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from the American Academy of Orthopaedic Surgeons

Diseases & Conditions



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Unicameral Bone Cysts

A unicameral, or simple, bone cyst is a common, benign (noncancerous) bone tumor that primarily occurs in children and adolescents.

Unicameral bone cysts (UBC) are cavities within bone that are filled with fluid. Although they can develop in any bone, UBCs usually affect the long bones — most often the upper arm bone (humerus) and the thighbone (femur).

In most cases, unicameral bone cysts are not painful, and are often discovered incidentally when an X-ray is obtained for another reason. Because these cysts can weaken the surrounding bone, fractures through UBCs do occur.

Treatment for a UBC is based on several factors, including the size and location of the cyst, and the risk for fracture. In some cases, surgery may be recommended.

X-ray of unicameral bone cyst on wrist

A unicameral bone cyst shows up clearly as a dark mass in this X-ray of the wrist.


Unicameral bone cysts 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.

Unicameral bone cysts do not metastasize (spread) to other parts of the body. They 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. 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. One theory holds that these cysts arise as an outgrowth of the growth plate or out of abnormal pockets of tissue that surrounds joints.


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

Some cysts are discovered after the bone breaks. Because it causes no symptoms, a cyst can expand and weaken the bone, creating an area susceptible to fracture. A bone weakened by a cyst can break with just a minor injury. This type of fracture is referred to as a pathologic fracture.

Unicameral bone cyst and 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 local area.

Doctor Examination

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


These tests 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.

X-ray and MRI scan of 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.

Differential Diagnosis

Sometimes, additional imaging tests are necessary to distinguish a unicameral bone cyst from another common noncancerous bone cyst: an aneurysmal bone cyst. Although it can resemble a UBC in an X-ray, an aneurysmal bone cyst is formed by a group of blood-filled cysts. This type of cyst can occasionally grow very large and cause serious deformity to the bone. Aneurysmal and unicameral bone cysts require different treatment methods.

In certain cases, a magnetic resonance imaging (MRI) scan or computerized tomography (CT) scan may be ordered to provide more detailed images of the cyst. An MRI scan of an aneurysmal bone cyst will show multiple cavities filled with fluid, as compared with a single cavity finding with a UBC.

If these tests do not adequately reveal the answer, a biopsy may be necessary. In a biopsy, a sample of the fluid within the cyst is taken and examined under a microscope.


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, and 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, then 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 in order to prevent another fracture.

Nonsurgical Treatment

Observation. In some cases of unicameral bone cyst, treatment consists of careful observation over time. The doctor may want to take regular X-rays to keep track of any changes in the cyst.

Activity Modification. If your child is not very active in sports or other high-risk activities, the risk for fracture will be less. The doctor may combine observation with maintaining a specific level of activity until your child is full-grown and the cyst resolves.

Surgical Treatment

The doctor may recommend surgery if the cyst is painful, growing larger, or 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.

Aspiration of unicameral bone cyst

(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 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 and patients are able to go home after a period of observation in the recovery area.


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.


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 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.

Research on the Horizon

The cause of unicameral bone cysts remains unknown, but continues to be investigated with ongoing research. In addition to identifying the cause of these cysts, research is also being done to better understand what might be done to prevent them. Research is also looking at better agents to inject into the cysts to prevent them from coming back.

Last Reviewed

April 2019

Contributed and/or Updated by

Rajiv Rajani, MDRobert H. Quinn, MDZachary A. Child, MD

Peer-Reviewed by

Stuart J. Fischer, MD

AAOS does not endorse any treatments, procedures, products, or physicians referenced herein. This information is provided as an educational service and is not intended to serve as medical advice. Anyone seeking specific orthopaedic advice or assistance should consult his or her orthopaedic surgeon, or locate one in your area through the AAOS Find an Orthopaedist program on this website.