Unicameral Bone Cyst

What is a 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.

What causes a unicameral bone cyst?

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.

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.