Subchondral bone cysts (SBCs) are sacs filled with fluid that form inside of joints such as knees, hips, and shoulders. The sac is usually primarily filled with hyaluronic acid. Hyaluronic acid is a liquid in joint fluid that lubricates the joint. SBCs aren’t technically cysts. Instead, they are fluid-filled lesions surrounded by bone. Sometimes doctors call them geodes.
SBCs are a sign of osteoarthritis (OA), a disorder in which the cartilage between joints wears away. You can have osteoarthritis without also have SBCs, however. Keep reading to learn more about SBCs.