Causes of knee swelling

Swelling within the joint may blur the kneecap’s outline and restrict bending. A more defined swelling can sit over a bursa or behind the knee as a Baker’s cyst. The speed of onset, recent activity, warmth, pain, and ability to bear weight help separate possible causes.

Rapid swelling after a twist, impact, or awkward landing can accompany an ACL or meniscus injury. A slower build-up after activity may reflect osteoarthritis or joint irritation. Gout can present with sudden intense pain, heat, redness, and marked tenderness in the knee.

A soft fullness behind the joint may be a Baker’s cyst, which often forms when another knee problem drives extra fluid. Local swelling at the front of the knee can occur with bursitis. A hot swollen knee with fever or feeling unwell raises concern for infection and needs urgent assessment.

Reduce weight-bearing activity that increases pain, elevate the leg, and apply a cloth-wrapped cold pack after a recent injury. Gentle bending and straightening can prevent stiffness if movement is comfortable, but avoid deep squats, running, and twisting until the cause is clearer.

Book an assessment for persistent, recurring, or unexplained swelling, especially when the knee catches, gives way, or cannot fully straighten. A clinician may examine the joint and decide whether imaging or joint aspiration would help identify blood, crystals, or infection in the fluid.

Get urgent medical care for a hot, red, very painful knee, particularly with fever, chills, a recent operation, an injection, or a break in the skin. Infection inside a joint requires rapid diagnosis and treatment.

Care should also be prompt after major trauma, when the knee looks deformed, or when you cannot bear weight. Sudden calf swelling or pain, chest pain, shortness of breath, or coughing blood requires immediate evaluation for a possible blood clot.

Ask whether fluid is mainly inside the joint or in surrounding tissue, which diagnosis fits the timing, and whether aspiration would provide useful information. Clarify what movements are safe and what change would justify earlier reassessment.