Vliv tepelného stárnutí na pevnost vazby ortodontického adhezivního systému bez přebytků Shear bond strength of a flash-free orthodontic adhesive system after thermal aging procedure

Abstrakt Cíl práce: Cílem této studie bylo porovnat pevnost vazby ke sklovině (SBS) bezpřebytkového (flash-free) předem naneseného ortodontického adheziva a ortodontického adheziva s kompomerem před a po termocyklování. Pro oba materiály byl také stanoven Index zbytků adheziva (ARI). Materiál a metodika: Celkem 120 lidských premolárů bylo náhodně rozděleno do dvou skupin (n = 60) podle použitého ortodontického ahdeziva: APC Flash-Free Adhesive Coated Appliance System (APC FF) a Transbond PLUS Color Change Adhesive (TP) jako kontrola. Poté byly vytvořeny tři podskupiny podle použité metody stárnutí: skladování ve vodě (37°C, 24h), termocyklování 10 000 nebo 20 000 cyklů. Následně byla testována SBS a vyhodnocen ARI pro každý vzorek. Výsledky byly analyzovány obousměrným ANOVA a Tukeyho Chi-square testy (p < 0,05).


Introduction
The use of adhesives and resin composites for bonding orthodontic brackets to etched teeth is considered a stan--dard procedure in the orthodontic practice (1). Usually, the process of bonding brackets is performed by the manual application of an adhesive to the bracket base. However, in an attempt to perform easier and faster bon-ding procedu-předem nanesené světlem tuhnoucí adhezivum (APM) (3M Unitek, Monrovia, CA, USA).
From then onwards, different developments of APC systems were introduced in the market by manufactu-rer. 3M Unitek began by launching the APC Adhesive Coated Appliance System followed by the introduction of APC II Adhesive Coated Appliance System in 2000. Both of them were resin-based adhesives, but the lat-ter was less viscous than its predecessor improving its handling properties and had a better blister package extending the expiration date and enhancing the pre-servation of the adhesive. The third development was introduced in 2002, which was the APC PLUS Adhesive Coated Appliance System (APC PLUS), which substan-tially modified the composition and properties of APC II Adhesive Coated Appliance System, being a compomer.
Because of this, APC PLUS provides a higher toleran-ce to moisture compared to its predecessors as well as fluoride release during treatment. Furthermore, it has a characteristic pink color before polymerizing (after light curing the color fades away) to facilitate flash removal. In 2013, the APC Flash-Free (APC FF) technology was launched (APC Flash-Free Adhesive Coated Appliance System, 3M Unitek) with the intention of further decrea-sing the time needed for the bonding process, by elimi-nating the need of flash removal around the bracket once it is placed. More importantly, the transition from APC PLUS to APC FF also meant a modification in the com-position of the adhesives, changing from a compomer to a low-viscosity resin. The APC FF orthodontic adhesive system is based on a nonwoven, polypropylene fiber ma-terial, which gets soaked by a low-viscosity resin. Once the bracket is placed, the compressible material lets the resin seep out, bonding the base of the bracket to the enamel (2). The difference between the chemical com-position of a compomer and a low-viscocity resin makes the evaluation of this novel flash-free system necessary, since the achievement of a stable bond strength in the enamel-bracket interface remains a priority for the co-rrect performance of the orthodontic treatment without an increase in bracket debonding rates (3). Moreover, the absence of flash around brackets when bonding, makes the evaluation of APC FF also interesting in order to decrease the formation of white spot lesion associated to adhesive excesses (4).
A limited amount of studies were found that evaluated APC FF (2,(5)(6)(7)(8)(9)(10)(11). Three of these confirmed some of the benefits anticipated by the manufacturer, such as time savings during bracket bonding (7,8,11), even up to one third compared with a conventional orthodontic adhesi-ve as has been recently stated by Grünheid et al. (11); or no need for flash removal after placement (7,(9)(10)(11). Three of the manuscripts (6,8,9) evaluated the SBS ob-taining high bond strength values. Those SBS tests were performed immediately after storing the samples in dis-tilled water for 24 hours. However, no study has been found that analyzed the aging effect in the bond strength of the APC FF low-viscosity type resin as this a more realistic application to the orthodontic treatment time. Hence, in accordance with Lee et al. (6) a thermocycling study is warranted to elucidate the role of aging on the bonding properties of APC FF, as it is still unknown. Since bonding efficacy (2,12), mechanical properties af-ter aging (13) and type of failures after debonding (2) are crucial sejmutí zámku (2) jsou rozhodujícími faktory pro ortodontisty při výběru ortodontonického adheziva, stávající studie byla zaměřena na: (a) srovnání SBS na rozhraní skloviny a zámku mezi APC FF a široce používaným konvenčním adhezivam, jako je lepidlo Transbond PLUS Color Change Adhesive (3M Unitek) (TP); b) stanovit vliv termocyklování na obě lepidla; a (c) vyhodnotit množství adhezivních zbytků na povrchu zubu po odstranění zámku.

Material and Methods
-Specimen preparation One hundred and twenty sound human premolars extrac--ted for orthodontic or periodontal reasons were stored in a 0. 1% aqueous solution of thymol at 4°C for no longer than 6 months. Their buccal enamel was sound with no damage due to the extraction process, and all the surfa-ces were cleaned from debris and soft tissue remnants before their storage. Roots were submerged in a self-cured acrylic cylinder (Special Tray, Dentsply Sirona, Ballaigues, Switzerland) to allow stability for posterior SBS testing, according to previous studies (2,3,5,6). The conditioning protocol was applied following the manufacturer's instructions. Firstly, the buccal crown surface of each premolar was brushed with fluoride-free pumice slurry for 15 seconds, and then rinsed and dried. Afterward, the enamel of the bonding surfaces was etched with 35% phosphoric acid (Transbond XT Etching Gel, 3M Unitek) for 30 seconds, rinsed for 30 seconds, and dried with oil-free and mois-ture-free air for 30 seconds. Then, a uniform coat of a light-cure adhesive primer (Transbond XT Primer, 3M Unitek) was applied. The teeth were randomly divided into two groups accor-ding to the bonding agent applied to bond the ceramic brackets used in the study (Clarity Advanced, 3M Uni-tek). The application mode and chemical composition of the tested materials are reported in Table 1 Stlačitelný netkaný materiál (síť polypropylenových vláken o průměru mikronů), napuštěný adhezivní pryskyřicí s relativně nízkou viskozitou. Když je přiložen na zub, stlačitelný materiál umožní pryskyřici prosáknout ven, aby vyplnila prostor mezi bází zámku a zubem. Compressible nonwoven material (web of mi-cron-sized polypropylene fibers), soaked with a relatively low viscosity adhesive resin. When is placed on a tooth, the compressible material lets the resin seep out to fill the space between the appliance bonding base and the tooth.
As the adhesive resin is already in the bracket base, brackets were placed immediately after primer application.

Group 2:
The orthodontic adhesive TP (3M Unitek), that requires operator application, was placed onto the brac-kets base, and they were positioned on the buccal ena-mel surface. The adhesive TP was directly applied from the syringe to the bracket base.
All brackets were positioned and bonded by the same operator (C.G-S.) in order to standardize the bonding procedure.
Once brackets were placed in the correct position (center of anatomic crown), they were pressed against the buccal surface of each premolar with the end of the bracket tweezers. For group 1, it was not neces-sary to remove the excesses around the bracket as APC FF does not generate flash. For group 2, flash was remo-ved with the help of an explorer before polymerizing. A LED unit (Elipar S10, 3M ESPE, St Paul, MN, USA) was used to polymerize both bonding agents. Both or-thodontic adhesives were polymerized for 40 seconds placing the end of the LED unit perpendicular to the bracket slot as recommended by the manufacturer, with an intensity of 1,200 mW/cm 2 . Specimens from both groups were randomly divided into three subgroups (n = 20) according to the aging pro-cedure carried out: storage in distilled water for 24 hours at 37°C and thermocycled for 10,000 cycles or 20,000 cycles. Thermocycling was performed between 5°C and 55°C with a dwelling time of 30 seconds. The rando-mization was performed by assigning a 6-digit number from a random number table to each of the three sub-groups, and then the premolars were randomly assigned to the previously mentioned subgroups. According to Gale et al., (14) 10,000 and 20,000 cycles thermocycling correspond to one and two years of cli-nical service respectively, which is the time usually re-quired to complete an orthodontic treatment. Moreover, the temperatures and dwelling time applied have been recently confirmed by the Academy of Dental Materials (15) as the most suitable way of testing dental materials under thermal aging procedures.
-Shear bond testing Afterwards, the acrylic cylinders were secured in a jig attached to the base plate of a universal testing machine (Instron 3345, Instron Corp., Canton, MA, USA). A chi-sel-edge plunger was mounted in the movable crosshead of the testing machine. It was positioned placing the lea-ding edge aimed at the bracket body, between the base and the bracket wings, before it was brought into contact exerting a force parallel to a flat interface in the occlu-sal-apical direction. A crosshead speed of 0.5 mm/min was applied. The force required to dislodge the brackets was measured in Newtons (N), and the SBS was calcu-lated in MegaPascals (MPa) by dividing the force values in N by the surface area of the bracket (mm 2 ).
-Type of failure and ARI Once brackets were debonded, enamel surfaces were analyzed by the primary investigator (C.G-S.) under a stereomicroscope (Olympus SZX7, Hamburg, Ger-many) at a magnification of 10X. The primary investigator previously received 20 hours of training on the use of the machine and how to obtain the To determine the type of failure, the ARI scores were deter-mined according to the Årtun et al. (16) classification. They were categorized as: 0 = no adhesive remaining on the tooth in the bonding area; 1 = less than half of the ad-hesive remaining on the tooth; 2 = more than half of the adhesive remaining on the tooth; 3 = all adhesive remai-ning on the tooth with a distinct impression of the brac-ket mesh. In cases where the adhesive remaining on the enamel surface was around 50%, to determine whether it was an ARI 1 or 2, a greater magnification was used (25X) as well as the evaluation of the bracket base with the same magnifications (10X and 25X) was performed. Finally, six specimens from each experimental subgroup were sputter-coated with gold (Bal-Tec Sputter Coater SCD 005, Witten, Germany) and observed under a scan-ning electron microscope (SEM) (Phillips XL30 ESEM, FEI Company, Hillsboro, OR, USA) to better analyze the type of failure of both adhesives.
-Statistical analysis Normality and homogeneity were checked with Sha-piro-Wilk and Levene tests, respectively. A two-way ANOVA was applied to analyze the influence of the orthodontic adhesive used and thermocycling procedu-re on SBS. Post-hoc comparisons were performed by Tukey's test. Chi-square (χ2) test was performed to as-sess differences in the amount of adhesive remaining on the enamel surface after bracket debonding according to the orthodontic adhesive tested and the aging procedure. All statistical tests were performed at a pre-set alpha of 0.05 using SPSS 22 for Windows software (IBM Corpo-ration, Armonk, NY, USA). Table 2 shows the SBS values obtained for the two adhe-sives evaluated after different thermal aging procedures. Two--way ANOVA revealed that the SBS values were significantly influenced by the thermal aging procedure (p < 0.01). Neither the orthodontic adhesive nor the in-teraction between both factors (adhesive type-thermocy-cling) significantly affected SBS results (p > 0.05).

Results
Both adhesives showed similar SBS values either without thermocycling (24 hour water storage) or when thermo-cycling was performed for 10,000 and 20,000 cycles. Thermocycling significantly decreased SBS values for the two adhesives tested. However, there was no signifi-cant difference in SBS mean values between 10,000 and 20,000 cycles for both groups.
Obě adheziva vykazovala podobné hodnoty SBS jak bez termocyklování (jen 24 hodinové skladování ve vodě), tak i po termocyklování 10 000 a 20 000 cyklů. Termocyklování významně snížilo hodnoty SBS u obou testovaných lepidel. Nebyl však nalezen žádný významný rozdíl v průměrných hodnotách SBS mezi 10 000 a 20 000 cykly pro obě skupiny adheziv.  The most prevalent type of failure for APC FF was at the enamel-adhesive level, with less than 50% of adhesive remaining on the enamel surface. Thermocycling did not alter this trend. On the other hand, with TP, bond failure occurred at the adhesive-bracket level, with all the ad-hesive remaining on the enamel surface in most cases; especially when thermocycling was performed.

Stárnutí
Percentages of ARI scores after debonding are shown in Table  3. According to Chi-square test results, there were statistically significant differences between the two adhesives tested (p < 0.001). Most of the brackets bon-ded with APC FF showed a score of ARI 1, regardless of the thermocycling procedure (Figs. 1a, b,  2a, b, 3a, b). However, thermocycling affected the failure mode when brackets were bonded with TP. Specimens not thermocycled obtained a score of ARI 2 in 65% of the cases (Fig. 1c-d).

Discussion
According to the results obtained, thermocycling in-fluenced SBS values of both adhesives and no significant differences were found between APC FF and TP. Fur-thermore, significant differences concerning ARI scores between both adhesives were found. This reveals that a high bond to enamel can be achieved with both systems, as previously reported by other authors (6,8,10,11,17), even taking into consideration that each material has a different composition and is applied differently ( Table 1). TP has been widely studied, either in its pre-coated (APC PLUS) (18) or conventional (TP) presentation (17). In this study, TP has been used as the control adhe-sive since it is considered an optimum orthodontic adhesive with high SBS values (17). The adhesion in the enamel-bracket interface ages throu--gh three different phenomena: mechanical, chemical and thermal changes. Although in vitro studies cannot exactly reproduce those conditions, they can, to some extent, simulate them through aging procedures (19). Thus, thermocycling is fully accepted and extensively used in experimental studies by the scientific community to reproduce the aging of dental materials produced by water and temperature changes in the oral cavity (14,15).
As noted, SBS values were significantly influenced by the thermal aging procedure. Several authors have described the deleterious effects of thermocycling in the adhesive interface after performing a SBS test (19)(20)(21). Hence, these number of cycles (10,000 and 20,000) are widely used in the scientific community when evalua-ting orthodontic adhesives (20,21).
Mean SBS results obtained with both adhesives were higher than 6 to 8 MPa that are the values considered as clinically accep-table for orthodontic purposes (22). The present results were higher than those obtained by Lee et al. (6) for both adhesives. This could be explained by differences within the methodology as they used a 0.016-inch stainless ste-el wire to debond the brackets instead of a chisel. Thus, the force applied was in a gingivo-occlusal direction, which was the opposite direction to the one used for the present study.
Obr. 2: Stereomikroskopický obraz a SEM mikrograf (35X) reprezentativních typů selhání pro testované experimentální skupiny. a, b) ukazují ARI skóre 1 u APC FF s 10000 cykly termocyklování; c, d) ukazují ARI skóre 3 u TP s 10000 cykly termocyklování. / However, the mean SBS value obtained by Ansari et al. (8) and Marc et al. (9) for APC FF (20. 13 MPa and 21.77 respectively), were very similar to the one obtained by us, being slightly lower. It is worth no-ting that although these studies were on SBS for APC FF, these studies did not evaluate thermocycling aging as the present study did. There is only one in vivo study that evaluated APC FF in an split-mouth randomized controlled clinical trial, although 2 manuscripts were pu-blished from it (10,11). After 1 year follow-up, they did not find any statistically significant difference between APC FF and a conventional orthodontic adhesive (APC II Adhesive Appliance System) with regard to bracket survival when ceramic brackets were used.
The continuous temperature changes performed in ther-mocycling have an influence on the bonding materials, causing the resin to expand and contract, as its thermal expansion coefficient is higher than that of the teeth; the higher the thermal expansion coefficient of a resin, the worse it would be for the adhesive interface as volume-tric changes of the resin will be greater. Moreover, the presence of water during this procedure causes hygros-copic expansion as well as chemical degradation of the resinous components, a process known as plastification (23). This deleterious effect of thermal changes and hy-drolytic degradation has been more relevant for the first 10,000 cycles, without a significant decrease in SBS va-lues when the number of cycles was doubled; similarly to what happened to Turk et al. (21) who did not find any significant difference between 10,000 and 20,000 cycles for Transbond XT Light Cure Adhesive (3M Unitek).
Not only is the time used in the bonding procedure clini-cally relevant, but so is the time in debonding (5); there-fore, the adhesive remaining on the tooth surface is also a key factor. When adhesive failure is produced at the ena-mel-adhesive interface, and is related to a lesser amount of adhesive on the tooth surface, there is a higher risk of enamel fracture (24), especially with ceramic brackets. Although at present APC FF is available with ceramic or metallic brackets, at the time of this study it was only available with ceramic brackets similar to other studies evaluating APC FF (2,(5)(6)(7)(8).

Conclusions
Within the limitations of the present study, the following conclusions were obtained: 1) both adhesive systems, APC FF and TP, showed similar bond strength to ena-mel, 2) thermocycling significantly decreased the SBS values of both adhesives, without any significant difference between them, 3) APC FF left significantly less amount of adhesive on the tooth surface when debon-ding, presenting the failure at the enamel-adhesive in-terface, while TP presented it at the bracket-adhesive interface. Thermocycling did not affect this pattern for APC FF, but it did for TP.