A subchondral cyst in the knee is a fluid-filled sac that develops in the bone just beneath the cartilage. This area, known as the subchondral bone, acts as a shock absorber for the joint. The cyst is a lesion containing joint fluid, forming a small pocket within the bone structure. These formations are not technically true cysts because they lack a distinct cellular lining.
Causes and Associated Symptoms
The development of subchondral cysts is most commonly linked to osteoarthritis (OA), a condition characterized by the breakdown of joint cartilage. As OA progresses, the cartilage wears down, increasing friction and pressure within the knee joint. This elevated pressure can force synovial fluid into microfractures in the subchondral bone, leading to the formation of a cyst.
While osteoarthritis is the primary cause, rheumatoid arthritis or direct trauma to the joint can also contribute to their formation. The symptoms experienced are often due to the underlying arthritis rather than the cyst itself. Patients report a deep, aching pain within the knee, which may worsen with activity and improve with rest.
Other common symptoms include swelling and stiffness in the joint, particularly after periods of inactivity. Some individuals may experience a clicking, grinding, or locking sensation during movement, accompanied by a reduced range of motion. It is important to note that many subchondral cysts are asymptomatic and are discovered incidentally during imaging for other reasons.
Diagnosis and Imaging
Diagnosis begins with a clinical evaluation, where a doctor assesses the knee’s range of motion, stability, and tenderness, while also reviewing the patient’s medical history. Imaging tests are then used to confirm the diagnosis and evaluate the overall health of the knee joint. The presence of a cyst can be an indicator of advancing joint disease.
An X-ray is often the first imaging study performed. It can reveal classic signs of osteoarthritis, such as joint space narrowing, and may show the cyst as a hollowed-out area in the subchondral bone. However, X-rays can sometimes miss smaller or less defined cysts.
For a more detailed view, magnetic resonance imaging (MRI) is the preferred method. An MRI provides clear, cross-sectional images of bone, cartilage, and other soft tissues, allowing doctors to visualize the cyst’s size and location. It is particularly effective at detecting smaller cysts and associated bone marrow edema, which is swelling in the bone.
Management and Treatment Options
Treatment for a subchondral cyst in the knee focuses on managing the symptoms of the underlying condition, usually osteoarthritis, as the cyst is often a secondary finding. Most treatment plans begin with conservative, non-surgical methods aimed at reducing pain and improving joint function.
Non-surgical management includes several strategies to reduce stress on the knee joint:
- Lifestyle modifications, such as switching from high-impact activities like running to lower-impact exercises like swimming.
- Over-the-counter nonsteroidal anti-inflammatory drugs (NSAIDs), like ibuprofen, to manage pain and reduce inflammation.
- Physical therapy to strengthen muscles surrounding the knee, which helps stabilize and support the joint.
- Corticosteroid injections into the joint to provide temporary relief from inflammation for more persistent pain.
Surgical intervention is considered when conservative treatments fail to provide adequate relief or if the cyst is large and contributing to joint instability or pain. One option is aspiration, where fluid is drained from the cyst with a needle. A surgeon may also perform an arthroscopic procedure to debride (clean out) the cyst and fill the cavity with a bone graft. In cases of severe joint destruction from advanced osteoarthritis, a total knee replacement is the most effective long-term solution to restore function.