Hello Dear. Swollen knee joint may be due to knee effusion or water in the joint. It may be due to several reasons. 1. Injury to knee joint. But as per your history we can rule it out 2. Osteoarthritis. Most common in old age. Your age is not mentioned here. 3. Septic and non septic bursitis. Knee joint has a cushion like covering called bursa. In case there is infection of it, it may lead to knee pain 4. Rheumatoid arthritis. Its an autoimmune disease that affects the delicate lining of the joints, rheumatoid arthritis can cause knee swelling, stiffness, pain, tenderness, and redness 5. Gout. A painful accumulation of microscopic uric acid crystals in the joint defines a gout attack. Knee swelling may occur rapidly and be accompanied by excruciating pain, redness, and warmth. These are the most common causes. There may be several other reasons. In any case please consult an orthopedician at the earliest and seek treatment. The doctor may first do an xray of the knee joint. And some specific blood tests if required Hope i have answered your question. Feel free to contact me if you have any more questions. I ll be glad to help you. All the best With warm regards Dr Sanjay Kini