"The surfaces of the knee joint can be thought of as being a bit like a Crunchie bar --- the deeper bone is like a honeycomb that has a very rich blood supply and which is full of bone marrow and stem cells. However, this is covered by a hard egg-shell like cortex of bone, called the subchondral bone plate, which acts as a barrier. The articular cartilage is like the chocolate layer on the surface of the Crunchie bar, but the hard layer of bone underneath acts as a barrier, and unfortunately the articular cartilage itself has no blood supply."