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PPlastic brackets were introduced in the orthodon-tic specialty in the 1970s, and initially were man-ufactured from unfilled polycarbonate.1,2 The firstgeneration of these appliances presented two funda-mental deficiencies: A tendency to undergo creepdeformation, a viscoelastic behavior, upon transfer-ring torque loads generated by activated archwiresto the teeth35 and a high incidence of discolorationduring clinical use. Ceramic-reinforced, fiberglass-reinforced, and metal slot-reinforced polycarbonatebrackets were subsequently introduced to alleviatethe first problem, while novel syntheses were used

to overcome the esthetic concern associated withcolor absorption and subsequent discoloration.

Reinforced polycarbonate brackets became popu-lar in the 1990s, when the first evidence of enameldamage from ceramic brackets became available.6

The low modulus of these appliances facilitated safedebonding, presenting a peel-off effect (Fig 1) similarto that seen in debonding metal brackets7 and, thus,plastic brackets were well-received by the orthodon-tic community. Concurrently, advances in the manu-facturing process of these appliances allowed theuse of brackets without primers to prepare the basefor bonding, so that this clinical step would beavoided during bracket placement.89 These primerscontain low molecular weight dimethacrylates andmethyl methacrylate, producing a surface that facili-tates micromechanical interlocking (Fig 2). Also,metal slot-reinforced polycarbonate brackets thatefficiently generated the desired torque on teethunder clinical conditions became available; thesebrackets had previously shown problems with theintegrity of the slot periphery.

Plastic Brackets: Hardness and Associated Clinical Implications

Theodore Eliades, DDS, MS, Dr Med, PhD1/Christiana Gioka, DDS1/Spiros Zinelis, PhD1/George Eliades, DDS, Dr Dent1/Margarita Makou DDS, MS, Dr Dent2

Aim: To assess the Vickers hardness of plastic brackets, after in vitro aging. The morphol-ogy and structure of corresponding retrieved brackets were also studied to assess the relia-bility of in vitro set-up in imposing effects comparable to intraoral aging. Materials andMethods: Four types of plastic brackets were selected. Specimens were immersed in abuffer solution to induce artificial aging; brackets of two groups were retrieved after ortho-dontic treatment. The wings of brackets were subjected to Knoop hardness testing, andscanning electron microscopy was employed to study the morphology and microstructureof retrieved brackets. The results were statistically analyzed by two-way analysis of varianceand Tukey test ( = 0.05). Results: Differences were identified between the brands of brack-ets, but not between the new and in vitro-aged appliances. The retrieved appliancesshowed significant reduction in hardness when compared with their as-received counter-parts. Evidence of degradation was seen in the retrieved appliances. Conclusions: Theinability of ex vivo experimental configuration to simulate clinical conditions is stressed;several clinical implications related to torque control, friction, and tie-wing strength ofbrackets are discussed. World J Orthod 2004;5:6266.

1Biomaterials Laboratory, School of Dentistry, University ofAthens, Athens, Greece.

2Department of Orthodontics, School of Dentistry, University ofAthens, Athens, Greece

CORRESPONDENCEDr Theodore Eliades, 57 Agnoston Hiroon Str, N. Ionia 14231,Athens, Greece. E-mail: teliades@ath.forthnet.gr

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The purpose of this study was to assess the varia-tion of hardness of plastic brackets subjected to arti-ficial aging and clinical use. In addition, a compre-hensive review of physical and mechanicalproperties of plastic brackets is provided, withemphasis on associated clinical implications.

MAMATERIALS AND METHODSTERIALS AND METHODS

The brackets included in this study were: Brilliant(Forestadent, Pforzheim, Germany), Align (Fores-tadent), Leone (Leone, Firenze, Italy), and Silicon(American Orthodontics, Sheboygan, WI, USA). Eightspecimens of each brand were divided in two groups:Brackets in the first group were embedded in epoxyresin with their faces pointing upward, and the sec-ond group was aged in a 0.02 M acetate, 0.1 M KClbuffer (pH = 4.8) for 2 weeks. All brackets were thenembedded in resin. The specimens were ground with220- to 2,000-grit SiC papers under water cooling,polished up to 0.05 m with alumina suspensions(Bueler, Lake Bluff, IL, USA) in a grinding/polishingmachine (Ecomet III, Bueler), and cleaned in an ultra-sonic water bath for 5 minutes. The wing surfaceswere used for assessment of the Knoop hardness byusing a microhardness tester (HMV-2000, Shimadzu,Tokyo, Japan) with a 25-g load and 15-second con-tact time. The results of the hardness testing wereanalyzed with two-way analysis of variance (ANOVA),with brand and aging as the discriminating variables,while further group differences were investigatedwith Tukey multiple comparisons test at = 0.05level of significance. The results were analyzed withone-way ANOVA and the Tukey test ( = 0.05).

Brackets of two of the tested groups (Leone andAmerican Orthodontics) were retrieved following fullorthodontic treatment with a mean duration periodof 18 months. These were subjected to hardnesstesting as previously described. The morphology and

microstructure of representative specimens of theas-received, in vitroaged, and retrieved groups werestudied with scanning electron microscopy (SEM).For this purpose, brackets were bonded to aluminumstabs, vacuum-coated with a thin layer of conductivecarbon, and examined under an SEM unit (Quanta200, FEI, Eindhoven, Netherlands).

RESULRESULTS TS

The results of the Knoop hardness test are shown inFig 3. No differences were identified among the as-received and in vitroaged specimens, but differ-ences were seen between various brands of brackets.

No differences between the in vitroaged and theas-received morphologic and structural characteris-tics of brackets were observed. However, significantalterations were detected in the retrieved bracketgroup. Figure 4 shows a representative SEM imageof a retrieved bracket following intraoral exposure for18 months, demonstrating a wing fracture.

DISCUSSIONDISCUSSION

The introduction of the first generation of estheticplastic brackets stimulated concern about the clini-cal performance of these materials, as they pre-sented a decreased ability to transfer torque to thetooth because of the plastic deformation associatedwith their low modulus.10,11 Fiber reinforcement ofthe plastic-polycarbonate appliances increased theappliance stiffness, thus minimizing some of theclinical concerns.

The morphology of the slot wall surface mayimpose several important effects on the performanceof the appliance. Bracket slot manufacturing intro-duces inclusion of particles and formation of groovesand striations. Figure 5 demonstrates the profile

Fig 1 (Left) Secondary electronimage of a debonded plastic bracketindicating signs of plastic deforma-tion (skewed slot outline) duringdebonding, mimicking the peel-offeffect seen in debonded stainlesssteel brackets. (Original magnifica-tion 30.)

Fig 2 (Right) Secondary electronimage of a plastic bracket base sur-face following application of a primerto increase the effective surfacearea for the adhesive, thus facilitat-ing micromechanical interlocking.(Original magnification 400.)

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appearance of a metallic slot insert of a plasticbracket in the as-received condition, demonstrating arough surface with irregularities, which may affect thedimensional accuracy of the slot. Inclusion of particlesduring manufacturing, coupled with the low modulusof plastic brackets, may have several clinical implica-tions. First, the formation of micro-irregularities, projec-tions, and porosities precludes the complete engage-ment of wire to the bracket slot walls. This effect mayjeopardize the effectiveness of the bracket, which maybe unable to provide the appropriate spatial orienta-tion of the crown because the incomplete engagementof the wire to the bracket cannot transfer the pre-scribed buccolingual inclination of preadjusted appli-ances. This issue receives further significance sincethere are other factors, such as the bracket archwireplay and slot wall and wire moduli, that also contributeto variation in the torque expression of the appli-ance.12 In addition to reducing the torque prescribedin the bracket, a shallower wire position inside thebracket slot, arising from incomplete filling, leads to amore labial crown orientation. The second issue ofclinical interest associated with the increased rough-ness of slot walls derives from the association ofrough surfaces with friction. Although surfaces of lowmodulus may be relatively smoothed during toothmovement, generally, static friction is increased whenthe roughness-dependent coefficient of friction is high.

The fact that manufacturing of most plastic appli-ances involves the use of the same low-modulus rawmaterial for both the wing and base components ofthe brackets imposes several limitations on the per-formance of the appliances. In the area of metallicbrackets, the industry has adopted the manufactur-ing of a two-piece bracket, where the slot-wing com-ponent of the bracket is fabricated from stainless

steel alloy with increased stiffness, able to withstandthe loads during wire activation, while the base ismade from a low-modulus stainless steel type tofacilitate a peel-off effect during debonding.13 Earlyefforts to transfer this scheme to synthetic appli-ances gave rise to appliances with a plastic baseand ceramic face (Ceramaflex, TP Orthodontics,LaPorte, IN, USA). However, most appliances cur-rently are made from a single raw material.

Testing of the bracket wings was selected for sim-plicity and normalization purposes. Also tie-wingfracture is a major concern for the orthodontist,since the appliance becomes ineffective and ligationof the archwire to the bracket is no longer achiev-able (see Fig 4). Photoelastic studies and finite ele-ment analyses have shown that the base of the tie-wing is generally the location of concentrated stresswhen force is applied to the bracket (Fig 6).3 In clini-cal conditions, these fractures usually include theincisal wings of the mandibular brackets. However,maxillary brackets and gingival wing fractures alsooccur, probably attributed to hard food chewing (Fig7a) or complex masticatory loads (Fig 7b). Wing frac-ture may be aggravating since debonding of the frac-tured bracket is usually required, followed by rebond-ing of a new bracket, which necessitates extrachairside time, in addition to the risk of enamel dam-age involved in adhesive resin removal. Alternatively,a temporary management of this incident mayinclude formation of a shallow groove underneaththe wing of the bracket to allow for tying the wirewith a stainless steel ligature (Fig 8). Considering thelow modulus of the plastic brackets, it is expectedthat the load that will induce tie fracture may belower than that for ceramic or metallic appliances,although further verification is required for this

1816

1412

108642

0

15.114.1

10.29.2

10.2

12.?12.8 13.1

As-receivedIn vitroaged

B F L S

Fig 3 Results of the Knoop hardness test of as-received and in vitroaged retrieved plastic brackets.B: Brilliant; F: Align; L: Leone; S: Silicon.

Fig 4 Optical microscopy image of a plastic bracketwith a stainless steel slot insert in the as-received con-dition, illustrating the rough and irregular slot wall sur-face, which may jeopardize full wire engagement (pic-ture width 2 mm).

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hypothesis. In general, differences in the fractureloads may also arise from variations in the bracketdesign, but the single factor most affecting tie-wingstrength is the modulus of raw material.

The foregoing discussion finds application in thecase of fatigue of plastic versus metallic brackets,where fundamental differences may be observed.First, cyclic hardening or softening in metals dependsupon the composition, previous cold work, and tem-perature in metals, whereas polymeric materials

exhibit a cyclic softening effect.14 Also, loading rateand ambient temperature changes substantially alterthe stress-strain characteristics of polymers. In addi-tion, when cyclic loading involves increased strainrates, a thermal softening effect may be induced,leading to a reduction in the fatigue life. Further, thecomposition of the environment plays a major role inthe total life estimate of the polymer, a fact that gainsspecial meaning in the intraoral aging of plastic brack-ets in the severe environment of the oral cavity. More-

Fig 5 (Left) Secondary electronimage of a retrieved plastic bracketdemonstrating wing fracture. Thisdefect occurred intraorally and is notattributed to debonding procedures.(Original magnification 30.)

Fig 6 (Right) Image of a photoelas-tic bracket model depicting increasedstress concentration at the wing-basejoint. (Courtesy of Dr A. Caputo.)

Fig 7 Incisal wing fracture of a maxillary incisor bracket (a) and gingival wing fracture of a canine bracket (b) notdue to contact with opposing teeth or brackets.

Fig 8 Procedures to temporarily manage a wing fracture include the formation of a shallow groove with anultrafine diamond bur (a), adjacent to fractured wing, and tying with a stainless steel ligature (b). Care should betaken to not overextend the groove, as this would jeopardize appliance integrity or bonding of bracket.

a b

a b

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over, synergy among several factors, such as loadingand temperature, potent solvents, or pH fluctuations,leads to a reduced fatigue limit for a polymer speci-men. As a rule, polymers exhibit a largely inferior resis-tance to fatigue compared to metallic materials.

Generally, in vivo aging of materials involves a num-ber of potent parameters, which cannot be simulatedunder in vitro methodological approaches, includingapplication of complex, multi-axial loads arising fromthe engagement of the wire in the slot and the masti-catory loads; action of oral flora and its by-products,which may modulate local micro-environmental vari-ables, as in the case of the acidogenic Streptococcusmutans; enzymatic degradation; and temperaturevariations that impose a softening effect. The syner-gistic action of these factors has been found to alterthe surface integrity of stainless steel brackets. This isevidenced in the comparison of the hardnessbetween the in vitroaged and retrieved brackets.

Because safety precautions and associated legisla-tion have a taken pivotal role in the specialty recently,an emerging issue of critical significance is therelease of substances from orthodontic appliances.15

Although release of wear and degradation productsfrom orthodontic material mostly concerns the appli-cation of alloys and composite resins, recent effortshave focused on the qualitative and quantitativeassessment of the properties released from plasticbrackets. Evidence has been presented that indicatesbisphenol A, a precursor of Bis-GMA, may be releasedfrom polycarbonate appliances immersed in salinesolutions for 1 month.16 The significance of this find-ing relates to the potential estrogenicity of this factor,which is a constituent component of several polymerappliances and devices. Estrogencitity is the propertyof chemicals from the environment, termed xenoestro-gens, that mimic the hormone estrogen, which isimplicated in growth. The controversy surrounding theestrogenicity of dental resins has resulted in a singlepaper in the literature,17 which reported the release ofbisphenol A in the saliva from dental sealants. How-ever, the potency of bisphenol A to induce cell growthin cell cultures has been shown to be 1,000 timesless than that of natural estrogen.18 Thus, even if den-tal resins do release measurable levels of bisphenol Ain vitro, it is doubtful that this would have any biologicrelevance. Nonetheless, further evidence derived fromin vivo studies is required before a consensus on thisissue can be reached.

CONCLUSIONCONCLUSION

Plastic brackets present some disadvantages due tolow modulus and excessive wear arising from their

decreased hardness. These appear in the form of wingor bracket fracture during service, a fact that necessi-tates the debonding and rebonding of the appliance. Atemporary method to bypass the frequent incidence ofwing breakage may consist of formulating a grooveonto the body of the bracket which could facilitatetieing to the archwire without removal of the bracket.

REFERENCESREFERENCES

1. Cohl ME, Green LJ, Eick JD. Bonding of clear plastic orthodon-tic brackets using an ultraviolet-sensitive adhesive. Am JOrthod 1972;62:400411.

2. Miura F. Direct bonding of plastic brackets. J Clin Orthod1972;5:446454.

3. Aird JC, Millett DT, Sharples K. Fracture of polycarbonatebracketsA related photoelastic stress analysis. Br J Orthod1988;15:8792.

4. Alkire RG, Bagby MD, Gladwin MA, Kim H. Torsional creep ofpolycarbonate orthodontic brackets. Dent Mater 1997;13:26.

5. Aird JC, Durning P. Fracture of polycarbonate edgewise brack-ets. A clinical and SEM study. Br J Orthod 1986;14:192195.

6. Arici S, Regan D. Alternatives to ceramic brackets: The tensilebond strengths of two aesthetic brackets compared ex vivowith stainless steel foil-mesh bracket bases. Br J Orthod1997;24:133137.

7. Eliades T, Eliades G, Brantley WA. Orthodontic brackets. In:Brantley WA, Eliades T (eds). Orthodontic Materials: Scientificand Clinical Aspects. Stuttgart: Thieme, 2001:143173.

8. Blalock KA, Powers JM. Retention capacity of the bracketbases of new esthetic orthodontic brackets. Am J OrthodDentofacial Orthop 1995;107:596603.

9. Moser JB, Marshall GW, Green FP. Direct bonding of polycar-bonate orthodontic brackets: An in vitro study. Am J Orthod1979;75:7885.

10. Feldner JC, Sarkar NK, Sheridan JJ, Lancaster DM. In vitrotorque-deformation characteristics of orthodontic polycarbon-ate brackets. Am J Orthod Dentofacial Orthop 1994;106:265272.

11. Dobrin RJ, Kamel IL, Musich DR. Load-deformation character-istics of polycarbonate orthodontic brackets. Am J Orthod1975;67:2433.

12. Meling TR, Odegaard J. The effect of cross-sectional dimen-sional variations of square and rectangular chrome-cobaltarchwires on torsion. Angle Orthod 1998;68:239248.

13. Eliades T, Eliades G, Zinelis S, Athanasiou AE. Characteriza-tion of retrieved and recycled stainless steel brackets. J Oro-fac Orthop 2003;64:8087.

14. Suresh S. Fatigue of materials. Cambridge Solid State Sci-ence Series, Cambridge, UK: Cambridge University Press,1991:236.

15. Pichay TJ, Seifert LJ. Proposition 65 in the dental office. JCalif Dent Assoc 2001;29:501506.

16. Watanabe M, Hase T, Imai Y. Change in the bisphenol A con-tent in a polycarbonate orthodontic bracket and its leachingcharacteristics in water. Dent Mater J 2001;20:353358.

17. Olea N, Pulgar R, Perez P, Olea-Serrano F. Estrogenicity ofresin-based composites and sealants used in dentistry. EnvirHealth Perspect 1996;104:298305.

18. Wataha J. Principles of biocompatibility. In: Brantley WA, Eli-ades T (eds). Orthodontic Materials: Scientific and ClinicalAspects. Stuttgart: Thieme, 2001:271287.

COPYRIGHT 2004 BY QUINTESSENCE PUBLISHING CO, INC: PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY: NO PART OF THIS ARTICLE MAY BE REPRODUCED OR TRANSMITTED IN ANY FORMWITHOUT WRITTEN PERMISSION FROM THE PUBLISHER: COPYRIGHT 2004 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART OF THIS ARTICLE MAY BE REPRODUCED OR TRANSMITTED IN ANY FORMWITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.