A Baker’s cyst appears as a smooth, rounded bulge behind the knee, sitting in the hollow space where the joint bends. It looks like a golf ball-sized swelling on the inner side of the back of the knee, though its size can vary from barely noticeable to large enough to see from across the room. The skin over it usually looks normal, with no redness or discoloration unless the cyst has ruptured or become inflamed.
Where It Shows Up on the Knee
The bulge sits specifically in the soft depression behind the knee, slightly toward the inner (medial) side. This area, called the popliteal fossa, is the natural crease where your leg folds. The cyst forms when a small sac between two muscles at the back of the knee fills with fluid that has leaked out of the joint through a one-way valve in the joint capsule. Fluid flows in but can’t easily flow back out, so the sac gradually expands.
When you stand with your knee straight, the cyst tends to look more prominent and feels firmer to the touch. Bend your knee to about 45 degrees, and the bulge may soften, shrink, or even seem to disappear entirely. This shift in appearance with knee position is one of the cyst’s most recognizable features, and doctors use it as a clinical sign during examination.
What It Feels Like to Touch
Pressing on a Baker’s cyst, you’ll feel a smooth, round mass that has a slightly squishy, fluid-filled quality. Clinicians describe it as “fluctuant,” meaning it gives a little under pressure, similar to pressing on a water balloon. It may be mildly tender or completely painless. Many people discover one only because they notice a lump while shaving or rubbing the back of their knee, not because it hurts.
The cyst doesn’t feel hard like bone, and it doesn’t feel like a solid lump. If what you’re feeling is rock-hard or doesn’t change at all with knee bending, that’s worth getting checked to rule out something else.
How Big They Get
Baker’s cysts range widely in size. Small ones may be only a centimeter or two across and barely visible, producing more of a vague fullness behind the knee than an obvious bump. Larger ones can grow to several centimeters and create a clearly visible bulge that makes the back of one knee look noticeably different from the other. The cyst tends to enlarge gradually over weeks or months as more fluid accumulates, especially if an underlying knee problem like arthritis or a meniscus tear keeps producing excess joint fluid.
Size often tracks with symptoms. A small cyst might cause no discomfort at all, while a larger one can create a feeling of tightness or pressure behind the knee, particularly when you fully bend or straighten the leg.
Why the Cyst Forms
Baker’s cysts are almost always a secondary problem. The real issue is something going on inside the knee joint itself. Osteoarthritis and meniscus tears are the most common culprits. These conditions cause inflammation, and an inflamed knee produces excess joint fluid. That fluid gets pushed through a one-way valve in the back of the joint capsule and pools in a natural pocket between two muscles behind the knee. Once there, it can’t easily drain back, so the pocket swells into a cyst.
Research published in RMD Open found that moderate-to-large Baker’s cysts appeared in 23% of people with symptomatic knee osteoarthritis, compared to just 9% of those without it. Overall, about 30% of people in the study had some degree of cyst formation, with the largest cysts clustering in the osteoarthritis group.
What a Ruptured Cyst Looks Like
A ruptured Baker’s cyst creates a dramatically different appearance. When the cyst wall breaks open, joint fluid spills down into the calf, and the swelling migrates from behind the knee to the lower leg. Your calf may become visibly puffy and swollen, sometimes extending down to the ankle. Discoloration often follows: depending on your skin tone, you might see shades of red, purple, or brown spreading across the calf, similar to a large bruise.
People commonly describe a sudden sensation of water running down the inside of their leg at the moment of rupture, followed by sharp pain in the calf. The back of the knee, where the cyst used to be visible, may actually flatten out once the fluid drains downward. The combination of calf swelling, pain, and discoloration can look alarming, and it closely mimics a deep vein thrombosis (blood clot). This resemblance is so well known that doctors have a name for it: pseudothrombophlebitis. If your calf suddenly swells and changes color, getting an ultrasound to distinguish between a ruptured cyst and a blood clot is important, since the treatments are completely different.
How Doctors Confirm What They’re Seeing
If there’s any doubt about the lump behind your knee, ultrasound is the go-to first step. It shows whether the mass is a simple fluid-filled sac or something more complex, and it can measure the cyst’s exact size and its relationship to surrounding structures. Importantly, a technique called color Doppler imaging confirms that there’s no blood flow inside the mass, which rules out a popliteal artery aneurysm, a much more serious condition that can look similar from the outside.
MRI provides more detail when needed, showing not just the cyst but also the underlying knee problem driving it. On an MRI, the cyst appears as a bright, fluid-filled pouch on certain image types, sometimes containing loose debris or internal dividers called septations. The MRI can simultaneously reveal the meniscus tear, cartilage damage, or inflammatory condition responsible for the excess fluid.
How to Tell It Apart From Other Lumps
Not every lump behind the knee is a Baker’s cyst. A few features help distinguish it. A Baker’s cyst is soft and fluid-filled, changes with knee position, and sits in a specific spot on the inner side of the back of the knee. A solid tumor would feel firmer and wouldn’t change when you bend your leg. A blood clot in the calf produces diffuse swelling, warmth, and redness in the lower leg rather than a discrete lump behind the knee. A popliteal artery aneurysm can feel like a pulsating mass in the same location, but you’ll notice a rhythmic throb that a cyst won’t have.
The classic Baker’s cyst is the most common mass found in this part of the leg. If you’re looking at a smooth, squishy bulge behind your knee that gets firmer when you straighten your leg and softer when you bend it, and the skin over it looks normal, you’re likely looking at exactly what you think you are.