A Baker cyst is a fluid-filled swelling behind the knee, often associated with a problem inside the joint. It can cause fullness or tightness, but a new or unexplained lump needs assessment. If calf pain and swelling increase, seek urgent medical care to exclude a clot. A cyst and a deep vein thrombosis can look similar; a familiar label is not a safe reason to wait.
Prepared by The Wellness Team. Evidence checked 2 November 2026.
Ask about a planned individual assessment. This messaging route is not an emergency service.
Why does fluid collect behind the knee?
The knee normally contains lubricating synovial fluid. Conditions such as osteoarthritis or a meniscal problem may lead to excess fluid passing into a space behind the joint. The visible swelling can therefore be a consequence rather than the whole diagnosis. Treating the underlying knee problem is part of the discussion. A photograph cannot reliably show whether a lump is fluid-filled or something else.
What if the cyst seems to burst?
A ruptured cyst can release fluid into the calf, causing pain and swelling that resembles a clot. Do not diagnose rupture at home from a sudden change in the lump. AAOS advises medical care right away for increasing calf pain and swelling to rule out a blood clot. Chest pain, breathlessness, coughing blood or collapse requires emergency help through 999, not a routine knee consultation.
Which tests are useful?
The history and examination guide testing. Ultrasound can help establish whether a lump contains fluid and may be part of a separate clot-assessment pathway when indicated. MRI may be useful to examine structures within the knee. Ask what each test is intended to establish. Finding a Baker cyst does not automatically explain every new symptom or remove the need to assess a concerning calf.
Discuss the right appointment after urgent concerns have been addressed. Suitability, location, timing and the complete fee are confirmed individually.
Does every cyst need draining?
No. AAOS describes many cysts improving with nonsurgical care, sometimes without a procedure. Activity changes, appropriate symptom treatment and addressing the associated joint condition can be enough. Aspiration or other interventions may be considered for selected symptoms. Recurrent swelling deserves a review of the underlying cause, not a promise that drainage will permanently cure the problem.
Where does PRP fit?
An injection offered for an underlying knee diagnosis should be judged on evidence for that diagnosis, not marketed as a proven way to dissolve a Baker cyst. The AAOS cyst guide does not establish PRP as a cyst cure. A person with osteoarthritis may have a separate discussion about its treatment options and uncertainties. That conversation begins after the lump and any urgent calf symptoms are properly assessed.
What should a useful follow-up plan include?
Agree which symptoms should improve, how activity will be adjusted and when a persistent or recurrent lump should be reviewed. Ask whether the diagnosis was established clinically or with imaging. If pain becomes severe, the calf swells or the person feels unwell, use the appropriate urgent route. An old scan should not be used to explain away a materially different new symptom.
What makes a premium joint or tendon assessment worthwhile?
A careful assessment should establish which structure is causing symptoms and whether an injection has a defensible role. Ask how the diagnosis will be confirmed, what rehabilitation or other treatment belongs in the plan and what would trigger referral. Where imaging is useful, ask what question it will answer. A scan finding is not automatically the cause of pain, and a treatment sold for one diagnosis should not be transferred to another without evidence.
For published London market context, Chelsea Pharmacy Medical Clinic lists a single-joint combination of hyaluronic acid, PRP and steroid at £925. This is a different combined intervention, not a like-for-like standalone PRP quote or The London PRP Clinic tariff. A higher price does not establish a better outcome. The complete fee should reflect the assessment, professional expertise, any agreed imaging or procedure, and the review arrangements. Request an itemised quote before deciding and ask which elements are separate. A technically impressive injection is not good value if it delays the right diagnosis or a time-sensitive treatment. No PRP response is guaranteed. The London PRP Clinic is part of The Wellness; the team confirms the appropriate appointment and the service it can arrange for the individual concern.
What should I bring to the appointment?
Bring the injury timeline, previous scan reports, medicines and a description of the activities you need to recover. Explain whether symptoms began with a particular event, whether movement is blocked and whether strength has changed. Tell the clinician about fever, wounds, anticoagulants, previous surgery and other medical conditions. Do not stop prescribed medicines to prepare for an enquiry. An informed decision can be to use rehabilitation, seek a specialist opinion or have no injection at all.
Request a suitable assessment and a clear individual plan. If symptoms are urgent, use the urgent route described above rather than waiting for a reply.
Sources and limits
- AAOS Baker cyst assessment and treatment
- Mayo Clinic Baker cyst
- Published London joint-procedure fees and scope
This is general information, not a diagnosis or individual treatment advice. Evidence and guidance apply to the populations described; they do not promise a result for an individual. No appointment, investigation or procedure is confirmed by submitting an enquiry.