Some percentage of red blood cells injected under the kneecap will revert to stem cells and grow new cartilage. An acquaintance had it done to one of his knee

Some percentage of red blood cells injected under the kneecap will revert to stem cells and grow new cartilage. An acquaintance had it done to one of his knees decades ago and recently told me he needs it redone. The doctor punched holes in his kneecap and injected red blood cells into the space where new cartilage was needed. He he rested the knee for six weeks after which he slowly increased use to normal. According to my neighbor, the process was developed decades ago by veterinarians for horses and was then adopted by doctors of professional athletes. He personally knew a pro football player who'd had it done. It apparently cost "an arm and a leg", but his family could afford to pay for it. Animal medicine and medicine for professional athletes proceeds faster than the medicine the rest of us have access to. "Follow the money" I guess.