Health condition · Clinically reviewed
Baker's cyst in children, a benign lump behind the knee that usually settles on its own.
Unlike the adult version, a childhood Baker's cyst is a primary swelling that does not connect to the knee joint. Most resolve within 1 to 2 years with reassurance and review.
Why trust this guide
- 01
Clinically reviewed
Written by our editorial team and reviewed by a registered UK clinician before publication.
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Sourced from guidance
Checked against BSCOS, RCPCH and peer-reviewed paediatric sources you can see at the end.
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Current for 2026
Reflects current UK paediatric orthopaedic practice on childhood popliteal cysts.
Key facts
Childhood Baker's cyst at a glance.
The essentials, in plain English: what it is, how it differs from an adult cyst, and why most children need reassurance rather than intervention.
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What it is
A benign fluid-filled swelling in the gastrocnemius-semimembranosus bursa behind the knee, distinct from the adult Baker’s cyst.
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How it differs
In children the cyst is primary and does NOT communicate with the knee joint; it is not caused by internal knee pathology.
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Peak age
Most common between 4 and 7 years, with a male predominance.
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Natural history
Around 70% resolve spontaneously within 1 to 2 years, so observation is usually the right first step.
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Typical presentation
A painless, firm, mobile lump in the medial popliteal fossa, often spotted by a parent during bath time.
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First-line imaging
Ultrasound confirms a simple anechoic cyst and rules out solid or vascular masses; MRI is reserved for atypical features.
Why this guide matters
A calm plan, not an anxious scan trail.
Childhood Baker's cysts worry parents far more than they harm children. The three points below shape everything else on this page.
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It is not the adult condition
A primary bursal cyst without joint communication. It is benign, not caused by cartilage or meniscal damage and rarely a marker of anything serious.
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Reassurance is the treatment
Around 70% resolve within 1 to 2 years. Aspiration and surgery bring more downsides than benefits in most children.
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Ultrasound is usually enough
A well-defined anechoic cyst on ultrasound is the diagnosis. MRI, bloods and referral are reserved for atypical features.
How the diagnosis is made
From first noticed lump to a clear plan.
The steps a UK GP or paediatric orthopaedic team will normally follow, in order, so you know what to expect and why.
Phase 1 · Assessing
History, examination and joint review
Phase 2 · Confirming
Ultrasound, selective MRI and bloods
Phase 3 · Planning
Reassurance and follow-up interval
- 01
Assessing
Focused history
When it appeared, whether it changes size, any pain, recent illness, joint stiffness or morning symptoms suggesting arthritis.
- 02
Assessing
Clinical examination
A round, fluctuant, transilluminable swelling in the medial popliteal fossa, most prominent when the knee is fully extended (Foucher sign).
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Assessing
Full joint assessment
Range of movement, effusion, tenderness and other joints to exclude juvenile idiopathic arthritis.
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Confirming
Ultrasound scan
First-line imaging: confirms a well-defined anechoic cyst, no solid component and no abnormal blood flow. Usually diagnostic.
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Confirming
MRI if atypical
Reserved for pain, rapid growth, a suggestion of a solid mass, concerning ultrasound findings or before any planned surgery.
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Confirming
Bloods only if arthritis suspected
ESR and CRP are not routine; they are added if the joint examination or history raises concern for inflammatory arthritis.
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Planning
Shared plan with parents
A clear explanation of the benign nature, expected timeline, warning signs and follow-up interval, usually annually.
Typical timeline: a first visit to a settled plan in days to a few weeks.
Symptoms
What a childhood Baker's cyst looks like.
A quiet lump behind the knee, often spotted by a parent, that changes little day to day. And the features that mean it is time for a closer look.
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Painless popliteal lump
A firm, smooth swelling in the medial popliteal fossa, often noticed by a parent rather than the child.
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Mobile and well-defined
The lump feels rounded and fluctuant, moves slightly with the skin and transilluminates with a torch.
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More prominent when leg straight
The classic Foucher sign: the swelling becomes tighter with the knee extended and softens on flexion.
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Usually unilateral
One knee is affected far more often than both. Bilateral cysts are uncommon and deserve a careful look at the joints.
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Rarely painful
Most children are entirely comfortable. Occasional mild ache after a long day of running or football is not unusual.
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Activity is preserved
The cyst generally does not limit walking, running or sport; children keep up with peers.
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Slow, variable size change
The swelling can wax and wane over weeks and months, which often worries parents but is expected.
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Red flag – rapid growth or pain
Sudden pain, quick enlargement, a hard or fixed lump, or an unwell child needs prompt paediatric orthopaedic review.
Treatment
How childhood Baker's cyst is managed in the UK.
Reassurance and observation first. Aspiration, injection and surgery are reserved for the small number of children with persistent or atypical cysts.
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Reassurance and observation
The mainstay of care. Explain the benign nature, expect resolution in most children and review annually.
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Activity as normal
No restriction on sport, PE or play. Children can continue to run, jump and swim as they wish.
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Simple analgesia if needed
Occasional paracetamol or ibuprofen is enough for the rare child who feels a mild ache after long play.
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Ultrasound follow-up
Rescan only if the cyst changes in a worrying way, or before considering surgery. Routine repeat scans are not needed.
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Rheumatology referral
Reserved for children with joint stiffness, morning symptoms, other swollen joints or systemic features suggesting juvenile idiopathic arthritis.
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MRI for atypical features
Used selectively when pain, rapid growth, a possible solid mass or concerning ultrasound features warrant a closer look.
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Aspiration and steroid injection
Rarely used in primary paediatric Baker’s cysts and not routine, given the high rate of natural resolution.
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Surgical excision
Reserved for persistent, painful, very large or atypical cysts, or those that fail to resolve by around age 10. Recurrence rates are 30 to 40%.
What this guide is based on
The sources behind every claim on this page.
UK paediatric orthopaedic guidance and peer-reviewed radiology literature, 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 paediatric team knows your child and can tell you which parts apply. If in doubt, get seen.
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British Society for Children’s Orthopaedic Surgery (BSCOS). Guidance on paediatric popliteal cysts.
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Royal College of Paediatrics and Child Health (RCPCH). Assessment of the child with a limb lump.
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De Maeseneer M et al. Popliteal cysts in children: prevalence, natural history and imaging. Peer-reviewed radiology literature.
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European Society of Paediatric Radiology. Ultrasound approach to soft tissue swellings in children.
Red flags
When a popliteal lump needs urgent attention.
Most childhood Baker's cysts are entirely benign. These are the features that change that assessment and prompt urgent review.
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Rapid or hard enlargement
A fast-growing, firm or fixed swelling raises the possibility of a soft tissue tumour and needs urgent paediatric orthopaedic review with MRI.
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Constant or night pain
Pain that wakes a child from sleep is never typical for a benign popliteal cyst and must be investigated promptly.
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Sudden calf pain and swelling
A ruptured cyst is uncommon in children but can cause acute calf pain, bruising and swelling that mimics deep vein thrombosis.
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Fever, redness or heat
These features suggest infection or an abscess rather than a simple cyst and need same-day clinical review.
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Joint stiffness or effusion
Morning stiffness, other swollen joints or a persistent knee effusion should prompt assessment for juvenile idiopathic arthritis.
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Neurovascular symptoms
Numbness, weakness, a cold foot or absent pulses require urgent review, though these are rare in childhood popliteal cysts.
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Systemic upset
Weight loss, night sweats or an unwell child alongside a lump changes the differential and mandates urgent paediatric input.
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Bilateral or multiple cysts
More than one cyst, or bilateral disease, deserves a careful review for underlying arthritis or a syndromic cause.
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Failure to resolve by age 10
A persistent, symptomatic cyst in an older child may justify surgical review, accepting the meaningful recurrence risk.
Living with it
A benign lump, with a patient plan.
Four things that make the biggest difference for families day to day: know it is benign, know the warning signs, keep life normal and review once a year.
A quiet reminder
Time is the treatment for most children.
Watching a lump slowly settle over months rarely feels intuitive, but it is the right course for most childhood Baker's cysts.
- 01 Reassure
It is almost always benign
Primary Baker’s cysts in children are not caused by joint damage and usually settle on their own within a year or two.
- 02 Watch
Know the warning signs
Sudden pain, inability to weight-bear, calf swelling, fever or redness should prompt a same-day review.
- 03 Play
Keep life normal
Sport, PE and running are not restricted. Children do not need to sit out of activities they enjoy.
- 04 Review
Annual check-in is enough
Most cysts can be safely reviewed once a year. Extra scans or specialist visits are only needed if something changes.
Frequently asked
Everything parents ask about Baker's cyst in children.
Quick answers on natural history, ultrasound, activity and when surgery is considered.
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What is a Baker’s cyst in a child?
It is a benign, fluid-filled swelling that sits in the gastrocnemius-semimembranosus bursa at the back of the knee. In children it is a primary cyst that does not communicate with the knee joint and is not caused by internal knee damage, which is why it behaves very differently from an adult Baker’s cyst.
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How is it different from an adult Baker’s cyst?
In adults, a Baker’s cyst is usually secondary to knee joint pathology such as meniscal tears or arthritis, and the cyst communicates with the joint. In children, the cyst is primary and self-contained, is not linked to internal derangement, and most resolve spontaneously.
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Will my child’s Baker’s cyst go away on its own?
In most cases, yes. Around 70% resolve spontaneously within 1 to 2 years. That is why observation and reassurance are the mainstays of treatment rather than aspiration or surgery.
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Does my child need an MRI scan?
Usually not. Ultrasound is the first-line test and is enough to confirm a simple cyst in most children. MRI is reserved for atypical features such as pain, rapid growth, a suggestion of a solid mass, concerning ultrasound findings, or before any planned surgery.
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Can my child still play sport and take part in PE?
Yes. A typical primary Baker’s cyst does not limit activity, and there is no need to restrict running, football, swimming or PE. If pain does develop after long play, simple analgesia and a rest day are usually enough.
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When would surgery be considered?
Surgery is reserved for cysts that are persistently painful, very large, growing rapidly, showing atypical features that raise concern about a tumour, limiting function, or failing to resolve into later childhood. Parents are counselled about the recurrence rate of around 30 to 40% because the underlying bursal anatomy remains.
Related content
Keep reading.
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Baker’s cyst (adult)
The adult version and how it differs.
Learn more -
Juvenile idiopathic arthritis
Inflammatory arthritis in children.
Learn more -
Arthritis
Joint inflammation across the lifespan.
Learn more -
Meniscal tear
Adult knee cartilage tears.
Learn more -
Private MRI scan
Detailed knee imaging when needed.
Learn more -
Private ultrasound
First-line imaging for popliteal lumps.
Learn more -
Arthroscopic knee surgery
Related surgical option in older children.
Learn more -
Paediatric orthopaedic clinic
Specialist review for children’s bones and joints.
Learn more