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 - 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.