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What is an aneurysmal bone cyst?

Bones naturally grow and change as your child gets older. However, throughout a long process, abnormalities can sometimes occur. An aneurysmal bone cyst (which is often abbreviated ABC) is one such abnormality. ABCs are blood-filled, fibrous cysts that expand the bone and can cause pain, swelling, and fractures. They are benign cysts (non-cancerous) that don’t spread.

ABCs are most common during a child’s teenage years. They can occur in virtually any bone in the arms, legs, trunk, or skull, as well as the vertebrae and knee. While it’s benign, it can be quite destructive, because it deforms the bone and can cause fractures. ABCs don’t spread but can be quite damaging to the bone and can come back after they are removed.

With surgery, these cysts are highly curable, although they do grow back in some cases.

What are the types of aneurysmal bone cysts?

An aneurysmal bone cyst usually falls into one of two categories: active and aggressive.

An active ABC is one that could deform the bone it’s growing in, but remains contained in the bone, while an aggressive ABC extends beyond the bone to the nearby soft connective tissues. Rarely will aggressive cysts go away without treatment.

Both types can cause pain and swelling and, in rare cases, fractures in the involved bone. Aneurysmal bone cysts do not generally go away on their own.

Aneurysmal Bone Cyst | Symptoms & Causes

What are the symptoms of an aneurysmal bone cyst?

  • swelling
  • mild to severe neurological problems (if the cyst is in your child's spine)
  • a fracture caused by the cyst (rarely)

It’s important to understand that the symptoms of aneurysmal bone cyst may resemble other medical problems, some of them which are very common and easy to treat, others which could be more serious.

Your child may experience symptoms differently. Therefore, it is important to be evaluated by a physician to get an accurate diagnosis. Always consult your child's physician if you have concerns.

What causes an aneurysmal bone cyst?

The cause of these cysts is unknown and controversial. They’re believed to grow in response to a disturbance of the blood vessels in the involved bone. They may grow because of a pre-existing tumor. In half of all cases, a pre-existing tumor, such as fibrous dysplasia, non-ossifying fibroma, a solitary bone cyst, or osteosarcoma also exists.

Abnormalities in the chromosomes (karyotype) of the tumor cells have been described, but the significance of these findings is unclear.

There is no definitive explanation for why these cysts occur. It’s important to know that there’s nothing that you could have done (or not done) that would have prevented your child’s cyst from developing.

Aneurysmal Bone Cyst | Diagnosis & Treatments

How are aneurysmal bone cysts diagnosed?

The first step in treating your child is forming an accurate and complete diagnosis.

Your child’s physician might order a number of different tests to determine the exact type of cyst, its size, and location. These tests may include:

  • a physical exam, including checking the area where the cyst is located
  • x-rays to determine the size of the cyst and whether there is a risk of fracture
  • magnetic resonance imaging (MRI), which produces detailed images of the bone where the cyst is located and determines the internal characteristics of the cyst and the specific type of cyst
  • computerized tomography scan (CT or CAT scan), which is used in some cases to get a clearer picture of the bone lesion
  • a biopsy or tissue sample, collected from the cyst through a needle to provide definitive information about the type of cyst

After we complete all necessary tests, your clinician will review and discuss what they have learned about your child's condition. Then we will meet with you and your family to discuss the results and outline the best treatment options.

What are the treatment options for aneurysmal bone cysts?

Your child's physician will determine a specific course of treatment based on several factors, including:

  • your child's age, overall health, and medical history
  • the location of the cyst
  • the risk of fracture
  • whether the cyst has recurred

Aneursymal bone cysts expand and deform the involved bone. In some cases, this can lead to damage to joints or pathological fracture (a fracture that occurs without trauma). They also cause pain and swelling around the site of the cysts and do not generally go away on their own. To prevent permanent damage to the bone, surgery is needed to cure these cysts.

Treatment options vary greatly, depending on your child's situation. Your doctor and other members of your care team will discuss the options with you in-depth.

There is potential for excessive blood loss during an operation to remove aneurysmal bone cysts because of the abnormal blood vessels that may be involved. As a result, surgery involves very careful planning.

Before surgery, a radiologist may perform a procedure called arterial embolization. This blocks the blood flow in the abnormal vessels that are involved with the cyst. This reduces the risk of excessive bleeding during surgery. To determine whether your child can undergo arterial embolization without complications, your doctor may use an angiogram, a special x-ray that allows doctors to visualize the involved blood vessels.

Treatment for the cyst will likely involve one or more of the following surgeries performed by a pediatric orthopedic surgeon.

Curettage and bone grafting

The most common treatment for an aneurysmal bone cyst, this is an operation during which the cyst is scraped out of the bone with a special instrument called a curette that has a scoop, loop, or ring at its tip. The procedure itself is called curettage.

The remaining cavity is then packed with donor bone tissue (called an allograft), bone chips taken from another bone (autograft), or other materials.

Extended curettage

In some cases, our orthopedic surgeons perform an “extended” curettage using a special instrument (a curette) to remove additional layers of cells around the bone cyst. This is simply a more aggressive type of the procedure described above. It is done to reduce the risk that the cyst will grow back.

Marginal or wide excision

This operation removes the part of the bone involving the cysts at its margins or beyond its margins (known as a wide excision). A wide excision is recommended when the cyst is located in bones considered expendable, such as the ribs or fibula. It is avoided whenever possible when the cyst occurs in a location that could compromise the bone's function.


This is a promising non-surgical technique for healing aneurysmal bone cysts. Instead of surgery to remove the cyst, doctors inject special chemicals into the cyst in order to promote the creation of scar tissue. This scar tissue eventually heals and hardens into bone, healing the cyst without the need for an open surgery. Read more about sclerotherapy.


Because it's possible that the cyst will grow back, this procedure, which surgically freezes the cyst, is sometimes used in addition to curetting and bone grafting. However, it is associated with complications such as fracture of the bone, nerve injury, and others.

What is the long-term outlook for a child with aneurysmal bone cysts?

Aneurysmal bone cysts return about 20 percent of the time. To treat cysts that return, our doctors provide follow-up care for every three months for the first two years after treatment. Recurrent aneurysmal bone cysts are treated using the same techniques, although your child's orthopedic surgeon may opt for a more aggressive treatment to prevent further recurrence.

How we care for aneurysmal bone cysts

Experts in Boston Children's Hospital's Bone and Soft Tissue Tumors Program within our Orthopedic Center provide comprehensive medical and surgical care for children with aneurysmal bone cysts and other bone and soft tissue disorders.

Our multidisciplinary approach to care ensures that your child’s case receives input from experts in several fields before your care team develops a personalized treatment plan.

Aneurysmal Bone Cyst | Programs & Services