Basically you've got a chronic problem and are f***, which is pretty obvious don't you think?
In addition to trying leg extensions, see if weighted step-ups are possible. I say this only because exercises that are similar can still sometimes be very different in terms of effect on the muscles and joints. Squats might aggrevate the knee while something similar might not; each variation has to be tried.
Then sample things that aren't related to lifting, all after extensive warmup of course. Climb stairs or hills with some extra weight, or cycle using decent resistance. Or try very very high rep squats only after pre-exhausting with stationary bike or stairs-try using resistance bands instead of weights for the squats, which have a different, smoother feel that can be easier on the joints. With these you might gain something from the extensive pumping, or at least create the impression by increasing definition.
