The knee is the largest joint in the human body. It includes a join between the femur and the tibia, and one between the femur and the patella.
The joint is surrounded by bursae or sacs that contain synovial fluid. Articular cartilage between the bones separates them and prevents damage. This wears down over time and has limited capacity for renewal. When this layer is worn down it causes rubbing between the bones causing pain and inflammation known as arthritis.