A subchondral cyst is a common finding in joints that have experienced wear, tear, or injury, frequently appearing in the hip, knee, and shoulder. This condition involves the formation of a fluid-filled sac within the bone structure that lies immediately beneath the joint surface. Although the word “cyst” suggests a primary disease, this finding is generally considered a secondary symptom, often developing in joints affected by conditions like osteoarthritis. The presence of a subchondral cyst often indicates underlying structural changes in the joint that require attention to maintain mobility and reduce pain.
Defining the Subchondral Cyst
The term “subchondral” literally means “beneath the cartilage,” precisely defining the location of these fluid-filled lesions, which are also sometimes referred to as geodes. Articular cartilage is the smooth, cushioning tissue that covers the ends of bones in a joint, allowing them to glide friction-free. The subchondral bone is the dense layer directly below this cartilage, acting as a shock absorber.
A subchondral cyst is a cavity that forms within this subchondral bone, typically filled with a viscous, gelatinous material composed mainly of hyaluronic acid, similar to synovial fluid. The cyst is not a true cyst because it lacks the epithelial lining characteristic of other types of cysts, instead being a lesion surrounded by bone. These cavities can vary significantly in size, but their presence indicates a disruption in the relationship between the joint cartilage and the supporting bone structure. The bone surrounding the cyst often becomes denser, a process known as subchondral sclerosis, as the body attempts to reinforce the damaged area.
Mechanisms of Formation
The development of subchondral cysts is closely linked to joint damage and is explained by two primary theories involving mechanical stress and fluid dynamics. One accepted mechanism, known as the synovial fluid intrusion theory, suggests that a defect in the overlying articular cartilage creates a pathway into the subchondral bone. Joint movement then forces pressurized synovial fluid into this opening, which carves out a cystic cavity within the bone marrow.
A competing explanation is the bone contusion or bone necrosis theory, which posits that repetitive mechanical stress leads to microfractures in the subchondral bone. This continuous, excessive loading can cause localized bone death (necrosis) and subsequent bone resorption, leading to the formation of a cystic cavity without a direct connection to the joint space. The bone resorption process is driven by the body’s inflammatory response to the damaged tissue, creating a void that eventually fills with fibrous or myxomatous tissue. Subchondral cysts are strongly associated with advanced joint degeneration, particularly osteoarthritis, where both cartilage loss and abnormal mechanical loading are prominent features.
Recognizing the Condition
Identifying a subchondral cyst involves evaluating a patient’s experience and utilizing medical imaging techniques. While many cysts are asymptomatic, larger or rapidly expanding cysts can cause localized pain, often worse during weight-bearing activities. Patients may also experience joint stiffness, limited flexibility, or tenderness directly over the affected joint line.
Medical professionals confirm the presence of a subchondral cyst using imaging studies. A standard X-ray is often the first tool used, where the cyst appears as a well-defined or poorly defined area of translucency within the bone near the joint. For a more detailed assessment, Magnetic Resonance Imaging (MRI) is utilized, as it is superior for visualizing the fluid content of the cyst and evaluating the surrounding soft tissues. MRI also helps in assessing the condition of the overlying articular cartilage and detecting associated bone marrow edema, which guides the overall diagnosis and treatment planning.
Management and Treatment Options
Treatment for a subchondral cyst is focused primarily on managing the underlying joint condition, such as osteoarthritis, and alleviating associated symptoms. Non-surgical management is the initial approach, involving strategies to reduce joint stress and control pain. Activity modification, such as switching from high-impact sports to low-impact exercises like swimming or cycling, can significantly reduce the load on the affected joint.
Nonsteroidal Anti-inflammatory Drugs (NSAIDs) are often prescribed to manage pain and reduce inflammation. Physical therapy plays a role by strengthening the muscles surrounding the joint, which can improve stability and reduce abnormal mechanical stress on the subchondral bone. Injections, such as corticosteroids or hyaluronic acid, may be used to treat generalized joint inflammation and pain, though they do not directly resolve the cyst itself.
If conservative measures fail and the patient experiences severe, persistent pain or the cyst is large enough to compromise the structural integrity of the bone, surgical intervention may be considered. One option is a procedure to drill into the cyst, decompress the fluid, and then fill the void with a bone substitute or a bone graft (autograft) to promote healing and structural support. This technique, sometimes called subchondroplasty, aims to stabilize the bone and prevent further collapse. If the overall joint damage from the underlying disease is too extensive, a total joint replacement surgery, such as hip or knee arthroplasty, may be the most definitive treatment option.