How come vp shunts don't set off the metal detectors at airports? will titanium plates/screws set them off? does it depend on the size of the metal?

Not iron: Vp shunts, although visible on x-ray, are not metallic and will not set off a magnetometer at the airport. Similarly, titanium is not ferromagnetic and does not set off these metal detectors as strongly as iron does. Some titanium implants are iron alloys and react more strongly in the detectors. And yes, the size of any implant makes a big difference in how much it sets off the detector.