Acute knee swelling: what it may mean and what to do

Timing gives useful clues: swelling immediately after a twist or impact can accompany bleeding inside the joint, while swelling that develops with heat, redness or illness raises concern for inflammation or infection. Sudden swelling without an injury also deserves assessment.

Fluid may collect inside the knee joint, within a bursa near the joint, or in soft tissues around it. Injury to a ligament or meniscus can produce a joint effusion. Gout can cause an abrupt, intensely painful inflammatory episode, while osteoarthritis may flare with extra joint fluid. A Baker's cyst usually forms behind the knee when joint fluid pushes into a small sac.

Examination may assess warmth, tenderness, motion, stability and weight-bearing. Depending on the pattern, a clinician may arrange imaging, blood tests or removal of joint fluid to check for blood, crystals or infection.

Pause running, jumping and movements that sharply increase pain. Rest the knee in a supported position, elevate it, and use a wrapped cold pack for short comfortable intervals if this feels helpful. Do not apply ice directly to skin, force a stiff knee to bend, or attempt to drain swelling yourself.

If you can walk safely, gentle movement can help prevent unnecessary stiffness. Arrange an assessment when swelling is substantial, motion remains restricted, the knee repeatedly gives way or locks, or symptoms persist. Treatment targets the cause and may include activity modification, rehabilitation, medicine or clinician-performed aspiration.

Seek same-day urgent assessment for a hot, red and very painful swollen knee, especially with fever, chills, feeling unwell, a recent wound, injection or surgery, or reduced immunity. Joint infection can damage the knee and requires rapid testing and treatment.

Urgent evaluation is also appropriate after injury if you cannot bear weight, the knee looks deformed, swelling expands quickly, the joint is locked, or the foot becomes cold, pale, numb or weak. Sudden calf swelling, chest pain or breathlessness requires emergency help because a blood clot must be considered.

Ask whether the fluid appears to be inside the joint or in nearby tissues, which diagnosis fits the timing and examination, and whether imaging or joint-fluid analysis would change care. Clarify how much weight-bearing is suitable, which movements to begin, and what new symptoms should trigger reassessment.