Why Would Patients Need to Wear Invisalign Longer Than 2 Years?

Invisalign would take 2 years if more advanced movements were needed. Significant crowding or rotating teeth can require more trays as well as someone who is not wearing them as recommend.  2 years of treatment is longer than average but does occur

 

If teeth are mostly straight, there is not much movement needed.  However, if teeth are very crowded, crooked or a LOT of movement is needed then treatment can last much longer.  Each aligner moves a tooth about 0.2 mm, so if a tooth needed to move 4 mm, then 20 aligners would be needed just for that movement alone.  Sometimes teeth need to be held in place while other teeth move, so more aligners are needed.  Rotations of teeth are unpredictable and can be very slow.  Some cases can last much longer than 2 years, so the 2 years is more of an average.