Enamel Bonding of Self-etch and Phosphoric Acid-etch Orthodontic ...

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140Enamel bonding of orthodontic adhesive systemsabenamel surface was selectively etched and easily identified.Micro-irregularity of the hydroxyapatitecrystals was apparent.Typical fracture surfaces on the enamel sideafter shear bond testing are depicted in Fig. 4.Partial adhesive failure occurred predominantly inthe BO, TB and KF groups, in which remnants ofthe adhesives remained on the enamel surface (Figs.4(a) to (c)). In the OS group, failures mainly occurredat the interface between the bracket and theadhesive in both TC-0 and TC-5000 conditions (Fig.4(d)). Distinct impressions of the bracket mesh wereobserved.Fig. 4 SEM photographs of typicalfracture surfaces on the enamelside after shear bond testing.a: Beauty Orthobond (TC-0) (2000); b: Transbond XT (TC-0)(2000); c: Kurasper F (TC-0)(2000); d: Orthomite Superbond(TC-0) (50). Partial adhesivefailure was observed, and remnantsof adhesive remained onthe enamel surface (a, b, c).Distinct impression of thebracket mesh was observed (d).Mode of fracture tendency ofeach adhesive was not differentbetween the TC-0 and TC-5000groups.cdDISCUSSIONDirect bonding of orthodontic brackets using theacid-etch technique has become a common techniquein the orthodontic field. Phosphoric acid etching producesa roughened enamel surface by dissolving calciumcomponents and forming enamel resin tags.Although enamel etching technique is a useful andaccepted procedure for bonding orthodontic brackets,there is a need to maintain clinically useful bondstrengths while minimizing the amount of enamelloss.Recent studies in operative dentistry have suggestedthat self-etching primers with lower decalcifyingability are less effective than phosphoric acidetching when used to bond to ground enamel or intactenamel 19) . Previously, Bishara et al. 11) found thatthe shear bond strength of orthodontic brackets afterPrompt L-Pop self-etching primer treatment was significantlylower than that after phosphoric acid etchingwith TB. Yamada et al. 20) also found that theshear bond strength after SE Bond self-etchingprimer (Kurary Medical) treatment was significantlylower than that after phosphoric acid etching withKF. Similarly, the present study demonstrated thatthe shear bond strengths of brackets bonded with theself-etching adhesive systems (BO and TB) were lowerthan those bonded with phosphoric acid-etching adhesivesystems (KF and OS) in the control groups.However, a bond strength of approximately 17 MPawas maintained after thermal cycling in the selfetchingadhesive groups, while the bond strength significantlydecreased after thermal stress in thephosphoric acid-etching groups. It should be pointedout that Martin and Garcia-Godoy 21)commented thathigh shear bond strength in orthodontics is not necessarilya beneficial property of an orthodontic adhesive,because enamel can be lost during the debondingprocedure as well as during the removal of residualresin. Bishara et al. 22) reported that a shear bondstrength of 7 MPa to the enamel was clinically acceptablefor bonding to the enamel surface in orthodontictreatment. Data obtained in this study thus
KITAYAMA et al. 141suggested that all the adhesive systems evaluatedwere acceptable for routine clinical use.The effects of thermal changes on the bondstrength of resin-based materials to hard dental tissues,as well as on their mechanical properties (i.e.,fracture toughness, elastic modulus), have been welldocumented 23,24) . Thermal cycling stresses the bondbetween resin and tooth substance and might affectbond strength 25) . Christensen 26) commented that impressivein vitro bond strengths were transient whensubjected to temperature changes in the mouth.In the case of self-etching adhesive systems, theacidic monomers in the self-etching primers are responsiblefor both etching and bonding. As such, thedepth of demineralized enamel corresponds to thedepth of penetration of the adhesive to be polymerized.This mechanism thus circumvents problems associatedwith insufficient penetration as well asimproves the quality of hybridization 27-29) , thereby ensuingan excellent mechanical lock.Phosphoric acid etching creates resin tags formechanical retention between enamel and resin 30) .However, the resin may not completely infiltrateetched enamel 31) . A region of unprotected enamelprisms might be susceptible to hydrolytic degradationafter thermal cycling. In addition, water diffusioninto the bonded interface between adhesive andtooth surface was found to cause the resin to swelland become plasticized 32) , as well as reduce enamelhardness due to loss of surface calcium 33) .Basically, there are two opinions on the remainingadhesive following bracket debonding. One opinionlargely favors bracket-adhesive interface failurewith the adhesive resin left mainly on the enamelsurface 5,34) , when a heavy-filled resin is used to bondorthodontic attachments. The microporosities createdby etching are filled with the resin and provide mechanicalretention. The second opinion favors failureat the enamel-adhesive resin interface, because thereis less adhesive left to remove from the enamel surfaceafter debonding 34) . Martin and Garcia-Godoy 21)suggested that a weaker adhesive with a lowerstrength value might be preferable so as to increasefailure rate at the enamel-adhesive resin interface.In this way, minimal clean-up would be needed withreduced likelihood of damage to the enamel.The predominant mode of bracket failure for BO,TB, and KF was at the enamel-resin interface withless than 50of the adhesive on the enamel surface.In the cases of BO and TB, scratch lines with 600-grit silicone carbide paper were observed afterdebonding, which indicated that the self-etching adhesivesystems did not damage the enamel surface duringdebonding.As for OS, the predominant mode of bracket failurewas at the bracket-adhesive interface with morethan 50of the adhesive on the enamel surface. OSis MMA-based, the mechanical properties of whichare weaker than the light-cured adhesive resins 35,36) .Hotta et al. 37) reported that 4-MET, a hydrolysisproduct of 4-META, promoted effective diffusion ofmonomers into enamel. SEM observations of theconditioned enamel surfaces revealed that the selfetchingprimers produced less enamel dissolutioncompared with phosphoric acid etching. Moreover,the morphological appearances of the enamel surfacewere different between KF and OS, which was probablydue to the different concentrations of phosphoricacid in the etchants. The 37phosphoric acid of K-etchant in KF was more aggressive than the 65solutionof Red Activator in OS 31) .Decalcification is a common side effect of fixedappliance orthodontic treatment 38) . On this note, orthodontictreatment with multibonded appliancesposes a significant caries risk 6,39) . To counter thisproblem, fluoride-releasing composites for bondingbrackets have attracted considerable attention andgarnered much interest. This is because they mayinhibit the decalcification of enamel around thebrackets by delivering fluoride to the affectedenvironment 40,41) . Furthermore, the remineralizationcapability and antibacterial property of fluoride mayhelp eliminate the risk of dental caries 42) . BO containsS-PRG filler for the release and uptake of fluorideions, and might thus prevent demineralizationbut facilitate remineralization of the surroundingenamel 6) .Orthodontic bracket bonding is performed on intactenamel. The intact enamel surface ishypermineralized and contains more fluoride thanground enamel. Prismless layer on enamel surface 43)is less conducive to bonding by conventional acid gelconditioning 44) and self-etching primer application 45) .Kanemura et al. 45)reported that bond strength wassignificantly reduced when self-etching systems werebonded to intact enamel. Similarly, Senawongse etal. 46)reported that self-etching adhesive systems exhibitedsignificantly lower bond strengths than thephosphoric acid-etching adhesive systems on intactenamel. However, no statistically significant differenceswere found between self-etching adhesive systemsand phosphoric acid-etching adhesive systems toground enamel. Furthermore, bond strengths of selfetchingadhesive systems to ground enamel were significantlyhigher than those to unground enamel,whereas phosphoric acid etching systems showed nosuch significant differences between intact andground enamel. Based on the results obtained todate, further research is indeed needed to clarifywhether self-etching primer adhesive systems couldprovide sufficient bond strength to intact humanenamel.
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