I agree you don't have to be upfront about the si or your motivation for not wanting the scalpels there... are they packaged? could you bring up them being sterile/clean if they are left out in the office? My dentist would set up trays needed for the procedure and leave everything out in the room before I got in. I mentioned the fact that they were no longer sterile/clean and they never showed up again until the dentist brought them in with himself... No doctor wants to run the risk of having their office look unclean or unsafe...
Or you can just say they make you uncomfortable... just find a way to ask that they not be there alone with you. And I like Courtney's suggestion of not taking/wearing anything you can slip one out in...
Good luck!