The knee joint is like any other large joint in the body, the bony surfaces are covered in a layer of articular cartilage, which allows the smooth running of the joint. Injuries to the knee can cause damage to the cartilage, either full or partial thickness.
This can produce flaps of cartilage that can loosen and float around free in the knee. These flaps or loose pieces can catch leading to pain or giving way. They can easily be removed and the surrounding cartilage smoothed down, with arthroscopic (keyhole) surgery.
Unfortunately cartilage does not heal, if the damage is full thickness, through to the underling bone, there are a number of procedures that can be performed. Microfracture, involves making small holes in the exposed bone to stimulate firbocartilage formation, which is less robust than articular cartilage but can help improve symptoms. Autologous cartilage transplantation (ACI) involves removing a piece of cartilage from a non-weightbearing area of the knee, sent to a laboratory to grow cartilage cells, which are then re-implanted into the damaged area.