A Baker’s cyst is a fluid-filled swelling that forms in the hollow at the back of your knee. It develops when excess joint fluid gets pushed out of the knee joint and pools in a small sac (called a bursa) nestled between two muscles behind the knee. Most Baker’s cysts are not dangerous on their own, but they almost always signal something else going on inside the knee joint.
How a Baker’s Cyst Forms
Your knee joint is lubricated by a slippery liquid called synovial fluid. When something inside the knee is damaged or inflamed, the joint produces more of this fluid than usual. The excess fluid gets squeezed toward the back of the knee, where it collects in a natural pocket between two muscles in your calf and thigh. Once the fluid enters that pocket, a valve-like mechanism makes it difficult for the fluid to flow back into the joint. Every time you bend and straighten your knee, the pressure changes actually push more fluid into the cyst rather than drawing it out.
When a torn meniscus is involved, the torn cartilage itself acts like a one-way valve, funneling fluid out of the joint. Over time, the trapped fluid thickens into a gel-like material, which is why Baker’s cysts often feel firm to the touch rather than squishy.
What’s Usually Going On Inside the Knee
A Baker’s cyst is rarely the whole story. In adults, it’s almost always a symptom of an underlying knee problem. Studies using MRI have found that 71% to 84% of knees with a Baker’s cyst also have a meniscal tear. One study of 46 knees with Baker’s cysts found that 94% had at least one internal knee problem, including meniscal tears (83%), cartilage damage (43%), or ligament injuries. Osteoarthritis is another common driver, with varying severity found in the majority of affected knees.
Children are the exception. In kids ages 2 to 14, Baker’s cysts are rare and almost always form on their own without any internal knee damage. About 95% of pediatric cases are considered isolated and typically resolve without treatment. Boys develop them about twice as often as girls.
What It Feels Like
Small Baker’s cysts often cause no symptoms at all and are discovered incidentally on an MRI or ultrasound done for another reason. When a cyst grows large enough to notice, you’ll typically feel a firm, smooth lump in the crease behind your knee. It may feel tight or uncomfortable, especially when you fully bend or straighten the leg. Some people describe a sensation of pressure or fullness that worsens with activity.
Larger cysts can limit your range of motion, making it harder to fully bend the knee. Standing for long periods or walking uphill may aggravate the discomfort. The cyst itself is usually painless to light touch, but when it presses on surrounding structures, it can cause aching that radiates into the upper calf.
When a Cyst Ruptures
A Baker’s cyst can burst, releasing its fluid into the calf. This causes sudden, sharp pain behind the knee and into the calf, along with swelling, redness, and warmth in the lower leg. A ruptured cyst can also produce bruising behind the knee or around the ankle, a pattern known as the crescent sign, where fluid tracks downward by gravity.
The tricky part is that a ruptured Baker’s cyst looks almost identical to a blood clot in the leg (deep vein thrombosis, or DVT). Both cause calf swelling, tenderness, and pain when you flex your foot upward. This resemblance is so well known in medicine that a ruptured Baker’s cyst is sometimes called “pseudothrombophlebitis.” Because a DVT is a medical emergency and a ruptured cyst is not, an ultrasound is typically the first step to tell the two apart. The ultrasound can confirm whether a blood clot is present and can also identify fluid collecting between the calf muscles, which points to a ruptured cyst.
How It’s Diagnosed
Your doctor can often feel a Baker’s cyst during a physical exam, but imaging confirms the diagnosis and checks for underlying knee damage. Ultrasound is the most common first-line tool. A meta-analysis of diagnostic studies found that ultrasound detects Baker’s cysts with 97% sensitivity when compared against surgical findings, and 94% sensitivity when compared against MRI. Its specificity is essentially 100%, meaning false positives are extremely rare.
MRI is typically reserved for cases where the doctor suspects an internal knee problem like a meniscal tear or cartilage damage. It provides a more detailed look at all the structures inside the joint and helps guide treatment decisions.
Treatment Options
Because Baker’s cysts are usually secondary to another knee problem, the most effective long-term approach is treating the underlying cause. Fixing a torn meniscus or managing arthritis-related inflammation often causes the cyst to shrink or disappear on its own. When the underlying condition was removed surgically but the cyst itself was left alone in older studies, recurrence depended heavily on whether the root cause was fully addressed. One study found that cysts recurred in 63% of knees when the underlying joint disorder wasn’t adequately treated, reinforcing that the cyst alone isn’t the real problem.
For cysts that cause significant discomfort, ultrasound-guided aspiration (draining the fluid with a needle) combined with a steroid injection is a common option. During the procedure, the doctor uses ultrasound to guide a needle into the cyst and withdraws the fluid. Volumes vary, but a typical aspiration might remove around 20 to 25 mL of yellow-tinged fluid. A steroid is then injected to reduce inflammation and slow fluid re-accumulation. One study of 32 patients treated this way found a recurrence rate of about 19%, with all recurrences occurring in patients whose cysts had a more complex structure on ultrasound.
Surgical removal of the cyst is generally considered a last resort, reserved for cases where the cyst keeps coming back, causes persistent symptoms, or compresses nearby nerves or blood vessels.
Exercise and Rehabilitation
Physical therapy focuses on two goals: restoring range of motion and strengthening the muscles that support the knee. A typical program includes hamstring stretching to relieve tightness behind the knee and quadriceps strengthening to improve joint stability. These exercises are meant to be done multiple times a day. Most people notice a meaningful reduction in pain within six to eight weeks of consistent work.
Low-impact activities like swimming and cycling are generally well tolerated, while deep squats and high-impact activities that increase pressure inside the knee joint may aggravate the cyst. Keeping the knee mobile without overloading it is the key balance to strike during recovery.