I overextended my knee, had arthroscopy, how can I improve my knee strength?

Quads/Hamstring. You need to start working on quad/hamstring strengthening. There are many exercises to do this which you can do in the physical therapy, gym or even at home. You can look these up online.
Quads& hamstrings . Isometric quadriceps contractions 10 reps 3 sets 3 times a day takes 5 minutes and is safe. Terminal extension exercises ( squats but only the 1st 30-40 degrees, leg presses only bend 30-40 degrees, knee extension machine, again from full extension to 40 degrees of bend). Hamstring curls but careful on machines that put you in a hyperextended position.