Baker’s Cyst (Popliteal Cyst)
A Baker’s cyst is a fluid-filled swelling at the back of the knee, also called a popliteal cyst.
Most people notice it as a lump, or as a feeling of tightness and pressure behind the knee that is worse when bending or straightening it. Kneeling becomes awkward, the knee feels full rather than painful, and the swelling seems to come and go.
Many Baker’s cysts cause little or no trouble at all. But the cyst is usually not the problem itself — it develops because something inside the knee is producing more joint fluid than it should. That distinction matters, because it determines what actually needs treating.
Why does a Baker’s cyst develop?
The knee naturally contains a small amount of lubricating joint fluid. When the knee is irritated or inflamed it produces more, and that fluid can track backwards and collect behind the joint, forming a cyst.
Conditions commonly associated with a Baker’s cyst include:
- knee osteoarthritis
- inflammation within the knee joint
- meniscal or other internal knee problems
- inflammatory joint conditions
- previous knee injury
This is why draining a cyst without establishing why it formed often ends with the swelling coming back.
What does a Baker’s cyst feel like?
A Baker’s cyst may cause:
- a lump or swelling behind the knee
- tightness when bending or straightening the knee
- aching or a feeling of pressure behind the knee
- reduced knee movement
- discomfort after prolonged walking or exercise
The size can fluctuate depending on how much fluid the knee is producing at the time, which is why it may seem to appear and disappear.
Some cysts are found by chance during a scan for something else and need no treatment at all.
Is every lump behind the knee a Baker’s cyst?
No.
A Baker’s cyst is a common cause of swelling behind the knee, but other conditions can produce pain or swelling in the same area.
Assessment matters, particularly where the swelling has appeared suddenly or the diagnosis is uncertain.
Ultrasound assessment
Diagnostic ultrasound is well suited to assessing a suspected Baker’s cyst.
It confirms whether the swelling is fluid-filled, and shows its size, location and relationship to the structures around it. The knee itself can be assessed at the same time for joint fluid and other findings that help explain why the cyst developed — which is usually the more useful half of the examination.
Ultrasound cannot assess every structure inside the knee. Depending on the findings, an X-ray or MRI may sometimes be more appropriate.
Does a Baker’s cyst need treatment?
Not necessarily.
A small cyst causing no significant symptoms may need nothing done to it.
Where treatment is appropriate, it should usually focus on the underlying knee problem rather than the cyst alone. If osteoarthritis is causing the knee to produce excess fluid repeatedly, managing the osteoarthritis matters more than draining what that fluid has collected into.
Treatment may include physiotherapy, activity modification, medication and management of the underlying joint condition.
Ultrasound-guided aspiration
Where a Baker’s cyst becomes large, tense or uncomfortable, ultrasound-guided aspiration may be considered.
Ultrasound is used to see the cyst and guide a needle accurately into the fluid collection, and the fluid is then withdrawn through the needle. This can relieve the pressure and the tightness.
Aspiration does not remove the reason the cyst formed. It can refill or recur, particularly where the knee continues to produce excess fluid — so it is considered as part of managing the knee rather than as a cure in itself.
In selected cases a steroid injection, sometimes called a cortisone injection, may also be considered, depending on the underlying condition and the clinical findings.
The clinician who assesses your knee and scans it is the same one who performs any aspiration and then manages what follows, so the procedure is planned as one part of treating the knee rather than as an isolated drainage.
What about a ruptured Baker’s cyst?
Occasionally fluid from a Baker’s cyst leaks or ruptures into the calf.
This can cause sudden calf pain and swelling, and can closely resemble a deep vein thrombosis (DVT).
Because DVT is a potentially serious condition, new or unexplained calf pain and swelling should never simply be assumed to be a Baker’s cyst. Where DVT is suspected, urgent medical assessment and appropriate vascular investigation are required.
What does it cost?
Every appointment lasts a full hour. Where a diagnostic scan is needed, that appointment is charged at the scan rate, which replaces the standard consultation fee rather than being added to it. Current prices are on our prices page.
Baker’s cyst assessment in Weybridge
Finding a Baker’s cyst is only part of the assessment. The more useful question is usually why the knee is producing enough fluid to create one.
Meyer Physio assesses and treats knee problems at its clinic on Baker Street in Weybridge, Surrey, combining clinical assessment, diagnostic ultrasound, ultrasound-guided procedures and rehabilitation in one place. You can come to us directly — no GP referral is needed to book, and there is no waiting list.
Treatment may involve physiotherapy, management of the underlying knee condition, ultrasound-guided aspiration or injection, further imaging or specialist referral, depending on what the assessment shows.
Written by Suegnet Meyer, Advanced Practice Musculoskeletal Physiotherapist and MSK Sonographer. HCPC registered [PH55635]. Chartered Society of Physiotherapy member [66175]. Last reviewed: 29 October 2026.