What it is.
There is a normal potential space behind the knee, between two tendons, called the gastrocnemius-semimembranosus bursa. In many adults it connects to the knee joint itself through a small valve-like opening.
If the knee produces excess fluid, that fluid is pushed backwards into the bursa when the knee is loaded, and the valve makes it easier for fluid to get in than out. The bursa distends. That is a Baker's cyst, named after the surgeon who described it.
It is a symptom, not a diagnosis.
This is the single most important thing to understand about it. In adults a Baker's cyst almost always means the knee is producing excess fluid, and the usual reasons are osteoarthritis, a meniscal tear, or inflammatory arthritis.
The practical consequence is that draining the cyst without addressing why the knee is making fluid tends to be temporary. The cyst refills because the cause has not changed. This is why aspiration is often disappointing and is generally reserved for cysts large enough to be mechanically troublesome.
Children are the exception: a popliteal cyst in a child is usually a primary problem, not secondary to joint disease, and usually resolves on its own.
Symptoms.
Often none. Many cysts are found incidentally on a scan done for another reason. When symptomatic, the usual complaints are a lump or fullness behind the knee, tightness on full flexion, and aching that varies with how active the knee has been.
Rupture, and why it matters.
A cyst can rupture, releasing fluid into the calf. That produces sudden pain, swelling and sometimes bruising around the ankle, and it mimics a deep vein thrombosis closely enough that it should not be assumed to be a ruptured cyst.
Sudden calf pain and swelling needs same-day assessment and usually an ultrasound to exclude a clot. Getting that wrong in either direction matters: an untreated clot is dangerous, and anticoagulating a ruptured cyst is not without risk either.
Treatment.
Treat the knee, not the cyst. Where the cause is osteoarthritis, that means the usual management: exercise, weight and load management, and where symptoms justify it, knee replacement. Cysts frequently resolve after a knee replacement, because the joint stops producing excess fluid.
Aspiration, sometimes with a corticosteroid injection, is an option for a cyst large enough to restrict movement. Surgical excision is rarely performed and has a high recurrence rate when the underlying cause remains.
Common questions.
Should the cyst be drained?
Usually not as a first step. The cyst refills because the knee keeps producing fluid, so draining it without treating the cause is temporary. Aspiration is reserved for cysts large enough to be mechanically troublesome.
Is a Baker's cyst dangerous?
The cyst itself is not. The situation that needs care is rupture, which causes sudden calf pain and swelling and can look exactly like a deep vein thrombosis. That needs same-day assessment and usually an ultrasound.
Will it go away?
Often, if the underlying cause is treated. Cysts frequently resolve after a knee replacement because the joint stops producing excess fluid. In children, popliteal cysts usually resolve on their own.
Does a Baker's cyst mean I have arthritis?
In an adult it usually means something inside the knee is producing excess fluid, and osteoarthritis is the most common reason. A meniscal tear or inflammatory arthritis can do the same. A weight-bearing X-ray is the sensible next step.
Can it be removed surgically?
It can, but excision is rarely performed because recurrence is high when the underlying joint problem remains. Treating the cause is more effective than removing the consequence.
References.
- Fritschy D, Fasel J, Imbert JC, et al. The popliteal cyst. Knee Surg Sports Traumatol Arthrosc. 2006;14(7):623-628.
- Herman AM, Marzo JM. Popliteal cysts: a current review. Orthopedics. 2014;37(8):e678-e684.
This page provides general information about a medical condition and a surgical procedure. It is not individual medical advice and does not replace consultation with a qualified practitioner. Outcomes vary between individuals and all surgery carries risk. A referral is needed for a consultation with Professor Daevyd Rodda; without one, you can start with the practice nurse practitioner, who can provide it.