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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 - 60% of the time) or the upper part of the femur (25-30 % 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.
What causes a unicameral bone cyst?
The cause of a unicameral bone cyst remains unknown. Theories have been proposed but none have 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.
What are the symptoms of a unicameral bone 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.
How is a unicameral bone cyst diagnosed?
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:
Treatment for a unicameral bone cyst
Specific treatment for a unicameral bone cyst will be determined by your child's physician based on:
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:
What is the long-term outlook for a child with a unicameral bone cyst?
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.
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