I've got a bad knee myself. Initially just tore the ACL in '98 and had it repaired surgically, but have re-injured the ACL repeatedly since then in off-bike endeavors and now have chondromalacia patella, a loss of cartilage on the inside of the kneecap, due to the altered movement. I did some research on it and talked to some PTs, and while mashing will mess up your knee more, spinning at a lower ratio can strengthen the muscles around your knee reinforcing the structure. I'm pretty happy on my fixie, and ride 39x16. I don't go quite as fast as other people, but I get a good enough speed and my knee feels great.