (by accident). This happens when an X-ray or other test is performed for an unrelated problem and the cyst is seen on the X-ray.

Some cysts are discovered after the bone breaks. Bones are made of:

• A strong, dense outer tube called the cortex.
• A  squishy, spongy inner bone called trabecular bone. Trabecular bone is found inside of the intramedullary canal (also known as the intramedullary cavity or intramedullary space) of the bone. 

A cyst grows in the intramedullary space. As it grows, it can make the cortex thinner and weaken the bone over time, putting the bone at risk for fracture (break). Because it causes no symptoms, a  bone weakened by a cyst can break with just a minor injury, such as a fall from standing height. This type of fracture (broken bone) is called a pathologic fracture.

This X-ray shows a UBC and a pathologic fracture in the humerus (upper arm bone). 

Less commonly, a patient may notice a painless bump if the cyst has caused the bone to enlarge in a specific area.

To confirm a diagnosis of unicameral bone cyst, the doctor will order imaging tests — most important, X-rays.

X-rays

X-rays provide clear pictures of dense structures like bone and will usually show a UBC quite well. In most cases, a plain X-ray is the only imaging test a doctor needs to diagnose a unicameral bone cyst.

A unicameral bone cyst in the pelvis shows up clearly in the X-ray on the left, and in the MRI scan on the right.

To determine the appropriate treatment, the doctor will consider a range of factors, including:

• The size of the cyst
• The strength of the bone
• Your child's activity level
• Any symptoms your child may be experiencing

If a cyst is small and not causing symptoms, it often can be simply observed. If a cyst is large enough to cause concern about the bone breaking with mild force, surgery is an option to reduce the risk of fracture.

If the bone has already broken through the cyst, the fracture will typically heal without surgery. Some cysts (approximately 25%) will completely heal along with the fracture. If the cyst is still present after the fracture has healed, surgery may be recommended to prevent another fracture.

Surgical Treatment

The doctor may recommend surgery if the cyst is:

• Painful
• Growing larger
• In a location that may cause fracture

Several different types of operations have been shown to have good results.

Aspiration and injection. In this procedure, needles are inserted into the cyst to drain the fluid. This is called an aspiration. The cyst is then injected with substances to decrease the chance of the cyst coming back. Sometimes, a series of injections is performed over several months.

(Left) This X-ray taken in the operating room shows needles placed in the cyst for the aspiration. (Right) This X-ray shows the cyst after dye has been injected to help doctors make decisions about filling the cyst with materials to prevent recurrence.

Curettage and bone graft. In this procedure, the cyst is aspirated and then scraped out of the bone. After curettage, the doctor may fill the hole with a bone graft; this is bone taken from a donor (allograft) or from another bone in your child's body (autograft). The doctor may also use a bone cement mixture to fill the hole.

General anesthesia (your child is put to sleep) is most often used for both surgical options. Both options are also typically done as outpatient procedures, meaning patients are able to go home after a period of observation in the recovery area.

Recovery

How long it takes to return to daily activities after surgery will vary depending on the size and location of the cyst. Most patients can return to full activity within 3 to 6 months. The doctor will provide specific instructions to guide recovery.

Outcomes

Regardless of the treatment method chosen, unicameral bone cysts come back in the same location 25 to 50% of the time. Generally, the younger the child is when the cyst is treated, the higher the chance of the cyst returning. Your child's doctor will schedule regular X-rays and follow-up appointments to check for recurrence.

Once a child is full-grown, these cysts typically stop growing and fill in with bone.

• The cause of unicameral bone cysts remains unknown, but continues to be investigated with ongoing research.
• Other research is aimed at developing better agents to inject into the cysts to keep them from coming back.
  Researchers are also trying to discover ways to prevent UBCs from forming.