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.

SBCs were first discovered in 1940s, but doctors are still uncertain about the reasons they form.

SBCs occur in the subchondral bone, which is the layer of bone right under cartilage. OA causes blood to flow more quickly to the subchondral layer of the bone. This increased pressure and blood flow may lead to the formation of SBCs and subchondral sclerosis. Subchondral sclerosis refers to higher bone density.

People with OA are more likely to develop SBCs. Because of this, the risk factors for SBCs are the same as the risk factors for OA:

Being obese. A strong amount of research suggests that increased body mass puts significantly more pressure on the knee joints. That increases the risk of OA of the knee. Being overweight may also put more mechanical stress on other joints, such as the hip and even hands.

Smoking tobacco. Some of the chemicals found in cigarettes and tobacco promote the degradation of cartilage tissue. Over time, this leads to stronger symptoms of osteoarthritis.

Familial history. Some types of osteoarthritis may be inherited. That means you may be more likely to develop OA and subchondral bone cysts if a family member has the condition.

Abnormal joint alignment or unusual joint shape. This can lead to greater wear and tear on the joints.

Prior joint Injury, especially due to physical labor or high-impact physical activity. High-impact activity may damage cartilage over time.

You can manage the symptoms of a subchondral bone cyst through a variety of methods:

  • Anti-inflammatory drugs, such as ibuprofen (Advil, Motrin), may temporarily reduce symptoms.
  • Weight loss may decrease the symptoms of OA and slow down loss of cartilage. Cartilage loss promotes SBC formation.
  • Avoid activities that aggravate the joint that’s affected by OA.
  • Perform lower impact activities, such as swimming or cycling, as opposed to higher impact activities such as running and jumping.
  • Ultrasound therapy may help. More research is needed to determine the efficacy of this treatment.
  • If pain gets worse, ask your doctor about physical therapy or pain-relieving drugs.
  • You may eventually need a joint replacement if OA progresses a lot.

Doctors don’t recommend treating SBCs directly. Removing or cutting the area around the cyst may increase the risk of infection or cause difficulties with wound healing. Generally, it’s important to let the SBC run its natural course while you manage the underlying OA and symptoms. In more extreme cases where the cyst is large or restricting movement, orthopedists may decide to remove it.

One recent study suggests that in people with OA of the knee, subchondral bone cysts may indicate increased rate of cartilage loss and progression of OA. The study also found that people with these cysts have, on average, twice the likelihood of needing a knee replacement over a span of two years.

SBCs are a sign of osteoarthritis and, according to one study, are seen in approximately one-third of cases of OA. They may appear as a fluid-filled sac around the joint and cause pain over time. However, generally doctors think of them as a symptom of OA rather than a condition in themselves. The best way to treat SBCs is to manage the symptoms of OA. In more extreme cases, your doctor may recommend a procedure to surgically remove a large growth.

Q:

Can I have subchondral bone cysts without having osteoarthritis?

A:

Subchondral bone cysts can happen with any type of arthritis, so rheumatoid arthritis can also cause it. The thought is that it is caused by bleeding into the bone, so a hemarthrosis, that is bleeding into the joint, can cause it as well. There is not much medical literature about subchondral bone cysts. We mostly focus on the underlying problem in the joint rather than the cyst, which is mostly just a finding on X-ray that we see.

Suzanne Falck, MD, FACPAnswers represent the opinions of our medical experts. All content is strictly informational and should not be considered medical advice.