The small layer of cartilage surfacing our joints performs a critical function in allowing our joints to move but also to support our weight. For these reasons it must be both elastic and firm, and the cartilage must match on both sides of the joint in order to minimize friction as motion occurs. When it works well it is a beautiful thing.
There are many reasons for cartilage loss. Some causes are related to injury, some to repetitive sports or work related activities, some to a genetic predisposition to lose cartilage. Often in one individual there may be more than one cause.
This arthroscopic photo shows a hole in the center- down to bone- with a crumbly rim of cartilage just around it. This is a large full thickness cartilage loss, and unfortunately there is no prospect that this could heal with normal cartilage if left to its own. In fact, most methods of cartilage reconstruction, even if successful, will probably fill this hole with something less than “normal” cartilage- although much better than what you see at present.