Yesterday, I took my first trip through an airport since knee-day. And yes, the replacement sets off the alarms.
I live in a small college town, with a relatively small airport. It was an early flight --- 0530 AM --- and I was not yet caffeinated, and only marginally awake. Nonetheless, I walked through the magnetic gate THREE TIMES, taking off more and more things each time, before I remembered that I HAD A KNEE REPLACEMENT LAST FALL. This recognition --- that I am so comfortable with the new knee that I did not even remember that it was there --- made me chuckle all day long.
Did I carry my knee-replacement card? No. But I did ask the question "Would I have to go through this if I had a card from my doctor?" The answer was "Yes". You set off the alarm, you get patted-down and wanded. It adds all of about 3 minutes to the process, and is not at all invasive. At both the small and large airports, the process is the same. Sit down. Wand each leg. Stand up, feet spread, arms raised, palms up. Wanded again all around. Repeat over the areas which "beep". Pat down those areas which "beep". The TSA employees I encountered were gentle and polite.
Be aware that your "stuff" sits in the bins on the conveyor belt, waiting for you. If you are traveling alone, you might want to keep an eye on your things. If you are traveling with a companion (and a companion who does not need to get patted down at each security checkpoint), I'd recommend that your companion watch your coat / shoes / bags / bins for you.
The other revelation of my first major travel day post-knee-day was that for the first time in years, I could walk comfortably from one end of the airport to the other, as fast as I wanted to walk, and without pain. In comparison, during one of my last business trips pre-knee-day, I had to walk 5 minutes, and then rest for 5 minutes, because my knee would no longer support me. I stubbornly refused the courtesy carts, but was to the point where I was seriously considering them.