There are TSA procedures for dealing with just that. Offer to remove the brace if you're comfortable with that as it might make your experience better. Then you'll go through the WTMD with crutches and they'll probably wand you and swab the crutches and you for substances. If they go to swab you be sure to remind the TSO that your leg is extremely tender and is not to have direct contact with the hand as the swab can glide along your clothing/leg.