Do craniotomy screws show on metal detectors at cruise ship ports?

My son had a TBI (traumatic Brain Injury) 2 years ago. They performed a craniotomy (removed a portion of his skull and replaced it with screws 7 weeks later-once the swelling had subsided). We are going on a cruise in October. I was wondering if the metal detectors at the port will detect the hardware and if so do I need to get some paperwork from his physician for this? I have an artificial knee and a cage in my back from spinal fusion. Normal detectors do not detect either, but the scanners at airports do detect my artificial knee, I have a card from my orthopedic surgeon that I carry. Since we aren't flying, I don't have to worry about the airport scanners/metal detectors. Any info would be appreciated.