READERS' FORUM

Letter to the editor*

The apple of Sir Isaac Newton

The recent article by Xu et al1 that examined the ef-fectiveness of the popular straight-wire appliance

system (SWA) for space closure after tooth extractionis very important. Surveys suggest that the SWA sys-tem is the favored appliance among orthodontists,and its evaluation in clinical studies is needed.2,3

Our own recent surveys (unpublished) confirmed thatthe SWA is the dominating appliance system. In theRepublic of Macedonia (37 of 40 orthodontists,98%), the SWA is routinely used by 34 orthodontists(95%). The MBT bracket prescription system,4 basedon the SWA, is routinely used by 7 orthodontists. Sim-ilarly in Serbia (57 of 120 orthodontists, 47.5%), theSWA is routinely applied by 56 orthodontists (99%).The excellent study of Xu et al is complex and couldtake us astray; some minute explanations might beneeded.

There are 2 basic biomechanical strategies for spaceclosure: frictional and frictionless. The MBT systemapplies frictional mechanics. Therefore, it is probablymore accurate to state that frictional mechanics wasused, whereby retraction en masse vs 2-stage retrac-tion were compared. Incidentally, the important vari-ables influencing force systems that could in turninfluence tooth movement during space closure werenot defined (point of force application, direction, andmagnitude). When subjected to a given force system,a tooth is set in motion and is subject to the laws ofmechanics and is probably much less dependent onsome biologic traits. Describing the force systems isprobably essential.

We might imagine that Sir Isaac Newton would beimpressed to hear that the gravitational force actingon an apple falling from a tree depends more on theapple’s color or age of the apple tree than, how simple,the apple’s mass. The differences that were found inthis study between sexes and age groups make us believethat tooth movements depend on sex hormones and age.However, to prove this, the most important preconditionwould be that the mechanical factors are preciselycontrolled. Another argument in favor of the precisemeasurements would be that, if in clinical studies theforce systems of the appliances that are used are not

*The viewpoints expressed are solely those of the author(s) and do not reflectthose of the editor(s), publisher(s), or Association.

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defined, the results of the studies would point outmore what we believe than what is the case.

Alexander SpassovDragan Pavlovic

Greifswald, Germany

Am J Orthod Dentofacial Orthop 2011;139:7140889-5406/$36.00Copyright � 2011 by the American Association of Orthodontists.doi:10.1016/j.ajodo.2011.04.004

REFERENCES

1. Xu TM, Zhang X, Oh HS, Boyd RL, Korn EL, Baumrind S. Randomizedclinical trial comparing control ofmaxillary anchoragewith 2 retractiontechniques. Am J Orthod Dentofacial Orthop 2010;138:544:e1-9.

2. Banks P, Elton V, Jones Y, Rice P, Derwent S, Odondi L. The use offixed appliances in the UK: a survey of specialist orthodontists.J Orthod 2010;37:43-55.

3. Keim RG, Gottlieb EL, Nelson AH, Vogels DS 3rd. 2008 JCO study oforthodontic diagnosis and treatment procedures, part 1: results andtrends. J Clin Orthod 2008;42:625-40.

4. Bennett JC, McLaughlin RP. Controlled space closure with a pread-justed appliance system. J Clin Orthod 1990;24:251-60.

Authors’ response

Orthodontics and classical Newtonian mechanicsboth deal with forces, but the targets to which the

forces are delivered differ substantially. The specializedmechanics we use in orthodontics is called “biomecha-nics” because the response to the forces we applydepends not only on mechanics per se, but also on thepatients’ physiologic reaction at the specific sites of forceapplication. Even when applying forces of the samemagnitude and in the same direction to the teeth of 2patients, experienced orthodontists are not surprisedwhen the patients respond in different ways.

Two kinds of space-closing mechanics are usedin orthodontics—sliding mechanics and closing-loopmechanics. The MBT system in our study uses slidingmechanics, which inevitably involves friction betweenarchwire and slots of brackets when teeth are movedalong the archwire. Recently, the use of self-ligatingbrackets (which claim much less friction than conven-tional brackets) has also become popular. But thus far,there is insufficient evidence to establish the role ofthe reduction of friction in accelerating tooth movementduring orthodontic treatment.1,2 This fact implies fur-ther that the biomechanics of orthodontic tooth move-ment involves much more than classical mechanicsalone. Also, there is the question of scale. In classical me-chanics, acceleration is usually measured in feet or