I'm approaching 75 and had a serious knee injury in my early 20s - which was treated at the hospital I was working, in Innsbruck, and I'm 100% certain that if it'd been treated in any country where skiing injuries weren't as common, I would now be on my second or third knee or have the joint fused. As it is, it's still functional without too much bother although post-traumatic arthritis is taking its toll. Nothing I can't cope with, though. The achilles tendon that 'went' a few years ago when I was chasing a dog downhill on a steep, wet and slippery slope gives me more trouble BUT the cycling is helping both it and the knee, more than I expected!
It remains to be seen how much strength returns in that leg - very little so far! - but it is already behaving in a more stable fashion. The issue of returning strength is doubtful, even though it's early days yet, and might well see me getting an electric conversion kit by, say, next spring. Getting up even slight hills - which is all we have round here in the flatlands - is very hard work when only one leg can push effectively and the other is somewhat of a passenger when any real effort is required.
Over fifty years ago when I injured my knee, I remember the radiologist, the plaster technician, the orthopaedist and the physio all united in saying 'What a pity you didn't just break it instead of shredding your soft tissue - it'd've healed much better and faster!'
Best of luck and good healing vibes to all who face major repair surgery!