How Much Torque Can Aligners Produce? —A Biomechanics Point of View

It seems that the worldwide craving for invisible orthodontic treatment is so intense that as soon as the first clear aligner’s system was unveiled, the foundations of the profession had been shaken. Orthodontists were “hypnotized” by novel programs outcome depicting 3D crowns’ illustrations, moving teeth from the “malocclusion” to the “dreamocclusion.” It did not just look like a miracle. It was the first breakthrough system that placed the profession in the new world of high technology. The hype that followed the buzz was so intense that even experts in biomechanics, the core of the profession that is based on physics laws, remained silent. It cannot be denied that the “in silico” effect (experiment performed on computer or via computer simulation and seen on the screen) was and still is so persuasive that it affected each one of us. We were absolutely sure that all our problems in moving teeth were solved. This visual illustration, that later was transferred to future patients, achieved its goal by dispelling all doubts. Therefore, it is not surprising that the first ones to embrace and aggressively promote the new systems as a potential invisible or transparent orthodontic tool to solve all malocclusions, before there was one aligners’ related evidence based dentistry (EBD) study published in the reputed professional literature, were orthodontists. Even now, 20 years later, while the number of the companies offering full aligners’ treatment and the growth in the number of treated patients have burgeoned, yet there is hardly any EBD study published in the reputed professional literature which can demonstrate and prove, the clear aligner ability to move teeth in all needed directions.

During time, reality slapped on our face since the above described, so-called “mind magic” effect lasted shortly. It was found that some movements were fully executable; however, some were limited or even could not be fulfilled. There even were cases that demonstrated severe side effects such as serious intrusions that needed compound fixed appliances care to recover it. Real-life proved that moving teeth by aligners only, was not always as portrayed on the screen; however, the profession still embraces these over 20 years oldie emerging technologies, and those faults were and are still related to the innovative or novel characteristics of the aligners.

Since the laws of physics are eternal, no magic can be embedded in any orthodontic appliance that can change it. Therefore, we can say that the main orthodontic movements aligners can produce are those defined as single point application movements, and in orthodontic terms, these are the uncontrolled tipping. True, pure rotations, intrusion, and extrusions can be detected only when the parallelogram or the net force happens to act exactly along the vertical axis of the tooth (for extrusion or intrusion) or around it (for rotation). Those movements if occur, may last for very short time only, and therefore are very limited. ¹

Torque and torque derivative movements (bodily, root movements or controlled tipping for example) are the outcome of a couple (a pair of opposite forces in different parallel lines of action, whose vectors are equal in magnitude), or a couple in conjunction with force, respectively, and are considered to be more complicated orthodontic movements that enroll at least two points force application, where there is a firm and long lasting ’fusion’ between the moving and the driving parts. The couple, placed in the bracket, is the only system that can rotate the affected teeth, around an axis developed in the mid-bracket itself, and not at the center of resistance of the tooth, as other movements do. This movement is unique to fixed appliances and not to removable appliances including the aligners. Therefore, those movement cannot be accomplished by removable aligners, where the teeth are free to move in space, since the appliance is removable. Let us assume that we can develop a couple on a tooth crown surface by using aligners (with for example attachments and/or power ridges). This couple, if exists, may be present only momentarily, immediately following the wear of the aligner. Its immediate effect on the crown changes the spatial position of the tooth relative to the couple’s two points of application, and the pre-engineered program embedded in the aligner loses its “couple” qualities into a single point force application (tipping force). The removable plastic aligner cannot “trace” or follow the moving tooth, and therefore no persistent couple continuation can be delivered on the teeth. This cannot happen when fixed orthodontic appliance is designated to deliver torque. The spatial torque movement of the tooth that are intimately and forcefully connected to its neighboring members, namely the anchorage unit, will continue as long as the wires can deliver the assigned forces.

Although those rules are clear, the attempt to relate the torque movement as initially defined by the profession and later by Andrews, as well as torque derivative movements, to the aligners is continuing endless. Even lately, the AJODO ² published an article where the authors were trying to convince the readers that aligners can produce torque and other two to three point application movements; however, even they cannot point or depict how and when the physical system that might have produced a couple, has occurred. Hiding behind sentences such as “the way torque is produced by aligners is the outcome of massive data, algorithms and artificial intelligence, and these data are the property that belong to the manufacturer and are beyond the understanding of human beings,” are far from being scientific. The “black box” remains as a “black box.” ³ This paragraph can be summarized by a quotation from A.C. Clarke sayings: “Any sufficiently advanced technology is indistinguishable from magic.” We believe that a scientific publication or EBD studies, cannot be published unless it can explain the way it came to its conclusions in order to become repeatable.

In 2002 letter to the editor, Ackerman and Proffit, ⁴ wrote, without giving any references: “This devices have now limited capability and efficacy compared to the fixed appliances.” In different world they both could have become the child in the Hans Christian Andersen’s book the Emperor’s New Clothes. Although reality proved that they were right, but unlike the legend, the crowd, namely the profession, had decided out loud, to remain blind or almost blind. Actually, since 2002 almost nothing had changed in the biomechanics of the aligners’ ability to move teeth, and it seems to remain as is.^{1, 5}

Among the questions that might be raised like how come an orthodontist and the profession are ready to abandon its scientific assets or properties acquired over more than a century, in clinical work and in-depth research, to limited mechanical systems, or how come the profession became, so called, more “forgiving” in the treatments’ outcome? We definitely know that not all movements can be fully accomplished as predicted by the program; the alignment is not the expected alignment; the occlusion is not the occlusion we strived to; the level is not the anticipated one; the curve of Spee remains many times deep as it initially was; the teeth, especially the canines are not fully rotated; the parallelism of neighboring to extraction space teeth does not exist, and the amount of the enamel loss, due to interproximal reduction that sometimes it is needed but in many times it is due to interproximal “collisions” and friction which make tooth movement with aligners impossible, changes teeth morphology and the dentition perimeter. It further seems that finishing many cases with aligners only, according to the traditional goals, became very complicated very challenging, if not impossible to be executed. The incorporation of, for example, intermaxillary elastics, braces, torquing springs TADS and more, in the treatment process, clearly proves the weakness of the aligners by themselves, to perform the full treatment without assistance from elements of the fixed appliances systems world. Further, the need sometimes, for endless refinements’ procedures, is the winning proof of the inability of the aligners to perform all movement as fixed appliances do. Had the profession, in the last 20 years, changed its goals? And what is the patients and future patients effect on the profession in this matter?

The answers to those questions are beyond the scope of this letter, that might serve as an opening point to studies that have the potential to further enlighten the questions the profession did not answer yet, those that are related to the reaction of orthodontists as well as patients to the current use of aligners in orthodontics.