Meet the Meniscus
Yep, that's an exposed knee joint.
The meniscus consists of two C-shaped cushions around the outermost ring of the shallow sockets of the leg bone. These receive blood flow and hence can to a limited extent heal and regrow.
The middle of each meniscus cushion is filled with articular cartilage which does not regrow.
Note: I'm not a doctor, and have never played one on TV. The above information reflects no medical expertise (merely a dedicated evening of Googling) and should not be taken as medical advice of any kind.
Another note: This image was purloined from http://www.meniscustransplantation.org/ and modified for purposes of education only.