Fluid on the knee is usually an accumulation of the liquid that lubricates the knee joint. According to the University of Washington, "Everyone has fluid in all mobile joints, but usually the amount of fluid is very small, and under normal conditions you don't know it's there." The causes of excess fluid on the knee could be; injury, infection or osteoarthritis.
Your knee joint contains a very slippery liquid called synovial fluid to lubricate and cushion the cartilage and bones. This fluid also nourishes and keeps the joint healthy.
Symptoms of fluid on the knee due to arthritis include swelling and tenderness. If your knee is inflamed, swollen, red and hot when you touch it, you probably injured your knee or have an infection.
Knee injuries can be sudden or from overuse. Sudden injuries include broken bones, torn ligaments or torn cartilage. Overuse injuries are from constant pounding on the joint such as running, hiking or aerobics. Jobs where you are on your knees often can cause overuse injuries as well.
Fluid on the knee from an infection could be an accumulation of pus in your joint instead of synovial fluid. Your doctor can take a sample of the fluid in your knee and have it tested to see what is causing it.
Osteoarthritis is also known as degenerative joint disease. This condition occurs as the cartilage, which supports the knee, begins to break down. As the cartilage wears out, the bones rub together and cause pain and swelling.