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What are Acetabular Subchondral Cysts?

Acetabular subchondral cysts are fluid-filled sacs or spaces that form in the acetabulum (hip socket) side of the hip joint. Subchondral bone is the layer of bone just below the cartilage in a joint. The suffix ‘chondral’ means cartilage, while the prefix ‘sub’ means below. Subchondral bone acts as a shock absorber in a weight-bearing joint.

The hip joint is a ball-and-socket joint in which the head of the femur forms the ball and the pelvic acetabulum forms the socket. The joint surface is covered by a smooth articular cartilage that cushions and enables smooth movement of the joint. 

Acetabular subchondral cysts develop just under the cartilage (tough spongy tissue) that covers the acetabulum and may cause joint pain and discomfort. The cysts primarily consist of hyaluronic acid, a liquid in the joint fluid that lubricates the joint.

Causes of Acetabular Subchondral Cysts

Acetabular subchondral cysts are commonly caused by osteoarthritis (OA), a degenerative joint condition and the most common form of arthritis, in which the cartilage between the joint wears away due to wear and tear over time or because of an injury to the joint. This affects the normal, smooth, sliding of one bone against another in your joints, causing more friction.

In some instances, a joint injury might also result in a cyst without OA.

Acetabular subchondral cysts can also form in cases of rheumatoid arthritis, another common form of arthritis, in which your immune system attacks and inflames your joints.

Risk Factors for Acetabular Subchondral Cysts

Some of the common risk factors for acetabular subchondral cysts include:

  • Obesity
  • Hereditary/family history
  • Prior joint injury
  • Abnormal joint shape and alignment
  • Smoking
  • Female gender

Signs and Symptoms of Acetabular Subchondral Cysts

The cysts by themselves do not seem to cause any symptoms, other than protrusion of soft tissue in the acetabulum in rare cases. However, other symptoms may occur as a result of the underlying disease conditions such as osteoarthritis and rheumatoid arthritis. Some of the signs and symptoms of acetabular subchondral cysts include:

  • Pain
  • Swelling
  • Stiffness
  • Inflammation
  • Reduced range of motion
  • A protruding sac from the joint

Diagnosis of Acetabular Subchondral Cysts

In order to diagnose acetabular subchondral cysts, your physician will review your symptoms and conduct a thorough physical examination of the hip to determine which motions are restricted or painful. During the examination, if pain, swelling, or stiffness is noted and your physician presumes it to be a sign of osteoarthritis, rheumatoid arthritis, or joint injury, your physician may order tests such as X-rays, MRIs, and CT scans for a detailed evaluation of hip joint structures and to confirm the presence of acetabular subchondral cysts.

Treatment for Acetabular Subchondral Cysts

Acetabular subchondral cysts may not cause any symptoms. However, in some cases, the cysts may continue to grow and affect the way your joint functions. If this happens, your physician may recommend a procedure called a  fine needle aspiration in which a thin needle is inserted into the cyst under ultrasound or fluoroscopic guidance to drain the cyst.

Other than that, your physician may try to treat the underlying cause of your cysts such as osteoarthritis or a joint injury through conservative treatments such as nonsteroidal anti-inflammatory drugs (NSAIDs) to alleviate pain and inflammation, weight loss programs to reduce pressure on the hip joint, low-impact exercises to improve or retain joint flexibility, ultrasound therapy, and physical therapy.

If none of these treatment measures help in relieving symptoms or provide the desired results, your physician may recommend a joint replacement surgery to replace the affected acetabulum with a prosthesis as a final solution.