A Baker’s cyst is considered “active” when it is producing symptoms, typically swelling, pain, and stiffness behind the knee. This distinguishes it from Baker’s cysts that exist quietly without causing any noticeable problems. Many people have small cysts behind their knee and never know it. An active Baker’s cyst is one that has grown large enough, or become inflamed enough, to make itself felt.
How a Baker’s Cyst Forms
Behind your knee, there’s a small fluid-filled sac (called a bursa) that sits between two tendons. This sac naturally communicates with the inside of your knee joint through a slit-like opening in the joint capsule. When you bend your knee, that opening widens and allows fluid to pass from the joint into the bursa. When you straighten your leg, the surrounding tendons compress the opening and close it off.
Under normal conditions, this system works fine. But when something inside the knee produces excess fluid, like a torn meniscus, arthritis, or cartilage damage, more fluid gets pushed into the bursa than can drain back out. The bursa swells, and a Baker’s cyst forms. The cyst itself isn’t really the problem. It’s a side effect of whatever is going wrong inside the knee joint.
What Makes a Cyst “Active”
A Baker’s cyst becomes active when it starts causing noticeable symptoms. The most common ones are swelling behind the knee (sometimes extending down the leg), knee pain that worsens with activity or prolonged standing, and stiffness that limits how far you can bend or straighten the knee. Some people feel a tight, pressure-like sensation in the back of the knee, especially after being on their feet.
Cysts can range from less than 1 cm to as large as 10 cm, with the average falling around 3.7 to 3.9 cm. Size alone doesn’t determine whether a cyst is active. A smaller cyst in a tight space can cause more symptoms than a larger one that has room to expand. What typically flips a cyst from quiet to active is a change in the underlying knee condition: a new flare of arthritis, a fresh cartilage injury, or increased inflammation from overuse.
Common Underlying Causes
In adults, Baker’s cysts almost always trace back to a problem inside the knee. Degenerative meniscal tears are one of the most common triggers. Osteoarthritis and rheumatoid arthritis also frequently drive cyst formation, as does any condition that causes ongoing inflammation in the joint. Less commonly, infections or direct knee trauma can be responsible. Baker’s cysts occur most often in adults aged 35 to 70.
This is why treating only the cyst itself often doesn’t solve the problem. If the underlying cause keeps producing excess joint fluid, the cyst refills. In one study of patients who had their cysts drained, about 13% experienced recurrence, and every one of those recurrences was linked to advanced arthritis or a complex cyst with thickened walls and internal debris.
When a Cyst Ruptures
An active Baker’s cyst can occasionally rupture, leaking fluid down into the calf. This is uncommon but worth knowing about because it closely mimics a blood clot in the leg, a condition called deep vein thrombosis (DVT). The similarity is so well recognized that a ruptured Baker’s cyst is sometimes called “pseudothrombophlebitis.”
A ruptured cyst typically causes sudden calf swelling, redness, and pain that gets worse when you flex your foot upward or squeeze the calf muscle. You may also notice bruising behind the knee or around the ankle. That ankle bruising, sometimes called the “crescent sign,” happens when leaked fluid tracks downward through the calf tissues. Because these symptoms overlap so heavily with DVT, imaging (usually ultrasound) is needed to tell the two apart. If you develop sudden calf swelling and pain, getting it checked promptly matters, since a true blood clot requires different and more urgent treatment.
How an Active Cyst Is Treated
Treatment for an active Baker’s cyst works on two levels: relieving the cyst symptoms and addressing whatever knee condition is driving the excess fluid.
For symptom relief, the first-line approach for a painful cyst is typically ultrasound-guided aspiration, where a needle drains the fluid, followed by an injection of a corticosteroid and local anesthetic. This combination has been shown to produce a durable reduction in pain for most patients. It’s a quick outpatient procedure, and many people feel significant relief within days. Surgery is reserved for very large cysts that compress nearby nerves or blood vessels, or for patients who are already getting a knee replacement for the underlying joint disease.
The more important piece is treating the root cause. If you have a torn meniscus, addressing that tear reduces the fluid production that feeds the cyst. If osteoarthritis is the driver, managing the arthritis with appropriate therapies keeps the joint calmer and reduces the chance the cyst comes back.
Exercises and Daily Management
Gentle exercise can help manage an active Baker’s cyst by improving the strength, flexibility, and stability around the knee. Walking, yoga, and Pilates are good low-impact options. Physical therapists commonly recommend a handful of specific exercises:
- Seated hamstring stretches: Sit with your heel on the floor and knee straight, then gently lean forward until you feel a stretch behind your knee and thigh.
- Wall squats: Lean against a wall with your feet about a foot in front of you. Slowly slide down, hold for a few seconds, then return to standing.
- Standing calf stretches: Face a wall with one leg behind you, knee straight. Lean forward into the wall until you feel a gentle stretch in the back of the straight leg.
- Side-stepping with a resistance band: Place a band around your ankles, keep your knees slightly bent, and take slow steps to the side while maintaining tension in the band.
Footwear also plays a role. If you have very flat feet or high arches, a supportive shoe insert can reduce stress on the knee. Wearing a lightweight compression knee sleeve during activity may help encourage the cyst to reabsorb more quickly. The goal with all of these strategies is to keep the knee moving without aggravating the inflammation that feeds the cyst.