What is a Bone Cyst?

A unicameral, or simple, bone cyst is a common, noncancerous (benign) bone tumor. They most often occur in children and adolescents. Bone cysts are one of the more common benign tumors that affect children. They are found more frequently in boys than girls and they do not spread to other parts of the body. 

Unicameral bone cysts (UBC) are cavities inside a bone that are filled with fluid. They can develop in any bone but most often affect the long bones near a joint or growth plate. All children have growth plates at the end of their long bones. These areas of developing cartilage expand and harden into bone as a child grows. UBCs are typically found in the upper arm (humerus) and the thigh bone (femur). As the bone grows, the size and shape of a bone cyst can change. The cyst will stop growing when the child is full-grown, at which time it will gradually fill with normal bone and disappear. 

There are two classifications of unicameral bone cysts:

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

Symptoms of Bone Cysts

Most UBCs are found incidentally because they do not cause symptoms. For example, a bone cyst may be found when an x-ray or other test is performed for an unrelated problem. Some cysts are discovered after a bone breaks. Because there are no symptoms, a bone cyst can grow large enough to weaken the bone, creating an area that is susceptible to fracture. 

Causes of Bone Cysts

It is not known what causes unicameral bone cysts to form. 

Diagnosis of Bone Cysts

If a bone cyst is suspected, your doctor will order imaging tests, most importantly x-rays. X-rays provide clear pictures of bones and will usually show a UBC quite well. In most cases, an x-ray is the only imaging test needed to diagnose a bone cyst. In fact, many bone cysts are discovered incidentally when a bone is x-rayed for another problem. 

Sometimes, additional imaging tests are needed to distinguish a UBC from another type of noncancerous bone cyst: an aneurysmal bone cyst. An aneurysmal bone cyst is formed by a group of blood-filled cysts. This type of cyst can grow very large and cause serious deformity. 

Treatment for Bone Cysts

Treatment for a UBC is based on the size of the cyst, its location and the risk for fracture. To determine appropriate treatment, your doctor will consider the size of the cyst, the strength of the bone, your child’s activity level and any other symptoms your child may be experiencing. 

Small cysts are often just observed over time with regular x-rays. 

Cysts that are painful, growing larger, or in a location likely to cause fracture may require surgery. The most common surgical treatments for bone cysts are:

• Aspiration and injection: Needles are inserted into the cyst to drain the fluid. This is called an aspiration. The doctor then injects a substance into the cavity to decrease the chance of the cyst coming back. Sometimes a series of injections are required over several months. 
• Curettage and bone graft: In this procedure, the cyst is aspirated and then scraped out. After that curettage, the doctor may fill the hole with a bone graft – bone taken from a donor or from another bone in your body. Your doctor may also use a substance called bone cement to fill the cavity. 

General anesthesia is required for these surgical treatments. Both are typically done as outpatient procedures and your child will be able to go home the same day. 

Regardless of treatment, unicameral bone cysts will return in the same location 25-30% of the time. Generally, the younger a child is during the initial treatment, the higher the chance of the cyst returning. Once a child is full-grown, UBCs typically stop growing and fill in with bone.