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what is a subchondral cyst

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.

There aren’t many distinctive symptoms of SBCs. They are more commonly thought of as a symptom of OA. In addition to symptoms of OA, you may experience:

a small, fluid-filled sac protruding from the joint
discomfort and mild to moderate pain
limited joint flexibility
SBCs can be diagnosed using an X-ray. If a cyst isn’t clear on an X-ray image, your doctor may order an MRI of the affected joint. In addition to these images, your doctor will ask about your medical history, symptoms of osteoarthritis, and risk factors. That information along with images can help your doctor correctly diagnose subchondral bone cysts.

Having OA doesn’t mean you’ll definitely have SBCs. In one study, researchers looked at X-rays of 806 people with OA, and only identified SBCs in about 30 percent of people in the study group.