Is Invisalign a Risk for Root Resorption on a Tooth That Has Been Knocked out and Had a Root Canal?

Make sure the treating dentist is aware of the history of your tooth. He should discuss the details with the Doc that did the Root Canal when the tooth was originally knocked out. Then, any orthodontic movement should be slow and light force.

A root canal tooth should move and behave exactly like a normal tooth. I do have a concern that the tooth still feels loose...after a month or so it should have returned to normal...of course every tooth shows a little bit of movement if you try to move it...are you sure that the movement is worse than the other teeth?

If the tooth root anatomy is normal then the only concern is whether or not the periodontal ligament around the root of the tooth is normal.  The periodontal ligament is the tissue space between the root of the tooth and the bone which supports the tooth.  This space contains blood vessels, nerve fibers and connective tissue attachment fibers.  The concern would be a connection or "ankylosis" of the tooth to the bone.  This could be in a small area or over the entire root of the tooth. The health of this space (periodontal ligament) can be determined by x-ray, percussion, and testing for slight mobility.  Assuming these all indicate a healthy periodontal ligament there is no concern for moving the tooth using Invisalign.  In any case Invisalign is far superior to traditional Orthodontics in safely moving teeth and avoiding root resorption.