Lessee, arthroscopy on left knee (torn medial meniscus, torn anterior cruciate ligament). Right knee 'goes out' if I do anything that involves torque under load.
After almost a year of fixed gear riding 46x17 in a somewhat hilly part of SoCal, knees have never felt better.
Advice here is good... take the ratio down low enough to be comfortable, ramp it up later as you get stronger. Keep both brakes on the bike and use them to stop, since it's much more stress to try to pull against the pedals (someone quoted 3x on these forums before). Really, really work the 'fit' on your bike until it feels like an extension of the body. Just try it for a while, maybe it'll work for you... if not, hopefully the bike you get can be converted into a coasting single-speed.