Bone Cyst A bone cyst is a benign (non cancerous), fluid-filled cavity in the bone, which weakens the bone and makes it more likely to fracture (break). It can occur in both children and adults. The most common type of bone cyst is the simple (unicameral) bone cyst. They normally occur in the long bones (legs and arms) of growing children, and are more common in boys than girls. In older children and adults bone cysts usually occur is usually in flat bones, such as the pelvis, jaw, rib cage or skull. A relatively rare type of bone cyst is the aneurismal bone cyst. This affects about one person in a million. The large majority (75%) of aneurismal bone cysts occur in 10-20 year olds, and they are more common in women than men. They tend to appear in the spine and the long bones, and differ from the simple bone cysts because they are filled with blood. Glossary Benign Benign refers to a condition that should not become life-threatening. In relation to tumours, benign means not cancerous. Blood Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart. Spine The spine supports the skeleton, and surrounds and protects the delicate spinal cord and nerves. It is made up of 33 bones called the vertebrae.