I've had knee issues for years (torn menisci and arthritis)due most likely to sports injuries. I strongly recommend PT as opposed to surgery-I've had both. In 2013 an MRI showed a big meniscus tear in my rt. knee, but I decided against surgery as I had already lost the other meniscus in that knee and didn't want to lose any more cartilage. I did the excercises religiously and in one month things had turned around to the extent that I was able to finish NH without too much issue a couple months later. The beauty of PT is that when I start to have problems again I start the excercises and so far it has kept straightening things out. I also wear a knee brace when hiking, usually just a neoprene sleeve, but take a heavy duty hinged one with me just in case. I never had ACL issues, but hopefully this info is helpful.
Bottom line-my knee issues have never stopped my AT hiking-I've had to get off and heal on occasion, but only temporarily.