When I ran track in college, I somehow developed focal cartilage defects in both knees. This brought my running career to a halt and made walking extremely painful for about a year. In an attempt to fix this, I had a type of surgery known as the OATS procedure performed. This is where the surgeon takes a plug of undamaged articular cartilage from a low load bearing region of the knee and swaps it with the damaged cartilage. Walking is mostly pain-free for me now, but it still hurts too much if I attempt to run.
As though one cartilage injury weren't enough, I somewhat stupidly decided to take up weight lifting after I couldn't run anymore and attempted to set down a barbell that was way too heavy for me. In doing so, I triggered a mild lower lumbar disc herniation. So now I have two permanent injuries. Luckily, neither injury is very severe, so some days I don't even notice the pain while other days it approaches mildly annoying "background noise".
These types of cartilage injuries are common, and arthritis is even more common. But the issue with cartilage is that once it's damaged, it doesn't heal on its own because cartilage has no vascular system. You can break all the bones you want and eventually they will heal, but damaged hyaline cartilage will not. The best that your body can do is to produce "low-quality" fibrocartilage in place of the damaged hyaline cartilage.
Fortunately, there's been a lot of research over the last decade on using mesenchymal stem cells (taken from your own bone marrow) to regrow true hyaline cartilage as opposed to fibrocartilage. The stem cells have actually been shown to differentiate into hyaline cartilage. For me, this has the potential to permanently alleviate both knee and back pain. Moving this research away from clinical trials seems to be taking forever for some reason though...