A Baker’s cyst is a fluid-filled swelling that forms behind the knee, creating a visible or palpable bump in the soft hollow at the back of the joint. Also called a popliteal cyst, it develops when excess lubricating fluid from inside the knee pushes into a small pouch (called a bursa) between two tendons behind the knee. Some people never notice theirs, while others experience pain, stiffness, and limited movement.
How a Baker’s Cyst Forms
Your knee joint naturally contains a small amount of synovial fluid, a slippery liquid that reduces friction when you bend and straighten your leg. When something irritates or damages the joint, your body often responds by producing extra fluid. That excess fluid can migrate to the back of the knee, where it pools between two tendons and stretches the surrounding tissue into a cyst.
The underlying knee problem is almost always the real issue. The most common culprits are osteoarthritis and meniscus tears, though inflammatory arthritis, cartilage injuries, and other forms of joint damage can also trigger overproduction of fluid. In people with symptomatic knee osteoarthritis, about 23% develop a moderate or large Baker’s cyst, compared to roughly 9% of people without osteoarthritis. The cyst itself is a secondary effect, essentially a pressure-relief valve for an overloaded joint.
What It Feels Like
The most obvious sign is a bump behind your knee. It can range from marble-sized to as large as a golf ball, and it often feels firm when the knee is fully extended but softer when the knee is bent. You might also notice:
- Stiffness or tightness behind the knee, especially after sitting for a while
- Difficulty fully bending the knee, as though something is blocking the last bit of motion
- Aching pain that worsens with activity or prolonged standing
- Swelling in the knee, and sometimes extending into the thigh or calf
Many Baker’s cysts are completely painless. People sometimes discover them by accident during an imaging scan for an unrelated issue, or by running a hand behind their knee and feeling an unfamiliar lump.
When a Cyst Ruptures
In rare cases, a Baker’s cyst can burst. When it does, the fluid inside leaks down into the calf, causing sharp, stabbing pain behind the knee or in the upper calf, noticeable swelling in the lower leg, and sometimes a sensation of warm water trickling down the inside of your leg. The skin over the calf may become red or discolored.
A ruptured cyst can look and feel remarkably similar to a deep vein thrombosis (DVT), which is a blood clot in the leg. Both cause calf swelling and pain. The key difference is that DVT typically produces warmth and redness concentrated in the affected area, while a ruptured cyst tends to cause more diffuse swelling and that characteristic “water running down” feeling. Because a missed blood clot can be dangerous, sudden calf swelling and pain should be evaluated promptly. An ultrasound can quickly distinguish between the two.
How It’s Diagnosed
A doctor can often identify a Baker’s cyst through a physical exam, feeling the characteristic lump behind your knee and checking how your range of motion is affected. Ultrasound is the most common next step. It’s fast, noninvasive, and extremely accurate for this purpose. Studies comparing ultrasound to MRI have found 100% accuracy when the imaging shows fluid collecting in the specific space between the two tendons behind the knee.
MRI is more useful when your doctor suspects a deeper joint problem, like a meniscus tear or cartilage damage, that might be driving the excess fluid production. The cyst itself shows up clearly on MRI as a bright, fluid-filled pocket.
Treatment Options
Because a Baker’s cyst is usually a symptom of another knee problem, treatment focuses on addressing the underlying cause. If the root issue improves, the cyst often shrinks or resolves on its own.
Nonsurgical Approaches
For mild cases, rest, ice, and compression can reduce swelling and discomfort. A cortisone injection into the knee joint can lower inflammation and shrink the cyst, though the effect isn’t always permanent. If the cyst is large or particularly painful, a doctor can drain the fluid with a needle in a procedure called aspiration, typically guided by ultrasound for precision. Aspiration and a cortisone injection are sometimes done together, draining the fluid first and then injecting the steroid to slow its return.
Neither draining nor injecting guarantees the cyst won’t come back. If the underlying joint condition persists, the knee will likely keep producing excess fluid.
Addressing the Root Cause
If a torn meniscus or significant cartilage damage is fueling the cyst, treating that problem surgically or through targeted rehabilitation is the most effective long-term solution. Repairing the structural issue reduces fluid overproduction, which takes the pressure off the back of the knee.
Exercises That Help
Gentle exercise can reduce stiffness, support the knee, and help manage cyst-related discomfort. The goal is to strengthen the muscles around the knee and hip while maintaining flexibility. Five commonly recommended exercises include:
- Seated hamstring stretches: Sit with one heel resting on the floor, knee straight, and gently lean forward until you feel a stretch behind your knee and thigh. This helps restore full extension if it’s been limited.
- Heel slides: Lie on your back and slowly slide your heel toward your buttock, bending the knee as far as comfortable. Hold briefly, then return. This gently works on knee bending range.
- Standing calf stretches: Face a wall with one foot stepped forward and the back leg straight. Lean toward the wall until you feel a stretch in the back calf. This relieves tightness that can develop when you’ve been favoring the affected leg.
- Wall squats: Stand with your back against a wall and slide down into a partial squat, hold for three seconds, and slide back up. This builds quadriceps strength, which is essential for stabilizing the knee.
- Banded side steps: Place a resistance band around your ankles, keep your knees slightly bent, and step sideways. This targets the glute muscles, which help stabilize both the hip and knee during movement.
Low-impact activities like walking, yoga, and Pilates also support overall knee stability. The key is to stay active without repeatedly aggravating the joint. Exercises that cause sharp pain behind the knee should be scaled back or modified.