Swelling Under the Knee: Possible Causes and What to Do

A fullness behind the knee can occur with a Baker's Cyst, which may develop when extra joint fluid moves into a small space behind the knee. Swelling below or beside the joint can involve a bursa or tendon. ACL Tear and Meniscus Tear may cause swelling with instability, catching, or difficulty straightening the knee. Knee Osteoarthritis can bring stiffness and activity-related discomfort. Sudden heat, redness, and strong tenderness may occur with Gout. The exact position, timing, and associated symptoms help distinguish these possibilities.

Reduce squatting, kneeling, running, and lifting until you know which movement aggravates the area. Elevate the leg when resting and use a covered cold pack after a recent injury or flare. Avoid squeezing or puncturing a lump behind the knee. Keep track of size, pain, warmth, range of motion, and any calf symptoms. A clinician can examine the knee and may recommend imaging or Joint Aspiration depending on the findings.

Seek prompt care after a significant injury, if the knee locks or gives way, or if walking becomes difficult. Urgent help is needed for a hot red knee with fever, sudden severe pain, marked calf swelling, or a pale, cold, or numb leg. See Knee Swelling for related guidance.

Ask whether the swelling is behind the joint, in a bursa, or part of a knee effusion. Ask which tests are useful, how much activity is sensible, and what symptoms require earlier review.