Unicameral Bone Cyst

A unicameral bone cyst, otherwise known as a simple bone cyst, is a fluid-filled cavity in the bone, lined by compressed fibrous tissue. It usually occurs in the long bones of a growing child, especially the upper part of the humerus (50 to 60 percent of the time) or the upper part of the femur (25 to 30 percent of the time). Other bones, however, can be affected.

These cysts usually affect children primarily between the ages of 5 to 15, but can affect older children or adults. In older children and adults, they tend to occur in flat bones (such as the pelvis, jaw, skull, or rib cage) or in the large heel bone (calcaneus)

Unicameral bone cysts are considered benign. They do not metastasize (spread) beyond the bone. Some heal spontaneously, while others enlarge. More invasive cysts can grow to fill most of the bone's metaphysis (the transitional zone where the shaft of the bone joins the end of the bone) and cause what is known as a pathological fracture. A more invasive cyst could also destroy the bone's growth plate, leading to shortening of the bone. Shortening in the upper arm (humerus) usually does not cause a functional problem, but it may produce a cosmetic problem if it occurs in early childhood.

These cysts are sometimes classified as either "active" or "latent."

• An active cyst is adjacent to the growth plate and tends to enlarge, causing the problems mentioned above.
• A latent cyst is one that is more apt to heal with treatment because the growth plate has migrated away from the cyst.

Unless there has been a fracture, bone cysts are without symptoms. They may occasionally be discovered by chance on X-rays obtained for other reasons. There is no mass or tenderness unless there is a fracture. There may be an abnormal angulation of the limb secondary to the fracture or shortening of the limb if the adjacent growth plate is involved.

Keep in mind that each child experiences symptoms differently. The symptoms of a unicameral bone cyst may resemble other conditions or medical problems. Always consult your child's physician for a diagnosis.

The cause of a unicameral bone cyst remains unknown. Theories have been proposed but none has been definitively proven. One of these theories is that the cysts result from a disorder of the growth plate. Another is that the cysts result from problems with circulation that are caused by a developmental anomaly in the veins of the affected bone. The role trauma plays in the development of these cysts is unknown. Some speculate that repeated trauma puts the bone at risk for developing a bone cyst. This, however, has not been proven.

In addition to a performing a complete physical examination and taking your child's medical history, the doctor will likely take a simple X-ray of the affected area, which can be used alone to diagnose a unicameral bone cyst. The following diagnostic tests are usually not required, except if the cyst is found in an unusual location, such as the pelvis:

• Magnetic resonance imaging (MRI): a diagnostic procedure that uses a combination of large magnets, radio frequencies, and a computer to produce detailed images of organs and structures within the body. This test is done to help document the extent of the cyst, how aggressive it is, and distinguish it from other types of bone cysts.
• Computerized tomography scan (also called a CT or CAT scan): a diagnostic imaging procedure that uses a combination of X-rays and computer technology to produce cross-sectional images (often called slices), both horizontally and vertically, of the body. A CT scan shows detailed images of any part of the body, including the bones, muscles, fat, and organs. CT scans are more detailed than general X-rays. This test can also help document the extent of the cyst, and distinguish it from other bone cysts.
• Bone scans: a nuclear imaging method to evaluate any degenerative and/or arthritic changes in the joints; to detect bone diseases and tumors; to determine the cause of bone pain or inflammation. This test is to rule out other cysts (which are quite unusual).

Specific treatment for a unicameral bone cyst will be determined by your child's physician based on:

• Your child's age, overall health, and medical history
• Extent of the disease
• Your child's tolerance for specific medications, procedures, or therapies
• How your child's physician expects the disease may progress
• Your opinion or preference

Treatment is aimed primarily at preventing recurrent fractures. If it is decided that the bone is relatively strong, observation may be recommended. If the child's activities are such that fracture is unlikely (especially in the humerus) observation may be recommended.

The decision to treat is sometimes difficult and the risks of the surgery must be compared to the likelihood of fracture without treatment. There is no right answer for everyone. If treatment is opted for, it will likely involve one or a combination of the following surgical procedures performed by a pediatric orthopaedic surgeon:

Curettage/bone grafting

Curettage describes a surgical scraping of the cyst with a special instrument called a curette that has a scoop, loop, or ring at its tip. For this procedure, surgeons make an incision in the bone to create a window. The fluid in the cyst is aspirated and the lining tissue is completely curetted. The remaining cavity is then packed with donor bone tissue (allograft), bone chips taken from another bone (autograft), or other materials depending on the preference of the surgeon.

Steroid injection

Injection of the steroid methylprednisone acetate into these cysts can help heal the cyst, sometimes without any other therapy. The steroid's healing ability is not fully understood, although it is believed that steroids can reduce the levels of prostaglandin, a type of fatty acid found in the cyst fluid. Prostaglandin is believed to reduce the cyst's ability to reabsorb into the bone.

For this procedure, spinal or bone marrow biopsy needles are placed into the cyst and the fluid is aspirated. The cyst is injected with radiographic contrast, which helps doctors determine whether the cyst can be filled. Then, the steroid is injected and the needles are removed. The cyst will likely be reinjected at regular intervals every several months, until there is adequate healing, which may take six to 12 months. Overall, the results of steroid injection are similar to curettage and the procedure is much easier on the child, so it is usually preferred to curettage as the first treatment. The risks of this treatment are minimal and generally restricted to the risk of general anesthesia, infection, fracture, and recurrence or persistence of the cyst. There is one case report of air getting into the blood stream. This is a potentially dangerous complication, but it is very unlikely to occur.

Bone marrow injection

Some evidence suggests that bone marrow aspirated from the pelvis above the hip through a small needle and injected in the cyst will help the healing. This is sometimes used in conjunction with demineralized bone gel to stimulate bone formation. The demineralized bone is from donor (allograft) bones. The calcium is removed in these preparations, but the bone proteins that stimulate bone formation remain, and it is believed that this may stimulate healing. The preparation is tested for transmissible diseases such as AIDS (HIV), hepatitis, and bacteria, and is believed to be very safe. Whether or not this approach is better or inferior to simple steroid injection is unknown.

Prognosis for a unicameral bone cyst is generally good. Most of these cysts do heal with proper treatment and if left alone, most heal spontaneously by the time the skeleton ceases to grow. Recurrence can, however, occur. Continuous follow-up care is essential for the successful treatment of this kind of bone cyst. A schedule of follow-up care should be determined by your child's physician and other members of your care team to monitor ongoing response to treatment and possible late effects of treatment.