What is behind knee swelling? A swollen area that can be felt behind the knee can be caused by several different conditions. A Baker’s cyst is a collection of synovial fluid (fluid that is present within joint spaces) that bulges out through the back of the knee joint and can be felt in the back of the knee as a lump or enlarged area. Knee injuries, arthritis, damage to the cartilage of the knee, and other problems are known to cause Baker’s cysts. Swelling behind the knee may be painful or may not produce any other symptoms. In some cases, you may notice tenderness, warmth, difficulties moving your knee joint, muscle weakness, or bleeding or bruising. Injuries to the knee, including sprains (damage to the stabilizing ligaments of the joint) can lead to diffuse swelling of the knee area. Other possible causes of swelling behind the knee include abscesses (infections); tumors of the skin, soft tissue, or bones; bleeding; deep venous thrombosis (blood clot in the deep veins of the legs); or deformity of the joint after a fracture of any of the bones of your knee joint. Swelling behind the knee may be associated with injury and may be accompanied by more-serious injuries to the joint. Seek immediate medical care (call 911) for serious symptoms, such as paralysis, loss of sensation, absent pulses in the feet, the inability to move the knee joint, severe bleeding, chest pain, difficulty breathing, or uncontrollable pain. If your behind knee swelling is persistent or causes you concern, seek prompt medical care.