I've busted up my legs many a times and use a cane. I've had wooden and metal canes, wooden and metal crutches. Never once have I had a problem flying international and domestic. The worse thing with braces on is that they do the extra screening on you, take my braces off and put it through the scanner, put the cane through the scanner, etc... One leg / knee brace is a warm weather brace that has all exposed metal posts and slats on it which are sharp and could easily be used as a weapon as it can be broken apart easily, but never have I had a problem with it. Last time used was two months ago international and domestic. If you can hobble through the scanner and don't have a metal brace on, then put the cane on the scanner and hobble through if you want to try to avoid the extra services TSA offers.
Any cane or crutch you get from a hospital or medical store will be fine. Unless you get the joke ones with a sword inside or flask inside you'll be fine.
They've only ever questioned me on the medication I had on me, which I had a copy of medical note on me as I use a pill carrier not a prescription bottle. Something about a handfull of pills makes them more worried then a cane or metal brace
One thing you can do... call the airline ahead of time, after surgeries or such when I really can't move much the airlines wheeled me to the security, got me right through it easily (or easier I guess then doing it myself) and to the gate. It's best not to make the situation (injury) worse and use the services they have available. Plus if they know you have a leg injury, if I wasn't already upgraded, I got upgraded or bumped up the stand by list for upgrade or put in a better seat to keep the leg safe.
