Knee swelling, also called a knee effusion or water on the knee, can make the knee feel unstable by increasing the space between the femur, tibia, and patella. There are many causes of knee swelling including injuries such as ligament tears, meniscus tears, patella dislocations, and fractures. Infections of the knee can cause significant swelling. Inflammation of the knee can also cause swelling. Many causes of knee swelling respond well to nonsurgical treatment. In some cases, however, the problem causing the swelling may need to be addressed surgically.