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Friction does not increase anchorage loading Thomas E. Southard,a Steve D. Marshall,b and Nicole M. Groslandc Iowa City, Iowa Conventional wisdom suggests that orthodontists must apply added force to overcome friction during canine retraction (sliding mechanics), the result of which can be increased anchorage loading and anchorage loss. However, for a frictional force to be exerted mesially by the archwire against a canine during retraction, the archwire must be compressed between the canine and the anchor molar, and an equal but opposite force must be applied distally against the molar by the archwire. In other words, the frictional force that reduces the force of retraction on the canine must also reduce the protraction force on the molar. Emphasis on employing reduced-friction (eg, self-ligating) brackets during sliding mechanics to prevent added posterior anchorage loading is unwarranted and based more on bracket salesmanship than on orthodontic biomechanics. (Am J Orthod Dentofacial Orthop 2007;131:412-4)
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commonly used orthodontic technique to close interdental spaces is termed “sliding mechanics,” in which the bracketed tooth, in effect, slide along an archwire. In orthodontic cases including maxillary first premolar extractions, a distally directed force is typically applied against the maxillary canines to move them distally along the archwire, and a reciprocal, mesially directed force, is applied against the posterior anchor teeth. During sliding mechanics, a frictional resistance results between the bracket and the archwire. This frictional force acts in a direction tangential to the plane of contact between the bracket and the archwire, opposes the sliding motion of the tooth along the archwire, and is proportional to the normal force transmitted across the plane of contact.1 Many articles in the orthodontic literature have dealt with friction and explored various factors that can affect frictional resistance during sliding mechanics. These factors include bracket slot width, bracket composition, wire size, wire shape, wire composition, wire-to-slot ligation method, bracket/wire surface conditions, interbracket distance, saliva, and relative interface motion between bracket and archwire.2-9 Recently, increasing emphasis has been placed on the design of brackets to decrease friction. Redlich et al10 evaluated the static friction force created between From the University of Iowa, Iowa City. a Professor and head, Department of Orthodontics. b Adjunct associate professor, Department of Orthodontics. c Assistant professor, Department of Biomedical Engineering. Reprint requests to: Thomas E. Southard, Department of Orthodontics, College of Dentistry, University of Iowa, Iowa City, IA 52246; e-mail,
[email protected]. Submitted, June 2006; revised and accepted, September 2006. 0889-5406/$32.00 Copyright © 2007 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2006.09.037
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archwires and reduced-friction brackets during sliding mechanics. They demonstrated significant differences among the groups of brackets tested and concluded that not all brackets provided reduced friction, even though the manufacturers describe them as doing so. Cacciafesta et al11 measured the frictional resistance generated between stainless steel self-ligating brackets, polycarbonate self-ligating brackets, and conventional stainless steel brackets using 3 orthodontic wire alloys. They reported that stainless steel self-ligating brackets generated significantly lower frictional forces and betatitanium archwires had higher frictional resistances than stainless steel or nickel-titanium archwires. Tecco et al12 reported similar findings. Why are orthodontists concerned with friction during sliding mechanics? Conventional wisdom states that an orthodontist must apply added force to overcome friction, the result of which can be increased anchorage loading and subsequent anchorage loss (Fig 1).7,9,13,14 In other words, for maxillary canine distal retraction through a first premolar extraction space, not only must a reciprocal force be applied between the canine and the anchor molar, but also an additional load must be applied by the anchor molar to overcome archwire friction as the canine moves distally. This additional load against the anchor molar can increase anchorage loss. This concept has motivated our specialty to seek techniques to reduce friction and, consequently, reduce the potential for increased anchorage loss.9 But is it true? Does archwire/bracket friction increase anchorage loading during sliding mechanics? Let us explore this concept carefully. First, examine a frictionless orthodontic system shown in Figure 2. Here, a 100-g elastic force is being applied between the canine and the molar brackets. The force is equal, but
American Journal of Orthodontics and Dentofacial Orthopedics Volume 131, Number 3
Fig 1. To retract a canine by sliding it along an archwire (in this case with 100-g distal force), conventional wisdom dictates that additional force, beyond what is required to move the tooth, is necessary to overcome friction (in this case also assumed to be 100 g). Some authors suggest that additional frictional force increases loading on anchor molar to value equal to canine retraction force plus frictional force (in this case to 200 g) and, consequently, increases molar anchorage loss.7,9,13,14
Fig 2. In a frictionless orthodontic system, in which equal-but-opposite 100-g elastic force is applied between canine and molar brackets, entire 100-g elastic force is transmitted directly to roots of both teeth. Furthermore, teeth can slide freely along the archwire, and the archwire is not compressed.
opposite, at the canine and the molar. Because there is no friction, the entire 100-g force being applied to the bracket of both teeth is transmitted directly to the roots of the teeth. In an orthodontic system with friction (Fig 3), the molar and the canine are still free to translate along the archwire, but part of the 100-g retraction force applied against the canine is reduced by the frictional force; less force is available for retracting the canine root. If one assumes a frictional force of 20 g, then the retraction force applied to the canine root is reduced from 100 to 80 g. Now, for this 20-g frictional force to be applied
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Fig 3. In this simplified diagram illustrating an orthodontic system with friction (frictional force between canine bracket and archwire assumed to be 20 g), the archwire between canine and molar is under compression; ie, for 20-g frictional force to be applied by the archwire against canine, equal but opposite force must be applied distally against the molar by compressed archwire. Frictional force that reduces force of retraction on the canine root to 80 g must also reduce force of protraction on the molar root to 80 g.
mesially against the canine, there must be an equal, but opposite, 20-g force applied distally against the molar. Explained from a different perspective, for the archwire to exert a 20-g mesial force against the canine, the archwire must be under compression between the canine and the molar, and the compressed archwire must push distally an equal amount against the molar. Simply put, if a frictional force opposes the distal slide of the canine, it will reduce the force applied to the roots of both the canine and the molar by an equal amount (in this case by 20 g). Next, assume that the orthodontist is not content with having 80 g of retraction force applied against the canine root. So he or she increases the elastic force applied at the bracket of the canine and the molar from 100 to 120 g (Fig 4). In this case, assuming that the frictional force remains at 20 g, the retraction force applied against the root of the canine (and the protraction force applied against the root of the molar) increases back to the 100-g force initially found in the frictionless orthodontic system (Fig 1). Finite element analysis confirmed these conclusions. A simplified 3-dimensional model was developed with an archwire sliding freely between brackets on a canine and a molar. A 100-g load was applied between the teeth. With wire/bracket friction values of 0 to 0.5, the resulting forces acting against the canine and the molar roots were found to be equal.
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Fig 4. Increasing the elastic force applied against the brackets of a canine and a molar to overcome friction and restore retraction force against root of a canine to 100 g concomitantly increases protraction force against molar anchor to 100 g. In other words, increasing interbracket elastic force to 120 g to overcome frictional force of 20 g essentially returns force system against roots back to original frictionless system.
What does this mean for the practicing orthodontist? Emphasis on using reduced-friction (eg, selfligating) brackets during sliding mechanics to help preserve posterior anchorage is unwarranted and based more on bracket salesmanship than on orthodontic biomechanics. Was conventional wisdom true? Does friction between brackets and archwires result in increased anchorage loading during sliding mechanics? No. If the teeth are free to slide along the archwire, friction between brackets and archwires does not increase anchorage loading. REFERENCES 1. Shames I. Engineering mechanics. Vol. 1. 2nd ed. Englewood Cliffs, NJ: Prentice-Hall; 1966.
American Journal of Orthodontics and Dentofacial Orthopedics March 2007
2. Frank C, Nikolai R. A comparative study of frictional resistances between orthodontic bracket and arch wire. Am J Orthod 1980;78: 593-609. 3. Kusy R, Whitley J, Mayhew M, Buckthal J. Surface roughness of orthodontic arch wire via laser spectroscopy. Angle Orthod 1988;58:33-45. 4. Drescher D, Bourauel C, Schumacher HA. Frictional forces between brackets and arch wire. Am J Orthod Dentofacial Orthop 1989;96:397-404. 5. Kapila S, Angolkar P, Duncanson M, Nanda R. Evaluation of friction between edgewise stainless steel brackets and orthodontic wires of four alloys. Am J Orthod Dentofacial Orthop 1990;98:117-26. 6. Yamaguchi K, Nanda R, Morimoto N, Yoshihito O. A study of force application, amount of retarding force, and bracket width in sliding mechanics. Am J Orthod Dentofacial Orthop 1996;109: 50-6. 7. Braun S, Bluestein M, Moore K, Benson G. Friction in perspective. Am J Orthod Dentofacial Orthop 1999;115:619-27. 8. Loftus B, Årtun J, Nichollis J, Alonzo T, Stoner J. Evaluation of friction during sliding tooth movement in various bracket-arch wire combinations. Am J Orthod Dentofacial Orthop 1999;116: 336-45. 9. Taylor N, Ison K. Frictional resistance between orthodontic brackets and archwires in the buccal segments. Angle Orthod 1996;66:215-21. 10. Redlich M, Mayer Y, Harari D, Lewinstein I. In vitro study of frictional forces during sliding mechanics of “reduced-friction” brackets. Am J Orthod Dentofacial Orthop 2003;124:69-73. 11. Cacciafesta V, Sfondrini F, Ricciardi A, Scribante A, Klersy C, Auricchio F. Evaluation of friction of stainless steel and esthetic self-ligating brackets in various bracket-archwire combinations. Am J Orthod Dentofacial Orthop 2003;124: 395-402. 12. Tecco S, Festa F, Caputi S, Traini T, Di Iorio D, D’Attilio M. Friction of conventional and self-ligating brackets using a 10 bracket model. Angle Orthod 2005;75:1041-5. 13. Articolo L, Kusy R. Influence of angulation on the resistance of sliding in fixed appliances. Am J Orthod Dentofacial Orthop 1999;115:39-51. 14. Proffit W, Fields H, Ackerman J, Bailey L, Tulloch J. Biomechanics and mechanics. In: Contemporary orthodontics. 3rd ed. St Louis: Mosby; 2000. p. 346-7.