Orthodontics and classical Newtonian mechanics both deal with forces, but the targets to which the forces are delivered differ substantially. The specialized mechanics we use in orthodontics is called “biomechanics” because the response to the forces we apply depends not only on mechanics per se, but also on the patients’ physiologic reaction at the specific sites of force application. Even when applying forces of the same magnitude and in the same direction to the teeth of 2 patients, experienced orthodontists are not surprised when the patients respond in different ways.
Two kinds of space-closing mechanics are used in orthodontics—sliding mechanics and closing-loop mechanics. The MBT system in our study uses sliding mechanics, which inevitably involves friction between archwire and slots of brackets when teeth are moved along the archwire. Recently, the use of self-ligating brackets (which claim much less friction than conventional brackets) has also become popular. But thus far, there is insufficient evidence to establish the role of the reduction of friction in accelerating tooth movement during orthodontic treatment. This fact implies further that the biomechanics of orthodontic tooth movement involves much more than classical mechanics alone. Also, there is the question of scale. In classical mechanics, acceleration is usually measured in feet or meters per second squared; in orthodontics, total tooth displacement is measured in millimeters per month.
The fact that orthodontists are coming to a greater realization of the importance of real-world clinical experiments is an important measure of progress in our field. We no longer just follow what we were taught to believe based merely on hearsay or purely “theoretical” grounds. Instead, we are paying more attention to clinical studies that test individual differences in tissue reactions to the same mechanics in various real-world situations. Clinical studies that examine real-world clinical treatment rigorously from different perspectives are difficult to conduct and are highly likely to be imperfect. But we believe that they are also the most promising source of the evidence-based knowledge base of future orthodontics.