Swollen Knee
An accumulation of excess fluid within or around the knee joint, resulting in stiffness, tightness, and puffiness.
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What does this symptom mean?
Swelling in the knee is caused by an accumulation of synovial fluid (the joint's natural lubricant) or blood inside the joint capsule, often referred to as 'water on the knee'. It can cause the knee to look puffy, feel tight, and restrict bending. Wording is cautious: swelling is a response to irritation or injury, and its rapid or gradual onset helps guide the diagnosis.
Anatomical Overview
Important — Educational Information Only
Pain location alone cannot determine the cause of knee symptoms. This map provides educational information about anatomical structures and possible associations. It is not a diagnostic tool. A clinical examination by a qualified specialist is required to determine the cause of your knee pain.
Knee Pain Location — Educational Guide
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Select a numbered hotspot on the Front (Anterior) photograph or use the Prev/Next buttons below to browse.
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Front (Anterior) View
Around the Kneecap (Anterior)
Pain around or behind the kneecap (patella). Often worsened by stairs, squatting, or prolonged sitting.
Outer Joint Line (Lateral)
Pain along the outer joint line. Often related to sports injuries, pivoting movements, or lateral compartment problems.
Inner Joint Line (Medial)
Pain along the inner side of the knee joint line. The medial compartment bears the majority of weight-bearing load.
Back (Posterior) View
Behind the Knee (Posterior)
Pain, swelling, or fullness in the hollow behind the knee (popliteal fossa). Often secondary to an internal joint problem.
Inner (Medial) View
Medial (Inner) Compartment
Pain along the inner side of the knee. The medial compartment is the most commonly affected by arthritis and meniscal tears.
Outer (Lateral) View
Lateral (Outer) Compartment
Pain along the outer knee compartment. Lateral problems are less common but frequently sports-related.
Symptom-to-Recovery Pathway
Standard Patient Journey Pathway
How the symptom may present
Patients describe this symptom in various ways depending on its trigger and severity:
Possible causes
The possible causes listed below are not exhaustive. One symptom can have several underlying causes, and a clinical assessment is typically required to confirm a diagnosis.
Knee Arthritis
Degeneration of the knee joint cartilage leading to progressive pain, stiffness, and restricted mobility.
Meniscal Tear
A tear in the shock-absorbing C-shaped cartilage discs of the knee, often caused by twisting forces or age-related changes.
ACL Injury
A sprain or rupture of the Anterior Cruciate Ligament, a primary stabilizer inside the knee joint, common in pivoting sports.
Cartilage Injury
Focal damage to the protective articular cartilage lining the bones, leading to localized pain, clicking, or swelling.
Baker's Cyst
A fluid-filled swelling that develops at the back of the knee, usually arising from an underlying joint problem like arthritis or a meniscus tear.
When to seek urgent advice
Certain symptoms (red flags) indicate the need for prompt clinical evaluation. Seek immediate assessment if you experience:
- The swelling is accompanied by a high temperature or feeling unwell
- The joint is hot, red, and extremely painful to touch
- You cannot bear any weight on the leg
- The knee swelled immediately (within 2 hours) of an injury
How the symptom is assessed
Your consultant will carry out a thorough assessment to understand the triggers, location, and functional impact of the symptom.
- Review of onset timing (immediate vs. delayed) and injury history
- Examination of swelling volume, temperature, and joint mobility
- Laxity and stability checks to evaluate ligament integrity
- Assessment of pain location and functional limitations
Investigations that may be considered
Depending on the findings of your clinical assessment, your consultant may recommend imaging scans:
Standard X-rays
evaluate bones and joint space
Ultrasound scan
confirm fluid collection or popliteal cysts
MRI scan
assess internal soft-tissue structures
What may help initially
For non-urgent presentations, simple self-care and load management can help settle symptoms:
- Following the RICE protocol: Rest from heavy tasks, Ice the area, apply gentle Compression, and Elevate the leg
- Avoiding heat packs or hot baths in the first 48 hours, as this can increase swelling
- Gentle knee bends while lying down to encourage fluid clearance
When to arrange a professional consultation
You should arrange a specialist consultation if you experience any of the following:
- Swelling persists for more than a few days without improvement
- The swelling is associated with joint locking or giving way
- The knee repeatedly swells after normal physical activities
Frequently Asked Questions
Would you like an individual knee assessment?
A consultation can help clarify the possible cause of your symptoms and discuss appropriate next steps.
Face-to-face appointments are arranged through the reception team at the chosen clinic. Video consultations use Google Calendar where available.
Next Step: Explore Possible Conditions
Similar symptoms can occur with several different knee conditions. Learn about common diagnoses, how they are assessed, and how force transmission affects joint health.
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Medical Disclaimer
This website provides educational and general information about knee symptoms, conditions, and treatments. The content is for informational purposes only and does not constitute medical advice, diagnosis, or a treatment plan. It should not be used as a substitute for a professional consultation, examination, or clinical decision-making by a qualified orthopaedic specialist.
If you are experiencing symptoms or are concerned about a knee problem, please arrange a clinical consultation. If you have urgent symptoms, severe pain, or are unable to put weight on your leg, please read our urgent advice guidance or contact emergency services immediately (dial 999 or NHS 111).