the guidelines by insurance companies, courts, or others. Thismight have caused the original document to be somewhatwatered down. Nonetheless, I strongly support the case madein the editorial. The AAO should get the guidelines out, dustthem off, and rewrite them as necessary for the protection ofthe public and for the good of our specialty.

Thomas J. CangialosiNew York, NY

Am J Orthod Dentofacial Orthop 2007;131:698-90889-5406/$32.00Copyright © 2007 by the American Association of Orthodontists.doi:10.1016/j.ajodo.2007.04.017

Friction might increase anchorageloading

The recent article, ”Friction does not increase anchorageloading” (Southard TE, Marshall SD, Grosland NM. Am JOrthod Dentofacial Orthop 2007;131:412-4), is a much-needed educational study that shows, clearly and concisely,the basic principles of biomechanics. That it was muchneeded, as evidenced by its publication in the highest-rankingorthodontic journal in the world, is unfortunate, because itindicates that our orthodontic educational system is stilllacking in biomechanics, even decades after the concentratedefforts of such pioneers as Mulligan, Burstone, and Melsen(apologies to others not mentioned). I want to congratulate theauthors for the clarity of their explanation, which can leave nodoubt about the validity of their conclusion. Nevertheless, inreal life, matters are never as simple as we would like, andsome additional points need to be made.

The authors assume that friction is equal at the 2 teeth.This might not be the case. Due to tipping of the canine andthe smaller mesiodistal width of the canine bracket, morebinding of the wire might occur there, resulting in increasedfriction or even complete binding. Then the canine will stillbe retracted, but it will drag the whole wire with it, and thewire will slide through the distal end of the molar tube. Theforces will look a little different than those shown by theauthors, because now we should consider our system asconsisting of 2 rather than 3 objects—ie, the canine and thewire as 1 object, and the molar as the other. The conclusionis still the same: both roots ”feel” equal forces.

However, implicit in this discussion and the authors’article is the assumption that the wire is a sectional wirerunning from canine to molar. This is seldom true in slidingmechanics. The wire usually goes from molar to molar. Insuch a case, the wire is not free to slide. Friction at theincisors and the curved shape of the archwire prohibitsignificant sliding. So, if the canine exhibits larger frictionthan the molar, the force of the elastic will be transferred tothe wire and, through the wire, to all anterior teeth. The molarcontinues to ”feel” the 100-g force, but the canine root feelsonly part of this. Posterior anchorage is sure to be compro-mised, and bracket salesmanship is vindicated.

One solution to this situation is to make sure that thefriction at the canine can never get larger than the friction at

the molar. This is easily accomplished by placing an archwirestop just mesial to the molar tube. With a mesial stop, evencomplete binding of the canine will not affect posterioranchorage; the teeth will just remain stationary, even underthe influence of the elastic force. Unfortunately, either amesial stop or relatively greater friction at the molar tube willtransfer the elastic force to the incisors. This has the benefit ofincreasing molar anchorage but at the expense of potentiallabial incisor movement, which must be reversed later bytaxing on additional molar anchorage.

The orthodontic literature is replete with articles studyingfriction between brackets and wires, but basic clinical ques-tions remain unanswered (but at least we are trying). What isworse, basic (should I say elementary?) principles of biome-chanics still elude us. Isn’t it disappointing that, more than100 years after the establishment of this specialty, we stillneed to publish articles such as this?

Demetrios J. HalazonetisKifissia, Greece

Am J Orthod Dentofacial Orthop 2007;131:6990889-5406/$32.00Copyright © 2007 by the American Association of Orthodontists.doi:10.1016/j.ajodo.2007.04.021

Authors’ responseThe basic tenet of our article was that frictional loading

during canine retraction can occur only if the archwire betweenthe canine and the molar is under compression (canine and molarare free to slide along archwire). This compressive force de-creases the retraction load on the canine but, at the same time,decreases the protraction (anchorage) load on the molar.

Several of Dr Halazonetis’s points warrant comment.“The authors assume that friction is equal at the 2 teeth.”

We stated that the frictional load applied by the compressedarchwire at the canine and the molar is the same. In otherwords, even if the coefficient of friction at the caninebracket/archwire differs from the coefficient of friction at themolar bracket/archwire, or even if the canine is tipping morethan the molar, the frictional load applied to the canine and tothe molar by the compressed archwire must be equal inmagnitude and opposite in direction (Newton’s third law).

“ . . . [I]mplicit in this discussion . . . is the assumptionthat the wire is a sectional wire . . . . The wire usually goesfrom molar to molar. . . . [T]he wire is not free to slide.” Weused a simplified system (sectional archwire) to illustrate thebasic concept because more complex orthodontic systemsquickly become indeterminate. With a continuous archwire,during retraction between the canine and the molar, the caninecan still slide distally, and the molar can still slide mesiallyalong the archwire, and our basic tenet still holds.

“Isn’t it disappointing that, more than 100 years after theestablishment of this specialty, we still need to publisharticles such as this?” In fact, our Short Communicationillustrates that our specialty is an exciting and ever-evolvingdiscipline in which a single orthodontist can have a simple

American Journal of Orthodontics and Dentofacial OrthopedicsVolume 131, Number 6

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