ONLINE ONLY ARTICLE

Effect of bracket base conditioningAndreas Faltermeiera and Michael Behrb

Regensburg, Germany

Introduction: The aim of this study was to compare the effect of a silicoating system, the influence ofsandblasting, and the effect of a silane-coupling agent after sandblasting on the shear bond strength of stainlesssteel foil-mesh brackets. To simulate the oral environment, all specimens were thermocycled (6000 times at 5°Cand 55°C) in a mastication device before testing. Methods: Four bracket groups were tested: group 1consisted of 20 metal brackets that were sandblasted on the base; group 2 contained 20 brackets that weresandblasted, and a silane-coupling agent was applied; in group 3, the surface of the base of 20 metalbrackets was treated by using a tribochemical system; and group 4 was the control group. The brackets werebonded with a light-curing adhesive to extracted third molars, and the shear bond strength and the adhesiveremnant index score were determined. The brackets of group 1 were reconditioned after debonding withsandblasting and tested again (group 5). Results: Sandblasting and tribochemical treatment of bracketsimproved the shear bond strength of stainless steel brackets. Combined sandblasting and silane-couplingtreatment offers no benefit of increased in-vitro strength. Conclusions: The bond of resins to tribochemicallysilicoated stainless steel brackets seems to be sufficient to strengthen the bond between the adhesive andthe metal bracket. This treatment is mainly indicated for low-compliance patients or teeth that are difficult to

bond. (Am J Orthod Dentofacial Orthop 2009;135:12.e1-12.e5)

The material most commonly used for brackets isstainless steel. Nevertheless, many efforts havebeen made to improve their esthetic appearance,

such as making them smaller or making them fromceramic and plastic. However, metal brackets performcloser to the ideal than their nonmetal counterparts.

Various types of steel, including American Iron andSteel Institute types 303, 304, 304L, 316, 316L, and317 are used for orthodontic brackets.1,2 Most orth-odontic brackets are made from type 304L, whichcontains 18% to 20% chromium and 8% to 10% nickel,with small amounts of manganese, silicon, and carbon.3

Direct bonding of orthodontic brackets to enamel isstate of the art in orthodontics. Since the basic investi-gations of Buonocore4 in etching tooth surfaces withphosphoric acid, satisfactory bonding between enameland adhesive is achievable.5,6 There are still problemswith retention loss between the bond of the bracket andthe resin. For that reason, most brackets offer mechan-ical retention—eg, a foil-mesh structure on the base ofthe bracket. In addition, sandblasting the brackets7 or alaser-structured base8 are methods to improve the bond

From the University of Regensburg, Medical Center, Regensburg, Germany.aAssistant professor, Department of Orthodontics.bAssistant professor, Department of Prosthetic Dentistry.Reprint requests to: Andreas Faltermeier, Department of Orthodontics, Univer-sity Clinics, Franz-Josef-Strauss-Allee 11, D-93042, Regensburg, Germany;e-mail, Andreas.Faltermeier@klinik.uni-regensburg.de.Submitted, December 2006; revised and accepted, March 2007.0889-5406/$36.00Copyright © 2009 by the American Association of Orthodontists.

doi:10.1016/j.ajodo.2007.03.034

between bracket and resin. Sandblasting can generate amicroretentive topography and increase the surfacearea.9,10 This method uses a high-speed stream ofaluminium oxide particles to remove unfavorable con-taminants and oxides.7

Today, not only mechanical retention betweenmetal and resin is possible, but also a chemical linkbetween alloy and adhesive can be achieved. Flame-pyrolytic or tribochemical systems can cover the alloysurface with a coat of silicate.11 In addition, a silane-coupling agent is necessary; it bonds to the silicatedalloy surface and the polymer.12,13 A chemical linkbetween metal and resin can attained by this procedure,which keeps away water from the bonding surface.

For that reason, the question arises whether silicoat-ing can improve the bond between stainless steelbrackets and a light-curing adhesive. Our aim in thisstudy was to compare the effect of a silicoating system(Rocatec, 3M Espe, Seefeld, Germany), the effect ofsandblasting, and the influence of a silane-couplingagent after sandblasting on the shear bond strength(SBS) of stainless steel foil-mesh brackets. The effectsof these conditioning methods were compared withuntreated foil-mesh brackets. To simulate the oralenvironment, all specimens were thermocycled (6000times at 5°C and 55°C) in a mastication device beforetesting.

Frequently, orthodontists are faced with the prob-lem of debonded or inaccurately positioned brackets.Therefore, another concern of this study was to inves-

tigate the influence of bracket reconditioning on the

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12.e2 Faltermeier and Behr

SBS by using sandblasting to remove the adhesive onthe base of the brackets.

MATERIAL AND METHODS

Eighty recently extracted third molars were col-lected and stored in 0.5% chloramine-T. The teeth wereexamined with a magnifier (10 times) to ensure theabsence of caries, hypoplastic areas, and cracks. Afterremoval of the roots, the crowns were embedded inauto-polymerization acrylic resin so that the facialsurface of each tooth was parallel to the base of thepolymer. After preparation, the specimens were storedin 0.5% chloramine-T at 37°C for a week. The teethwere cleaned with nonfluoridated pumice paste andrubber cups. Because 4 bracket groups had to be tested,the teeth were randomly assigned to 4 groups of 20teeth each. Group 1 consisted of 20 Ormesh brackets(Ormco, Glendora, Calif) that were sandblasted on thebase with 120 �m aluminium oxide for 20 seconds at 2bar and cleaned in an ultrasonic device (Sonorex, ADJensen, Zwolle, the Netherlands) before bonding.Group 2 had 20 Ormesh brackets that were treated asdescribed for group 1; additionally, a silane-couplingagent (Espe Sil, 3M Espe) was applied to the sand-blasted base of the bracket. In group 3, the surfaces ofthe base of 20 Ormesh brackets were treated by usingthe Rocatec device. First, the area was blast cleaned for10 seconds (pressure, 2.8 bar) with 110 �m of alumi-nium oxide (Rocatec Pre, 3M Espe). Then, a tribo-chemical coating was added by using Rocatec Plus (3MEspe) with a pressure of 2.8 bar for 13 seconds. Thedistance between the base of the brackets and the blastoutlet was 10 mm. A silane-coupling agent (Espe Sil,3M Espe) was applied. Group 4, the control group,included 20 new and untreated foil-mesh brackets. Thesurface area of the Ormesh bracket bases was deter-mined by measuring length and width and computingthe mean area. The surface area of these brackets was12.77 mm2.

The enamel surface of each tooth was etched with20% phosphoric acid (Gluma Etch 20 Gel, HeraeusKulzer, Hanau, Germany) for 30 seconds, rinsed withwater for 20 seconds, and dried with an oil-free airsource. An orthodontic resin-based light-activatedbonding system, Transbond XT (3M Espe) was usedaccording to the manufacturer’s instructions. All brack-ets were placed centrally on the flat buccal surfaces ofthe teeth. The excess resin was carefully removed fromthe tooth with a dental probe. The samples were thenlight-cured with a light emitting diode curing device(Ortholux LED, 3M Unitek, Monrovia, Calif) for 20seconds. All brackets were bonded by the same opera-

tor (A.F.).

Twenty-four hours after preparation, all sampleswere thermocycled to simulate the moisture of salivaand the temperature changes in the oral environment.Therefore, all bracket groups were alternately floodedevery 2 minutes with warm (55°C) and cold (5°C)distilled water for 6000 cycles in a mastication de-vice.14

The SBS was measured with a universal testingmachine (1446, Zwick, Ulm, Germany) at a crossheadspeed of 1 mm per minute–1. The embedded tooth and theadhesively fixed bracket were positioned in the testingmachine so that the bracket slot was horizontal. A knife-edge shearing rod was used to deliver the shear force atthe bracket base-enamel interface. All brackets weretested to failure. The SBS was determined by using theformula �shear � Fmax/Abracket base surface (MPa).

To study the influence of recycling artificially agedbrackets, the brackets of group 1, which were sand-blasted before bonding, were reconditioned with sand-blasting (120 �m aluminium oxide, 20 seconds, 2 bar)to remove the adhesive. After this, the recycled brack-ets were cleaned in the ultrasonic device and againbonded as described before on the cleaned tooth sur-face. The SBS was determined again (group 5).

Medians and standard deviations were calculated.To determine statistical differences, the Kruskal-Wallisand Mann-Whitney U tests were used. The level ofsignificance was set at � � 0.05.

To compare the amount of adhesive on thebrackets, the adhesive remnant index (ARI) wasrecorded (Table I).

RESULTS

After sandblasting, a significant increase of SBSwas determined (P � 0.005) compared with the controlgroup (Fig 1, Table II). The use of a silane-couplingagent after sandblasting showed a significant (P �0.005, Table II) enhancement of SBS compared withthe untreated brackets (control group). However, the

Table I. Adhesive remnant index (ARI) scores of thebracket groups

ARI score 0 1 2 3

Control group (untreated) 0 0 4 16Sandblasted brackets 0 1 4 15Sandblasted and silane-treated brackets 0 1 3 16Tribochemically treated brackets 0 1 6 13Sandblasted recycled brackets 0 1 3 16

ARI scores: 0, no composite remained on the enamel; 1, less than50% of the composite remained on the enamel; 2, more than 50% ofthe composite remained on the enamel; 3, all composite remained on

the enamel.

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silane-coupling agent after sandblasting did not changethe SBS significantly when compared with the sand-blasted brackets (without the silane-coupling agent).The highest SBS median values (7.72 � 0.88 MPa)were measured after tribochemical treatment of thebrackets (Table III). The SBS enhancement after thetribochemical process was significant (P � 0.005)compared with the control group (Fig 1), but notsignificant if compared with sandblasting treatment ofthe brackets (with and without the silane-couplingagent).

The ARI showed no statistical difference (P �0.05)between the test groups (Table II). Nevertheless, a trendfor improved adhesion between bracket base and resinof the tribochemically treated brackets was noticedafter the ARI results (Table I).

After recycling the used brackets with sandblasting,no statistical difference between the SBS and the ARIscores was determined compared with the originally

Fig 1. SBS (in MPa) of the bracket groups.

Table II. Statistical analysis (P values from Kruskal-Wallis and Mann-Whitney U tests) of SBS and ARIscores compared with the control group

Bracket surface modification SBS ARI

Sandblasted brackets 0.005 NSSandblasted and silane-treated brackets 0.005 NSTribochemically treated brackets 0.005 NS

NS, Not significant (level of significance, P � 0.05).No significant difference (P �0.05) between modified bracket groupswas measured.

sandblasted brackets (Fig 2).

DISCUSSION

In orthodontic literature, bonding brackets toenamel is well documented.4,8,15-19 Nevertheless, theinfluence of silicoating and silane treatment of stainlesssteel brackets compared with sandblasting has not beensufficiently investigated. Atsü et al20 described higherbond strengths in both metal and ceramic brackets aftersilica coating followed by silanization. Newman et al21

examined the effect of adhesion promoters on the bondstrength of metal brackets. They described the in-creased bond strength of 80-gauge metal mesh bracketsafter applying the promoters. However, they used onlya self- adhesive, and the accelerated aging process wasonly 1500 thermocycles. We used the light-cured ad-hesive Transbond XT, and the accelerated aging pro-cess was 6000 thermocycles (5°C and 55°C) in amastication device.

In-vitro studies of SBS testing often have theproblem that, in the oral environment, saliva penetrates

Fig 2. SBS (in MPa) of brackets before and aftersandblasting and recycling.

Table III. SBS median values and standard deviations ofthe bracket groups

Group Treatment SBS (MPa)

1 Sandblasted new brackets 7.43 � 0.622 Sandblasted and silane-treated brackets 7.42 � 0.663 Tribochemically treated brackets 7.72 � 0.884 Control (untreated new brackets) 5.86 � 0.605 Sandblasted recycled brackets 7.15 � 0.57

between the surfaces of bracket and resin; this is

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difficult to simulate. Therefore, in-vitro studies in a dryenvironment are seldom comparable with clinical con-ditions. In this study, an artificial oral environment witha mastication device was chosen to simulate moistureand temperature changes in the oral cavity. The tem-perature in the mastication test chamber varied between5°C and 55°C for 6000 cycles. Our results are fairlydifficult to compare with other in-vitro investigations,because most studies about the SBS of brackets useddry conditions and disregarded the influence of waterpenetration into the bracket, resin, and tooth interface.

The influence of combined sandblasting and silane-coupling treatment on the bond strength of plasticbrackets was discussed by Guan et al.9 They concludedthat combined sandblasting and silane-coupling treat-ment increased the bond strength of plastic brackets.The reason for their findings could be that, aftersandblasting, a higher level of inorganic glass fillerswith silicon dioxide were exposed on the surface.Therefore, the bond between resin and glass fillerscould be strengthened by using a silane-coupling agent.

Silane-coupling agents were applied in dentistry forbonding organic materials to ceramic materials or forstrengthening the bond between metal and resin after atribochemical or flame-pyrolytic process. A silane-coupling agent can connect silicon dioxide groups onactivated metal or ceramic surfaces with an adhesiveconsisting of a methyl-methacrylate or a 2,2-bis[p-(3-methacryloxy-2-hydroxypropoxy)phenyl]propane sys-tem because of its bipolar structure. In our study, nostatistical difference between bond strength of bracketssandblasted without the silane-coupling agent and withsilane treatment could be found. Nevertheless, the bondstrength of brackets after Rocatec (tribochemical) treat-ment showed a slight improvement compared withsandblasted (with or without the silane-coupling agent)brackets. An explanation for these findings might bethat tribochemical systems cover the alloy surface witha coat of silicate. A silane-coupling agent can connectthe silicated alloy surface with the resin because of itsbipolar structure. This procedure offers a chemical linkbetween metal and polymer that keeps water from thebonding surface.11 Sandblasting treatment only seemsunable to activate the metal surface as required by thesilane-coupling agent, because the silicoating process ismissing. Therefore, no enhancement of bond strengthbetween bracket and adhesive could be determinedafter the use of a silane-coupling agent on the surfaceafter sandblasting compared with only sandblastedbracket surfaces. However, all modified surfaces of thebrackets (sandblasted with or without the silane-cou-

pling agent and tribochemical treatment) improved

significantly the SBS compared with the untreatedcontrol group.

The ARI scores might be of interest to clinicians.Even though no statistical differences between testgroups were found, a trend was obvious: adhesiveresins seem to remain more frequently on silicoatedbracket bases. The explanation for this finding is thata silicoated and silane-treated stainless steel surfacemight improve the bond to the adhesive resin;therefore, after debonding, less resin remains on thetooth.

In our investigation, we used only 1 type of cement(Transbond XT) to guarantee that any variations of SBSand ARI scores were attributable to the surface modi-fication methods. The use of Transbond XT, a highlyfilled resin cement, is well established in orthodontics.The SBS of orthodontic brackets depends on severalvariables: bracket material, surface structure of thebrackets, type of adhesive, and enamel quality. Perhapsmore distinctive improvements of the SBS of bracketsand the ARI could be made if a lower filled or unfilledadhesive had been used, because a more liquid cementcould better penetrate the grooves and undercuts of thevarious surface-modification methods.

In accordance with our results, Sharma-Sayal et al16

stated that sandblasted brackets are reliable when re-used, and, if damage to the bracket base from sand-blasting is minimal, the SBS is not compromised.Huang et al1 compared the ions released from recycledand new brackets and found that recycled bracketsreleased greater amounts of ions—eg, nickel and iron.Cacciafesta et al22 investigated the bond failures ofrecycled vs new stainless steel brackets in a clinicalstudy. They found no statistical difference between thetotal bond failure rates of recycled and new brackets.The in-vitro findings of our study confirm their results.Even though bracket recycling is popular, the clinicianmust be satisfied with clinical performance, and therecan be no infection risk between patients.23 However,the clinician must determine whether it is cost effectiveto reuse brackets, because it takes more chair-side timeto clean and recycle them.16

It is a common belief that clinically adequate bondstrength for a stainless steel bracket to enamel shouldbe 6 to 8 MPa.17,24,25 Even though bond strengthincreased distinctly after silicoating and sandblastingtreatment compared with the untreated control group inour study, median SBS values did not exceed 8 MPa.Thus, it can be concluded that these surface modifica-tions raise the SBS significantly and in a clinically

acceptable way.

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CONCLUSIONS

Sandblasting and tribochemical treatment of stain-less steel brackets improves their SBS. Combinedsandblasting and silane-coupling treatment offers nobenefit of increased in-vitro strength. Bonding resins totribochemically silicoated stainless steel bracketsseems to be sufficient for strengthening the bondbetween the adhesive and the metal bracket. Thistreatment is mainly indicated for low-compliance pa-tients or teeth that are difficult to bond.

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