I have a brand new knee replacement (this year). When I travel, I notice some airport detectors go off and others don't. Why is that?

Many people have reported differences not only from airport to airport but even from time to time through the same security detector. To solve this puzzle, Dr. Robert F. Ostrum, Chief Orthopaedic Surgeon at Cooper University Hospital in Camden, New Jersey did a little research on the topic.

He found that airport metal detectors generate a brief magnetic field. When a metal object passes through the detector, the magnetic field is reversed and a sharp electrical spike sets off the alarm. Many other factors come into play as well.

For example, the more metal you have (say from multiple implants) or the larger the pieces (mass), the more likely it is that the detector will sound the alarm. The type of metal can also make a difference. Anything with iron in it or other type of metal that can be magnetized increases your chances of detection.

Detection units can also be set for higher sensitivity. So on high alert days, you are more likely to set off the alarm when you walk through the archway compared to a low-alert status.

Some of the walk-through archways only have a detection device on one side. If your implant is on the opposite side, you are less likely to set off the alarm. The handheld wand detectors are more likely to detect metal implants but these are not used routinely.