Subchondral cysts, often referred to as geodes or subarticular pseudocysts, are fluid-filled sacs that develop in the bone tissue immediately beneath the cartilage of a joint. These non-cancerous lesions are common in weight-bearing joints, such as the hip and the knee. Their presence indicates mechanical stress and degeneration within the joint. While not a disease themselves, subchondral cysts are frequently identified during the diagnostic workup for chronic joint pain, marking joint wear and tear.
Anatomy and Formation of Subchondral Cysts
A subchondral cyst forms within the subchondral bone, the dense layer located directly beneath the articular cartilage. This region, typically found in the epiphysis or metaphysis of long bones, is subjected to immense mechanical forces. The cysts are not lined with epithelial cells like true cysts, which is why they are sometimes termed pseudocysts. Their contents are heterogeneous, often composed of fibrous connective tissue, myxoid material, or fluid rich in hyaluronic acid.
Two primary theories explain their formation. The synovial intrusion theory posits that defects in the overlying cartilage create a pathway for pressurized synovial fluid to enter the subchondral bone. Driven by joint loading pressure, this fluid acts like a one-way valve, forcing its way into the bone marrow and causing bone resorption, which carves out the cystic cavity. The second mechanism, the bone contusion theory, suggests that repetitive microtrauma and abnormal mechanical stress cause small areas of bone tissue to die (necrosis). The body’s attempt to clear this necrotic tissue leads to the formation of a cavity, which subsequently fills with fibrous or fluid-like material.
Underlying Causes and Associated Conditions
Subchondral cyst formation is linked to conditions that disrupt the integrity of the joint surface and underlying bone structure. Osteoarthritis (OA) is the most frequent and significant associated condition, with cysts often considered a secondary feature of advanced joint degeneration. The chronic, abnormal mechanical loading and cartilage loss characteristic of OA trigger the cystic process.
The development of these cysts is tied to the body’s pathological response to joint injury, including increased blood flow and localized bone remodeling. While OA is the primary culprit, other joint-affecting diseases can also lead to their development. These include inflammatory conditions like Rheumatoid Arthritis, where an aggressive inflammatory process can erode the bone, and Avascular Necrosis, which involves the death of bone tissue due to a lack of blood supply.
Diagnosis and Symptom Presentation
Subchondral cysts are often asymptomatic, meaning their presence is frequently an incidental finding on imaging performed for other reasons. When symptoms do present, they usually overlap with the pain, stiffness, and limited range of motion caused by the underlying joint disease. Large or rapidly expanding cysts can sometimes cause localized pain through direct pressure on surrounding soft tissues or nerves, resulting in deep-seated discomfort or a persistent ache within the joint.
Diagnosis relies heavily on advanced imaging. Plain X-rays serve as the initial screening tool to identify the characteristic bony lesion. Computed Tomography (CT) scans provide detailed cross-sectional images, allowing clinicians to accurately measure the cyst’s volume and assess surrounding bone sclerosis. Magnetic Resonance Imaging (MRI) is the method of choice for comprehensive evaluation due to its ability to visualize soft tissue. MRI clearly delineates the cyst’s fluid-like content and its association with adjacent features, such as bone marrow edema-like lesions (areas of fluid accumulation that often precede or accompany cyst formation).
Treatment Approaches
Treatment for a subchondral cyst focuses on managing the underlying joint condition, as the cyst is a symptom of a broader problem. Conservative management is the first approach and includes lifestyle modifications like weight loss and low-impact physical activities. Nonsteroidal anti-inflammatory drugs (NSAIDs) are often prescribed to reduce pain and inflammation.
Physical therapy strengthens supportive muscles and improves joint flexibility and stability. For localized pain relief, joint injections, such as corticosteroids or hyaluronic acid, may be administered.
Direct surgical intervention is reserved for cysts that are large, rapidly growing, or severely symptomatic despite conservative measures. Procedures like subchondroplasty involve injecting a bone substitute material into the cavity to provide structural support and promote healing. Alternatively, a surgeon may perform cyst curettage and bone grafting to remove the lesion and fill the defect. When severe osteoarthritis significantly limits function, joint replacement surgery is the ultimate treatment, often leading to the spontaneous regression of the subchondral cysts due to restored biomechanical loading.