Knee Swelling: Causes, Warning Signs and Treatment

Knee swelling means extra fluid or inflammation around the joint, and its timing, pain, heat and injury history help guide diagnosis.

A swollen knee may look puffy around the kneecap or feel full and tight inside the joint. Bending can become difficult, and the knee may feel heavy, warm, stiff or tender. Swelling that appears soon after an injury differs from swelling that builds gradually, so note when it began and what you were doing.

Compare the knee with the other side, but do not repeatedly press it or force movement. Pain, bruising, warmth, fever, difficulty bearing weight and a feeling of instability are useful details for a clinician.

Seek urgent medical care for a hot, very painful or rapidly enlarging knee, especially with fever or feeling unwell. After an injury, prompt assessment is appropriate if swelling appeared quickly, the knee is deformed, you cannot bear weight, or it cannot fully move. Sudden calf swelling or breathlessness also needs urgent medical attention.

These causes are in alphabetical order, not ranked by likelihood. The same visible swelling can have different explanations, so examination and timing matter.

An ACL Tear can present with rapid swelling after a twisting or pivoting injury, often with instability. A Baker's Cyst can present as fullness behind the knee and may become more noticeable when standing. Gout can present with sudden intense pain, warmth and swelling from crystal-related inflammation. Knee Bursitis can present with swelling over a pressure point around the knee. Knee Osteoarthritis can present with activity-related swelling and stiffness. A Meniscus Tear can present with swelling, joint-line pain or catching after twisting. Septic Arthritis can present with a hot, swollen, severely painful joint and fever and requires urgent evaluation.

A clinician asks about injury, activity, previous episodes, fever, medical conditions and medicines. They inspect the swelling, warmth and skin, test movement and stability, and examine nearby areas. X-rays may assess bones and arthritis; ultrasound can assess fluid or a cyst; magnetic resonance imaging can examine ligaments, menisci and cartilage.

If infection, crystals or another fluid-related cause is a concern, joint aspiration may remove a small fluid sample for testing and sometimes ease pressure. The test choice depends on the examination and urgency.

Until assessed, protect the knee from aggravating activity, elevate it when comfortable and use a cold pack wrapped in cloth for short periods if that helps. Avoid forcing a swollen joint through painful movement. A clinician can advise on suitable pain relief and whether a brace or crutches are appropriate.

ACL or meniscus injuries may need structured rehabilitation and sometimes orthopaedic review. A Baker's Cyst is treated by addressing the knee problem driving the fluid. Gout needs clinician-guided treatment for the flare and prevention plan. Bursitis often improves with pressure reduction and rehabilitation. Osteoarthritis care may combine strengthening, pacing and movement support. Suspected septic arthritis needs urgent medical treatment. Joint aspiration may be used when fluid testing or pressure relief is indicated.

Increase exercise load gradually and practise controlled landing, turning and strengthening if your activities involve pivoting. Keep thigh and hip muscles conditioned, vary repetitive kneeling or pressure, and use protective equipment suited to the activity. Treating an underlying joint condition can reduce recurrent fluid build-up.

Do not return to demanding sport while the knee remains swollen, unstable or unable to move normally. A physiotherapist can help set progression markers based on strength, balance and control.

Pain, heat, redness, bruising, stiffness, locking, clicking, instability and fever help distinguish patterns. Swelling behind the knee or into the calf should be described clearly, particularly if it appeared suddenly.

A non-urgent review can identify whether swelling is inside the joint, in a bursa or behind the knee and can match care to the cause. Take note of the onset, triggers, injury mechanism and any fever or inability to bear weight so the clinician can triage you safely.