作者对读者砖头的回答

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 is dependent not only on mechanics per se, but also on the physiologic reaction of the individual patient at the specific sites of force application. Even when we apply a force of the same magnitude in the same direction to the teeth of two patients, experienced orthodontists are not surprised when the two patients respond in different ways.

Two different kinds of space-closing mechanics are used in orthodontics -- sliding mechanics and closing loop mechanics. The MBT system employed 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 (1, 2). 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 reaction to the same mechanics in different 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.

Reference

1.       Andrew T. DiBiase, Inas H. Nasr, Paul Scott, and Martyn T. Cobourne:  Duration of treatment and occlusal outcome using Damon3 self-ligated and conventional orthodontic bracket systems in extraction patients: A prospective randomized clinical trial.  AJODO, 2011,139:e111-e119

2.       Padhraig S. Fleming, Andrew T. DiBiase, and Robert T. Lee: Randomized clinical trial of orthodontic treatment efficiency with self-ligating and conventional fixed orthodontic appliances. AJODO,2010,137:738-742