I carefully read the article “Forces and moments generated by removable thermoplastic aligners: Incisor torque, premolar derotation, and molar distalization” (Simon M, Keilig L, Schwarze J, Jung BA, Bourauel C. Am J Orthod Dentofacial Orthop 2014;145:728-36), and I believe that there might be some misunderstanding or probably misuse with the different interpretations of torque and moment terminology. The term torque in orthodontics was defined in 1959 by Rauch, who described it as “a moment generated by the torsion of a rectangular wire in the bracket slot.” A simpler definition defines torque as movement of the root either buccally or lingually (palatinally). The term moment in orthodontics is the resultant force applied to the bracket multiplied by the distance between the point of force application (center of the bracket) and the center of resistance of the tooth. Since, in most force applications, we cannot place the force directly on the center of resistance, moment is almost always developed. Therefore, I believe that in this article the use of “incisor torque” means “incisor moment.”
It is known that to move the root of a maxillary incisor in a palatal or a labial direction with an edgewise appliance that is bonded to the crown of the tooth without moving the area of the center of the bracket, which plays as the center of rotation, we have to develop a couple in the bracket itself. A couple is a system of forces with resultant moment but not resultant force. In orthodontics, it is generated by 2 equal forces placed in equal distance from the center of resistance of a body. Since the “arm” of the bracket is about 0.5 mm, for example, to receive a moment of 200 g-mm (∼2 N-mm) from a couple, about 400 g (∼4 N) of force is needed; if a moment of 750 g-mm is needed (eg, to nullify the moment developed by 150 g of force to drive the tooth with 10 mm distance from the center of the bracket to the center of resistance of the tooth bodily), 1500 g (∼15 N) of force is needed to be placed inside the miniature slot of the bracket. Those forces or forces close to them must remain there constantly without decay to keep the pure torque/bodily movements.
Several major parameters are missing in removable thermoplastic aligner (RTA) systems: eg, the teeth are “floating” in the appliances, and therefore currently there is no way to develop a real orthodontic torque (as it defined) in the system. In addition, the amount of force the RTA can develop is limited due to the materials it is built from. The force developed at the edge of the teeth is much higher than that developed close to the gingiva; therefore, no real couple can be developed, and as a result no orthodontic torque (again, as it is defined in the professional literature) can be a part of the force system of the RTA.
For many years, it has been well known that the differences between in-vitro systems—as sophisticated, analytical, and intelligent as one may imagine—and real-life in-vivo scenarios raise many questions. In this case, the answer to one of them is probably the definition of torque vs moment. Finally, The RTA system is a popular, useful, and effective treatment module for many reasons; however, it has several limitations, and, as for now, orthodontic torque is one of them.