Anyone familiar with shorter crank arms to help with limited knee range of movement? [View all]

I'd really like to get back into biking but since my last knee operation I'm left with a knee flexion of maybe 90 degrees on a real good day. I'd rather retrofit my Trek 7300 if such parts exist but when I asked at my local bike shop a couple years ago they looked at me like I was from Mars (which I ought to be used to by now, wherever I go...).

I've stumbled across a couple of more technical articles on the web about benefits of different lengths, which tell me that such a mod must be possible, but little information along the lines of "I made this change for my bad knee, it helps a lot, and this is where I got the parts."
So if anyone here has any advice I'd love to hear it.

Thanks!
GNC