Best of luck to you man. I just had surgery and went through therapy in January for torn meniscus cartilage in my right knee. The best advice I think I can give everyone out there is to avoid shitty exercises like leg extensions. They are tough on the joints. Stick with squats and leg presses and some hacks.
After my surgery and rehab I could tell just how tough extensions are on the knee joints, due to my right knee being more sensitive to awkward movements. My tear was incurred at work, however I stay away from leg extensions now and stick with squats, presses and hacks. I only use a light super slow movement on extensions to warm the knees up.
I know you can come back from this man. Just dig deep and use all the will and fight you put into your workouts to overcome this obstacle. I am now stronger than ever in the squat and my injury is what made me determined to get to where I am now. Granted my squat is not ass to the floor anymore (I squat at parallel) but I still am concentrating on dealing with my limitation there but putting up bigger and bigger weights. Good luck to you man !!!!