Artwork showing the structure of the knee joint which articulates the femur (upper centre) with the tibia and fibula (bottom). The knee is a strong hinge joint held in alignment by ligaments (light purple) and muscles; it is capable of flexion, extension and very slight rotation. The synovial membrane (blue) forms a large pouch, also known as bursa, which contains a clear fluid that lubricates the joint and eases its movement. The thin horizontal (purple) feature at centre between the femur and tibia shows the position of the menisci; these are discs or cartilaginous tissue which also reduce friction during joint movement. The patella is seen at centre right.