A Baker’s cyst, also known as a popliteal cyst, is a fluid-filled sac that develops behind the knee. It is a symptom resulting from an underlying problem within the knee joint, such as osteoarthritis or a cartilage tear. Excess synovial fluid—the natural lubricant of the knee—builds up due to joint irritation and bulges backward into the popliteal space, forming the cyst.
The Natural Course of an Untreated Baker’s Cyst
Leaving a Baker’s cyst untreated often means waiting for the cyst to change on its own while addressing the underlying joint issue. Many cysts will fluctuate in size or may even resolve completely without specific intervention. This spontaneous resolution is more common in children, but it also occurs in adults.
When a cyst persists, symptoms are intermittent and localized to the back of the knee. Individuals may experience tightness, fullness, or mild aching, which becomes more noticeable with physical activity or prolonged standing. The size of the cyst changes depending on the severity of knee inflammation, often shrinking when the underlying condition is quiescent and swelling during a flare-up.
The continued presence of the cyst indicates that the primary knee pathology, such as arthritis or a meniscal tear, remains active. A large cyst can cause discomfort and stiffness, prompting a person to seek treatment. The natural timeline for a cyst to dissipate can range from a few months to a few years, depending on the body’s response to the knee condition.
The Acute Risk of Cyst Rupture
The most concerning outcome of an untreated Baker’s cyst is rupture, which occurs when the internal fluid pressure becomes too high. When the cyst ruptures, the synovial fluid leaks out and dissects down into the calf muscle and surrounding tissue. This sudden fluid release causes immediate, sharp pain in the calf or back of the knee, often described as a tearing sensation.
The leaked fluid triggers an inflammatory response in the calf tissue, leading to pseudothrombophlebitis. Symptoms include significant swelling, redness, and warmth in the lower leg, which can be clinically indistinguishable from a Deep Vein Thrombosis (DVT). This similarity makes immediate medical evaluation necessary to rule out DVT, as DVT treatment is vastly different and inappropriate care can worsen a ruptured cyst.
Imaging tests, most commonly ultrasound, are used to confirm whether the swelling is a ruptured cyst or a DVT. In some cases of rupture, bruising may appear around the ankle, known as the “crescent sign,” as the fluid tracks downward due to gravity. While acute symptoms are intense, they typically resolve over several weeks with conservative management like rest and anti-inflammatory medication.
Chronic Impacts on Mobility and Joint Health
A large, persistent Baker’s cyst can impose chronic limitations on physical function and quality of life. The mechanical presence of the fluid-filled mass behind the knee restricts the full range of motion. This limitation primarily affects the ability to fully flex the knee, making activities like squatting, kneeling, or climbing stairs difficult.
The chronic tightness and stiffness caused by the cyst can alter a person’s gait and muscle use, potentially aggravating the pre-existing joint condition. A large cyst may compress nearby structures, causing pain or, in rare instances, numbness or tingling if it pressures a nerve. This continued pressure contributes to chronic pain that limits daily activities and the ability to exercise.
Over time, this sustained functional impairment can indirectly affect overall joint health by discouraging movement and leading to muscle weakness around the knee. A large cyst can sometimes compress the popliteal vein, leading to swelling in the leg below the knee.