A synovial cyst is a non-cancerous, fluid-filled sac that develops near a joint. These growths are outpouchings from the synovial membrane, the tissue that lines joints and produces lubricating fluid. While often benign, a synovial cyst can cause significant discomfort or severe pain if it expands and presses on nearby structures.
Defining Synovial Cysts and Common Locations
A synovial cyst is a benign, enclosed sac filled with viscous synovial fluid, the natural lubricant found within joints. The fluid is typically clear or straw-colored and reduces friction during movement. Cysts form when the joint capsule weakens, allowing the synovial lining to herniate and create a balloon-like structure connected to the joint space.
Synovial cysts can develop near any joint, but they are most frequently encountered in two primary areas. Over 90% of spinal synovial cysts occur in the lumbar region, arising from the facet joints, where they are often called juxtafacet cysts.
They also form in peripheral joints, such as the knee, ankle, or hip. It is important to distinguish them from ganglion cysts, which are also fluid-filled masses near joints. A true synovial cyst contains a lining of synovial cells, while a ganglion cyst (often found in the hand or wrist) is a non-epithelial, mucin-filled sac lacking this true lining.
Identifying Symptoms and Diagnostic Methods
The presentation of a synovial cyst varies significantly depending on its size and location. Peripheral cysts, such as those in the knee or ankle, may present as a palpable, soft lump under the skin. These can cause localized discomfort or stiffness when the joint is moved, though many small peripheral cysts cause no noticeable symptoms.
When a synovial cyst develops on the spine, it can extend into the spinal canal and press on the spinal cord or nerve roots. This nerve compression results in radicular pain, often radiating down the leg (sciatica). Patients may also experience numbness, tingling, or muscle weakness in the affected limb.
A common pattern for spinal cysts is neurogenic claudication, characterized by cramping or heaviness in the legs that worsens with standing or walking and is relieved by sitting or leaning forward. Medical imaging is necessary for diagnosis to confirm the cyst’s presence and its relationship to surrounding structures. While X-rays can reveal associated joint degeneration, a Magnetic Resonance Imaging (MRI) scan is the most effective tool to visualize the fluid-filled sac and potential nerve root compression.
Underlying Causes and Risk Factors
Synovial cysts form as a direct consequence of joint degeneration, which destabilizes the joint structure. The mechanism involves chronic friction and wear on the joint’s cartilage and capsule. As the joint attempts to compensate for this damage, it often produces an excess of synovial fluid.
This increased fluid volume combined with the compromised joint capsule integrity allows the synovial membrane to bulge outward. This herniation forms the cyst. Spinal instability, such as spondylolisthesis (a slight slippage of one vertebra over another), can accelerate this process by placing irregular stress on the facet joints.
Age is the most significant risk factor, as the likelihood of developing a synovial cyst increases with cumulative joint wear, making it most common in individuals over 60 years old. Pre-existing degenerative joint diseases, particularly osteoarthritis, are strongly associated with cyst formation due to the chronic inflammation and structural changes they cause. Previous joint trauma or injury that alters the joint’s biomechanics can also predispose an individual to developing a synovial cyst.
Management and Treatment Options
Management of a synovial cyst begins with conservative treatment, especially for those with mild or intermittent symptoms. Initial steps include rest, the use of non-steroidal anti-inflammatory drugs (NSAIDs) to manage pain and inflammation, and physical therapy to strengthen supporting muscles. This approach aims to reduce mechanical stress on the affected joint, potentially allowing symptoms to stabilize or resolve.
If symptoms persist or significantly interfere with daily life, minimally invasive interventions are considered. One common procedure is percutaneous aspiration, where a needle guided by fluoroscopy or CT imaging is used to drain the fluid from the sac. This draining is frequently combined with a corticosteroid injection into the joint or the cyst itself to reduce inflammation.
Aspiration and injection provide substantial short-term relief, but this treatment has a notable recurrence rate because the neck of the cyst remains open. Surgical intervention is reserved for cases causing severe, intractable pain or neurological deficits, such as significant muscle weakness. The standard surgical procedure involves a laminectomy (removing a portion of the vertebral bone) and the complete excision of the cyst to decompress the trapped nerve root.
Surgical removal is associated with a high success rate for symptom resolution, though a small risk of recurrence exists following decompression alone. For patients with underlying spinal instability, cyst excision may be accompanied by spinal fusion. This procedure permanently stabilizes the motion segment to prevent further degenerative changes and minimize the chance of the cyst returning.