I don't understand why sanding teeth is necessary for Invisalign.

Dear Itaylor1234,Thank you for posting your question on Real Self.  Sanding (IPR-Interproximal Reduction) is used in Invisalign treatment and traditional braces to make space to align teeth.  Spacers are placed to create space to allow molar bands to fit and slide over your back teeth.  I hope that this answers your question.Best, Dr. Jenn

In order to make room for the teeth to move (assuming they are crowded over the teeth are overly forward positioned), sanding or Interproximal Reduction (IPR) is needed to create some room to align the teeth. This technique is commonly used in both Invisalign and traditional orthodontic treatment with metal braces. 

This is a common concern for our patients.  So I will answer to the reason for "sanding" or interproximal reduction(IPR) during orthodontic treatment.There are many cases for which we want to use orthodontics to fix the position of teeth.  Often the problems occur because of a lack of space... for example crowding.  To fix the problem we need to create space.Space is created in several ways.  We can push teeth forward, thus opening space, we can sometimes move teeth backwards, we can remove/extract teeth or we can reduce the size of teeth thus making more room.  The same techniques are true when we use traditional braces and when we use Invisalign aligners.  The spacers in traditional orthodontics are used to create space for inserting the rings around the teeth not for resolving problems.  IPR is one of the most conservative ways of managing space.  For example, instead of removing a tooth we can do reduction to 6 lower teeth, the reduction is a total of 0.25mm on either side of a tooth which has on average about 1.0mm of enamel.  As you can see interproximal reduction or "slenderizing" is a useful tooth we have for treating spacing issues.Hope this helps.

The spacers were to make room for the bands to be slipped over your molar teeth.  Sanding (interproximal reduction) is used so that the upper and lower teeth can fit together properly when just a little space is needed to accomplish that -- it is an orthodontic thing, not an invisalign thing.  It is also used to reshape teeth so when straightened there is not a gap at the gumline  like a black triangle where the gums don't fill up that space even though the teeth are touching their neighbors, especially in someone who's gums may have receded a bit.