Swelling behind the knee

Also called: Baker cyst; popliteal cyst

Relief and prevention Strength Stretch Massage

A soft swelling in the hollow behind the knee that becomes tighter when the knee is straight. It is joint fluid tracking backward into a bursa - a symptom of what is happening inside the knee, not a problem in itself.

How common: Found in around 20% of adults with knee osteoarthritis and often incidentally on imaging

Open the Knee Pain/Bulletproofing routine All problems

What it is

A Baker's cyst is not really a cyst. It is a bursa behind the knee that communicates with the joint through a one-way valve, and it fills when the knee produces excess fluid. The cyst is the visible consequence of something inside the joint irritating the lining.

That is why treating the swelling alone rarely works. Draining it gives temporary relief and it refills, because whatever is producing the joint fluid - arthritis, a meniscal tear, inflammation - is still doing so. Treating the knee treats the cyst.

What it feels like

A soft, sometimes tense swelling in the hollow behind the knee
Tighter and more obvious when the knee is straight, softer when bent
A feeling of fullness or tightness at the back of the knee
Discomfort with deep bending and kneeling
Often no pain from the cyst itself, with the knee's own symptoms being the real complaint

What causes it

Most people have more than one of these at once, and they feed each other. Read the list looking for the two or three that sound like your week rather than for the one true answer — the ones that describe you are the ones worth acting on.

Excess joint fluid from anything inside the knee: The cyst is a downstream consequence. Whatever makes the joint produce extra fluid - arthritis, meniscal tearing, inflammation - fills the bursa behind the knee through a one-way connection.
Knee osteoarthritis: The commonest underlying cause in adults. An arthritic knee produces more synovial fluid, particularly during flares, which is when the cyst becomes noticeable.
Meniscal tears: A torn meniscus irritates the joint lining and increases fluid production. Baker's cysts are frequently found alongside degenerative meniscal tears.
Inflammatory arthritis: Rheumatoid and other inflammatory arthritis produce substantial joint effusions and cysts, sometimes large ones.
The one-way valve mechanism: Fluid passes from the joint into the bursa more easily than it returns. That is why the cyst persists and why it enlarges with each flare rather than settling in between.
Previous knee injury: Any injury producing ongoing joint irritation can lead to a cyst, sometimes months or years afterwards.
Gout and crystal arthritis: Produce acute joint inflammation with substantial effusion and can present with a rapidly developing cyst.
In children, a different story: Popliteal cysts in children are usually not associated with any joint problem, are benign and typically resolve on their own without treatment.
Rupture, which mimics something serious: A cyst can burst and track fluid down the calf, producing a swollen, painful, red calf that looks exactly like a deep vein thrombosis. It is a recognised diagnostic trap and it needs excluding properly rather than assuming.

Who tends to get it

Anyone with knee osteoarthritis, in whom it is common
People with a meniscal tear or a previous knee injury
Anyone with rheumatoid or another inflammatory arthritis
Adults over forty, in whom joint pathology becomes common
Children, in whom it is benign and usually unrelated to any joint problem

What makes it worse, and what settles it

Makes it worse

Ignoring the underlying knee problem, which is what keeps refilling it
Repeated aspiration without treating the cause, since it refills
Activity that flares the knee, which increases fluid production
Deep kneeling and squatting, which compresses it
Assuming a swollen painful calf is a burst cyst without excluding a clot

Settles it

Treating the underlying knee problem, which is what actually resolves the cyst
Quadriceps and hip strengthening for the knee, which reduces the irritation driving it
Managing activity to reduce knee flares
Massage and gentle range work behind the knee for symptomatic tightness
Aspiration or injection where the cyst itself is large and limiting, alongside treating the knee

What actually helps

The short version: Joint fluid tracking into the popliteal bursa; it is a symptom of what is happening inside the knee, not a problem in itself

Strength work: Legs: Quads; Glutes: Med; Legs: Hamstrings - treat the underlying knee problem

Stretching: Legs: Hamstrings; Legs: Calves

Massage: Legs: Hamstrings; Legs: Calves - avoid direct pressure on the cyst

Also worth doing: Manage the underlying knee osteoarthritis or meniscal problem

What the evidence says: Treat the knee, not the cyst. Aspiration without treating the cause means it refills.

The names above are the Targets qFIT tracks — the Muscle Guide explains what each one does, and Quick Add switches on the Exercises you already have.

The qFIT routine for this

Ready-made routineKnee Pain/Bulletproofing

qFIT already has this one built. It runs massage first, then stretching, then strength — the order that works on a body that is already sore — and every activity in it carries the reason it is there.

The button below opens your Checklist with this routine already selected, so instead of your whole list you land on just its Categories, Targets and Exercises, each with its checkbox, ready to tick as you work through them.

Open this routine in my Checklist

Have more than one problem? Tick yours under My Problems on My Profile and your Checklist's Routine list gains Recommended for my problems — every routine for the problems you picked, merged into one. Your picks stay private; that filter is the only thing they feed.

Get it checked if…

A ruptured cyst mimics a DVT exactly - sudden calf pain and swelling needs a scan before anyone massages it

And whatever the body part: chest pain or unusual breathlessness on exertion; a fever alongside the pain; unexplained weight loss; pain that is constant and unrelated to how you move; new numbness, weakness or loss of bladder or bowel control; or one calf that is swollen, warm and painful. Any of those, stop and get assessed.

How it usually goes

The cyst follows the knee: as the underlying problem settles, the fluid production falls and the cyst shrinks. Cysts in children resolve spontaneously and need no treatment. Treating the swelling alone gives temporary relief at best. A suddenly painful, swollen, warm calf needs assessment for a deep vein thrombosis rather than being assumed to be a burst cyst, since the two look identical and only one is dangerous.

Prevalence basis: Knee imaging studies

Others the same routine covers

These share the Knee Pain/Bulletproofing routine, so the work is the same. If one of them also sounds like you, that is a reason to expect more from it, not less.

This is general information, not a diagnosis, and it cannot examine you. If it matches you, use it to ask better questions of a clinician rather than to replace one.

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