What should you know about fluid and swelling in knee?

A knee can swell after a twist, impact, overuse, or inflammation. A ligament injury such as an [ACL Tear](/conditions/acl-tear/) or a [Meniscus Tear](/conditions/meniscus-tear/) can irritate the joint. A [Baker's Cyst](/conditions/bakers-cyst/) may create fullness behind the knee rather than a general joint effusion. [Knee Bursitis](/conditions/knee-bursitis/) affects a cushioning sac, while [Knee Osteoarthritis](/conditions/knee-osteoarthritis/) can cause recurring fluid with stiffness. A hot, suddenly swollen joint can also occur with [Gout](/conditions/gout/). A swollen knee is a presentation of several conditions; the swelling alone does not identify which one is present.

An examination may include movement checks and, when needed, imaging or testing of the fluid. The pattern of swelling, fever, injury, and ability to walk helps guide that decision.

Reduce activities that increase swelling, elevate the leg, and use a wrapped cold pack for short periods. Keep gentle movement within comfort so the knee does not become stiff. Do not repeatedly press or puncture the swelling yourself. A clinician may discuss [Joint Aspiration](/treatments/non-surgical-orthopedics/joint-aspiration/) when fluid needs testing or pressure relief.

Seek urgent assessment for a hot or red knee with fever, severe pain, inability to bear weight, a locked joint, major deformity, or swelling that appeared rapidly after injury. Calf swelling or breathlessness alongside leg symptoms also needs urgent attention. Arrange a routine review when swelling persists, returns repeatedly, or limits stairs, walking, or sleep.

Ask whether the fluid is inside the joint or in a bursa, what examination or imaging is appropriate, and which activities are safe while it settles. Tell the clinician about recent injury, fever, skin changes, and previous attacks of joint inflammation.