Ganglion cysts are common, non-cancerous lumps that typically form near joints and tendons. These masses are benign, appearing as small, fluid-filled sacs just beneath the skin’s surface. While most frequently observed around the wrist and hand, they can also occur near the ankle and foot. They may fluctuate in size or disappear entirely on their own, often causing no symptoms unless they press on an adjacent nerve.
Occurrence and Specific Locations Near the Knee
Ganglion cysts can occur near the knee, though this location is less common than the hand or wrist. They are recognized as both extra-articular (outside the joint capsule) and intra-articular (inside the joint capsule) masses in the knee region. Extra-articular cysts often appear in the surrounding soft tissues, including the joint capsule, tendons, muscles, or nerves. Specific locations for these external cysts have been documented as anterior, lateral, medial, and posterosuperior to the joint.
A common site for an extra-articular cyst is near the proximal tibiofibular joint, located on the outer side of the knee. Cysts in this location may compress the common peroneal nerve, causing pain or weakness in the lower leg and foot. Intra-articular knee ganglion cysts are rarer but frequently arise from the cruciate ligaments, particularly the anterior cruciate ligament (ACL). If these internal cysts grow large enough, they can interfere with normal knee motion, causing pain or tightness.
Less commonly, intra-articular cysts may be found in the infrapatellar fat pad, which sits just below the kneecap. It is important to distinguish a true ganglion cyst from a Baker’s cyst (popliteal cyst), which is swelling in the back of the knee. Baker’s cysts usually arise due to underlying joint issues like arthritis or meniscal tears, causing an overflow of fluid into the bursa. Ganglion cysts are not typically associated with these degenerative conditions.
What Causes These Fluid-Filled Lumps
The formation of a ganglion cyst is linked to a mechanism involving connective tissues and joint fluid. The primary physiological explanation suggests the cyst forms due to the mucoid degeneration of connective tissue, where the tissue breaks down and becomes gelatinous. This process is often linked to repetitive microtrauma or chronic irritation to a joint or tendon sheath.
Structurally, a ganglion cyst is a sac-like growth that arises from a joint capsule or tendon sheath, often described as having a stalk. The sac is filled with a thick, sticky, jelly-like fluid. One theory suggests that a weakness or tear in the joint capsule allows the synovial tissue to herniate, forming the cyst.
Another concept involves a one-way valve mechanism: joint fluid is pushed into the cyst sac but cannot easily flow back. This trapped fluid gradually thickens, creating the characteristic viscous material. When a cyst is connected to the knee joint, changes in pressure can influence the size and extension of the cyst, explaining why activity often causes enlargement while rest may lead to shrinking.
How Doctors Confirm the Diagnosis
Diagnosis begins with a physical examination to assess the lump’s characteristics. The doctor checks the mass for firmness, mobility, and tenderness, noting any changes in size with joint movement. A classic physical test is transillumination, where light is shone through the mass. Since the cyst is fluid-filled, the light passes through, confirming its cystic nature and differentiating it from a solid tumor.
Imaging techniques are crucial for a definitive diagnosis, especially for deep or non-palpable cysts. Ultrasound is often the initial modality, as it confirms the mass is fluid-filled and assists in guiding aspiration procedures.
Magnetic Resonance Imaging (MRI) is the most reliable tool, particularly when surgery is considered. An MRI provides detailed soft-tissue visualization necessary to determine the cyst’s exact anatomical origin, such as its connection to a cruciate ligament or the joint capsule. MRI also helps rule out other potential knee masses, including meniscal cysts, lipomas, or rare conditions like synovial cell sarcoma. The findings typically show low signal intensity on T1-weighted images and high signal intensity on T2-weighted images.
Managing and Treating Knee Ganglion Cysts
Management is determined by whether the cyst is causing symptoms, such as pain or functional limitations. For asymptomatic or mildly bothersome cysts, the first line of treatment is observation. Many ganglion cysts spontaneously resolve over time without intervention. During this period, temporary immobilization may be recommended to reduce joint movement, as activity can cause the cyst to grow.
If the cyst causes persistent symptoms, minimally invasive treatment may be considered. A common non-surgical option is aspiration, where a needle is used to drain the thick fluid from the cyst. This procedure is often performed under ultrasound guidance and may be followed by a corticosteroid injection to reduce inflammation. While aspiration provides immediate relief, it carries a high risk of recurrence because the cyst wall and the stalk connecting it to the joint remain intact.
Surgical excision, or ganglionectomy, is reserved for symptomatic cysts that have failed aspiration or recurred multiple times. The goal of surgery is to remove the entire cyst, including the stalk that connects it to the joint capsule or tendon sheath. For intra-articular cysts, this is often performed arthroscopically. Removing the connecting stalk is essential to minimize the recurrence rate.