I shattered my left knee on a motorcycle almost 20 years ago. 3 months in hospital and 18 months off work (Thank god I still got full pay). I had three ops over that time and it took me almost 3 years to get enough strength and stamina back to be able to run 5kms in a decent time.
If I run too much now, then after a week or two it starts to ache all the time. If I back off the running it comes good again. When I started snowboarding it caused me lots of grief. Swelling, aching, ACL aching and numbness in my foot. I found out later the numbness was caused by the swelling in the knee trapping the foot nerves that ran through it. Swelling went down and the foot numbness went away about 5 days after boarding.
I learnt four things.
1. ABSOLUTELY do all your Physio exercises and do them correctly, it makes a HUGE difference.
2. For boarding, get to the gym or something and do leg work to build up the supporting muscles.
3. ACL's can be re-injured easily
4. Avoid Cortisone if at all possible, Yes, it relieves pain, but it also weakens the tendon strength which can lead to the ACL pulling off of the bone.
I was really suprised at how well my knee was this year in Japan, it hardly ached at all and I put this down to pre-snowboard trip training. Lots of squats and leg extensions, jogging and cycling, lunges and stretching. I got the exercises off of YouTube believe it or not.
I will board until I need a new knee, which according to my ortho surgeon, should have been done 20 years ago when I shattered the knee joint. I am hoping by the time I have to, that the docs will have come up with something knew to fix knees like mine. If they can spin silk from DNA altered Goat's milk, then growing knee joints can't be too far off.