What did he do to his teeth?

I didn't notice that, I just think his over all health has made everything look a little different.

He had some work done in 2009 before filming The Back Up Plan but nothing recently as far as I can tell. It looks liek he had veneers put on before he did The Back Up Plan. Looking at screencaps of recent episodes (post 2.16), it kind of looks like Alex may have chipped one of his front teeth or one of his veneers fell off or chipped/broke. His teeth look more uneven than they did before 2.16. To me it looks like he chipped a tooth and hasn't had it fixed yet.

I hadn't noticed, but now it makes me want to go back and check

On the other hand, in the episode where they were at the spiffy function with the Governor, and Steve went over the balcony to save the guy, didn't Steve tell Danny that he was fine except for a chipped tooth? I was wondering about that at the time, why they would say Steve chipped his tooth if it didn't really happen. It's not like I expected anyone to put a fake chipped tooth on Steve, so maybe he chipped it during filming and they decided to write it in to the script?

That's what I was thinking too. A chipped tooth is kind of a random pick for Steve's injuries in the car accident and it would take a lot of work for makeup to make his tooth look like that. Alex probably chipped a tooth noticeably so the wrote it in.

I spend all day looking at other peoples teeth as a dental hygienist so I notice little things like all the time lol.

That's right, the car accident, I had the right episode but wrong scene. I just remember Steve mentioning the chipped tooth and ribs to Danny, and that was an episode where Steve was injured twice, more or less. Although I'm going to assume that it was a stuntman really hitting the windshield of that car :-)