[Periodontal limits of orthodontic tooth movement]

It is believed that the limits of orthodontic tooth movement are defined by strict physiologic and anatomic boundaries of periodontal tissue. Excessive tooth movement may result in alveolar bone dehiscence and/or fenestration, or root resorption. Andrews' six elements of orofacial harmony theory may serve as an efficient tool to guide orthodontists to decide proper torque of all tooth, so as to keep the roots from insulting cortical bone. It is worth noting that when palatal expansion is executed, the proper torque control of posterior segments is of importance and, when incisor retraction is executed, the proper torque control is of importance as well. Individualized torque control protocol is needed when such kind of tooth movement is carried out.