Subchondral bone cysts are sacs of fluid that form inside a person's joints. The cysts occur in the subchondral bone, the layer of bone just under the cartilage.

The subchondral bone acts as a shock absorber in weight-bearing joints, such as the knees, hips, and shoulders. The liquid inside subchondral bone cysts (SBCs) is hyaluronic acid, a component found in synovial fluid, which is the thick substance that lubricates joints, allowing the bones to slide past one another without friction.

Risk factors for SBCs include obesity and smoking, but the precise cause is unknown. Symptoms include joint pain and discomfort. Lifestyle changes help symptom relief and may prevent the formation of other SBCs.

Fast facts on SBCs:

  • Initially discovered in the 1940s, SBCs are a sign of osteoarthritis (OA).
  • The exact cause of SBCs is not known.
  • Engaging in high-impact activities can contribute to injury and damage to the joint and cartilage over time, possibly leading to SBCs.
  • SBCs are diagnosed using imaging tests, such as X-rays.

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A swollen joint may be a symptom of a subchondral bone cyst.
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Carrying excess weight may be a risk factor for SBC.

Some experts believe that SBCs are a precursor to osteoarthritis (OA), which itself is a painful condition affecting more than 30 million adults in the United States.

OA causes the cartilage and bones within a joint to gradually break down. SBCs are considered one of the four cardinal or key radiological findings for OA.

However, in one large study of 806 people with OA in the knee, SBCs were found in only 30.6 percent of them. Other conditions besides OA, such as rheumatoid arthritis, also cause cysts to form on the bone joints.

Though the cause is unknown, it is thought SBCs are the result of repeated stress to the bone. This stress is caused by increased pressure from quicker blood flow to the subchondral bone, something that is seen in people with OA.

Certain characteristics and behaviors can increase the risk of developing an SBC. These include:

  • Sex - women may have a higher risk than men of SBC formation, according to some research.
  • Heredity - as certain forms of OA may be hereditary, people with a family member with the condition are more likely to develop OA and SBCs.
  • Joint alignment and shape - abnormal joint shape or alignment can increase friction, leading to greater damage and a higher risk of cyst formation.
  • Obesity - carrying excess weight puts additional pressure on the joints of the body, increasing the risk of joint problems.
  • Smoking - tobacco contains chemicals that contribute to cartilage damage. Some research has shown that male smokers with OA experience greater cartilage loss.
  • Activity and injury - joint injuries increase the risk of joint problems later, including the development of SBCs.

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Low-impact activities may help to treat and manage SBC and OS.

It is recommended that SBCs are not treated directly. Due to the risk of infection, these cysts should not be removed. However, they can regress on their own.

As a result, treatment typically involves making lifestyle changes and providing symptom relief. Some people may benefit from replacing the joint if problems are ongoing or progressive.

Treatments for SBCs include the following:

Nonsteroidal anti-inflammatory drugs (NSAIDs)

These over-the-counter painkillers, such as ibuprofen and aspirin, may reduce symptoms of SBCs. It is always advisable to check with a doctor before taking NSAIDs. Long-term use should be avoided.

Low-impact activities

It may be recommended to choose low-impact activities such as swimming, aqua aerobics, and cycling. These put less pressure on the knees and hips joints than high-impact activities, such as running and jumping, which can exacerbate the symptoms of OS and SBCs and lead to further joint damage over time.

Weight management

Maintaining a healthy weight reduces excess stress on the joints and may reduce the rate of cartilage loss.

Quit smoking

As this is a risk factor for the development of osteoarthritis, quitting smoking and avoiding secondhand smoke may reduce the symptoms of SBCs and OA.

Ultrasound therapy

At least one study suggests that delivering localized ultrasonic therapy to cartilage and subchondral bone may help treat OA. However, much more research is needed in this area.

Physical therapy

According to the American Academy of Family Physicians, mild degenerative joint disease, of which SBCs may be a feature, can be treated with physical therapy.

If present, symptoms of SBCs can include:

  • discomfort
  • pain
  • lack of flexibility in the joint
  • swelling or bulging around the joint

There are only a few symptoms associated with SBCs, as they are typically considered to be a symptom of OA, particularly progressive OA.

SBCs are considered a symptom of OA or other joint conditions. They may resolve on their own or persist long-term. SBCs may cause pain and contribute to disease progression.

The best way to treat these cysts is to manage the symptoms of OA and other joint conditions.