What Is a Baker’s Cyst? Causes, Symptoms & Treatment

A Baker’s cyst is a fluid-filled swelling that forms behind the knee, in the space known as the popliteal fossa. It develops when excess synovial fluid, the slippery lubricant inside your knee joint, gets pushed through a one-way valve into a bursa at the back of the knee and becomes trapped there. The cyst is almost always a symptom of something else going on inside the joint, such as a meniscal tear or arthritis, rather than a standalone problem.

How the Cyst Actually Forms

Between the tendons of two muscles at the back of your knee, the gastrocnemius and the semimembranosus, there is a small sac called a bursa. In a large share of adults, a natural opening connects the inside of the knee joint to this bursa. Cadaver studies have found this opening in roughly 40% to 54% of healthy adult knees.

This opening acts like a one-way valve. When you bend your knee, the valve relaxes and fluid can flow from the joint into the bursa. When you straighten the leg, the surrounding muscles tighten and press the valve shut, trapping whatever fluid has already passed through. Researchers have confirmed the one-way nature of this valve by injecting dye into cysts and showing that fluid moves from the joint into the cyst but cannot flow back the other way.¹ If your knee is producing extra fluid because of injury or disease, the bursa slowly fills and distends, forming the visible lump most people notice.

Interestingly, this arrangement may serve a protective purpose. Because the cyst volume tracks with the amount of fluid inside the knee, the one-way valve could act as a pressure-relief system, reducing hydraulic stress on the joint itself.¹ That is a small consolation when you have a golf-ball-sized bulge behind your knee, but it helps explain why these cysts are so common in people with knee problems.

What Causes the Knee to Overproduce Fluid

A Baker’s cyst is rarely the primary issue. In adults, it is overwhelmingly linked to an underlying knee condition that triggers excess synovial fluid production. The most frequently identified culprits are meniscal tears, osteoarthritis, and cartilage damage.

Meniscal tears have a particularly strong association. Research has found that Baker’s cysts are significantly linked to meniscal injuries, and that even a partial tear, not just a complete one, can be enough to set the process in motion.² Tears of the medial meniscus, the cartilage pad on the inner side of the knee, show a particularly consistent connection. One ultrasound study of patients with symptomatic knees found Baker’s cysts in about 23% of cases, and the cysts were significantly associated with medial meniscal tears even after adjusting for other factors like age.³

Osteoarthritis is another major driver. The chronic joint inflammation it causes produces a steady stream of excess synovial fluid, which feeds the cyst over time. MRI studies have confirmed a direct statistical relationship between the presence and quantity of synovial fluid in the knee and the existence of a Baker’s cyst.⁴ Cartilage degeneration and certain structural features inside the joint, such as medial plicae (folds of tissue on the inner knee), have also been linked to larger cyst volumes.⁵

Rheumatoid arthritis, gout, and other inflammatory joint diseases can also produce enough effusion to generate a cyst. The common thread is excess fluid in the joint pushing through that one-way valve. Treat the reason the knee is swollen, and the cyst often shrinks on its own.

Baker’s Cysts in Children

The picture is somewhat different in younger patients. Children can develop popliteal cysts without any identifiable joint problem, a pattern that is much less common in adults. In pediatric cases, the cyst sometimes appears as an incidental finding and tends to resolve without treatment over months or years. However, children with underlying conditions like juvenile arthritis or hypermobility syndrome are considerably more likely to have them. One study of pediatric patients found popliteal cysts in roughly 57% to 58% of knees affected by arthritis or hypermobility, compared with about 28% of knees without those risk factors.⁶ That same study noted that cysts in children with arthritis were more likely to contain echogenic (thickened, debris-filled) fluid, suggesting more active inflammation. Communication between the cyst and the joint space was visible in the majority of cases on imaging.

What a Baker’s Cyst Feels Like

Many Baker’s cysts cause no symptoms at all and are discovered accidentally during an MRI or ultrasound ordered for something else. When they do cause problems, the most common complaint is a feeling of tightness or pressure behind the knee, sometimes accompanied by a visible bulge. The swelling tends to be most noticeable when you are standing.

A classic clinical sign, known as Foucher’s sign, explains why the cyst feels harder and more uncomfortable when you straighten your leg. In extension, the gastrocnemius and semimembranosus muscles squeeze toward each other and compress the cyst against the deep tissue. In flexion, those muscles relax and the cyst softens.⁷ This is why sitting with a bent knee often provides relief, while standing or walking can make things worse.

Larger cysts can limit your range of motion, making it difficult to fully bend or straighten the knee. Pain, when present, is usually a dull ache that worsens with activity. Some people describe the sensation as feeling like they have something wedged behind the joint.

When a Baker’s Cyst Becomes a Problem

Most Baker’s cysts are harmless, but a few complications are worth knowing about because they can be genuinely alarming.

The most common complication is rupture. When a cyst bursts, the synovial fluid leaks down into the calf, causing sudden pain, swelling, and redness in the lower leg. The problem is that this looks almost identical to a deep vein thrombosis (DVT), and distinguishing the two based on a physical exam alone is extremely difficult.⁸ This mimicry is common enough to have its own name: pseudothrombophlebitis syndrome. Because a missed DVT can be dangerous, anyone presenting with sudden calf swelling typically gets an ultrasound to rule out a blood clot before the ruptured cyst is treated.⁹

In rare cases, a large or unusually positioned cyst can press on nearby structures. There are reports of popliteal cysts compressing the popliteal vein and artery, causing blood flow problems in the lower leg.¹⁰ Giant cysts that extend laterally can also compress nerves, leading to numbness, tingling, or weakness in the foot and lower leg. One case report documented a large multi-chambered cyst compressing both the tibial and common peroneal nerves simultaneously, an exceptionally unusual scenario.¹¹ These complications are rare, but they underscore why new nerve symptoms or vascular changes in the lower leg deserve prompt medical attention.

How Baker’s Cysts Are Diagnosed

If your doctor suspects a Baker’s cyst based on the location and feel of the lump, imaging confirms it. Ultrasound is usually the first step because it is fast, inexpensive, and highly accurate. One study comparing ultrasound to MRI found that when ultrasound showed fluid between the semimembranosus and medial gastrocnemius tendons, it achieved 100% sensitivity and specificity for diagnosing Baker’s cysts.¹²

MRI comes into play when the doctor needs to see more than just the cyst itself. It can reveal the underlying joint problem driving the effusion, whether that is a torn meniscus, damaged cartilage, or inflamed synovium. MRI also helps rule out other masses behind the knee, including tumors and vascular lesions, which is important when the presentation is atypical.¹³ When evaluating risk of rupture, MRI can assess features like the cyst’s internal structure, its dimensions, and its relationship to the surrounding tendons.¹⁴

Non-Surgical Treatment

Because a Baker’s cyst is almost always secondary to a joint problem, the first line of treatment is usually addressing the underlying cause. If arthritis is producing the excess fluid, managing the arthritis with anti-inflammatory medications, physical therapy, or intra-articular injections often reduces the effusion and, with it, the cyst. If a meniscal tear is the driver, treating the tear may be necessary before the cyst will stay away.

For the cyst itself, the most common in-office procedure is ultrasound-guided aspiration, where a needle is inserted into the cyst under real-time imaging to drain the fluid. This is frequently combined with a corticosteroid injection to reduce inflammation and slow re-accumulation. The procedure is considered safe, relatively quick, and effective at providing relief.¹⁵ In patients with knee osteoarthritis, percutaneous aspiration and treatment produced significant decreases in cyst volume that correlated with meaningful clinical improvement.¹⁶

A longer-term follow-up study tracked patients who underwent ultrasound-guided aspiration, fenestration (making small punctures in the cyst wall), and injection as their sole treatment. Pain, stiffness, and physical function scores all improved significantly. The recurrence rate was about 13%, with only six of 47 patients needing a repeat aspiration. No infections or other complications were reported.¹⁷ For many people, this approach resolves the problem without surgery.

When Surgery Is Considered

Surgery enters the conversation when a cyst keeps coming back despite aspiration and treatment of the underlying joint problem, or when complications like nerve compression make it necessary. There are two broad approaches: arthroscopic and open.

Arthroscopic surgery works from inside the knee joint. The surgeon inserts a camera and instruments through small incisions to address the internal pathology (repairing a meniscal tear, cleaning up damaged cartilage) and, in many cases, widens or excises the one-way valve to allow free drainage back into the joint. This prevents new fluid from being trapped. One study of 30 patients who underwent arthroscopic cystectomy found that none experienced clinical recurrence at two years. The study also confirmed that about a third of patients had a surgically visible one-way valve, and roughly three-quarters had concurrent cartilage and meniscal damage.¹⁸

Open posterior cystectomy, performed through an incision at the back of the knee, is sometimes used for particularly large or complex cysts, or for cysts that have failed arthroscopic treatment. A study evaluating this approach in recalcitrant cases reported meaningful improvements in knee function and comfort scores after the procedure.¹⁹

A recurring theme in the surgical literature is the importance of treating the intra-articular pathology at the same time as the cyst. Studies have found that recurrence rates go up when concurrent meniscal or cartilage injuries are left unaddressed during cyst decompression.²⁰ Removing the cyst without fixing the reason the knee is swollen is a bit like mopping a floor while the faucet is still running.

Exercise and Rehabilitation

Regardless of whether you have aspiration, injection, or surgery, structured exercise is a consistent part of recovery. Quadriceps strengthening and calf stretching are the staples, both aimed at supporting the knee joint and reducing the mechanical load that contributes to effusion. In one treatment protocol, patients began these exercises three days after receiving intra-articular injections, and the vast majority, about 88%, achieved good to excellent improvement in pain and function.²¹

The reasoning behind exercise is straightforward. A stronger quadriceps helps stabilize the knee, reduces strain on the meniscus and cartilage, and may lower the amount of inflammatory fluid the joint produces. Flexible calf muscles reduce the compressive forces on the popliteal fossa. Neither of these eliminates a cyst that already exists, but they can help slow re-accumulation after treatment and ease symptoms in cysts that are being watched rather than treated aggressively.

Emerging Approaches

A few newer treatments are being explored, though the evidence for them is still limited to case reports and small series rather than large trials.

Platelet-rich plasma (PRP) injections have shown up in the Baker’s cyst literature. One case report described a patient with post-traumatic osteoarthritis and a popliteal cyst who reported complete resolution of both pain and cyst size after leukocyte-rich PRP injections.²² The thinking is that PRP may address the underlying joint inflammation that drives fluid production, hitting the problem upstream rather than just draining the result. However, one successful case is a long way from a proven treatment, and PRP remains experimental for this specific indication.

Sclerotherapy, where an irritant is injected into the cyst cavity to collapse and scar the walls shut, is another option being tested. One case report described successful treatment of a Baker’s cyst with distal extension using a combination of PRP and doxycycline-based sclerosis, with symptom resolution within three weeks and sustained improvement after a second lower-dose session.²³ Sclerotherapy has a longer track record for other types of cysts and may prove useful for Baker’s cysts that recur after standard aspiration, but more data are needed before it becomes a routine recommendation.

Where the Name Comes From

The cyst is named after William Morrant Baker, a British surgeon who described the condition in 1877. Baker was not actually the first to identify popliteal swellings. Robert Adams described them before Baker, and researchers have traced descriptions matching the condition all the way back to the Galenic Corpus, the ancient medical texts attributed to Galen of Pergamon in the second century.²⁴ Baker’s contribution was connecting the cyst to joint pathology and new sac formation outside the knee, which is why his name stuck. In medical literature, you may see the terms “Baker’s cyst” and “popliteal cyst” used interchangeably; they refer to the same thing.