It is important to remember that Invisalign is just a tool to move teeth the same as braces. The quality of the job is going to depend on the person, his education and his experience. Invisalign will make aligners (trays) to anyone who has taken a two day course regardless of any understaning of orthodontic technique. Let me try and explain by an example.
I had taken diagnostic records for my 15 year old son. I submitted his case with a presciption to Invisalign. The treatment plan I received in return was a treatment that would have taken 32 weeks for my son to complete. I was rated by invisalign as an "easy" case to treat. They would allow a general dentist to treat this case. The only problem the finished case was terrible. It would basically align 6 upper and lower front teeth and the deep bite, excessive overjet and noncoincident midlines would still remain. It took 5 modifications to make the case an acceptable case. The revised case would take76 weeks to treat. Unfortunately the average dentist and the general public would have not known the difference. It is unfortuate that while Invisalign can rate there case as to the ease at wich they can expect a person to do a terrible job, there are no criteria for Inviaslign and their technicians to do the same.
The point I am making is that there are many criteria that would govern the ultimate fee. If fee is your only criteria beware, you will probably get what you pay for.
