A soft lump or feeling of pressure behind the knee can be alarming. One common cause is a Baker’s cyst, also called a popliteal cyst.
A Baker’s cyst is a pocket of joint fluid that collects behind the knee. It is usually not a separate disease. In adults, it is more often a sign that something inside the knee is producing excess fluid, such as arthritis, a meniscus tear, inflammation, or another source of irritation.
The most useful question is therefore not simply, “How do we drain the cyst?” It is:
Why is the knee making extra fluid, and what treatment best addresses that cause?
What is a Baker’s cyst?
The knee normally contains a small amount of synovial fluid. This fluid lubricates the joint and helps the cartilage surfaces move smoothly.
When the knee becomes irritated, it may produce more fluid than usual. That fluid can pass through a small communication at the back of the knee and expand the bursa between the medial head of the gastrocnemius muscle and the semimembranosus tendon.
The result is a fluid-filled swelling known as a Baker’s cyst.
The American Academy of Orthopaedic Surgeons explains that Baker’s cysts commonly develop when an underlying knee problem causes excess fluid production. Common associated conditions include osteoarthritis, rheumatoid arthritis, meniscus tears, and ligament injuries. Read the AAOS patient guide.
What does a Baker’s cyst feel like?
Some Baker’s cysts cause no symptoms at all. They may be discovered incidentally during an ultrasound or MRI performed for another reason.
When symptoms occur, patients may notice:
- A soft or firm lump behind the knee
- Tightness when straightening the leg
- Pressure or fullness behind the knee
- Aching after walking or standing
- Reduced ability to bend the knee fully
- Swelling that changes in size
- Pain extending into the upper calf
- General knee swelling from the underlying joint problem
Symptoms often become more noticeable after activity because the knee produces more fluid when it is irritated.
Is the cyst itself causing the pain?
Sometimes yes, but not always.
A large cyst can create localized pressure behind the knee. It may make deep bending uncomfortable and can occasionally irritate nearby nerves or veins.
However, many patients feel pain primarily from the underlying knee condition, not from the cyst itself. Arthritis, a meniscus tear, synovitis, cartilage injury, or another problem may be the true pain generator.
This distinction matters. Draining a cyst may reduce pressure temporarily, but the cyst can refill if the knee continues producing excess fluid.
What causes a Baker’s cyst in adults?
Common causes include:
Knee osteoarthritis
Arthritis can irritate the joint lining and increase synovial fluid production. A cyst may enlarge during an arthritis flare and become smaller as inflammation improves.
Meniscus tears
A torn meniscus can create inflammation and recurrent swelling. Degenerative meniscus tears are especially common in adults with early or moderate arthritis.
Inflammatory arthritis
Rheumatoid arthritis, gout, and other inflammatory conditions can produce substantial joint fluid.
Ligament or cartilage injuries
ACL injuries, cartilage defects, and other internal derangements may also lead to swelling and cyst formation.
No obvious cause
Occasionally, particularly in children, a Baker’s cyst may appear without a clearly identifiable internal knee problem.
Is a lump behind the knee always a Baker’s cyst?
No. A new lump behind the knee should be evaluated rather than assumed to be harmless.
Other possibilities include:
- A blood clot
- An aneurysm or other vascular abnormality
- A solid soft-tissue mass
- A meniscal cyst
- A ganglion cyst
- A localized hematoma
- A tumor, although this is uncommon
Ultrasound is particularly useful because it can determine whether a lump is fluid-filled or solid and can evaluate nearby blood vessels.
Baker’s cyst versus a blood clot
A ruptured Baker’s cyst can cause sudden calf pain and swelling that resembles a deep vein thrombosis, or DVT.
Seek urgent medical evaluation for:
- Rapidly increasing calf swelling
- New calf tenderness
- Warmth or redness
- Shortness of breath
- Chest pain
- Unexplained weakness or numbness
The NHS and AAOS both advise prompt assessment when calf pain and swelling could represent a blood clot. See the NHS Baker’s cyst guidance.
A ruptured cyst is often treated without surgery, but a DVT requires a very different treatment plan. It is important not to diagnose the cause at home.
How is a Baker’s cyst diagnosed?
Diagnosis begins with a history and physical examination.
During the examination, the clinician evaluates:
- The location and consistency of the swelling
- Knee range of motion
- Joint-line tenderness
- Signs of arthritis
- Meniscus symptoms
- Ligament stability
- Calf swelling and circulation
- Whether the mass changes with knee position
X-rays
A Baker’s cyst usually does not appear directly on an X-ray. X-rays can still be valuable because they may reveal arthritis, alignment problems, bone changes, or other causes of recurrent knee swelling.
Ultrasound
Ultrasound can often confirm that the mass is a cyst. It also allows the clinician to assess:
- Cyst size and complexity
- Internal septations or thick fluid
- Nearby blood vessels
- Joint fluid
- Whether aspiration can be performed safely
Ultrasound guidance is especially useful when a procedure is being considered because the needle can be watched in real time.
MRI
MRI may be recommended when the diagnosis is uncertain or when the clinician needs to evaluate the underlying knee structures. MRI can show:
- Meniscus tears
- Cartilage loss
- Ligament injuries
- Synovitis
- Bone marrow lesions
- Loose bodies
- The communication between the joint and cyst
An MRI is not necessary for every Baker’s cyst. The decision should depend on the symptoms, examination, X-rays, and whether the result would change treatment.
Do Baker’s cysts go away on their own?
Many do improve without direct cyst treatment.
A small, minimally symptomatic cyst may shrink as the underlying knee inflammation settles. Observation is reasonable when the diagnosis is clear and the cyst is not causing significant pain, nerve symptoms, or vascular compression.
AAOS recommends nonsurgical care first for most patients. Options may include activity modification, anti-inflammatory medication when medically appropriate, treatment of the underlying joint condition, aspiration, or injection. Surgery is rarely the first step. AAOS: Baker’s Cyst Treatment.
First-line treatment
Treatment is individualized, but common early measures include:
Activity modification
Temporarily reduce activities that repeatedly increase swelling, such as high-impact exercise, deep squatting, and prolonged standing.
This does not necessarily mean complete rest. Gentle movement is often helpful unless another injury requires protection.
Ice and elevation
Short periods of ice may reduce discomfort. Elevating the leg can help when swelling extends into the calf or ankle.
Medication
Anti-inflammatory medication may be considered when it is safe for the patient. Kidney disease, ulcers, blood thinners, heart conditions, and other medical issues can affect whether an NSAID is appropriate.
Acetaminophen may help pain but does not directly reduce inflammation.
Physical therapy
Physical therapy may help improve motion, strength, mechanics, and tolerance for activity. Therapy is generally directed at the knee condition rather than trying to “exercise the cyst away.”
Learn more about physical therapy at Pacific Bone & Joint.
Treating the underlying knee problem
When the cyst is related to arthritis, a meniscus tear, or another structural problem, addressing that condition may reduce the knee’s tendency to produce excess fluid.
Should a Baker’s cyst be drained?
Aspiration may be considered when the cyst is large, painful, or limiting motion.
During ultrasound-guided aspiration, the skin is numbed and a needle is advanced into the cyst while the clinician watches the procedure on the ultrasound screen. Fluid is withdrawn, and an injection may be considered depending on the diagnosis and treatment plan.
Potential benefits include:
- Reduced pressure behind the knee
- Improved ability to bend the knee
- Short-term pain relief
- Confirmation of the fluid’s appearance
Important limitations include:
- Thick or complex cyst fluid may be difficult to remove completely
- The cyst can refill
- Aspiration does not automatically correct arthritis or a meniscus tear
- Infection, bleeding, and injury to nearby structures are uncommon but possible procedural risks
Small studies suggest ultrasound-guided aspiration and corticosteroid injection can improve pain and function in selected patients with knee osteoarthritis and a symptomatic Baker’s cyst. However, recurrence remains possible, especially when the underlying joint continues producing fluid. Di Sante et al., prospective study and Smith et al., long-term follow-up.
Should the injection go into the cyst or the knee joint?
That depends on the source of symptoms.
If the knee joint is inflamed from arthritis, an intra-articular injection may reduce joint inflammation and fluid production. If the cyst itself is creating substantial pressure, aspiration and treatment of the cyst may also be considered.
The best approach is based on:
- The location of pain
- Cyst size and complexity
- The amount of knee-joint swelling
- Arthritis severity
- Meniscus findings
- Previous treatment response
- Patient-specific risks
There is no single injection strategy that is correct for every patient.
What about PRP or other biologic injections?
Evidence for biologic treatment of Baker’s cysts is still evolving.
A small randomized trial published online in 2026 compared aspiration alone with aspiration followed by an intracystic platelet-rich plasma injection in 36 patients with mild-to-moderate knee osteoarthritis. The PRP group reported greater improvement in pain and function at 12 weeks, but the study was small and does not establish a universal standard of care. Read the 2026 randomized trial.
This is an example of early evidence, not proof that every Baker’s cyst should receive PRP. Larger studies, longer follow-up, and better-defined patient selection are still needed.
My clinical perspective is that orthobiologic treatment should be considered only after identifying the actual pain generator. Treating a cyst without understanding the arthritis, meniscus, cartilage, alignment, or inflammatory condition can lead to disappointment.
Learn more about our regenerative medicine options.
Why does a Baker’s cyst come back?
Recurrence is common because the cyst often communicates with the knee through a one-way valve-like opening.
The cyst is more likely to return when:
- Arthritis remains active
- A meniscus tear continues irritating the joint
- The cyst is complex or divided by septations
- The knee repeatedly swells after activity
- The underlying inflammatory condition is not controlled
Recurrence does not necessarily mean aspiration was performed incorrectly. It may mean the knee is still generating excess fluid.
When is surgery needed?
Surgery is uncommon for an isolated Baker’s cyst.
It may be considered when there is:
- Persistent severe pain despite appropriate nonsurgical treatment
- Repeated recurrence after aspiration
- Nerve or blood-vessel compression
- A mechanical knee problem that is appropriate for surgery
- Diagnostic uncertainty about the mass
When surgery is recommended, the goal is usually to address the intra-articular cause and, in selected cases, the valve or cyst itself.
Open excision of the cyst alone may have a meaningful recurrence risk if the underlying knee problem is not addressed.
Can I exercise with a Baker’s cyst?
Often, yes, but activity should be guided by symptoms and the underlying diagnosis.
Lower-impact activities may be better tolerated, including:
- Stationary cycling with a comfortable range of motion
- Pool exercise
- Flat-surface walking
- Gentle strengthening
- Controlled mobility work
Stop and seek reassessment if activity causes rapidly increasing swelling, sharp calf pain, instability, or inability to bear weight.
Dr. Morton’s approach
When evaluating a Baker’s cyst, I focus on three questions:
- Is the lump truly a Baker’s cyst?
- What is causing the knee to produce excess fluid?
- Which treatment is most likely to provide durable relief?
Ultrasound can be very useful in the office because it helps confirm the diagnosis and assess whether the cyst is simple or complex. X-rays and MRI may be added when arthritis, a meniscus tear, cartilage damage, or another structural cause is suspected.
Some patients need only reassurance and time. Others benefit from medication, physical therapy, treatment of arthritis, or an image-guided procedure. Surgery is reserved for selected situations.
Baker’s cyst care in Hawai‘i
Pacific Bone & Joint evaluates posterior knee swelling, Baker’s cysts, knee arthritis, meniscus problems, sports injuries, and persistent knee pain at our locations in Honolulu, Waipahu, Hilo, and Kona. Kona visits are by appointment only.
Our team can combine examination, X-rays, diagnostic ultrasound, MRI review, physical therapy, bracing, image-guided injections, and surgical consultation when appropriate.
Call (808) 439-6201 to request an appointment or learn more about our sports medicine services in Hawai‘i.
This article is educational and does not replace an individualized medical evaluation.
Sources and further reading
- American Academy of Orthopaedic Surgeons. Baker’s Cyst (Popliteal Cyst). OrthoInfo.
- NHS. Baker’s Cyst. Updated 2025.
- Di Sante L, et al. Ultrasound-guided aspiration and corticosteroid injection of Baker’s cysts in knee osteoarthritis: a prospective observational study. American Journal of Physical Medicine & Rehabilitation. 2010.
- Smith MK, et al. Treatment of Popliteal (Baker) Cysts With Ultrasound-Guided Aspiration, Fenestration, and Injection: Long-term Follow-up. Sports Health. 2015.
- Akarsu O, et al. Ultrasound-Guided Aspiration of Baker’s Cyst Combined With Intracystic Platelet-Rich Plasma Injection in Knee Osteoarthritis: A Randomized Controlled Trial. American Journal of Physical Medicine & Rehabilitation. 2026.
