Only people with heart pacemakers are exempt from the security detection units (walk-through or handheld wand). They are, however, still required to be searched via a pat-down exam.
Carrying a surgeon's certificate, card, or letter no longer gives you special privileges. Since 911, increased airport security requires that all passengers participate in routine airport screening. You may still want to carry this documentation for your own peace of mind.
No harm can be done to your implant from any airport metal detectors. These devices operate on the basis of a pulse induction eddy current generation and electromagnetic field distortion.
There's a coiled wired in the detector. It generates short bursts of current forming a magnetic field used to detect metal. No change occurs to the metal object that is detected. Some types of implants are more likely to set off the alarm. That's because the type of metal or the amount of metal makes it easier to magnetize and set off the alarm.
Talk to your surgeon if you still have any doubts or concerns. He or she can tell you what kind of implant materials you have and how likely they are to set off security alarms.