A subchondral cyst is a fluid-filled sac that develops in the bone just beneath the cartilage of a joint. These cysts, also known as bone cysts or geodes, are most frequently found in weight-bearing joints like the hip or knee but can also occur in the shoulder. These formations are benign (non-cancerous) and are considered a secondary sign of an underlying joint problem rather than a primary disease.
Causes of Subchondral Cysts
The formation of subchondral cysts is most commonly linked to osteoarthritis (OA), a disease characterized by the breakdown of joint cartilage. When this protective cartilage wears away, the underlying subchondral bone is exposed to increased pressure and friction. This stress can lead to the development of tiny fractures in the bone.
Synovial fluid, the lubricating liquid within the joint, is then forced through these small cracks under high pressure. Over time, this intruding fluid creates a cavity within the bone, which becomes a fluid-filled sac, or cyst. These cysts are filled with components of synovial fluid, such as hyaluronic acid.
While osteoarthritis is the primary cause, subchondral cysts can also arise from other conditions. Rheumatoid arthritis, an autoimmune disorder, can lead to the cartilage and bone erosion that facilitates cyst formation. Additionally, significant joint trauma can damage the cartilage and subchondral bone directly, creating the conditions for a cyst to develop without widespread arthritis.
Recognizing Symptoms and Diagnostic Methods
Subchondral cysts themselves are often asymptomatic. The symptoms a person experiences, such as joint pain, stiffness, and reduced mobility, are caused by the underlying joint disease, like osteoarthritis. The pain from OA tends to be intermittent at first, often worsening with activity, and can become more constant as the condition progresses.
As the joint’s cartilage continues to deteriorate, individuals may notice increased swelling and a grinding or scraping sensation during movement. The pain associated with OA in the hip or knee can be felt in the groin, thigh, or buttocks. A person might only become aware of a subchondral cyst after seeking medical advice for these persistent joint symptoms.
Diagnosis begins with a physical examination and a review of the patient’s medical history and symptoms. To confirm a diagnosis, a physician will order imaging tests. An X-ray is often the first step; it can reveal signs of osteoarthritis and may show the cyst as a hollowed-out area in the subchondral bone. For a more definitive view, a magnetic resonance imaging (MRI) scan is used to clearly visualize the cyst, the condition of the surrounding cartilage, and any bone damage.
Available Treatment Options
The management of subchondral cysts focuses on addressing the underlying joint condition, not the cyst itself. The primary goal is to alleviate pain, improve joint function, and slow the progression of the disease. Treatment plans are tailored to the patient and categorized as either non-surgical or surgical.
Non-surgical management is the first line of defense and often involves a combination of treatments.
- Over-the-counter nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, to help manage pain and reduce inflammation.
- Physical therapy with tailored exercise programs to strengthen muscles, improve flexibility, and enhance joint stability.
- Lifestyle modifications, including weight loss to reduce pressure on joints and switching to low-impact activities like swimming.
- Corticosteroid injections directly into the joint to offer temporary reduction of inflammation and pain.
Surgical intervention is reserved for severe cases where non-surgical methods have failed to provide adequate relief or when there is significant joint damage. One minimally invasive option is arthroscopy, where a surgeon can clean out the joint and fill the cyst with a bone graft. For large cysts causing mechanical issues, a surgeon might drain the fluid with a needle (aspiration). In end-stage osteoarthritis, a total joint replacement (arthroplasty) is the most definitive solution, replacing the entire damaged joint with a prosthesis.