I don't think you can lose cartilege in one jump. You can strain, twist, break, blowout, hyperextend, tear ... but absence of cartilege sounds like a chronic thing, not an impact injury.
I got the feeling they were trying to get at bodily wear-and-tear from years of pushing himself over his limits, not a one-time impact injury. You can rehab an injury over 8-years ... you can't rehab a chronic condition. His years-of-service are a little vague -- I tend to think its longer than some do -- but jumping off rooftops and drop-kicking criminals for a living is bound to beat-up the knees.
SnakeDoc
