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Health condition · Clinically reviewed

Baker's cyst, a swelling behind the knee that tells you what the joint is doing.

The cyst is usually not the problem. Treating the underlying knee - arthritis, meniscal tear or cartilage damage - is what makes it settle.

Jump to treatment
A radiographer guides a patient onto the bed of an advanced 3 Tesla MRI scanner in a London imaging suite

Why trust this guide

  • 01

    Clinically reviewed

    Written by our editorial team and reviewed by a registered UK clinician before publication.

  • 02

    Sourced from guidance

    Checked against NICE CKS, BOA and peer-reviewed orthopaedic sources you can see at the end.

  • 03

    Current for 2026

    Reflects modern UK practice on ultrasound-first imaging, MRI staging and treating the underlying joint.

Key facts

Baker's cyst at a glance.

The essentials, in plain English - what it is, why it forms, and how it is treated in the UK today.

  • What it is

    A fluid-filled swelling behind the knee - the joint capsule extending into the gastrocnemius-semimembranosus bursa.

  • Who gets it

    Adults, almost always as a secondary sign of something happening inside the knee joint itself.

  • Usual cause

    Osteoarthritis is the most common driver - followed by meniscal tears, rheumatoid arthritis and cartilage damage.

  • How it feels

    Fullness or pressure behind the knee, often worse after activity or prolonged standing. Some cysts cause almost no symptoms.

  • When it ruptures

    Sudden calf pain, swelling and bruising - can mimic a DVT and always needs a Doppler scan.

  • Foundation of care

    Treat the underlying knee - meniscal repair, steroid injection, weight loss and, for advanced OA, joint replacement.

Why this guide matters

The cyst is a symptom, not the diagnosis.

Three ideas run through everything else on this page - what actually drives the cyst, why imaging matters, and when to act.

  • It is almost always secondary

    In adults, a Baker’s cyst nearly always reflects something inside the knee - most often osteoarthritis or a meniscal tear.

  • Imaging changes the plan

    Ultrasound confirms the cyst and rules out mimics. MRI shows the joint - and the joint is usually where the answer lies.

  • Treat the knee, not just the cyst

    Aspiration and injection can help in the short term, but lasting relief comes from addressing the underlying joint pathology.

How the diagnosis is made

From swelling behind the knee to a clear plan.

The steps a UK GP, physiotherapist or orthopaedic surgeon will normally follow, in order.

  1. 01

    Assessing

    Clinical exam

    A round, fluctuant swelling in the medial popliteal fossa, most obvious with the knee fully extended - Foucher sign.

  2. 02

    Assessing

    History of the joint

    Any prior knee injury, arthritis, meniscal trouble, gout or inflammatory disease helps point to the underlying cause.

  3. 03

    Assessing

    Exclude DVT if calf involved

    If the calf is swollen, painful or bruised, a Doppler ultrasound comes first - a ruptured cyst can look identical to a clot.

  4. 04

    Confirming

    Ultrasound - first-line imaging

    Confirms a septated, anechoic cyst and separates it from a popliteal aneurysm, DVT or solid tumour.

  5. 05

    Confirming

    MRI of the knee

    The gold standard - shows the cyst and, more importantly, the meniscal tear, chondral defect or arthritis driving it.

  6. 06

    Planning

    Aspirate if diagnostic doubt

    Rarely needed for diagnosis, but useful when infection or inflammatory arthritis is suspected.

  7. 07

    Planning

    Plan around the joint

    Findings on MRI shape the treatment plan - a cyst without an underlying lesion behaves very differently from one with a torn meniscus.

Typical timeline: a first visit to a confirmed plan in a couple of weeks.

Symptoms

What a Baker's cyst actually feels like.

A mix of visible swelling, pressure behind the knee, and features of the underlying joint. And the signs that mean it is time to act.

  • Posterior knee swelling

    A visible or palpable lump in the crease behind the knee, often most obvious when standing with the leg straight.

  • Fullness and pressure

    A sensation of tightness or bulging behind the knee - many people notice it more than they feel pain.

  • Discomfort with activity

    Aching after walking, running or long periods of standing - eased by rest and elevation.

  • Reduced knee bend

    A large cyst can limit full flexion - it feels as though something is blocking the movement.

  • Minimal or no symptoms

    Small cysts are often silent and picked up incidentally on an MRI done for another reason.

  • Signs of the underlying joint

    Joint-line tenderness, catching, locking or morning stiffness point to the real driver - meniscal or arthritic disease.

  • Ruptured cyst

    Sudden calf pain, swelling and bruising - a crescent of bruising below the malleolus is a classic sign.

  • Red flag - possible DVT

    A hot, swollen, tender calf should always trigger a Doppler ultrasound before anything else.

Treatment

How a Baker's cyst is treated in the UK.

Conservative care first, joint-directed injection or arthroscopy next, and knee replacement for advanced arthritis - guided by what the MRI shows.

  • Treat the underlying knee

    The single most effective step - addressing meniscal tears, arthritis or cartilage damage often resolves the cyst for good.

  • Activity modification

    Short-term reduction in aggravating loads - long standing, running, deep squats - while the joint settles.

  • NSAIDs

    A short course of ibuprofen or naproxen calms synovitis inside the knee and reduces fluid production.

  • Physiotherapy

    Quadriceps and hip strengthening, gait work and range-of-motion drills support the knee and reduce recurrent effusions.

  • Intra-articular steroid injection

    Given into the knee joint itself, not the cyst - reduces synovitis and often shrinks the cyst indirectly.

  • Ultrasound-guided aspiration

    Drains the cyst and can be combined with steroid - relief is temporary if the underlying lesion is not treated.

  • Arthroscopic knee surgery

    Repair or trim a torn meniscus, remove loose bodies, address chondral flaps - often the definitive fix.

  • Knee replacement

    For advanced osteoarthritis, partial or total knee replacement treats the cause and typically resolves the cyst.

A note on ruptured cysts

A ruptured Baker's cyst is treated supportively - rest, elevation, NSAIDs and compression. A Doppler ultrasound is used to exclude a DVT before starting. If a secondary DVT is found, anticoagulation follows standard NHS protocols.

What this guide is based on

The sources behind every claim on this page.

UK national guidance and specialist society standards, current at the time of last review.

Key references

Guidelines and standards we relied on.

A quiet reminder

This guide is for information, not medical advice.

Your GP or orthopaedic surgeon knows your knee and history and can tell you which parts apply to you. If in doubt, get seen.

  • NICE Clinical Knowledge Summaries. Osteoarthritis and knee pain.

  • British Orthopaedic Association. Standards for knee assessment and surgical management.

  • Royal College of Radiologists. Guidance on knee ultrasound and MRI.

  • BMJ Best Practice. Baker cyst - diagnosis and management.

Red flags

When a knee swelling needs urgent attention.

Most Baker's cysts are manageable in primary care. These are the situations that are not - and where a specialist opinion is needed.

  • Suspected DVT

    Hot, tender, swollen calf with unilateral pitting oedema - urgent Doppler ultrasound and clinical review before assuming a ruptured cyst.

  • Sudden severe calf pain

    A cyst can rupture into the calf, causing dramatic pain, swelling and bruising - needs assessment to exclude a clot and to rule out compartment syndrome.

  • Popliteal aneurysm mimicking a cyst

    A pulsatile swelling behind the knee is a vascular problem, not a cyst - needs urgent vascular imaging.

  • Signs of infection

    Fever, spreading redness or a very tender swelling suggest infection - do not aspirate blindly, seek urgent orthopaedic advice.

  • Locking or true blocking of the knee

    A mechanical block often means a displaced meniscal tear or loose body - MRI and orthopaedic review are warranted.

  • Rapidly enlarging mass

    A cyst that grows quickly, especially in an atypical location, should be imaged to exclude a soft-tissue tumour.

  • Nerve symptoms

    Numbness, weakness or foot drop can rarely reflect nerve compression by a very large cyst - specialist review is needed.

  • Systemic inflammatory features

    Multiple joint swelling, morning stiffness and fatigue point to inflammatory arthritis - a rheumatology opinion helps.

  • Persistent effusion despite treatment

    Recurrent knee swelling after standard care usually means the underlying joint pathology has not been fully addressed.

Living with it

A treatable condition, once the joint is understood.

Four things that make the biggest difference day to day - sensible loading, quadriceps strength, healthy weight and acting on recurrence.

A quiet reminder

Strong quads protect the knee.

Consistent strengthening does more than any single treatment for most knee problems - stick with it for months, not weeks.

  1. 01 Load

    Manage the load

    Short walks, cycling and swimming keep the knee mobile without hammering it - long standing and deep squats often make things worse.

  2. 02 Strength

    Build the quads

    A strong quadriceps and glute complex protects the joint, reduces effusions and often shrinks a recurrent cyst.

  3. 03 Weight

    Every kilo counts

    Modest weight loss meaningfully reduces knee load and pain, and lowers the volume of joint fluid feeding the cyst.

  4. 04 Escalate

    Do not ignore recurrence

    A cyst that keeps coming back is telling you the joint needs attention - MRI and an orthopaedic opinion usually change the plan.

Frequently asked

Everything we get asked about Baker's cysts.

Quick answers on imaging, aspiration, rupture and knee surgery.

  • What is a Baker’s cyst?

    A Baker’s cyst, also called a popliteal cyst, is a fluid-filled swelling behind the knee. It forms when joint fluid from inside the knee tracks through a normal communication into the gastrocnemius-semimembranosus bursa. In adults it is almost always a secondary sign of something happening inside the knee itself - most often osteoarthritis, a meniscal tear or inflammatory arthritis.

  • How do I know if I have one?

    The classic sign is a round, soft swelling in the crease behind the knee, often most obvious when you stand with the leg straight. You may feel fullness, pressure or aching that is worse after walking or long standing. Many small cysts are silent and only picked up on a scan done for other reasons.

  • Is a Baker’s cyst dangerous?

    On its own it is not dangerous, but two situations need prompt attention. First, if the cyst ruptures, fluid tracks into the calf and causes sudden pain, swelling and bruising that can look identical to a DVT - always get a Doppler scan. Second, a large cyst can, rarely, compress the popliteal vein and cause a genuine secondary DVT.

  • How is it diagnosed?

    Clinical examination is usually the first step. Ultrasound is the first-line scan - it confirms the cyst and separates it from a popliteal aneurysm, a DVT or a solid tumour. MRI of the knee is the gold standard because it also shows the underlying meniscal, cartilage or arthritic problem driving the cyst.

  • How is it treated?

    The most effective treatment is aimed at the underlying knee. That may mean physiotherapy and weight loss for early osteoarthritis, an intra-articular steroid injection to calm synovitis, arthroscopic surgery for a torn meniscus or, in advanced arthritis, a partial or total knee replacement. Aspirating the cyst gives short-term relief but it usually returns if the joint problem is left alone.

  • Can I exercise with a Baker’s cyst?

    Usually yes, and gentle exercise helps. Low-impact activity like swimming, cycling or walking keeps the joint moving and builds the quadriceps and hip muscles that protect the knee. Ease off deep squats, heavy lifting and long periods of standing while symptoms settle, and stop any activity that reproduces sharp pain.

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