As someone who's had knee pain and known the definition of "arthroscopic" since before puberty, I have never incorporated squats into a routine. I did hack squats as a kid because that's the only thing I could do relatively pain free, and during my most recent lifting phase (prior to my 15 year high school reunion and living on a buoy tender with nothing else to do, what my family called the "what the hell are they feeding him" phase) I was putting up respectable weight on a leg press (as respectable as leg press numbers get), but even then I could never go past 90 without knee pain.
So having spent some time here and on various lifting sites I realized that I had never done a properly executed squat, perhaps because of my reluctance to stick my butt that far out away from the rest of me. After spending a couple hours reading about knees and squats, I became convinced that it was worth a shot. Another half hour or so of reading and videos about technique later, I tried a dozen or so face first against a wall and was surprised by both the awkwardness of the maneuver and at the total absence of pain. Off to the gym at school, and I settled into the squat rack and proceeded to go up the rack from an empty bar all the way to a paltry 155 doing deep squats for the first time in my life, with zero knee pain. I did somewhere north of eight sets, nuking the site from orbit just to be sure. No knee pain during or after.
I have zero reluctance about watching Brianna Trindall stick her butt out.
And truth be told this was actually five days ago, and now that I can move my legs again I've been experimenting and I'm pretty sure that my lifelong knee problems could have been easily solved by just changing habits. If I'm kneeling on the ground with my feet under my butt and stand up by extending my legs with my quadriceps, I get discomfort and crepitus you can hear from across the room. The closer I get to a proper squatting position with the tib/fib close to vertical, the quieter and easier standing up gets. I'm seriously amazed, and a little pissed it took me this long to figure it out.
I also suspect there may be an evolutionary component to this, as I am admittedly inclined to do.
If there's anyone else out there who thinks they can't do squats, I'd encourage them to give at least the first two articles some consideration and re-evaluate.
http://www.bodybuilding.com/fun/drsquat12.htm
http://www.bodybuilding.com/fun/no_pain_squat.htm
http://squatform.com/squat-form-how-to-squat-with-correct-technique/