Subchondral Bone Cysts

Fluid-Filled Sacs in Joints Associated With Osteoarthritis

Knee, semiology elderly person
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A subchondral bone cyst (SBC) is a fluid-filled sac that forms within a joint space such as in the hip, knee, or shoulder. They can develop in people with osteoarthritis and are usually not treated.

How Osteoarthritis Progresses

Osteoarthritis is caused by the breakdown of cartilage in the joints. Cartilage serves as a cushion between joint bones, allowing them to glide over each other and absorb the shock from physical movements.

Osteoarthritis typically develops in stages:

  1. At the onset of disease, the space between the joint bones will begin to narrow due to cartilage degeneration.
  2. As the joint tries to repair itself, the remodeling of bone can often be haphazard and lead to the formation of bone spurs (osteophytes) and the development of subchondral sclerosis (the stiffening of the joint due to increased bone mass).
  3. As cartilage loss continues, SBCs may begin to form and protrude from the joint.
  4. The increasing depletion of cartilage will eventually cause bone to rub against bone, triggering pain and the loss of mobility.

Causes and Symptoms

Subchondral bone is the layer of bone just below the cartilage. With osteoarthritis, the blood supply to these areas is typically increased as the body tries to repair the joint damage. The increased pressure is largely responsible for the formation of capsulated pockets of hyaluronic acid which we refer to as SBC.

Together with bones spurs and increased bone mass, the formation of SBCs can further complicate the symptoms of osteoarthritis. From a symptomatic standpoint, SBCs are not all that distinctive and may cause mild to moderate pain and/or affect joint flexibility.

Not all people with osteoarthritis will develop SBC, and it is not even entirely clear why some people develop the condition and others don't.

Diagnosis

SBCs are diagnosed with an X-ray. They will appear as fluid-filled capsules in the joint space but are often poorly defined and difficult to spot. In such case, a magnetic resonance imaging (MRI) scan may be ordered to provide the doctor a more three-dimensional view.

This, along with a review of the person's symptoms and risk factors, is usually enough to confirm the diagnosis.

The risk factors for SBC include:

  • Obesity
  • Smoking
  • Abnormal joint alignment
  • Abnormal joint shape
  • Prior joint injury
  • A family history of osteoarthritis

Treatment

SBC is not treated directly as the lancing of the cyst can lead to infection and a worsening of the joint condition. Instead, the doctor will find ways to help alleviate the acute and long-term symptoms of osteoarthritis.

Options may include:

If none of these options provide relief, the doctor may recommend a joint replacement surgery.

Sources:

Guangyi, L.; Yin, J.; Gao, J. et al. "Subchondral bone in osteoarthritis: insight into risk factors and microstructural changes." Arthritis Res Ther. 2013; 15(6):223. DOI: 10.1186/ar4405.

Han Xinyun, A.; Hamid Rhammalah, B.; and Tan, A. "The Truth Behind Subchondral Cysts in Osteoarthritis of the Knee." Open Orthop J. 2014; 8: 7-10. DOI: 10.2174/1874325001408010007.