Interactive edgewise mechanisms: Form and function comparison with conventional edgewise brackets

American Journal of ORTHODONTICS and DENTOFACIAL ORTHOPEDICS Volume 111 Number 2 February 1997

Founded in 1915

Copyright © 1997 by the American Association of Orthodontists

SPECIAL ARTICLE

Interactive edgewise mechanisms: Form and function comparison with conventional edgewise brackets John C. Voudouris, DDS (Hons), DORTH, MSC* Toronto, Ontario, Canada As the frequency of use of the interactive twin (I-twin) edgewise mechanisms and techniques become increasingly prevalent, it is important to consider how they compare with conventional twin (c-twin) edgewise bracket systems. Optimum intrabracket and interbracket forces in I-twins balance with capillary blood pressures. An unbiased, experimental in vitro, scanning electromicroscopy and in vivo clinical investigation of 83 patients was conducted to compare the frictional resistance of three different I-twin bracket systems, type A (Sigma, American Orthodontics), type B (Interactwin, Ormco Corp.), and type C (Damon, A-company) with three c-twins counterparts respectively types D, E, and F. The three interactive twin brackets were each self-seating by an integrated arm component and made significant incremental improvements to the conventional twins in three different ways. First, the I-twin types reduced frictional resistance by using arm engagement with a lower coefficient of friction and a reduced seating force against the arch wire. The reduced seating force friction produced initially small interbracket arch wire deflections for biocompatible tooth movement and, combined with a wide twin bracket, produced accurate rotation corrections. Reduced friction within the I-twin bracket consequently permitted the effective use of light forces for flowing biomechanics that maximized anchorage. In evaluating friction, two distinctly different interaction forces acting to seat the arch wire were also identified. Type A demonstrated active interaction with round arch wires that resulted in a low functional seating force responsible for early and complete tooth control in comparison to the high seating force of c-twins. Types B and C both showed passive interaction with seating force friction approximately equal to zero that required large rectangular dimension arch wires for full bracket expression. Second, a significant reduction in the time taken to change arch wires was found, improving clinical time management. Third, I-twins assisted bracket placement with both center-slot Jdentification markers and coordinated bracket and bonding pad reference planes in three dimensions. In addition, they improved bracket hygiene compared with c-twins by minimizing the use of plaque-retentive conventional ligatures. Despite the reduced elastomer use, experimental and clinical results showed I-twins effectively conserved the traditional four tie-wing design for ease of colored elastomer placement where an increase in friction for anchorage was selectively required and to enhance patient acceptance and motivation. Finally, interactive twins could be used by the practitioner as conventional twins without the interactive arms making them fail-safe. (Am J Orthod Dentofac Orthop 1997;111:119-40.)

This article reflects the current state of interactive technology at the time of publication. For further details on cases presented in this article, please consult with the Interactive Foundation for Research. The Sigma and Interactwin will be available after 1 year; Damon is currently available. From the Graduate Department of Orthodontics, University of Toronto. *Associate in Orthodontics; recipient of the AAO Milo Hellman Research Award, 1989. Reprint requests to: Dr. John C. Voudouris, Suite 707, 2300 Yonge St., Toronto, Ontario M4P lEA, Canada. E-mail: [email protected] Copyright © 1997 by the American Association of Orthodontists. 088%5406/97/$5.00 + 0 8/1/77814

Interactions of a biologic or biomechanical nature caused by the edgewise appliance have been of interest to both orthodontic researchers and practitioners since the time of Edward H. Angle. More highly interactive roles for edgewise brackets are being proposed to marry with the larger advancements in orthodontic arch wires. Today, the changes in both the form and the function of several types of edgewise brackets have relieved them of 119

120

Voudouris

American Journal of Orthodontics and Dentofacial Orthopedics Februaly 1997

Fig. 1. A, Initial interactive edgewise placement 3x, photographic magnification type A,

actively engages 0.014-inch NiTi arch wire with resilient arm without any form of conventional ligation at all canines, first and second premolars (0.022 × 0.028-inch arch wire slot dimension, 5-5). System of fully bonded mechanisms were placed after first molar distalization. Arm accurately locks arch wire in all or none manner. Minuscule functional friction in conjunction with low intrabracket and interbracket arch wire deflections (white arrow) result in active dental autorotations (black arrow). Relatively ligature-free, hygienic brackets are also indicated for gingival and periodontal considerations. B, Perspective of A, 2× magnification. C, 2 x magnification, treatment time elapsed 7 weeks. Second premolar bracket components show wheeling along arch wire-arm complex. Crimpable stop marker tangent to mesiobuccal cusp of right first molar shows free distal flow of arch wire secondarily assisting posterior movement of engaged canines and first premolars. Selective and controlled use of high friction in same system is shown, with figure 8 conventional metal ligatures around four distinct tie-wings of each I-twin is indicated for anterior anchorage. D, Articulated initial study models of boy, age 12 years, with severe Class II, Division 2 skeletal malocclusion characterized by hyperactive neuromusculature and moderate overjet-overbite relationship (postorthodontic records completed).

binding ligatures while uniquely maintaining the majority of the traditional edgewise form. The intrabracket kinetic forces generated from the activated edgewise mechanism provides return communication for an interchange of forces with the arch wire. This has given rise to the use of a highly energized system of miniaturized and independent edgewise machines that use light clinical forces compared with inert conventional edgewise brackets

that require external ligatures. In addition, substantial esthetic, hygienic, and time efficiency improvements have produced direct benefits for the patient and the practitioner by improving compliance and minimizing root resorption and decalcification. The synergistic interchange of biomechanical forces between the interactive edgewise mechanism and highly interactive arch wire appears to have the potential to generate physiologic forces consistently

American Journal of Orthodontics and Dentofacial Orthopedics Volume 111, No. 2

Voudouris

121

Fig. 2. SEMs of type A, active interaction twin sample. A, Active engagement of 0.019 × 0.025-inch STS arch wire with resilient arm for active torque correction. Contact of arm with arch wire is minimal for low functional friction. Closed arm rests in V-shaped deflection area. B, Four tie-wing undercuts are open and accessible for improved cleaning. C, Buccolingually narrow vertical slot for lingual arm component in twin design. D, Vertical component of arm in open position in vertical slot. E, Arm in open position flexed under small ledge near arch wire slot to prevent both inadvertent closure and fall-out. F, Closed arm showing two occlusal guide surfaces resting on two stops of oeclusal wings to prevent reopening. Precision arm extends mesiodistally for rotation control.

and reproducibly. Halderson 1,2 extended the results of Moyers et al. 3 and demonstrated that minuscule forces equal to the capillary blood pressure of 25 gm or 0.24 N could be generated by the edgewise appliance to produce positive responses in the periodontal ligament. Intrabracket and interbracket forces decline generally in magnitude with the progression of corrective dental movements similar to arch wire force diminution. 4 The light intrabracket force seating the arch wire toward the slot base should not ideally approach zero in interactive mechanisms to provide the minuscule functional friction force to maintain (fm) three-dimensional control throughout treatment (see Figs. 1,

A, and 7, B and D). The previously elusive balance between minuscule friction and high control with conventional brackets is a fait accompli with interactives. In addition, a balance is reached between the light forces of the appliance and biologic force. Interactive edgewise mechanisms transduce optimal biomechanical forces that are synchronous with physiologic capillary forces and other interactions, including bioelectric and biochemical. 5 In comparison, a rigid conventional edgewise bracket system requires the arch wire to work relatively alone with heavy conventional ligatures that can amplify mechanical forces in the periodontium unnecessarily.

122

Voudouris

Current interactive mechanisms have b e e n able to combine effectively for the first time the o p t i m u m clinical forces of Burstone 6"7 with the microcirculation studies o f the periodontal ligament of Halderson 1,2 by making use of light and continuous orthodontic forces. T h e biologic interactions begin at the cellular level of the periodontium, however, edgewise mechanisms m a y have positive far-reaching effects at the level of b o n e metabolism, the neuromuscular s and the facial response.

REPRESENTATIVE CLINICAL CASE (Fig. 1) During the early stages of Class II treatment where rotated second premolars are effectively distalized, a unique turbine phenomenon is observed with active interaction. First, the twin mechanism uses the wider bracket body and the associated larger moment of force (compared with narrow single brackets) to assist the turbine effect distally along the arch wire, especially in a ligaturefree environment. Second, the arm also begins to self-seat the arch wire deeper into the slot base, accentuating the movement of the palatal cusp distally (Fig. 1, C). The reciprocating and functional friction of the flexed arm whirls the mechanism effectively in a counterclockwise direction toward the labially deflected arch wire and arm. In relative terms, the base of the arch wire slot moves toward and with the arch wire in a buccat direction. Third, the distal movement of the excess arch wire acts similar to an undulating water current to also move the mechanism distally similar to a turbine, since the mechanism latches on with functional friction. Both the arch wire and flexed arm component can be considered in relative terms the undulating current for the travel path of the bracket component. The premolar distalization by the wide turbine effect and distally moving arch wire is also assisted by transeptal fiber pull that combine to offset the undesirable mesially directed moment of force. This occurs because the distalizing force exerted by the arch wire is labial to the center of resistance tending to move the palatal cusp in an undesirable mesial direction (similar to the initial wheel movement effect of the water). In comparison to I-twins, overcorrected first molars shown in Fig. 1, A and B, upright and recover some of the overrotation C. This is due to the nonseating property of the conventional, convertible molar bracket-tube showing near-zero friction but also minimal early control in ligature-free and passive interaction brackets. Favorable osseous changes have been elicited through the application of minuscule functional forces in histologic investigations of direct resorption. 9'1° In conventional bracket systems, the forces of metal or polyurethane elastomers are also external to the bracket near the arch wire. The heavy external forces used with c-twins over short durations have demonstrated obstructed periodontal circulation, 11 resulting in hyalinization and undermining resorption. In contrast, there has also been some speculation in the past to the contrary that

American Journal of Orthodontics and Dentofacial Orthopedics February 1997

heavy forces may be less damaging to the PDL than light, continuous forces. 12 Friction is highly relevant to orthodontic investigations of clinical anchorage requirements because they impact the treatment effects and the time efficiency of millions of orthodontic patients worldwide on a daily basis. Interactive edgewise mechanisms adopt strongly the results of friction investigation. Currently, the engineering science of friction or tribology (pronounced tri'vo, the Hellenic term for friction) initiated by Leonardo da Vinci, and originally documented by the French physicists Amontons and de Coulomb in the 17th and 18th centuries, respectively, is changing rapidly. The laws of friction were largely derived from dry and often straight-line sliding of materials. Orthodontic research combines the laws of friction with physiology (tribobiology). Static and dynamic friction measurements, require a consideration of the biologic sciences, because they are being applied in a highly individual, wet, rhythmic, and undulating environment during biomechanical tooth movement that flows (rather than sliding mechanics). Anchorage taxation that often occurs because of friction between the triad bracket slot, arch wire, and engagement method has the potential to slow treatment progress (similar to ceramic versus metal brackets). In current Class II treatment mechanics without headgear, interactives have the ability to reduce friction significantly in anterior and middle brackets to prevent anchorage taxation posteriorly by using lighter clinical force from the first molar mechanisms. In addition, the bracket slot mesiodistally has been made convex to further reduce friction in the past. Interactive Siamese twin mechanisms are a lineage of brackets derived from the standard edgewise appliance that have a high frequency of use in the fixed therapeutic modalities of orthodontics and dentofacial orthopedics. The evolution of a self-seating, interactive twin bracket form is the result of the consistent functional demand on conventional twin appliances to combine the clinical improvements of greater freedom for translatory tooth movement with greater bracket hygiene, and greater time efficiency for anchorage-based and flowing biomechanics (Fig. 1). The interactive twin bracket systems (Figs. 2, 3, and 4) are the most recent series of edgewise improvements that originally began with Angle's development of the narrow standard edgewise appliance with two straight tie-wings in 1928.13 Steiner 14 subsequently modified the two tie-wings in the single bracket to include undercuts and Begg 15 incorporated external pins to his ribbon arch appliance to reduce friction. Charles Tweed 16 contributed a wide single bracket for greater rotational moments, and a well-delineated anchorage-based appliance technique to control three orders of movement. The wide single bracket was later transformed by Brainerd Swain to a wide Siamese twin. 17 Further, Andrews' development of the Siamese twin straight-wire appliance is derived from research on nonorthodontic normal patients 19 and Roth's overcorrection refinements 2° for gnathologic, end of treat-

American Journal of Orthodontics and Dentofacial Orthopedics Volume 111,No. 2

Voudouris

123

Fig. 3. SEMs of type B, passive interaction twin sample. A, Passive engagement of 0.019 x 0.025-inch STS arch wire with arm in closed position for passive torque control. Note space above arch wire results in near-zero friction. B, External third layer with beveled occlusal lip located on labial aspect spring-locks arm into body. C, Three layers of arm where two layers wrap around central middle layer. D, Four tie-wing undercuts are accessible for conventional ligature application. E, Compressed labial layer of arm prevents inadvertent closure and lateral extrusions of middle layer prevents arm fall-out when in open position. F, Closed arm extends approximately 1/2 of slot width to produce greater interbracket distance and arch wire flexibility to facilitate arch wire placement. ment goals resulted in widespread use of a preprogrammed twin appliance system. Finally, Rickett's involvement in the development of a visual treatment objective (VTO) that included growth prediction emphasized the importance of maximizing efficiency by relating computerized edgewise appliance design to the fourth dimension--time. Of interest, however, to both researchers and clinicians was one separate biologically based school of thought. It uniquely encapsulated the results of optimum biologic force investigations while also working parallel with edgewise bracket development. Interactive single bracket (I-single) systems used successfully hygienic, nondegrading, and nonbinding arch wire engagement while

they simultaneously maintained a mild seating force on active arch wires for complete tooth control and optimal time efficiency. The sustained I-single advancements developed by Wildman, 22 Hanson, Pletcher, and others 23-27 that were recorded as early as 1935 were clinically significant and occurred in almost the same time frame of development as the single edgewise appliance. The synthesis of the distinct functional characteristics of the interactive single philosophy with the preadjusted Siamese twin form has created the new art and science of interactive Siamese twins. Interactive twin systems can therefore be differentiated from conventional twin systems by their adoption of certain technological criteria that, until recently, were limited to the interactive single

124

Voudouris

American Journal of Orthodontics and Dentofacial Orthopedics Februa~ 1907

Fig. 4. SEMs of type C, passive interaction twin sample. A, Passive engagement of 0.019 × 0.025-inch STS arch wire with arm in closed position for passive torque control. Note space above arch wire results in near-zero friction. B, Labial aspect of arm locks into body with resistance fit. C, Mesial and distal guides of arm component in open position showing wraparound labial aspect of bracket body. D, Four tie-wing undercuts are accessible for conventional ligature application, fi, Open arm position shows two small V-shaped notches in gingival interwing area and two metal protrusions on arm for special instrument application. F, Closed precision arm extends entire width of slot for rotation control. edgewise brackets. As a consequence, I-twins bring incremental improvements to c-twin bracket function. The interactive single bracket progenitors appear to have an international presence such as Edgelok (Ormco), SPEED (Strite Industries Ltd.), Activa (A Company), and, more recently, Time (Adenta GmbH.) (Fig. 5). Interactive twins consist of two main components: (1) an underlying independently functional twin bracketbonding pad component, and (2) an internal engaging component, called a precision arm, as opposed to an external elastomer. The arm component has two functions: (1) to lock the arch wire into the slot by using a low friction interface area with the arch wire, and (2) to interact kinetically with a mild seating force on the arch wire to produce excellent control of rotations and other

first-, second-, and third-order movements (Fig. 2, A). Conventional elastomers produce a form of active interaction, but the polyurethane material distorts and the force decays permanently over time making force prediction 28 and bracket hygiene difficult. Design changes purport to transform the Siamese twin multibracket system into one that is more hygienic and interactive than c-twins with elastomeric or metal ligatures. The I-twin system reciprocates back on arch wires with a nondegrading resiliency and flexibility. The use of the term interactive twin in the scientific literature is therefore suggested to replace the clinical misnomer self-ligating used in the past. A further classification of interaction based on the seating force of the arm in a twin bracket is proposed here. Active interaction is used for the system where the seating forces

American Journal of Orthodontics and Dentofacial Orthopedics Volume 111, No. 2

Voudouris

125

Fig. 5. A, Interactive single bracket example defined as having three or less tie-wings. Maxillary right lateral incisor with tooth number marking on arm of Time bracket produces active interaction with the arch wire. Also note, labially positioned circular opening for instrument access, straight bracket body and rhomboid-like pad is observed. B, Lateral, three-quarter view of Time bracket in open position showing translation and rotation of arm in gingival tie-wing region.

on round arch wire are continuous as opposed to passive interaction where the forces are intermittent (Fig. 6) and correspond to the biology of tooth movement. The data on the effects of bracket width on the level of frictional resistance appear to be contradictory in the literature. In the more accurate in vitro study to date, wide brackets were found to produce less frictional resistance than narrow brackets where premolar retraction was replicated in a simulated dentoalveolar environment. 29 These results were confirmed by others. 3°,31 Some studies also found that either narrow brackets decreased frictional resistance, 32-34 or that there was little difference related to bracket widthY ,36 Despite the conflicting resuits on the effect of bracket width on friction, the new I-twin embodies the design principles of the c-twin bracket predecessors and have a mesiodistally wide bracket body and wide bonding pad for the purpose of improving rotational moments and bond strength. The characteristic low arch wire seating force of interactive brackets has been found to have an even greater effect on axial moment production than the bracket width for rotation correction, by producing significantly reduced but continuous initial moments 37 with smaller arch wire deflections (Fig. 7). In comparison, metal ligations of moderate rotations bind the arch wire and can deform it, whereas degrading elastomers stretch away from the bracket and also produce high moments. Unlike their c-twin predecessors, I-twins are designed to be independent of conventional elastomeric or metal ligation to achieve relatively low-friction and low-moment tooth movement. In addition, I-twin systems can also continue to readily receive colored elastomers and other auxiliaries completely, over all four tie-wings for selective anchorage requirements, or diagonally for patient acceptance while reducing arch wire contact. Clinical force application has also been found to directly affect friction. Approximately 200 gm or greater of

relatively harsh, anchorage-taxing elastomeric chain force continues to be used during the Class II canine retraction with c-twins to overcome the friction of the elastomerie plus the metal ligature for antirotation? 8'39 Much of the recent orthodontic literature indicates that interactive bracket systems were designed primarily to decrease this static and dynamic frictional resistance with the arch wire. When I-single brackets were compared with c-twins, in vitro, they produced inherently lower levels of friction with stainless steel (STS) arch wire that permitted the consistent use of desirable light force application, 7"35'4° Another in vitro study, showed that heavy force application led to higher levels of friction for narrow I-singles compared with wide c-twins because of the I-single bracket binding with the arch wire. 4a However, the forces used in this study were high compared with forces applied clinically for I-singles confirming the significance of using light force with interactive brackets. Heavy or moderate force application is generally contraindicated for interactive edgewise mechanisms used for flowing biomechanics. Finally, the reproducibility of the results of recent studies indicating that interactive brackets produce measurable time savings were tested. One investigation of I-single brackets found that the time needed to change arch wires when compared with conventional ligation was reduced by up to three timesY A previous I-single study also suggested that friction reduction was associated clinically with reduced treatment time. 4° Another similar time-in-motion study found that arch wire changes were reduced fourfold when interactive bracket mechanisms were used instead of conventional brackets. 42 STATEMENT OF THE PROBLEM Overall, a comprehensive in vitro frictional resistance study and in vivo clinical investigation was designed to examine and to quantify the frictional resistance levels,

126 Voudouris

American Journal o f Orthodontics and Dentofacial Orthopedics February 1997

PassiveInteraction

ActiveInteraction

Active Arch Wire

Active Arch Wire .....

!

.. • ~-

:....

:!

~:.:lI

~!!: B,

•:-: • ::~I ..... •i .: . .',

7"

Mild Seating Force

C.

Minimum Seating Force

D,

Fig. 6. Active versus passive interaction. A, Occlusal view of rotated and lingually displaced mandibular lateral incisor with type A I-twin with active interaction, used as a reversibly convertible tube. Arch wire is seated into base of slot to effect highly efficient alignment of mandibular crowding. Low resistance within I-twins permitted excess interbracket wire adjacent to rotated teeth to slide through low resistance system with distal force vector past end brackets (arrow), contributing to uninhibited early alignment during flow mechanics. Distal vector of force in type A also facilitates distal flow of teeth into extraction spaces. B, Lateral view of A illustrating labial force vector of arch wire against resilient arm. C, In type A, 0.018-inch arch wire is actively seated into base of slot closing space lingual to arch wire producing early correction of lingually displaced incisor. Note in type A position of round arch wire self-seated completely to occlusolingal effecting early, accurate, and automatic vertical leveling corrections of overerupted lateral incisor. Arm in type B does not actively seat arch wire into base of slot leaving space lingual to arch wire and uses larger dimension arch wires for displacement correction.

physical dimensions, and clinical characteristics of interactive twin brackets. This study used three I-twin types: type A, the Sigma System (American Orthodontics); type B, the Interactwin System (Ormco Corporation); and type C, the Damon System (A-Company), all in phase II clinical trials (Figs. 2, 3, and 4). The conventional twin counterparts used for comparison were type D, American Master Series; type E, Ormco Diamond; and type F. A-company Twin. In vitro frictional resistance measurements, as well as scanning electron microscopy (SEM) and clinical evaluations, were conducted to assess and to

compare interactive twin mechanisms with conventional twin brackets to determine the extent of improvement on conventional twins, if at all. The friction created by the 1-twin arms was compared with that of elastomeric and metal ligation (due to their prevalent use) of c-twins. This was used to test the hypothesis that the I-twins produced a reduced level of frictional resistance by two methods: (1) with the lower coefficient of friction of a stainless steel engagement arm opposed to polyurethane to interface with the arch wire in twin brackets, and (2) by reducing the seating force on the arch wire over a smaller interface

Voudouris

American Journal of Orthodontics and Dentofacial Orthopedics Volume 111, No. 2

127

aI

AI

/ I

t

CI

1

OI

f F

÷

Fig. 7. A, Low moments with interactive twins. Active interaction in lower incisor of type A arm on round arch wire showing reversibly deflected arm on distal aspect (arrow) actively seating arch wire with (force f) into base of slot. Note all brackets open in occlusal direction. B, Arm returns to undeflected position where force is centralized. C, Low moments (m = fxdl, medium arrow) and mild initial arch wire deflections are created in interbracket regions for biocompatible tooth movement during initial rotation control with interactive arm. White outline of same arch wire shows it fully ligated with metal ligature. This produces high deflection moments (M -- FxD1, large arrow), tending toward greater patient discomfort and higher susceptibility for debonding during forced ligation. D, After passivation of movement, frictional maintenance force (fm, small arrow) maintains control effectively with 0.018 or larger arch wire in 0.022 x 0.028-inch arch wire slot size.

area with the arm. The area of the interface was not directly related to seating force because the macroscopic interface area is independent of friction. Several orthodontic factors must be differentiated and combined to understand the interactive edgewise mechanism including differences among interactive singles, twins, and the con-

ventional bracket counterparts, distinctions between intrabracket and interbracket forces, to combining the findings of scientific studies with the results of clinical investigations. Finally, this study has further clinical relevance because I-twins may provide other clinical advances such as

128 Voudouris

American Journal of Orthodontics and Dentofacial Orthopedics February 1997

Universal Instron Apparatus Conventional Twin (Type F: A-Company Twin)

m

Conventional Twin (TypeE: Orrnco Diamond)

m

Conventional Twin (TypeD: Amer MasterSeries)

m _

97.07 gms

=ml BBI

Active Interactive Twin (TypeA: American Sigma)

54.12 gms II~t,;IP~01* lille

Passive Interactive Twin (TypeB: On'nco Miniline)

0.29 gm 0,24 grn

Passive Interactive Twin (TypeC: A-Company Darnon)

n

.020,,

[]

Conventional Twins

[]

.019" x ,025"

[]

Interactive Twins

0.25 gm 0.19 gm

0

B

20

40

6O

80

100

120

140

160

Fnctiona~ Resistance (gms, 1N=lO2gms)

Fig. 8. A, Universal Instron apparatus. B, Graph showing comparison of passive, active interaction frictional resistance levels and conventional ligation in Siamese twins with 0.020-inch and 0.019 x 0.025-inch STS arch wires at 0 ° angulation.

distinct improvements in bracket hygiene, bracket placement, and bracket width. These factors would elevate the standards of conventional twin function and warrant clinical application.

METHODS AND MATERIALS In vitro dynamic frictional resistance measurements were completed by using maxillary left canine I-twin brackets with 0.022 × 0.028-inch slots for each type A, B,

American Journal of Orthodontics and Dentofacial Orthopedics Volume 111,No. 2

Voudouris

129

p < 0,0001 Figure of 8 Elastomer (Type D C-Twin)

Extreme High Friction

~28.63 gms

O-pattem Elastomer (Type D C-Twin)

High Friction

Metal Tie (Type El C-Twin)

,

Active Interaction (Type A I-Twin)

Moderate Friction

Low

31,0 gm

Friction

Passive Interaction (Type C I-Twin)

30.18 gm

Extreme Low Friction

Passive Interaction (Type B I-Twin)

00.10 gm

Extreme Low Friction

0

30

60

90

120

150

Frictional Resistance (gms, 1N=lO2gms) Fig. 9. Graph showing comparison of passive, active interaction frictional resistance and conventional ligation friction in Siamese twins with 0.018-inch STS arch wires. Frictional resistance is also shown for figure 8 elastomeric ligation and standard O-pattern metal ligation of type D conventional twins.

and C, and type D, E, and F. Three different stainless steel arch wire sizes (GAC International) sizes, 0.018 inch (0.457 mm), 0.020 inch (0.508 ram), and 0.019 × 0.025 inch (0.483 × 0.635 ram) were tested with each of the six bracket types. Eight sample brackets from each of the six types were tested for each of the three wire sizes. For the conventional twin types D, E, and F, elastomers (0.030 inch Dentalastics, Dentaurum) were ligated in a regular O-pattern. In addition, figure 8 elastomer patterns and 0.011-inch (0.279 mm) metal ligatures were tested with 0.018-inch arch wire size using type D. Each bracket was bonded and light-cured (Transbond XT 3M/Unitek) to acrylic blocks, wiped with alcohol, and allowed to dry. The Universal Instron tensile apparatus (Instron Corp.) with a 2000 gm load cell, accurate to the 0.1 gm level, was used for testing (Fig. 8). Measurements were completed under dry conditions, because no significant differences in frictional resistance and adhesive effects were found when artificial saliva was used in vitro. 25,43-45Each sample was moved straight along a 0.315-inch (8 mm) distance that represented a premolar extraction space closure, at a crosshead speed of 0.394 inch (10 mm) per minute. A new bracket and arch wire were used for each test to avoid distortion. The elastomeric material in types D, E, and F was placed over the arch wires 24 hours before testing to allow for adaptation. The mean and standard

of the mean was calculated from six readings of dynamic frictional resistance. The data were statistically analyzed with the analysis of variance. The bracket dimension study was completed on eight brackets of each type A, B, and C. The Sigma, Interactwin, and Damon brackets were examined under SEM at 25×, 30×, and 50× magnification, and with stereomicroscopy (Leitz, WILD Oobersung Photografica). The relationship of the self-seating arm to the bracket body was examined with SEM. In addition, the physical contact relationship of 0.018-inch, 0.020-inch, 0.019 × 0.025-inch NiTi (Neosentalloy, GAC International) and STS arch wires with each bracket was studied. The overall mesiodistal, labiolingual, and buccolingual bracket pad and arm dimensions of each of the I-twins in types A, B, and C were measured and compared with the dimensions of each of the conventional twin brackets, types D, E, and F, respectively, with colored elastomers. In addition, four types of I-singles were measured for comparison. The third part included a bracket hygiene investigation that used three-dimensional measurement of SEM frontal and profile views at 25× magnification of the contact areas and where the seating force was applied over 0.020-inch arch wire. Two types were compared for size and shape of contact areas contributing to bracket hygiene: eight type A I-twin interface areas with 0.020error

130

Voudouris

American Journal of Orthodontics and Dentofacial Orthopedics February 1997

Table I. Differences in I-twin, c-twin, and I-single dimensions and characteristics (0.022 × 0.028" slot, R ot h prescription me a s ure d at largest dimension for each group) B-L (mm)

M-D (ram)

Arm (mm)

Interacth,e twin Type A, Sigma Type A, Interactwin Type C, Damon

1.98 2.42 2.31

3.18 2.91 3.09

3.05 1.65 3.94

Lingual Labial Labial

Active Passive Passive

Conventional twins Type D, Master Series Type E, Diamond Type F, A-Co twin

1.80 1.80 1.80

3.30 3.52 3.84

----

Labial Labial Labial

Active elastomer (degrading) Active elastomer (degrading) Active elastomer (degrading)

Interactive sb~gles Edgelok SPEED Activa Time

2.42 2.31 2.54 2.32

2.36 2.33 2.65 2,73

2.32 1.92 2.81 2.43

Lingual Lingual Lingual Labial

Passive Active Passive Active

Bracket type

inch round STS arch wire, and eight type D c-twins with bilateral elastomeric contacts and 0.020-inch STS. Finally, the clinical time to disengage and remove 0.018-inch STS mandibular arch wire, and the time to engage a 0.020-inch STS by three different orthodontic operators was measured. Each operator studied 25 mandibular arches with type B I-twin brackets and 25 mandibular arches with type E c-twin brackets. Ligation in each c-twin sample arch included four metal ligatures placed on all mandibular canine and lateral brackets with a total of six elastomers for the remaining brackets. RESULTS

Frictional resistance was significantly reduced in all three I-twins tested in comparison to the respective c-twins with conventional ligation (Figs. 8 and 9). Friction also increased with greater arch wire dimensions. The classification of the interaction force of the arm with the arch wire for type A (Fig. 10) was different than types B and C collectively (Figs. 11 and 12). The active engagement system of type A contained a labial arm component that partially extended into the arch wire slot in an arc producing a low seating force over a small interface area on the arch wire when dimensions greater than or equal to 0.018-inch round arch wire were used. This produced a constant base level of functional friction above zero that was found to be lower than the high friction observed for c-twins (Fig. 10, D). Clinical evaluation revealed that when active 0.018inch arch wire exerted a labial vector of force within the bracket slot small interbracket arch wire deflections were observed in comparison to c-twins with metal ligatures. The resilient arm then reciprocated with a continuous seating force that diminished

Arm position (Lab or Li)

Active or passive interaction

gradually as the tooth was accurately derotated into alignment. The low friction was also used to maintain postrotation tooth control including the time when tooth movement was passivated. Active rotation control, arch alignment, and leveling movements were found to occur early with the functional friction in type A that used 0.018-inch arch wire. The labial free-end component of the arm in type A contacted the arch wire at a tangent. The seating force had a perpendicular vector to the tangent and directed the arch wire into the base of the slot over time. In contrast, types B and C demonstrated rigid and passive arms that did not extend into the arch wire slot, resulting in a relatively straight, vertical closure of the slot. This produced minimal seating force and contact with initial round wires (Figs. 11 and 12) for rotation correction. As a consequence of the small passive arm with a fixed, labial position in relation to the 0.022 × 0.028-inch arch wire slot, types B and C produced minimal, near-zero frictional resistance (Figs. 8 and 9). Clinically, an active labial vector of arch wire force against the passive arms in types B and C produced a reaction (rather than direct action) by the arms to initiate tooth movement and effectively prevented arch wire release from the slot. Types B and C also exhibited low interbracket arch wire deflections during initial rotation corrections. This resulted in an intermittent force or partial interaction between the arm and the arch wire. However, after the passivation of tooth movement with initial bracket alignment, the arm generally did not demonstrate an active seating force on round arch wires. Clinically, the near-zero seating force provided mild arch wire contact and

American Journal of Orthodontics and Dentofacial Orthopedics Volume 111, No. 2

Voudouris

131

.125" (3.18mm)

lllll,l,I

.130" (3.30mm)

I,l,lllll Fig. 10. A, Stereomicrography at 13x magnification of type A, Sigma bracket with center-slot marker (red) for bracket placement reference and with closed arm engagement of 0.020-inch arch wire without need of ligatures. B, Rolled T-shaped arm contacts 0.020-inch (and 0.018-inch) arch wire showing active interaction for early and complete tooth rotation, displacement corrections and vertical alignment. C, Comparison with conventional twin counterpart type D, Master Series bracket and elastomer with high coefficient of friction over large contact area and producing high seating force friction on 0.020-inch arch wire. Types A and D show both rhomboid body and bonding pad that parallels all bracket reference planes for bracket placement and results in linear alignment of brackets with straight wire. D, Lateral view shows significantly larger contact area with 0.020-inch arch wire (arrows) versus mild tangential contact by arm in type A (B above) resulting in new low friction level for edgewise twin brackets with round arch wires for early tooth control.

required larger dimension 0.019 x 0.025-inch or 0.021 × 0.028-inch NiTi to effect reactive rotation control by the arm for complete alignment. Despite the differences, types A, B, and C as twins had STS arms that produced precise (nonde-

grading) forces and gave the practitioner the capacity to accurately preset the frictional resistance locally or throughout the system with either of the two new reduced levels, a functional low friction and an extremely low friction (near-zero, Fig. 9). In

139

Voudouris

American Journal of Orthodontics and Dentofacial Orthopedics February 1997

B.

,i

/

4

.115" (2.91mm)

lllllll,I

.139" (3.52mm)

IIILlllll Fig. 11. A, Stereomicrography at 13× magnification of type B, Interactwin interactive edgewise twin bracket in closed arm position with 0.020-inch arch wire without use of ligatures. B, Round 0.020-inch arch wire shows space labial to arch wire. This results in passive interaction that produces near-zero friction and uses rectangular dimension arch wires for complete tooth control. Type B bracket shows a straight bracket body and rhomboid pad that are not parallel reference planes for bracket placement but result in linear alignment of brackets with straight wire. C, Comparison with conventional twin counterpart type E, Diamond bracket with elastomer showing undesirably large contact areas (arrows) producing high seating force friction against 0.020-inch arch wire. Type E shows both rhomboid body and bonding pad similar to type D above. D, Lateral view shows elastomer contact covering approximately half circumference of 0.020-inch arch wire (arrows) versus near-zero contact with arm in type B (B above). Also note at gingival aspect, elastomer protrusion that also affects bracket hygiene.

comparison, metal ligation with 0.018-inch arch wire generally produced moderate friction, given its dependence on an unpredictable degree of tightening force 14 and tightening pattern. Plastic polyurethane elastomeric ligation created two levels of increased

friction compared to metal ligation, including high with the regular elastomer pattern and extremely high with the figure 8 pattern that degraded unpredictably, The levels of clinical friction available for the practitioner were expanded to include new

American Journal of Orthodontics and Dentofacial Orthopedics Volume 111, No. 2

Voudouris

133

.155" (3.94mm)

IIlllllll

Dm

.152" (3.84mm)

I,l,l,lll Fig. 12. A, Stereomicrography at 13x magnification of type C, Damon interactive edgewise twin bracket in closed arm position with 0.020-inch arch wire without use of ligatures. B, Round 0.020-inch arch wire shows space labial to arch wire. This results in passive interaction producing near-zero friction and type C brackets use rectangular dimension arch wires for complete tooth control. Type C shows straight bracket body with rhomboid bonding pad similar to type B above. C, Comparison with conventional twin counterpart type F, Twin bracket with torque in base and elastomer showing undesirably large contact areas (arrows) and also producing high seating force friction against 0.020-inch arch wire. Straight bracket body and straight bonding pad with diagonal angulation tips with straight wire bracket alignment. D, Lateral view shows elastomer contact covering approximately half circumference of 0.020-inch arch wire (arrows) versus near-zero with arm in type C (B above).

subthreshold levels of friction with the I-twins (Figs. 8 and 9, white regions). The two reduced friction settings were found to be more efficient biomechanically and more biocompatible for the periodontium. The overall dimensional measurements (Table I)

of the various brackets, showed the self-seating I-twin brackets were generally smaller in size than the c-twin counterparts, except in the buccolingual dimension. Another finding was that the arm width in type B was found to be narrower mesiodistally

134

Voudouris

=

,

American Journal of Orthodontics and Dentofacial Orthopedics February 1997

,

Fig. 13. A, Type A active interaction twin application in child. Pretreatment moderate maxillary and mandibular dental crowding with deep overbite because of overclosure. B, After 2 months of treatment, dental alignment and deep overbite correction occurs without use of intermaxillary elastics. Note bracket hygiene near surrounding enamel of I-twins in comparison to conventionally ligated mandibular lateral incisors that show proximity of elastomers near enamel. As consequence of low-friction interaction found between arch wire and I-twins, highly effective and early alignment, leveling and overbite correction were observed.

Table II. T i m e to d i s e n g a g e 0.018-inch a n d e n g a g e 0.020-inch a r c h wires: C o n v e n t i o n a l twin type E versus interactive twin type B

C-twin I-twin

Operator 1 *

Operator 2*

Operator 3*

Total average time'/"

159.0 17.7

97.7 22.5

140.0 40.7 28.0 21.0

61.1 132.2 = 193.4 20.6 22.7 = 43.3 Time Efficiency Ratio 4.47x

85.7 19.3

57.0 21.5

*Time to clisengage engage. ?Disengage + engage.

than the arms of types A and C and the majority of the I-single arms for interbracket flexibility during arch wire placement. The I-twins were found to be fully preadjusted (modified Roth prescription) with the coordination of rhomboid-shaped bracket bodies (type A) and rhomboid bonding pads (types A, B; and C) providing parallel siting references with the long axis of the clinical crown during bracket placement. Bracket placement was also simplified by using the color identification marker located on the center of the type A I-twin brackets. The I-twins demonstrated labial arch wire contact with either a narrow interface area (type A, Fig. 10, B) or arm contact that was near-zero (types C and D, Figs. 11, B, and 12, B) and both improved bracket hygiene. The arm in type A contacted the

0.020-inch arch wire fully (or with rotated teeth, partially) within the area of the arch wire slot, creating a relatively two-dimensional interface. The arm covered an average arch wire area of 12.09 × 10-5 square inches. In contrast, the c-twin elastomer engaged the labial circumference of 0.020inch arch wire external to the c-twin bracket on both mesial and distal aspects in a three-dimensional, wide, and semicircular band pattern curving toward the lingual and covering approximately half the circumference of the arch wire (Figs. 10, D, 11, D, and 12, D) with an average total contact area of 52.36 × 10-s square inches. This indicated that the use of the single, internally located and linear contact area of the I-twin resulted in less than 25% (0.23) of the contact area of c-twin

American Journal of Orthodontics and Dentofaeial Orthopedics Volume 111, No. 2

Vottdouris

135

brackets ligated with elastomers that retained plaque and calculus. This investigation also demonstrated that the time involved to disengage and engage arch wires of the I-twin brackets was significantly reduced by approximately a factor of greater than 4 when compared with the combined elastomeric and metal ligation methods (Table II). Finally, clinical distortion of the arm and calculus obstruction were found to be uncommon. Arm distortion and inadvertent arch wire release were related to operator technique and were prevented by seating the arch wire at both the mesial and distal ends of the arch wire slot before closing the arm and by avoiding direct I-twin contact with the patient's occlusion. When the arm was distorted, it was found usually to be of iatrogenic origin where heavy force was exerted. Where arm distortion occurred, the underlying body of the I-twin was easily ligated as a conventional twin until the arm was replaced. DISCUSSION

Although the in vitro study of friction does not exactly reproduce the in vivo environment,46when it is combined with the SEM and clinical investigation, three main observations and several other significant clinical results can be derived regarding the advantages of I-twins compared with c-twin bracket systems. First, the reduced frictional resistance in I-twins is closely associated with both a low frictional coefficient of the arm that interfaces minimally with the arch wire, and with a reduced seating force friction in comparison to conventionally ligated ctwins. Consequently, the lower friction and the nondegrading resiliency of the arm result in the reduction of interbracket arch wire deflections. This produces both gradual and precise tooth movement in severe rotations that lead to a biocompatible periodontal tissue response (Figs. 1 and 13). Further, interactive twins also permit consistent light force application that is clinically effective for canine retraction because of the reduced frictional resistance for anchorage-based flow (sliding) mechanics that includes mandibular lip bumper, light anterior J-pull or molar headgear that prevent incisor proclination. In the passive interaction of types B and C, the arch wire is the primary determinant of tooth movement with the rigid arm having a secondary and reactive effect on the arch wire and tooth movement. In comparison, the active interaction of type A demonstrated interplay initiated by the arch wire and followed by the arm for accurate tooth move-

Fig. 14. Lingual view of type A I-twin and type D c-twin showing similarity of size and rhomboid-shape of twin bonding pad design to maintain similar bond strengths between I-twins and c-twins.

ment. Type A I-twins have a small interface area and a low active seating force that is desirable since the low functional friction directs the arch wire precisely into the base of the slot. This produces early rotation control with 0.018-inch and 0.020-inch round arch wires. Further, it results clinically in crown alignment of local lingual and palatal tooth displacements, antirotation control during retraction mechanics with 0.018-inch arch wire application, and inclination (torque) with larger arch wires. In comparison, types B and C I-twins have a minimal interface and no active seating force with the arch wire, that produces near-zero friction and clinically uses the application of large rectangular dimension arch wires for early and complete rotation control. In c-twin, types D, E, and F, wide polyurethane (high coefficient of friction) to metal arch wire contact areas and large seating forces produce undesirably high frictional resistance in comparison to the I-twin types A, B, and C A 45% to 56% reduction in frictional resistance occurs in type A I-twins compared with type D c-twins with

136

Vottdouris

American Journal of Orthodontics and Dentofacial Orthopedics February 1997

Fig. 15. Initial bracket placement of type B (Interactwin) passive interaction twin system results in low frictional resistance and bracket hygiene for entire multibracket system that require no form of elastomeric or metal ligation. CuNiTi arch wire of 0.019 × 0.025-inch dimension inserted to fill slot in mild mandibular crowding because arm does not extend into large, four-walled 0.028 × 0.028 slot. Colored elastomers can be easily applied in O-pattern or figure 8 pattern selectively for greater local rotation control and high friction for anchorage, or diagonal-pattern limited to maxillary lateral incisors for patient acceptance to maintain low friction.

O-pattern elastomers with all three arch wire sizes tested. Type A I-twin friction with 0.018-inch arch wire is reduced 76% compared with type D c-twins with figure 8 elastomers. Second, the time-in-motion study also shows a significant reduction in the time requirement to change arch wires by decreasing the use of conventional ligatures for chairside assistance confirming previous findings.15,42 The third set of observations relates to the bracket-siting advantages of the rhomboid bracket shape, the rhomboid conventional pad, and the colored center-slot marker found in the I-twin. These three features collectively facilitate bracket placement and bracket handling4v during alignment along the vertical crown axis, and coordinate the center of the clinical crown with the I-twin centerslot marker due to the presence of the new labial component of the four-walled slot. In contrast, the labially located marker is not possible in the open three-walled slot of c-twins. Overall, this achieves ideal diagonal angulation of the crowns of the teeth

with a linear bracket alignment (Fig. 13) and good bond success rates, due to the wide conventional pad (Fig. 14). The resilient arm component also improves bond success because of the lower arch wire deflections in contrast to forced metal ligation with c-twins that are susceptible to inadvertent debonding. Finally, the clinical improvement in bracket hygiene is found immediately adjacent to the periphery of the I-twin bracket surfaces (Fig. 15). This is a direct result of selective but not complete elimination of plaque-retentive conventional ligation that leaves the four tie-wings unobstructed for accessible cleansing particularly for periodontally compromised and orthognathic surgery patients (Fig. 16). Heat-activated NiTi arch wires in the chilled and plastic martensitic phase are particularly compatible because they prevent arm distortion and require no change in the conventional arch wire sequence. The NiTi arch wire size sequence for types B and C is shorter including 0.014 inch, 0.016 x 0.025 inch, and

American Journal of Orthodontics and Dentofacial Orthopedics Volume 111, No. 2

Voudouris

137

Fig. 16. Type B active interaction application in adult. A, Pretreatment Class It, Division 2 malocclusion with moderate maxillary and mandibular dental crowding and severe

overbite. B, After 2 months of treatment, initial dental alignment and deep overbite correction is observed. Note fail-safe feature of interactive mandibular right lateral incisor bracket where arm was lost and was replaced with metal ligature because of presence of four tie-wings. Bracket hygiene improvement for periodontal health is also noted with I-twins compared with conventional metal ligation (mandibular right lateral incisor). Elastomeric chain in maxillary arch was positioned labially over arch wire (compared with previous interactive systems where elastic chain was applied before arch wire placement) away from enamel surfaces, resulting in reduced opportunity for decalcification. I-twin at lingually displaced mandibular left lateral incisor was used as reversible, convertible tube by closing arm first and then feeding 0.014 NiTi arch wire using curved end. C, Finishing stages of treatment with 0.021 x 0.025-inch arch wires.

0.019 x 0.025 inch to occupy the large four-walled rectangular slot early, for tooth control (Fig. 17). Continuous stainless steel arch wires are used after NiTi leveling and alignment of rotations, and when arch wire stiffness is required (overbite correction). The interactive twin brackets act as true, convertible tubes that are reversible for all orthodontically bonded teeth and can act as islets for the correction of severe lingual or palatal tooth displacements (Fig. 16). In comparison, the previous c-twin convertible tubes with brazed sheaths become irreversible brackets once the sheaths are removed and are limited to the first molars in c-twin systems.

The premise that a low functional level of friction is needed in biomechanics for tooth control and anchorage is satisfied by the three I-twins because they do not completely eliminate friction because of the ability to accept conventional ligatures. The practitioner is able to select a new platform of low friction levels with interactive twins. Conventional elastomer and metal ligature use is therefore reduced but not eliminated with I-twins for ideal biomechanics. Extreme or high friction and moderate friction can be obtained for maximum anchorage because the I-twins have the unique four tie-wing undercuts for selective ligation with conventional

] 38

Voudouris

American Journal of Orthodontics and Dentofacial Orthopedics February 1997

Fig. 17. Intraoral photographs showing type C, Damon brackets.

A,Q

i

•

O

B.'-

Fig. 18. Selective elastomeric chain placement was used to preset friction levels in system of biomechanics. Red markers represent high friction and green low friction (A). Canine retraction with Class I NiTi coil (B) or light-force elastomeric chain was attached to ball-hook for low frictional resistance setting. Elastomeric chain was placed behind arch wire (arrow) to prevent dislodgement by brushing or chewing forces and buccally over premolar and molar tie-wings for maximum posterior anchorage. Light Class II elastics (C) can be used effectively because canine bracket is preset at low resistance for distalization. When premolar I-twins were used to bond first and second molars (D), tie-back bends were facilitated.

colored elastomeric or metal ligatures. To derive the low friction locally in the same system of anchoragebased flow (sliding) mechanics the use of colored elastomeric chain for maxillary canine retraction is specifically limited to only the ball-hook of the self-seating I-twin canine bracket (Fig. 18). Light elastics can be used effectively because the canine bracket is preset at a low resistance setting for distalization, thereby maintaining anchorage at the lower incisors (Fig. 18, B). A lower frictional resistance setting in interactive edgewise systems was achieved at the canine bracket by avoiding circumferential ligation over the arch wire because it was not required for antirotation control. Closely adapted lip bumper and headgear (molar or anterior J-pull) were used to prevent mandibular and max-

illary incisor proclination during anchorage-based sliding mechanics (Fig. 18, C). In addition, when the premolar I-twins were used to bond the first and second molars, tie-back bends were facilitated because the arch wire was cinched before its placement in the second molar I-twin brackets (Fig. 18, D). The end brackets were opened, then closed as reversible tubes over precinched wire. In this manner, tie-back bends distal to the brackets were accurately made, extraorally reducing irritations of soft tissue mucosa and the I-twins precluded the need to later cut cinched-back arch wires for removal and adjustment. The significant difference is that this selection or choice of I-twin engagement to acquire the low functional friction setting has not been possible with ligature-free c-twins.

American Journal of Orthodontics and Dentofacial Orthopedics Volume 111, No. 2

Voudouris

139

CONCLUSIONS

FUTURE INVESTIGATIONS

In summary, interactive edgewise brackets, presented here for the first time in Siamese twin form, are capable of kinetically interacting in a nondegrading manner with resilient arch wires and without elastomers or metal ligatures to create a new reduced friction standard for biocompatible tooth movement. The arch wire-elastomer complex of conventional twins operates with extreme or high friction and in a degrading manner. This investigation produced scientific and clinical evidence showing that interactive twins, having self-seating arms, are hybrids of both conventional twins and interactive single brackets, that make significant incremental improvements to both previous systems. These combined improvements are as follows:

Further study of interactive edgewise appliances is required. Quantification of the bacteria accumulating proximal to conventionally ligated twin edgewise brackets compared with ligature-free interactive twins may further differentiate possible hygiene differences. Second, a modified Instron apparatus (according to the method of Drescher) that simulates multiple in vivo tooth movements is being developed. Canine retraction and premolar anchorage effects can be recorded with three-dimensional video imaging to better assess frictional resistance and arch flow, compared with relatively straight sliding. Finally, specialized coatings that bond to the twin bracket component and arm material made of thermally activated, ion-bombarded NiTi are being tested to prevent possible calculus binding.

1. Frictional resistance reduction from the low STS coefficient of friction and low seating force of the arm. 2. Resultant low arch wire deflections in the interbracket regions because of resilient arm. 3. Light clinical force application permits anchorage conservation because of low-friction properties. 4. Time savings in changing arch wires reduces chairside assistance time. 5. Center-slot marker on fourth labial wall of arch wire slot improves bracket placement. 6. Bracket hygiene improvement relates to small arm interface (contact) areas with arch wire. 7. Preadjusted, wide rhomboid bracket for rotation corrections and bracket placement references. 8. Four tie-wings accessible for colored elastomers, Kobayashi hooks, and rotation wedges. 9. Wide rhomboid-shaped conventional bonding pads for bond strength. 10. Fail-safe because it can be used as conventional twin brackets without the arms. Two classifications of interactive twin brackets are identified as active or passive and require different arch wire sequences. Active interaction twins are capable of seating round arch wires gradually and accurately for early and complete tooth control. A minuscule functional seating force found with active interaction was clinically desirable for automatic and early rotation corrections, in-out discrepancies, vertical leveling, antirotation during flow mechanics, and active torque expression by the flexible arm. Passive interaction twins require early application of larger dimension arch wires because of their near-zero friction with the stiff arm. The interactives require formal training and a light force technique for the arm, the arch wire, and the retraction auxiliaries to be effective. The results demonstrated that resilient twin arm is beneficial for patients, clinical assistants, and practitioners 48 because interactive edgewise mechanisms significantly improve the overall reduction in frictional resistance levels, anchorage requirements, infection control, hygiene, and time efficiency of current conventional edgewise bracket systems.

I gratefully acknowledge the generous support of the head of the Orthodontic Graduate Department, University of Toronto, Emile Rossouw for this investigation; and Professor Emeritus Dr. Donald G. Woodside, Dr. Jack G. Dale, who inspired this research. REFERENCES

1. Halderson H. Routine use of minute forces. Am J Orthod 1957:43:750-68. 2. Halderson H, Johns EE, Moyers RE. Selection of forces for tooth movement: a summary of our present knowledge. Am J Orthod 1953;39:25-35. 3. Moyers RE, Bauer JL. Periodontal response to various tooth movements. Am J Orthod 1950:36:572-80. 4. Creekmore TD. Straight wire: the next generation. Am J Orthod Dentofac Orthop 1993;104:8-20. 5. Davidovitch Z, Finkelson MD, Steigman S, Shanfeld JL, Montgomery PC, Korostoff E. Electric currents, bone remodeling and orthodontic tooth movement; II, increase in rate of tooth movement and periodontal cyclic nucleotide levels by combined force and electric current. Am J Orthod 1980;77:33-47. 6. Burstone CJ, Groves MH. Threshold and optimum force values for maxillary anterior tooth movement. J Dent Res 1961;39:695. 7. Burstone CJ, Baldwin JJ, Lawless DT. The application of continuous forces to orthodontics. Angle Orthod 1961;31:1-14. 8. McNamara JA. Neuromuscular and skeletal adaptations to altered function in the orofacial region. Am J Orthod 1973:64:578-606. 9. Reitan K. Continuous bodily tooth movement and its histological signifcance. Acta Odontol Scand 1947;7:115. I0. Reitan K. Some factors determining the evaluatiou of forces in orthodontics. Am J Orthod 1957;43:32-45. 11. Murrell EF, Yen EHK, Johnson RB. Vascular changes in the periodontal ligature after removal of orthodontic forces. Am J Orthod Dentofac Orthop 1996;110: 280-6. 12. Moyers RE. Handbook of orthodontics. 3rd ed, Chicago: Year Book Medical Publishers, 1973:434. 13. Angle EH. The latest and best in orthodontic mechanism. Dent Cosmos 1928;70: 1143-58. 14. Steiner CC. The Angle orthodontist. Brooklyn: EH Angle Society of Orthodontia, 1933. 15. Begg PR, Kessling PC. Begg orthodontic theory and technique. 3rd ed. Philadelphia: WB Saunders, 1977. 16. Vaden JL, Dale JG, Klontz HA. The Tweed-Merrifield edgewise appliance: philosophy, diagnosis~ and treatment. In: Graber TM, Vanarsdall RL Jr, editors. Orthodontics: current principles and techniques. 2nd ed. St Louis: CV Mosby, 1994:627-84. 17. Swain B. Clinical demonstration of the bull technique. Charles H. Tweed Foundation Meeting, Chicago, 1952. 18. Andrews LF. The straight-wire appliance. J Clin Orthod 1976:10:99-114. 174-95, 360-79. 19. Andrews LF. The six keys to normal occlusion. Am J Orthod 1972;63:296. 20. Roth RH. Five year clinical evaluation of the Andrews' straight-wire appliance. J Clin Orthod 1976;10:836-50. 21. Ricketts RM, Bench RW, Gugino CF, Hilgers JJ, Schulhof RJ. Visual treatment objective or VTO. In: Bioprogressive therapy. Book I. Denver: Rocky Mountain Orthodontics, 1980:35-54.

1 40

Voudoufis

22. Wildman AJ, Hice TL, Lane HM, Lee IF, Strauch EC Jr. Round table-the Edgelok bracket. J Clin Orthod 1972;6:613-23. 23. Hanson GH. The SPEED system: a report on the development of a new edgewise appliance. Am J Orthod 1980;78:243-65. 24. Sims APT, Waters, NE, Birnie DJ, Pethybridget RJ. A comparison of the forces required to produce tooth movement in vitro using two self-ligating brackets and a pre-adjusted bracket employing two types of ligation. Eur J Orthod 1993;15:377-85. 25. Shivapuja PK, Berger JA. Comparative study of conventional ligation and selfligation bracket systems. Am J Orthod Dentofac Orthop 1994;106:472-80. 26. Stolzenberg J. The Russell attachment and its improved advantages. Int J Orthod Dent Children 1935:9:837-40. 27. Stolzenberg J. The efficiency of the Russell attachment. Am J Orthod Oral Surg 1946;32:572-82. 28. Wong AK. Orthodontic elastic materials. Angle Orthod 1976;46:196-205. 29. Drescher D, Bourauel C, Schumacher H. Frictional forces between bracket and arch wire. Am J Orthod Dentofac Orthop 1989;67:397-404. 30. Tidy D. Frictional forces in fixed appliances. Am J Orthod Dentofac Orthop 1989;96:249-54. 31. Thurow RC. Edgewise Orthodontics. 3rd ed. St Louis: CV Mosby, 1982:167-8. 32. Nicolls J. Frictional forces in fixed orthodontic appliances. Dent Pract 1968;18: 362-6. 33. Frank CA, Nikolai RJ. A comparative study of frictional resistance between orthodontic bracket and arch wire. Am J Orthod 1980;78:593-609. 34. Kapila S, Angolkar PV, Duncanson M, Nanda RS. Evaluation of friction between edgewise stainless steel brackets and orthodontic wires of four alloys. Am J Orthod Dentofac Orthop 1990;98:117-26. 35. Andreasen GF, Quevedo FR. Evaluation of friction forces in the 0.022 × 0.028 edgewise bracket in vitro. J Biomechanics 1970;3:151-60. 36. Peterson L, Spencer R, Andreason GF. Comparison of frictional resistance of Nitinol and stainless steel wires in edgewise brackets. Quintessence Int Digest 1982;13:563-71.

American Journal of Orthodontics and Dentofacial Orthopedic~ February 1997 37. Bednar JR, Gruendeman BW. The influence of bracket design on moment production during axial rotation. Am J Orthod Dentofac Orthop 1993:104:254-61. 38. Hershey HG, Reynolds WG. The plastic module as an orthodontic tooth moving mechanism. Am J Orthod 1975;67:554-62, 39. Annello JC. Force degradation characteristics of orthodontic colored elastomeric chain-a comparative study. [Masters thesis.] San Francisco: University of the Pacific, 1993:1-85. 40. Berger JL. The influence of the SPEED bracket's self-ligating design on force levels in tooth movement: a comparative study. Am J Orthod Dentofac Orthop 1990:97:219-28. 41. Bednar JR, Gruendeman BW, Sandrik JL. A comparative study of frictional forces between orthodontic brackets and orthodontic wires of four alloys. Am J Orthod Dentofac Orthop 1991;100:513-22. 42. Maijer R, Smith DC. Time savings with self-ligating brackets. J Clin Orthod 1990:24:29-31. 43. Kusy R, Whitley J, Prewitt M. Comparison of the frictional coefficients for selected arch wire bracket slot combinations in the dry and wet states. Angle Orthod 1991;61:293-302. 44. Garner L, Allai W, Moore B. A comparison of frictional forces during simulated canine retraction on a continuous edgewise arch wire. Am J Orthod Dentofac Orthop 1986:90:199-203. 45. Stannard J, Gau J, Hanna M. Comparative friction of orthodontic wires under dry and wet conditions. Am J Orthod Dentofac Orthop 1986;89:485-91. 46. Vaughan JL, Duncanson MG, Nanda RS, Currier GF. Relative kinetic frictional forces between sintered stainless steel brackets and orthodontic wires. Am J Orthod Dentofac Orthop 1995;107:20-7. 47. Harridine NWT, Birnie DJ. The clinical use of active self-ligating brackets. Am J Orthod Dentofac Orthop 1996;109:319-28. 48. Roth RN. Treatment mechanics for the straight-wire appliance. In: Graber TM, Vanarsdall RL Jr., editors. Orthodontics: current principles and techniques. 2nd ed. St Louis: Mosby-Year Book, 1994:685-711.