A bone cyst is a cavity within the bone that is filled with fluid. This is usually joint fluid, but in the case of some bone cysts, it can be blood. Bone cysts are not cancerous, but they can cause damage to the bone by making them weak, which also makes them more at risk for a fracture. There are different types of bone cysts, and most are often diagnosed at a young age (before 20 years old), but can be found in adults as well.
Simple Bone Cysts
A simple bone cyst, also referred to as a unicameral bone cyst, is a cavity within the bone that's filled with fluid. The American Academy of Orthopaedic Surgeons reports that simple bone cysts are most often diagnosed in persons under the age of 20, and most often appear in the thighbone and upper arm, although they can also appear in the heel, pelvis and ankle. As of 2009, it is not known what causes bone cysts to develop.
Subchondral Bone Cysts
A subchondral bone cyst is often seen in arthritis patients. According to General Electric Healthcare Medical Encyclopedia, a subchondral bone cyst can be caused by pressure on the cartilage and the underlying bone (subchondral bone), which causes the fluid between the joints (synovial fluid) to leak into the bone. The swelling and inflammation associated with arthritis can put extra pressure on the cartilage.
Aneurysmal Bone Cysts
An aneurysmal bone cyst is filled with blood. According to the Children's Hospital Boston, the cyst appears as a tumor, and can expand the bone. It is not cancerous, and does not spread; however, it can cause damage to the bone it is in. As of 2009, there is no specific cause for the development of an aneurysmal bone cyst.
