Posterior knee swelling: causes and warning signs

A soft fullness behind the knee can occur when joint fluid pushes into a Baker's cyst. Knee osteoarthritis, a meniscus tear, gout, and other forms of joint inflammation may increase fluid inside the knee. Knee bursitis can affect nearby cushioning sacs, while an ACL tear may cause swelling after a twisting injury, often with instability or a clear injury story.

The appearance of swelling does not identify its cause. A clinician compares both knees, checks movement and stability, feels for tenderness, and considers imaging. A knee swelling page can help you describe the symptom, but it cannot replace an examination when the cause is unclear.

Reduce running, jumping, kneeling, and deep bending if they increase the swelling. Rest the leg in a comfortable position, use a cold pack wrapped in cloth for a short period, and elevate it when resting. Do not puncture or press firmly on a lump behind the knee. Note whether it changes with bending, follows an injury, or is paired with locking, giving way, or warmth.

A clinician may treat the underlying joint problem first. Joint aspiration can sometimes remove fluid and help clarify what is happening, but whether it is suitable depends on the examination and suspected cause.

Get emergency help for sudden calf or leg swelling with chest pain or breathlessness. Seek prompt care for a hot, red, very painful knee, fever, inability to bear weight, a rapidly enlarging swelling, a cold or pale foot, or new numbness. Arrange assessment when swelling persists, recurs, or follows a twisting injury with instability or locking.

Ask: Is the fullness inside the joint or in a cyst or bursa? Could my injury involve a meniscus or ligament? Do I need imaging? Would aspiration help diagnosis or comfort? Which activities are safe while swelling settles? What change should prompt urgent review?