Permanent loss of root structure related to orthodontic treatment occurs primarily at the:
It is estimated that root resorption occurs in at least 12% of all orthodontic patients presently being treated by competent orthodontists. 
The apex of the root is more susceptible than the other parts. 
Deviating root form is more susceptible to post orthodontic root resorption. Convergent apical root canal is considered to be an indication of high root resorption potential. 
The degree of root resorption in teeth with blunt or pipette-shaped roots was significantly higher than in teeth with normal root form. 
The pipette-shaped root was shown to be the most susceptible root form to root resorption.

​Textbook of ORTHODONTICS Sridhar Premkumar
Coronal third
Middle third
At apex
All of the above