I was 14 and had bilateral orthoscopic knee surgeries and the DR. scraped out more arthritis than the average 65 year old. I asked my parents if I had every been in an accident. I hadn't other than my knees often 'gave out' on me. And I think after so many years of that, it was trauma enough.
My dad had a knee replacement shortly after I had my knee surgeries. He had his knee crushed by a towmotor at work. He doesn't have the best luck with it b/c of such a traumatic injury and they had to wait so long for workman's comp to cover it. B/c of that he only has anout 70% range of motion to it. Of course I think I probably have that also!! B/c I THINK the arthritis is coming back.