The science behind elastics

[Kipepeo]

So I'm wondering, if the ortho wants to pull one or two teeth down and decides to use elastics, how are the bottom teeth that are anchors for the elastics prevented from coming up? How do the teeth "know" which ones are supposed to be following the elastics?

[compac22]

I have been thinking exactly the same thing. In my first month and havent got elastics yet... Im thinking that the sooner i get them the sooner ill have the braces off.

[Marigold]

I've been wearing elastics for over a year now and I'm still confused about them. My understanding is that they're used to align the bite, and that bringing a tooth down is done by bending a wire. But that's only based on my personal experience, so perhaps they have other purposes. I do know this - my upper canines where the hooks are attached look longer now, and my bite hits there first. So the elastics do seem to be pulling them down. I told my ortho about it and he said it was because of the elastics, so I take that to mean it's an expected side effect (for lack of a better word). How he plans to correct this remains to be seen.

[orangeracer]

How do the teeth "know" which ones are supposed to be following the elastics?

They don't. The elastics pull from each attachment point with the same force and all the teeth involved will move to some degree. The trick is in the anchorage of the teeth, how many roots they have and how much space there is to move. Teeth with less anchorage will move faster and further than teeth with more anchorage. So teeth such as your incisors and canines will move into their desired location before the better rooted and tighter spaced molars can get too far out of whack. After the elastics are done the molars can settle back and the teeth that were supposed to move can be tweaked and fine tuned in their new home.