I injured my knee originally skiing. The ironic thing for me was that I was a fairly good competitive skier who raced through university with my only career injuries a broken wrist, and a broken collarbone. I first tore my ACL free skiing on a fairly easy hill while out of shape and fooling around. Once injured, my one knee kept getting re-injured. I got a DVT after ACL surgery (over 30 years ago) so they had to cut the cast off and as a result I feel the graft was never tight enough. So over the years, with the excess movement and additional cartilage injuries and scopes I developed arthritis. There is not enough cartilage and at my age - no surgeon will try a repair (and I’ve seen Olgivie-Harris who is one of the best sports knee surgeon around)- he told me replacement is the next option.
I agree that a good sports rehab therapist can really help. I find biking** and weights (leg press and extension on a machine) the best for my knee. I take glucosamine supplements and regularly work on stretching and range of motion. I used to sleep with a pillow under my knee but that is actually not a good practice. Now I use a band to try to straighten my knee as much as possible and then bend it as much as possible. It hurts when you do it, but helps relieve pain and increase range of motion. For pain relief, by far the best (for me) has been Voltaren Extra strength applied all around the knee. For any sport where my knee might be at risk, I wear a custom brace (a Don Joy). Because of advances in equipment, I feel that I can actually free ski as well as ever on groomed runs, but avoid bumps.
** edit to add for biking : Start off slowly and pay attention to saddle position and height. When you have bad knees even 1/4 inch adjustment can make a big difference. If you feel knee pain stop - usually it is because of incorrect saddle position. Fix this before biking some more (if necessary get a bike fit done). If you are a serious biker with knee problems, also check crank length. I went with shorter cranks last year and it helped eliminate knee pain (pedalling smaller circles helps at both the top and bottom of the pedal stroke).