A joint is the point at which two or more bones come together. Where the bones meet, they are covered in cartilage, a complex and specialised connective tissue that enables the bones to move smoothly against each other. Knee cartilage is a durable substance that allows large forces to be transmitted across the end of the thigh bone and the top of the shin bone. It provides a shock absorbing effect and allows the bones to move smoothly when in motion. The knee is the largest joint in the human body and this complex joint is subjected to huge daily forces. Due to accidents, sporting injuries or unusual twisting movements, sometimes an area of cartilage can be damaged or torn off. Cartilage has a very limited ability to heal itself and this self-reparing mechanism decreases further as we age.