A Baker’s cyst (also called a popliteal cyst) is a fluid-filled sac that forms behind the knee, creating a noticeable lump or sensation of fullness in the back of the joint. It develops when excess joint fluid gets pushed into a small bursa (a natural cushioning pocket) located between two muscles at the back of your knee. Most Baker’s cysts are caused by an underlying knee problem like arthritis or a cartilage tear, and they range from painless and barely noticeable to large enough to limit your movement.
How a Baker’s Cyst Forms
Your knee joint naturally contains a small amount of synovial fluid, the slippery liquid that lubricates the joint and reduces friction when you move. When something irritates the knee, whether it’s arthritis wearing down cartilage or an acute injury, the joint responds by producing extra fluid. That excess fluid needs somewhere to go.
Behind your knee, there’s a bursa nestled between the calf muscle and one of the hamstring tendons. In many people, this bursa connects to the knee joint through a small opening in the back of the joint capsule. The connection works like a one-way valve: fluid flows from the joint into the bursa but has difficulty flowing back. Over time, this traps more and more fluid in the bursa, causing it to swell and form the characteristic bulge. The cyst can stay small for months or grow progressively larger as fluid continues to accumulate.
Common Causes
In adults, a Baker’s cyst is almost always secondary to another knee condition. The two most common triggers are osteoarthritis and meniscus (cartilage) tears. Roughly 22% of patients with knee osteoarthritis have a Baker’s cyst detectable on ultrasound. Rheumatoid arthritis, gout, and other inflammatory joint conditions can also drive excess fluid production. Knee injuries, including ligament tears and fractures, are another frequent cause.
In children, Baker’s cysts sometimes appear without any underlying joint problem. These primary cysts tend to resolve on their own over time.
What It Feels Like
Many Baker’s cysts cause no symptoms at all and are discovered incidentally during imaging for another issue. When symptoms do appear, the most common ones include:
- A feeling of fullness or tightness behind the knee, especially when you straighten your leg
- A visible or palpable lump at the back of the knee that feels firm when the leg is extended and soft when the knee is bent
- Stiffness that makes it harder to fully bend or straighten the knee
- Swelling in the knee or upper calf
If the cyst grows large enough, it can press on nearby blood vessels or nerves. This may cause pain, swelling in the lower leg, weakness, or even numbness below the knee.
When a Cyst Ruptures
A Baker’s cyst can rupture, releasing its fluid into the surrounding calf tissue. When this happens, you’ll typically feel a sudden, sharp pain behind your knee followed by swelling, redness, and tenderness that spreads down your calf. The sensation is often described as feeling like water running down the back of your leg.
This presentation is important because it closely mimics a deep vein thrombosis (DVT), a blood clot in the leg. The symptoms overlap enough that doctors sometimes call a ruptured Baker’s cyst “pseudothrombophlebitis.” Because a DVT is a medical emergency and a ruptured cyst generally is not, getting the right diagnosis quickly matters. If you develop sudden calf swelling and pain, it’s worth having it evaluated promptly rather than assuming it’s the cyst.
How It’s Diagnosed
A Baker’s cyst can often be identified through a physical exam alone. Your doctor will press on the back of your knee and may ask you to straighten and bend the leg, checking whether the lump changes firmness with movement. However, because the symptoms can resemble a blood clot, an aneurysm, or even a tumor, imaging is frequently ordered to confirm the diagnosis.
Ultrasound is typically the first-line imaging tool. It’s quick, noninvasive, and effective at showing a fluid-filled cyst and confirming that the deep veins are open and free of clots. When more detail is needed, particularly about the internal structure of the knee or the size of a very large cyst, an MRI provides a more comprehensive picture. X-rays don’t show the cyst itself but can reveal arthritis or other bony changes that may be driving the problem.
Treatment Options
Because a Baker’s cyst is usually a symptom of something else going on in the knee, treatment focuses on the underlying cause. If arthritis is driving excess fluid production, managing the arthritis often reduces or eliminates the cyst. If a torn meniscus is the culprit, repairing the tear addresses the root issue.
Nonsurgical Approaches
For mild or moderate cysts, the initial approach is usually conservative. Rest, ice, compression with a lightweight knee sleeve, and elevation can reduce swelling and discomfort. A compression sleeve worn during activity may also help the cyst reabsorb fluid more quickly. If the cyst is painful or large, a doctor can drain it using a needle (aspiration), sometimes guided by ultrasound for precision. A corticosteroid injection into the knee joint may follow to reduce inflammation and slow fluid reaccumulation.
Surgery
Surgery is reserved for cases where the cyst is very large, keeps coming back after drainage, or where a significant knee injury like a ligament tear needs repair. Traditional open surgical removal of the cyst carries a notable drawback: a high recurrence rate, because the one-way valve mechanism that created the cyst in the first place still exists. Arthroscopic surgery, which uses small incisions and a camera to address both the cyst and the internal knee problem causing it, has become increasingly preferred. Arthroscopic approaches report near-zero recurrence rates in some studies, compared with a recurrence rate of around 5% or higher with open excision alone.
Managing Symptoms at Home
Gentle exercise can help maintain range of motion and support the knee joint while you’re living with a Baker’s cyst. The goal isn’t to “fix” the cyst through exercise but to keep the surrounding muscles flexible and strong, which reduces strain on the joint.
A standing calf stretch is a good starting point. Face a wall, step one foot forward, and keep your back leg straight as you lean in until you feel a gentle pull along the back of your calf. Seated hamstring stretches, where you extend one leg with your heel on the floor and lean forward gently, help loosen the muscles directly around the cyst. Wall squats build strength in the quadriceps without high impact: lean your back against a wall, slide down until your knees are slightly bent, hold for a few seconds, and slide back up.
Low-impact activities like walking, yoga, and Pilates support overall flexibility and stability around the knee. If you have very flat feet or high arches, a shoe insert can reduce stress on the knee and ease discomfort. Avoid high-impact activities or deep knee bends that significantly increase pressure behind the joint, as these can aggravate the cyst.