So in the 80's the BBC had a program called Tomorrow's World. They had a race horse which had worn its cartilage which would have meant retirement. Surgeon opened front knee enough to drill up into bone from where the cartilage had worn our, bone marrow (stem cells) spilled out, sealed up knee and the stem cells adopted/became the tissue it was next to which included new cartilage. Horse recovered quickly, like allowed to walk around in a day or two and was back running when stiches were removed. No visible signs of pain. Hailed as a success for worn cartilage.

I don't understand why the medical professionals use metal alloys for joint replacements considering how the metal is attacked by the immune system and deposited elsewhere in the body. Doesn't make sense.