In my case, it's age and old injuries. I had a chunk of cartilage removed from the right one (which I'd dislocated years previously) and the doc tells me that what's left is gradually wearing away.
Right now, I'm managing on cortisone. Works a treat... I get a shot and I'm good for 3-4 months. However, they tell me it'll eventually get to the point where it simply doesn't work any more, and then it's new-knee time.
Riding is one of the few things I can do pain-free... The doc recommends it. But I do keep the gearing low and the rpm high.
Right now, I'm managing on cortisone. Works a treat... I get a shot and I'm good for 3-4 months. However, they tell me it'll eventually get to the point where it simply doesn't work any more, and then it's new-knee time.
Riding is one of the few things I can do pain-free... The doc recommends it. But I do keep the gearing low and the rpm high.