clinical and laboratory perspectives of improved orthodontic bonding to normal, hypoplastic, and...

8/10/2019 Clinical and Laboratory Perspectives of Improved Orthodontic Bonding to Normal, Hypoplastic, and Fluorosed Ena

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Clinical and Laboratory Perspectives ofImproved Orthodontic Bonding to Normal,Hypoplastic, and Fluorosed Enamel

William A. Wiltshire and James Noble

Success in fixed orthodontic treatment is highly dependent on the main-

tenance of the bond between orthodontic attachments, and etchedenamel for the duration of treatment. Bracket debonding can significantlyincrease treatment time, operator time, material costs, and patient dis-comfort. It is therefore essential for the orthodontist to be able to obtainreliable bonding to enamel at the initial bonding appointment. At times,the orthodontist may need to bond to compromised enamel surfaceswith the standard acid-etch protocol. The advent of adhesion promotershas provided orthodontists the possibility to potentially increase the

bond strength of orthodontic attachments to these compromised enamelsurfaces. The current paper presents a summary of investigations of bondstrengths of orthodontic attachments to normal, hypoplastic, and fluo-rosed enamel as well as recent advances in biomaterials technology andtheir impact on adhesivity. (Semin Orthod 2010;16:55-65.) 2010 ElsevierInc. All rights reserved.

Direct bonding in modern day orthodonticclinical practice owes thanks to the pio-

neering work of Buonocore (1955)1 on the acid-

etch technique, as well as the introduction of achemically cured composite resin to dentistry byBowen (1962).2 This resulted in the introduc-tion of resin-bonded brackets in orthodontics 2years later.3 Three years after introducing com-posite resin to dentistry, Bowen4 described asurface-active comonomer, N-phenylglycine gly-cidyl methacrylate, with alleged chemical bond-ing to enamel. In the past 44 years since theintroduction ofN-phenylglycine glycidyl methac-rylate, many advances and innovations in adhe-

sion and bonding have occurred, resulting inthe introduction of products based on halophos-phorus esters of bis phenyl-A-glycidyl methacry-

late (bis-GMA) and hydroxyethylmethacrylate(HEMA), both withclaims of adhesivity to calci-fied tooth structure.5

Most of the research on adhesivity has been onrestorative dentistry. Orthodontics has often be-come a beneficiary of this work, particularly withthe quest for improved bonding to dentin in amoist environment. Initially, multistep procedureswith the new adhesion promoters heralded newvistas in dentin bonding, notwithstanding time-consuming and technique sensitive procedures.The coapplication of these new products, with im-

proved chemical adhesive properties and modi-fied technical procedures to orthodontic bonding,was an obvious progression for compromisedenamel bonding situations. The potential to bondorthodontic brackets to compromised enamel sur-faces with these new and improved products, de-spite some of their disadvantages, remains too re-markable to resist, in the orthodontists search forsuperior adhesives in difficult bonding situationsand/or in a moist oral environment.

From the Division of Orthodontics, University of Manitoba,Winnipeg, Canada.

Address correspondence to William A. Wiltshire, BChD, BChD(Hons), MDent, MChD(Orth), DSc (Odont.), F.R.C.D. (C)(Orth),Division of Orthodontics, Faculty of Dentistry, University of Mani-toba, 780 Bannatyne Ave., Winnipeg, MB, Canada. R3E 0W2;E-mail:[email protected]

2010 Elsevier Inc. All rights reserved.1073-8746/10/1601-0$30.00/0doi:10.1053/j.sodo.2009.12.005

55Seminars in Orthodontics, Vol 16, No 1 (March), 2010: pp 55-65
mailto:[email protected]:[email protected]


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Orthodontists have continued in their searchfor a user-friendly, one-step, reliable, stable, andaffordable, adhesion promoter that can bondsuccessfully to compromised enamel surfaces,in a moist oral environment, with adequate im-

mediate bond strengths that are maintainedthrough the course of treatment. Furthermore,the adhesion promoter should provide for quickand easy debonding, without damage or micro-fractures in the enamel surface and result in anenamel-resin surface with minimal resin residue,which is quick and easy to clean and polish.Resin tag remnants within the etched enamel atthe time of debonding, should be stain-resistantin the long term so that it doesnot compromiseesthetics after bracket removal.6

Bonding Challenges to Fluorosed,Hypoplastic, and HypomineralizedEnamel in Orthodontics

There appears to be an increased incidence offluorosed teeth in everyday clinical practice,even in geographic areas without an overabun-dance of fluoridated water, possibly due to theuse of fluoride supplementation and artificialfluoridation of community water supplies.7 It isbelieved that fluorosed enamel may be moreresistant to acid etching, resulting in decreasedbond strengths of orthodontic attachments to

the enamel.8,9 Because of the increased porosityof fluorosed enamel, its physical strength maysuffer10 and this may result in enamel damageduring debonding, particularly if high bondstrengths to enamel are achieved on certain ar-eas of the tooth. Patients who present to theorthodontist with dental fluorosis (Fig 1) need

to be advised about the difficulty and risks inbonding attachments to their teeth.

Fluorosed teeth manifest as an extensive hy-pomineralized subsurface layer underneath anouter well-mineralized acid-resistant surface

layer that varies between 50 and 100 m indepth.8 It is this outer acid-resistant hypermin-eralized layer that prevents conventional 37%phosphoric acid from effectively etching the sur-face, resulting in inconsistent etch patterns andan unreliable enamel surface for orthodonticbonding. Increased etchant concentrations andincreased etching time, of up to 2 minutes, haveprovided inconclusive results with respect to im-proved bonding.9 The subsurface hypomineral-ized layer of fluorosed enamel manifests itself asunsightly white or brown discolorations occur-ring as pits, striations, or white opaque lines (Fig1) posing an esthetic concern for patients whooften choose to receive composite or laminateveneers after the completion of orthodontictreatment.

Miller8 reported that orthodontic bondingfailure to fluorosed teeth occurs almost univer-sally at the enamel-resin interface, which in-creases the risk of enamel fracture.

The advent of adhesion promoters with a re-liable chemical bond would benefit orthodon-tists in clinically challenging situations, such asbonding to hypoplastic and hypomineralized

enamel, enamel surfaces with developmental de-fects and opacities, and patients with conditionssuch asamelogenesis imperfecta.

Effectiveness of Adhesion Promoters inBonding to Hypoplastic and FluorosedEnamel

An adhesion promoter provides the ability to usea chemical dimension during bonding, whichmay provide more predictable outcomes. Thepromoter consists of the primer, which is often

an aqueous solution of HEMA and a polyalk-enoic acid, which is believed to assist with mois-ture control. The primer allows the resin layer toflow or wet the etched surface. The adhesive isoften a Bis-GMA and HEMA resin combinedwith a blend of amines, which can provide a fast,10-second cure when activated by a visible lightcuring unit. It is claimed that this chemical ad-hesion to enamel results in less microleakageand a superior hermetic seal.11

Figure 1. A patient with fluorosis requiring orth-odontic bracket bonding. (Color version of figure isavailable online.)

56 W.A. Wiltshire and J. Noble


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Wiltshire et al12 bonded orthodontic attach-ments in vitro with Transbond (3M Unitek, St.Paul, MN) to moderately fluorosed teeth etchedwith 37% phosphoric acid for 60 seconds, withand without the use of an adhesion promoter

(Prime and Bond 2.0, Dentsply, Milford, DE).The bonded test assemblies were stored in waterin an incubator at 37C and 100% relative hu-midity for 24 hours to permit adequate watersorption equilibration before debonding. Nostatistically significant differences in shear bondstrengths (SBS) were recorded, whether an ad-hesion promoter was used or not (P 0.05).However, the mean SBS was slightly higher whenthe adhesion promoter was used. More impor-tantly, the standard deviation was less when theadhesion promoter was used on the teeth withfluorosed enamel (Table 1).

Inanother in vitro study, Schirmer and Wilt-shire9 compared the SBS of the orthodontic ad-hesive Right-On (TP Laboratories, Inc., LaPorte, IN) to normal and moderately fluorosedenamel using both buccal and lingual surfaces ofhuman premolars under the same test condi-tions as the aforementioned study.12 The resultsof this study are presented in Table 2. Theseauthors found no statistically significant differ-ences in the SBS values between fluorosed andnormal teeth (P 0.05). However, focusing onlyon mean values masks the large variation in SBS

values that were found, particularly in the lowerrangeof SBSs observed among certain fluorosedteeth. In fact, the range for fluorosed teeth wasstartlingly inconsistent and varied between 0.35and 29.71 MPa, whereas the nonfluorosed teethprovided more consistent SBSs ranging from6.22 to 22.38 MPa. The difference in the rangeof values is an indication of the complexity andvariability of the surface of fluorosed enamel asa bonding substrate. Another interesting obser-vation was that it was not possible, by clinicalappearance alone, to predict whether an orth-

odontic attachment would successfully bond to afluorosed tooth.

In contrast, it was shown in a recent study,where resin alone was bonded to fluorosedteeth, that the micro-SBS was not influenced bythe severity of the fluorosis when 2 adhesionpromoters were used.13

Accordingly, the use of adhesion promoters,originally developed for improved bonding todentin, may provide advantages in the search toattain superior and more consistent bondingprocedures when faced with the challenges ofcompromised enamel.

The question that arises is: would an adhesionpromoter with claims of chemical adhesion totooth structure and a consequent advantage ofpotentially reduced microleakage, be clinicallyadvantageous where enamel surfaces are intactand normal in appearance? Alternatively, would

the strength of the bond be so high that difficultdebonding procedures, patient discomfort, mi-crofractureand damage to enamel would occur?Wiltshire14 investigated the first question (simi-lar to the Ratnaweera et al study13 done 15 yearslater), by evaluating the tensile bond strengths(TBSs) of Concise Orthodontic Bonding System(3M, St. Paul) bonded to etched human enamelusing either a conventional Enamel Bond adhe-sive (3M, St. Paul) or Scotchbond (3M, St. Paul),an adhesion promoter based on halophospho-rus esters of Bis-GMA.

Although the TBSs were higher in magnitudewhen Scotchbond was used, the differences werenot statistically significantly different (P 0.05)(Table 3).

The TBSs in the lowest range of magnitude(8 MPa) were very similar between the 2groups and Scotchbond provided no apparentadvantage in increasing the TBS in the lowestranges, when bonding to normal enamel sur-faces. It must be borne in mind that these prod-

Table 1. Shear Bond Strength of Transbond (3MUnitek, Monrovia, CA) and Prime and Bond2.0 (Dentsply, Milford, DE) to Moderately FluorosedTeeth12

Fluorosed enamel without adhesionpromoter

14.94 8.80 MPa

Fluorosed enamel with Prime &Bond 2.0

17.27 2.33 MPa

Table 2. Shear Bond Strength of Right-On (TPLabs Inc., La Porte,IN) to Normal and ModeratelyFluorosed Enamel9

Nonfluorosedbuccal enamel

12.96 3.30 MPa (range, 8.43-20.68)

Nonfluorosed

lingual enamel

14.46 4.10 MPa (range, 6.22-22.38)

Fluorosed buccalenamel

13.03 4.30 MPa (range, 3.73-20.95)

Fluorosedlingual enamel

11.34 7.19 MPa (range, 0.35-29.71)

57Improved Orthodontic Bonding to Normal, Hypoplastic, and Fluorosed Enamel


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ucts were among the earlier generations of ad-hesion promoters.

Newer Generations of AdhesionPromoters and Self-Etching Primers

Since Fusayama15 advocated the etching of den-tin in conjunction with a second-generationadhesion promoter containing a hydropho-bic monomer (phenyl-P), which reacted withHEMA (Clearfil Bond System F, Kurary Dental,Kurashiki, Japan), development is ongoing inthe field of dentistry with regard to adhesionpromoters for bonding to dentin. The seventhgeneration of adhesives was introduced in 2002and includes products, such as iBOND (HeraeusKulzer, Hanau, Germany) and G-Bond (GC, Al-sip, IL), which are one-step, one-component,no-mix systems with a simplified one-step clini-cal procedure and a user-friendly application.These new systems replace the more compli-cated multistep procedures involving etching,

rinsing, conditioning, priming, and then curing,which have the potential to introduce more er-ror and contamination and subsequently in-creased risk of bond failure.

Self-etching primers (SEPs) in orthodonticsgained popularity because they save chair timeby reducing the need for an independent etch-ing step. They are also less aggressive in theiretching capability, which would theoretically re-sult in less enamel loss and there is a decreasedneedto remove residual adhesive after debond-ing.16A concern with SEPs other than increased

costs, is the adequacy of their bond strength,which remains controversial.17 However, in arecent randomized clinical trial,16 using 2 differ-ent SEPs, only 26 bracket failures were recordedfrom 480 brackets placed on 24 patients andfollowed up over a 12-month period. This high-lights the clinical success of SEPs in an orth-odontic application.

Of particular interest to orthodontists isearlybond strength, which must be sufficient to permit

the arch wire to be tied in immediately. SEPs useweaker acid concentrations, which may preventenamel from being effectively etched. This canbe of greater concern in enamel that is alreadycompromised. In addition, the single systems

acidic adhesive produces an oxygen-inhibitedresin layer. This oxygen-deprived layer is oftenincompatible with self-cure and dual-cure resinsand product selection thus becomes important.The universal effectiveness of all SEPs on com-promised enamel surfaces therefore remainsquestionable.

To address the concern of the potentiallyweaker bond strengths of SEPs, Wiltshire andKaraiskos18 evaluated and compared the SBSs of3 new SEPs to human enamel (Table 4). Trans-bond plus SEP (3M Unitek, Monrovia, CA) wasalready being marketed as an orthodontic SEP,but iBOND (Heraeus Kulzer, Hanau, Germany)and One Coat SE Bond (Coltne Whaledent,Altsttten, Switzerland), were not. Transbondplus SEP produced the highest SBS (18.8 3.5MPa), followed by iBOND (16.8 3.4 MPa),and then One Coat SE bond (14.4 4.5 MPa).No statistically significant differences existed be-tween the 3 SEPs (P 0.050664). However, thePvalue (P 0.050664) was very close to the 5%level of significance (P 0.05), which wouldhave shown a significant difference. All 3 SEPsproduced greater values than what has been

considered acceptable for orthodontic bond-ing (6-8 MPa).19 Alhough the Wiltshire andKaraiskos study was undertaken in vitro,18 theconcern that SEPs result in weaker SBS, were notfound. The fact that SEPs can perform ade-quately, has subsequently been proven in theclinical setting for Transbond plus SEP (3MUnitek, Monrovia).16 However, the same cannotbe said for Clearfil Protect Bond (Kuraray Den-tal, Kurashiki, Japan).16

Table 3. Tensile Bond Strength Comparing EnamelBond (3M, St. Paul, MN) and Scotchbond (3M, St.Paul) to Etched Human Enamel13

Concise Orthodontic Conciseenamel Bond

13.3 3.5 MPa(range, 8.0-19.1)

Concise Orthodontic

Scotchbond

17.8 5.7 MPa

(range, 8.5-24.2)

Table 4. Shear Bond Strengths of Three oftheNew Self-Etching Primers to Human Enamel16

Transbond Plus SEP(3M Unitek,Monrovia, CA)

18.8 3.5 MPa (range, 14.4-24.2)

iBOND (HeraeusKulzer, Hanau,Germany)

16.8 3.4 MPa (range, 11.7-21.8)

One Coat SE Bond(ColtneWhaledent,Altsttten,Switzerland)

14.4 4.5 MPa (range, 7.6-19.7)



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All SEPs do not appear to perform similarly inthe clinical environment and in vitro bondstrength studies, as well as clinically based stud-ies remain important to determine which newproducts to select forpatient care.

Recently, Ho et al20 investigated the immedi-ate mean SBSs of Transbond plus SEP (3MUnitek, Monrovia) and 2 of the new seventhgeneration SEPs (iBOND, Heraeus Kulzer andG-Bond [GC, Alsip]) to human teeth in vitro(Table 5). The results demonstrated thatG-Bond, produced statistically significantlyhigher mean SBSs (P 0.01) (8.3 MPa) thanTransbond plus SEP (5.32 MPa), but did notdiffer significantly from its other seventh ge-neration rival, iBOND (P0.05) (6.69 MPa).The conventional Transbond XT system (3MUnitek) served as the control in this study and

produced the highest mean SBSs (P 0.01)(11.22 MPa). Ho et al study20 demonstrated thatin normal enamel, conventional acid etchingand bonding outperforms the SEPs and from abond strength perspective alone, it seems thatSEPs offer no advantage in orthodontic bondingto normal enamel.

The aforementioned 2 studies18,20 were un-dertaken at intervals of 4 years, at the sameinstitution and used 2 of the same SEP materials,Transbond plus and iBOND, with the same test-ing machine (Zwick, Ulm, Germany) and the

same crosshead speed (0.5 mm/min). In 2005,18

the SBS tests wereconducted after 24 hours andin the 2009 study,20 the tests were conductedimmediately after bonding.

An interesting finding was that the matura-tion of the bonds after 24 hours is very markedwhen comparing the 2 studies. After 24 hours,the bond strength of Transbond SEP increasedby about 13 MPa and that of iBOND by about 10MPa. The clinical implication of bond matura-

tion because of water sorption equilibration,even after only 24 hours, was noteworthy. Whilethe immediate SBSs of orthodontic attachmentsbonded with SEPs are in the lower range ofmagnitude, they are nevertheless within what

may be considered acceptable limits for earlyarch wire ligation with light forces, such as thosepotentially produced by small dimension, lightinitial arch wires. Judging from these 2 studies,bond maturation seems to at least double ortriple the SBSs within the first 24 hours.18,20 Thissuggests that when the enamel surface is com-promised and the orthodontist is using SEPs, itmay be prudent to wait 24 hours after the brack-ets are bonded, when increased bond strengthsare present, before insertion and tie-in of thearchwire. This may, however, prove to be clini-cally challenging.

A more recent study by Minick et al21 investi-gated bond strengths of newer bonding systemswith either bioactive components (amorphouscalcium phosphate and an antibacterial mono-mer 12-methacryloyloxydodecylpyridinium bro-mide), or an SEP with a conventional bondingsystem (Transbond XT, 3M Unitek).

The newer bonding systems achieved SBSssignificantly lower than the conventional system(Transbond XT), which achieved SBSs of 10.1 0.8 MPa at 30 minutes and an almost identicalSBS of 10.1 1.0 MPa at 24 hours. The newer

systems provided varying SBSs at both 30 min-utes and 24 hours:

Aegis Ortho (Harry J. Bosworth, Skokie, IL),(5.3 0.5 MPa at 30 min; 7.2 0.7 MPa at24 h);

Clearfil Protect Bond (Kuraray Dental,Kurashiki, Japan), (7.1 0.8 MPa at 30 min;6.1 0.6 MPa at 24 h);

Clearfil SE Bond (Kuraray Dental, Kurashiki,Japan), (3.8 MPa at 30 min; 6.6 0.5 MPa at24 h), and

iBOND (Heraeus Kulzer), (3.9 0.4 MPa at

30 min; 3.9 0.3 MPa at 24 h)

The authors21 concluded that iBONDs SBS islower than that which might be acceptable forclinical usefulness.

In contrast, Wiltshire and Karaiskos18 re-ported much higher values for iBOND (HeraeusKulzer) at 24 hours (16.8 3.4 MPa; range,11.7-21.8 MPa) than reported byMinick et al21

(3.8 MPa). In addition, Ho et al20 also reported

Table 5. Shear Bond Strength to Human Enamelof Transbond Plus (3M Unitek, Monrovia, CA),iBOND (Heraeus Kulzer, Hanau, Germany)andG-Bond (GC, Alsip, IL) Self-Etching Primers17

Transbond XT(control)

11.22 1.98 MPa (range, 7.91-14.92)

TransbondSEP

5.30 1.81 MPa (range, 2.78-8.83)

iBOND 6.89 1.78 MPa (range, 2.84-9.84)G-Bond 8.30 2.42 MPa (range, 5.17-13.35)



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higher values at 24 hours (6.89 1.78 MPa;range, 2.84-9.84 MPa) compared with that re-ported by Minick and coworkers at 24 hours(3.9 0.3 MPa).21 However, Ho et al20 reportedvalues were lower than those reported by Wilt-

shire and Karaiskos.18

Although rigorous in vitro studies remain thecornerstone of biomaterials testing, they do notreplicate the clinical situation. In vivo investiga-tions are therefore a necessary and vital part ofproduct evaluation, particularly as the results ofin vitro investigations vary greatly.18,20,21

With variations in in vivo versus in vitro situ-ations in mind, a very recent randomized clinicaltrial evaluated the performance of a self-etchingsystem (SEP) (Transbond plus SEP, 3M Unitek)compared with a conventional system (Trans-

bond XT, 3M Unitek) over a 12-month period invivo. No significant differences were found be-tween the failure rates of the conventional(4.78%) and SEP (6.88%) groups. It was con-cluded that both systems were adequate for orth-odontic needs.22 Results, therefore, of rigorousindependent clinical testing of products, re-mains the most important indicator of productevaluation and selection. The translation of re-search from bench-side to clinical, cannot beoverstated.

A recent prospective clinical study on bond-

ing to fluorosed enamel11

compared the failureof brackets bonded to severely fluorotic teethusing an adhesion promoter (Scotchbond Mul-tipurpose Plus Primer, 3M, St. Paul) in combi-nation with acid-etching, with and without theadditional step of microabrasion of enamel with0.05 m of aluminum silicate for 5 seconds. Theauthors found no statistical significance betweenthe 2 groups, suggesting that the use of an ad-hesion promoter can provide the clinician withsuccessful bonding to fluorosed enamel, withoutthe need for the additional step of microabra-

sion. Although it has been suggested that micro-abrasion of fluorosed enamel concomitantlywith acid etching, improves bond strength,8,23

this study11 suggested that the use of an adhe-sion promoter can effectively replace microabra-sion. The drawbacks of microabrasion includedamage to enamel, the need to use a rubberdam, poor powder control, patient ingestion ofthe powder particles and potential from thepowder aerosol to cause facial trauma, increased

chair time and costs, and allergy to the alumi-num oxide or silicone carbide powder.

Further clinical trials evaluating the SEPs andadhesion promoters to both normal and com-promised enamel during orthodontic bonding is

necessary before any definitive conclusions canbe drawn.

Influence of Nanotechnology inOrthodontic Bonding

The advent of the nanotechnology era in dentalbonding is upon us. Individual, discrete particlesas small as 0.1-100 nanometers (nm) are usedwithout particulate agglomeration and the nano-particles are kept in a colloidal suspension. Po-tential advantages are longer shelf-life, increasedstability, increased strength, improved manipu-lative advantages, homogeneity, translucency,and polishabilty, as well as showing 50% lessshrinkage.24 Disadvantages include increasedcosts and the lack of laboratory and clinical stud-ies that assess their efficacy. Some of the afore-mentioned properties of nano-era adhesives maybe of interest in orthodontic bonding andin particular, bonding tocompromised enamelsurfaces. Bishara et al25 compared the SBSof a nano-hybrid restorative material, Grandio(Voco, Cuxhaven, Germany), to that of a con-ventional adhesive material (Transbond XT; 3M

Unitek) when bonding orthodontic brackets.The mean SBS for Grandio was 4.1 2.6 MPaand that for Transbond XT was 4.6 3.2 MPa.During debonding, 3 of 20 brackets (15%)bonded with Grandio failed without registeringany force on the Zwick recording machine.None of the brackets bonded with TransbondXT failed in a similar manner. The authors con-cluded that the nano-filled composite materialshowing only 15% catastrophic failure at testing,yet providing similar mean SBSs to a conven-tional resin, can potentially be used to bond

orthodontic brackets to teeth, if their consis-tency can be more flowable, to readily adhere tothe bracket base.

Voco GmbH (Cuxhaven, Germany) has nowreleased Universal Futurabond DC, which itrefers to as probably the first eighth-generationadhesive combining all the advantages of all pre-vious generations in one bonding system. It is anano-reinforced, dual-cured, single-step, self-etch adhesive, which is moisture-tolerant and



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can be universally used as a bonding agentand/or adhesion promoter with all compositeresins that are currently on the market. Its uni-versality with orthodontic resins has yet to betested.

Moisture-tolerant adhesives, however, are notnew in dentistry and have been of interest toorthodontics for several years. Nemeth, Wilt-shire, & Lavelle26 compared the in vitro SBSs of2 moisture-tolerant products, a cyanoacrylate(Smartbond, Gestenco, Sweden) and a poly-acidmodified composite adhesive (Assure, Reliance,Itasca, IL) to etched human enamel in either apostetched, washed, and dried state or wet salivaflooded state. Transbond XT (3M Unitek) adhe-sive served as the control SBS bonding product.SBSs were determined at 24 hours and 6months.

The overall mean SBSs for Smartbond variedfrom 0 to 8.98 MPa, Assure from 2 to 10.93 MPaand Transbond XT from 0 MPa (moist/wetstate) to 17.67 MPa, when tested in both wet anddry bonding conditions.

In moist bonding conditions, Assure outper-formed the other bonding test groups reachingmean SBSs of 6.03 MPa after 24 hours and ma-turing to slightly higher mean SBSs of 6.86 MPaafter 6 months. It was concluded that bonding tomoist or wet enamel is material specific. TheSBSs, which were achieved in the lower ranges of

magnitude and varied between 0 and 2.22 MPafor both of the moisture-tolerant materials(Smartbond and Assure), make these 2 productstoo unreliable for clinical application in orth-odontic bonding in the wet/moist state.

Glass Ionomers as Bonding Agents

Glass ionomers (GIs) have become increasinglypopular in dentistry during the past 2 decades asan alternative bonding agent, due to their anti-cariogenicity through fluoride release,27 chemi-

cal adhesion to tooth structure, and stainlesssteel and, in addition, the elimination of theneed to etch enamel before bonding. The abilityto release fluoride is not unique to GIs as F-releasing orthodontic resins and bonding agentshave been developed for their anticariogenicpotential.28,29,30

Wiltshire31 published the results of an in vitrostudy where he evaluated the SBS of a new an-hydrous GI (Orthocem B, PSP Dental, Co, Belve-

dere, Kent, UK) to human enamel, with andwithout etching. A no-mix adhesive resin (right-On, TP Orthodontics, Inc., La Porte, IN) wasused as a control resin to bond orthodonticattachments to enamel (Table 6). The GI ce-ments produced significantly lower SBSs thanthe no-mix adhesive resin with the lowest meanvalues (2.2 MPa) being recorded when noenamel etching was carried out (P 0.05). Evenwhen the enamel was etched, the lowest valuesattained (2.7 MPa) remained statistically signifi-cantly low (P 0.05), when compared with thecontrol. However, there were no statistically sig-nificant differences found between the 2 GIgroups, etched or not (P 0.05).

In the aforementioned study,31 the GI SBSs inthe lowest ranges of magnitude (2.2-2.7 MPa)would be a concern for the orthodontist, if di-rectly extrapolated to the clinical situation. Fa-

jen et al32 tested 3 different GIs and reportedsimilar mean SBS values. However, Cook &Youngsen33 found that the pretreatment ofenamel with polyacrylic acid, weakened the SBSsignificantly.

Despite the apparent low in vitro SBS of GIs,a 12-month clinical study34 using a light-acti-vated GI cement for direct bonding of orthodon-tic brackets in vivo, demonstrated that therewere no significant differences in failure ratesbetween a GI (Fuji II LC, GC America, Inc.,Alsip) (3.3%), and a conventional orthodontic

resin (System 1

, Ormco Corporation, Glen-dora, CA, USA) (1.6%), despite the GI failurerate being double that of the conventional Sys-tem1 resin.

Conflicting in vitro results when comparingmaterials clearly demonstrates that while inde-pendent laboratory studies provide importantand valuable product information about a mate-rials performance and allows interesting prod-uct comparisons, the in vitro data should always

Table 6. Shear Bond Strength of Orthocem B (PSPDental Co., Belvedere, Kent,UK) Glass Ionomer toEtched and Unetched Enamel21

Unetched GI 4.4 1.8 MPa (range, 2.2-7.3)

Etched GI 5.5 2.2 MPa (range, 2.7-10.5)Etched No-mix right-On

(TP Laboratories,Inc., La Porte, IN)control resin

26.0 6.0 MPa (range, 16.0-31.0)



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be extrapolated to the clinical situation, withextreme caution. Kusy35 commented on Wilt-shires GI study31 stating that stronger is notnecessarily better. This is true, particularly forGI bonding in orthodontics where mean SBS of

5 MPa measured in vitro appear to performadequately, clinically.

Consideration of OptimumOrthodontic Bond Strength

Newton (N) is the metric unit of force. One N isrequired to accelerate a mass of 1 kg at a rate of1 m/s2. The force of earths gravity on a small102 g apple is about 1 N. Forces are often ex-pressed in kiloNewton (kN) (1 kN 1000 N), orin pound (lb) or pound-force (lbf.) (1N 0.22481 lbf.).

36,37

In orthodontic bond strength testing terms,use is often made of the Pascal (Pa), or metricunit of pressure or stress (1 Pa 1 N acting onan area of 1 m2). Pound per square inch (psi) asa unit of stress is also frequently used in bondstrength reporting (1 psi 1 lbf/in

2) (1 psi 6894.76 Pa).36,37

MegaPascal (MPa) is currently generally ac-cepted as the preferred unit for reporting bondstrength.

Bond strengths can be converted as follows: 1MPa 1 000000 Pa 1MNm2 1N mm2

145.037743psi or lbf/in2.36,37Waters38 measured the maximum achievable

biting force on a single tooth as 265 Newton (N),but noted that normal occlusal loading was in amuch lower range of merely 3-18 N.

In orthodontics, the average force of mastica-tion on anterior brackets is approximately 5 MPaand approximately 20 MPa on posterior teeth.39

Masticatory forces delivered to orthodonticbrackets in vivo are thus probably a complicatedmixture of shear, peel, shear-peel, and tensileforces, which are difficult, if not impossible, to

replicate in the in vitro environment. Neverthe-less, laboratory studies give the clinician an un-derstanding of how biomaterials may perform inthe complex oral environment.

It is clear from the studies reported in theliterature that the bond strengths of orthodonticattachments to enamel vary greatly, dependingon the material used, the conditioning agent,the adhesive, enamel morphology, preparationof the enamel surface, and the test conditions.

Test conditions can vary widely between differ-ent investigators and include the varying use ofbovine, porcine and human teeth, extractederupted, or surgically removed impacted andunerupted third molars. Differences in testing

equipment, crosshead speed, load cell applica-tion, storage media, thermocycling, test method(tensile, shear), and variations in the site offorce application, make comparisons betweendifferent studies difficult or even impossible. Inaddition, data reporting as either debond forcein (kilograms [kg] or Newton [N]), orforce perunit area (MPa, N mm2, MN m2, lbf/in

2) of thebonding surface further complicates compari-sons of reported bond strengths from variousstudies. All these differences make direct com-parisons between the various investigations dif-ficult, even enigmatic, but highlight the value ofcomparing a series of studies performed underexact or similar test conditions.

The series of studies presented in this articlewere performed by 1 investigator and his col-leagues.6,9,11,12,14,18,20,26 Test conditions werecontrolled. The crosshead speed during testingwas standardized at 0.5 mm/min. All sampleswere stored at 100% relative humidity and 37Cin an incubator in distilled water simulating theoral environment. In addition all the test sam-ples were kept hydrated in distilled water beforeSBS testing, rather than the over-rigorous ther-

mocycling, which does not represent the normaloral environment. All sample SBS testing wasdone using the Bencor Multi T testing castle(Danville Engineering, San Ramon, CA) (Fig 2),which was first developed at the University ofPretoria, Republic of South Africa.40 A knife-edged shearing blade was applied at thetoothbracket interface as the bonded testassembles were tested to failure (Fig 3). Onlyunaltered human teeth with a correct anatom-ical form were used in the authors studies. Awide range of bond strengths to enamel were

nevertheless recorded, with the range 0-29.71MPa.9,12,18,20,26,31

With the generation of biting forces in theposterior region of the mouth approaching 20MPa,39 there may be an indication for the needto attain SBSs of bonded brackets of at least 20MPa on posterior teeth. However, because mas-ticatory forces are dynamic and complex, this isnot the case. It has been recorded that materialssuch as the GIs remain satisfactorily bonded in-



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traorally, even though when tested in the labo-ratory recorded SBSs may be as low as 2.2MPa.31,34

Enamel fractures have been observed withbond strengths as low as 9.7 MPa,41 at theadhe-sive-etched enamel interface. Reynolds19 pro-posed that clinically acceptable bondstrengthsshould be in the 6-8 MPa range.16 However,Reynolds study19 was published 30 years agoand testing systems, computerization, and prod-

ucts have changed significantly over the past 3decades. Higher bond strengths are not neces-sarily optimal clinically. Further, attempting tocorrelate in vitro and in vivo bond strengths isdifficult. It becomes even more complex, if notconfusing, to try and compare different studiesfrom different institutions using different mate-rials and methods.

Stronger SBSs are not always better35 andbond strengths that are too high may do nothing

more than create iatrogenic damage duringbracket debonding. Accordingly, the idealbond strength is difficult to define, as everypatient is unique with respect to the ability oftheir enamel to be etched and their individual-

ized masticatory and intraoral factors that mayaffect bonding and bond strength.

Indeed, the differences in the composition ofenamel within the teeth of each patient, and, thecomplex masticatory forces at work on bondedbrackets, while chewing a range of differentfoods, differing muscular patterns, bruxism andclenching habits, and the different forcespresent in the mouth as the teeth are beingmoved during treatment, make predicting bondstrengths a formidable task.

Again, rather than solely focusing on the mag-nitude of the bond strengths, clinicians need tobe continually mindful of possible iatrogenicdamage at the debonded interfaces, such asgross enamel fracture, enamel tearing, crazingand microfractures. Adhesive remnants shownto have tag-lengths o f up to 1 m penetrat-ing into enamel13 are also important as theirclean-up and removal during debonding proce-

Figure 3. The knife-edged shearing blade, which isattached to the Bencor Multi-T System (Danville En-gineering, San Ramon, CA) and applied at the bracket-enamel interface during shear testing. (Color version offigure is available online.)

Figure 2. The Bencor Multi-T System (Danville Engi-neering, San Ramon, CA) attached to a Zwick testingmachine (Ulm, Germany) in preparation of shearbond strength (SBS) testing. A 10-20 kN load cell isfrequently used in SBS testing and crosshead speedsgenerally vary between 0.5 and 5 mm/min in thereported literature. The authors SBS research alwaysused a knife-edged shearing blade at the bracket-enamel interface parallel to the mounted tooth sur-face in the tooth test-cups. (Color version of figure isavailable online.)



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dures may result in potential iatrogenic damageand different adhesives may take differentamounts of time to clean. Residual tags in theenamel may also be subject to staining.6

It seems reasonable to assume that for mini-

mal reliable clinical bond strengths to occur, invitro SBS testing should yield values of at least 3or 4 MPa for the lowest values in the range,generated in a bond strength test series. Prefer-ably, SBSs should be consistent along the testrange with low coefficients of variation (20%).When evaluating bond strength studies, it is im-portant to consider not only mean values butalso the range of values attained, which may bemore important when considering the effective-ness of a new product. In vitro testing shouldattempt to mimic oral conditions as much as is

possible with human teeth preferred over bovineor porcine teeth. Independent, unbiased labora-tory or university-based testing of all new bioma-terials should form the basis of product selectionin clinical orthodontic practice.

The search continues for the ideal orth-odontic adhesive. With new advances in nano-technology-based adhesives, we are at the fore-front of working with a more user-friendly,efficient, and effective adhesive that is univer-sally applicable to all bonding situations. Theneed for independent testing, universally andinternationally accepted testing standards andmore independent clinical research, remains anall important phase of product evaluation forthe future.

The placement of an arbitrary numericalclinically acceptable bond strength is not thepurpose of in vitro testing and solely looking atthis value oversimplifies these studies. Rather,rigorously designed, clinically simulated in vitroinvestigations should serve as the gateway toin vivo clinical trials.

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