i had the pleasure of travelling with the "frankenstein boot (ankle surgery & mine up to just below the knee) and since the metal buckles set off the wtmd.....

i would tell them that i can take it off so they can x-ray it but i have to sit while they do it. i was wanded and patted down-waist down while sitting with me pulling up my pant leg so they didn't have to touch my ankle and from the waist up while standing after i put the boot back on.

shouldn't be much of a delay and it might even get you to the front of the line and if they have the puffer, i'd opt for that as well but either way you should be patted down as above (or at least request the "sitting and standing" pat down if they tell you as it sure made it easier for me)

lastly, the airports i used while booted were sfo, ord, bur, lax iad & pbi