The ‘fender’ problem that makes MY knees (and my arthritic mom’s knees) is riding in a western saddle that does not have turned stirrup leathers.
This is also sometimes called a ‘Hamley twist’ or something similar.
A saddle with a twist in the stirrup leather will have a stirrup that hangs perpendicular to the saddle, so your foot just sets right in it:
http://www.freckerssaddlery.com/the-wade.php
If you look closely, the stirrup leather has been turned around at the Blevins buckle so the stirrup will hang properly.
There’s a closeup of a turned leather at the bottom of this page:
http://www.horsesaddleshop.com/turning-stirrups-western-saddles.html
(As a side note to the previous paragraph/website, I’ve tried a Stirrup Straight and found that they made the stirrup feel really unstable, and the metal dug into my shin. I wouldn’t personally recommend them.)
A saddle without the twist can hang like a Breyer saddle, with the stirrup parallel to, rather than perpendicular to, the saddle:
http://www.identifyyourbreyer.com/images/02018.jpg
Riding in a saddle like this, you have to make a twist in the fender (as you do in an english stirrup leather) to have the stirrup properly supporting across the ball of your foot. In a western saddle with a fairly stiff fender, this will make you put pressure on the stirrup to keep the tread secure across the ball of your foot- and THAT is what causes the knee pain.
You can dampen fenders, and put a broom handle through the stirrups so they get a twist in them, but this doesn’t work nearly as well as
turning the stirrups (which requires removing rivets, turning the blevins buckle over, resetting the rivets, and using a saddle string to put a bend in the end of the leather below the newly replaced rivets.) A saddlemaker can do this for about $40 on most saddles.