A Baker’s cyst behind the knee is a soft, fluid-filled lump in the hollow at the back of the joint. Most patients find it by accident, reaching down after a shower or noticing that one calf looks fuller than the other.
Here is the first thing I tell them: the lump is almost never the real problem. After more than twenty years and 700 joint replacements a year, I have watched it cause more worry than it deserves.
What is a Baker’s cyst behind the knee?
A Baker’s cyst is a pocket of joint fluid that has pushed out the back of the knee and inflated a small cushioning sac called a bursa. Doctors also call it a popliteal cyst, after the popliteal fossa behind your knee.
The mechanism is simple. Your knee is lined by a membrane that makes synovial fluid, the slick liquid that lubricates the joint. When something inside is irritated, that lining makes extra fluid.
Many people have a natural opening at the back of the knee capsule. Fluid pushes through under pressure and struggles to come back, so the bursa inflates and you feel a lump. Think of it as a pressure relief valve.
What causes a Baker’s cyst?
A Baker’s cyst is caused by anything that makes the knee produce extra fluid. In adults, that usually means arthritis or a meniscus tear.
The sources I see most:
- Knee osteoarthritis. Worn cartilage inflames the joint lining, and the lining answers with fluid. This is the most common cause in patients over 50.
- Degenerative meniscus tears. The meniscus is the C-shaped cartilage shock absorber. A frayed tear keeps the joint irritated, so the fluid keeps coming. Here is how a meniscus tear is diagnosed.
- Rheumatoid and other inflammatory arthritis. The immune system attacks the joint lining, which swells and pours out fluid.
- Gout. Uric acid crystals inflame the knee and drive the same response.
- Injury. Ligament tears, cartilage damage, and fractures cause swelling that pushes backward.
Children are the exception. Cysts in kids often appear in a healthy knee and settle on their own.
Is a Baker’s cyst a sign of arthritis?
Frequently, yes. In that study of 399 adults with knee pain, a cyst was found in 25.8 percent, tracking closely with osteoarthritis and fluid inside the joint. These cysts cluster between ages 35 and 70.
That is why I never treat the lump in isolation. The cyst is the smoke. The arthritis or the tear is the fire.
What does a Baker’s cyst feel like?
Most patients describe a tight bulge that feels like a small water balloon, more obvious standing with the leg straight and softer when the knee bends.
Along with the lump, you may notice:
- Knee stiffness or fullness behind the knee, worse after walking or standing
- Aching in the knee or upper calf
- Trouble bending the knee all the way
- Clicking or catching, which points to the underlying tear
- Swelling in the knee itself, and sometimes down into the calf
Plenty cause no symptoms and turn up as a surprise on an MRI ordered for something else.
One exam finding is worth knowing. A cyst feels firm with the knee straight and softens at about 45 degrees of bend. That change, called Foucher’s sign, separates a cyst from a solid mass in seconds.
How is a Baker’s cyst diagnosed?
A Baker’s cyst is diagnosed by physical exam first, with imaging used to confirm the fluid and find what is driving it.
- The exam. I feel the back of the knee straight and again at 45 degrees, check motion, and press the joint line for a meniscus tear.
- Standing X-rays. An X-ray does not show the cyst, because fluid is invisible on plain film. It shows the arthritis underneath, which is what changes your plan.
- Ultrasound. This confirms the mass is fluid rather than solid, and it is the fastest way to rule out a blood clot.
- MRI. Saved for seeing the meniscus and cartilage in detail, or for planning surgery.
Not every lump needs advanced imaging. Scanning earns its place when something does not fit: a hard mass, one that does not change with knee position, or rapid growth.
Can a Baker’s cyst be dangerous?
A popliteal cyst is benign. It is not a tumor and never a sign of cancer. The real risk is a rupture that leaks fluid into the calf and mimics a deep vein thrombosis (a blood clot in the leg).
Get evaluated the same day for:
- Sudden sharp pain in the calf or behind the knee
- Calf swelling, warmth, or a change in skin color
- A sensation of warm water running down the leg
- Calf tenderness that builds over hours
Call 911 for chest pain or shortness of breath along with leg swelling. That combination can signal a clot that has traveled to the lung.
How do you tell a ruptured cyst from a blood clot?
You cannot tell them apart by feel, and neither can I. The symptoms overlap almost completely, which is why an ultrasound is worth doing when calf symptoms start suddenly.
What is the best treatment for a Baker’s cyst?
The best treatment is treating the knee problem that created it. Fix the source, and the cyst usually takes care of itself. For most patients that starts this week:
- Activity adjustment. Back off deep squatting, kneeling, and long stretches on hard floors.
- Ice and elevation. Ten to fifteen minutes of ice, a few times a day, leg propped up.
- Anti-inflammatory medication. A short course of ibuprofen or naproxen, if your medical history allows it.
- Physical therapy. Quadriceps and hip strength take load off the joint, and calf flexibility eases the tightness behind the knee. This is the step people skip and later regret.
- Targeted injection. A corticosteroid injection into the joint can quiet the inflammation generating the fluid.
Beyond that, the plan follows the diagnosis. Our nonsurgical joint pain treatment options, including gel injections, settle a great many without an operating room.
Does draining a Baker’s cyst work?
Draining works fast and often does not last. Joint aspiration (pulling fluid out with a needle, usually under ultrasound guidance) relieves pressure the same day.
The catch is that the valve is still open and the joint is still making fluid, so cysts commonly refill. Recurrence runs higher in older patients and in knees with degenerative meniscus tears.
I do use aspiration when a cyst is large enough to limit therapy, paired with an injection and a plan for the joint. What I will not do is drain the same one repeatedly and call that a treatment.
When does a Baker’s cyst need surgery?
Surgery to simply correct the cyst itself is rare. Far more often it is aimed at the knee, and the cyst improves as a byproduct. Removing the sac carries a real recurrence rate when the arthritis underneath is left alone.
Arthroscopy is handled by my sports medicine partners at the Bone and Joint Institute of Tennessee. My practice is limited to hip and knee replacement, and I refer within our group when that fits.
What happens to a Baker’s cyst after knee replacement?
Most of these cysts shrink or disappear after knee replacement, though it takes longer than patients expect.
In a prospective study of patients with a confirmed cyst before total knee replacement, the cyst was still present in 85 percent of knees one year later. Symptoms told a different story: cyst-related complaints fell from 71 percent before surgery to 31 percent after.
The same group followed those patients to an average of 4.9 years. By then 67 percent of the cysts were gone. Size mattered: smaller ones resolved 83.7 percent of the time, larger ones 52.1 percent.
So if your arthritis is bad enough to need total knee replacement, the cyst is not a second operation and not a reason to delay. Give it a year or two.
I perform knee replacement through a muscle-sparing subvastus approach with Mako® robotic-arm guidance for precise, CT-planned implant positioning. (Disclosure: I serve as a paid consultant to Stryker, maker of that platform.) You can read more about robotic joint replacement.
When should you see a specialist about a lump behind the knee?
See a specialist for any new lump behind the knee, even a painless one. Rare masses here can look and feel like a cyst, and only an exam plus ultrasound is certain.
Come in sooner if the knee locks, gives way, stays swollen for weeks, or will not straighten. Those point to the joint itself, and persistent stiffness rarely improves on its own.
I see patients from Franklin, Nashville, Columbia, Spring Hill, and across Middle Tennessee at the Bone and Joint Institute of Tennessee, 3000 Edward Curd Lane, Franklin, TN 37067. Most leave with a plan that does not involve surgery.
To schedule with Dr. Cory Calendine, call (615) 791-2630 or request a visit online. No referral required.
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Medical disclaimer: This article is for education only and does not replace a personal evaluation. Dr. Cory Calendine is a board-certified orthopaedic surgeon whose practice is limited to hip and knee replacement. Individual results vary.
References
- Picerno V, Filippou G, Bertoldi I, et al. Prevalence of Baker’s cyst in patients with knee pain: an ultrasonographic study. Reumatismo. 2014;65(6):264-270. PubMed
- Leib AD, Roshan A, Foris LA, Varacallo MA. Baker’s Cyst. In: StatPearls. Treasure Island, FL: StatPearls Publishing; updated August 4, 2023. NCBI Bookshelf
- Frush TJ, Noyes FR. Baker’s cyst: diagnostic and surgical considerations. Sports Health. 2015;7(4):359-365. PubMed Central
- Hommel H, Perka C, Kopf S. The fate of Baker’s cyst after total knee arthroplasty. Bone and Joint Journal. 2016;98-B(9):1185-1188. PubMed
- Hommel H, Becker R, Fennema P, Kopf S. The fate of Baker’s cysts at mid-term follow-up after total knee arthroplasty. Bone and Joint Journal. 2020;102-B(1):132-136. Bone and Joint Journal
- Van Nest DS, Tjoumakaris FP, Smith BJ, Beatty TM, Freedman KB. Popliteal cysts: a systematic review of nonoperative and operative treatment. JBJS Reviews. 2020;8(3):e0139. PubMed
