This research was approved (certificate n. ###) by the Animal Use Ethics Committee of the ###. Non-probabilistic intentional samples were used because the quantity of n = 10 is a standard value for laboratory studies in dentistry with good sample power [3, 9, 11, 14]. In the present study, the power of the sample was calculated using the website < www.openepi.com> (OpenEpi, version 3.01).
First, 40 permanent lower bovine incisors, visually free of cracks and/or fractures and with a post-extraction time of less than one month were used for the SBS test [16]. Adhered soft or hard tissues were removed with periodontal curettes. The roots were sectioned with double-sided diamond disc (Microdont, São Paulo, SP, Brazil) and discarded (Fig. 1A). The crowns were stored in distilled water at 4ºC [16].
Each crown was included in chemically activated acrylic resin (Vipiflash® and Ortocor®, Vipi, Pirassununga, SP, Brazil) using a prefabricated rectangular industrial silicone mold (Silicone Master®, Talmax, Curitiba, PR, Brazil). The buccal face of the crown was in contact with the mold base.
Taking into consideration ISO/TS 11405 (2003) [16], an area was minimally and carefully planned to even out the enamel surface without removing it excessively (Figs. 1B-1C) using sandpapers of different granulations progressively (#200, 400 and 600) for 10 seconds/each and under constant water irrigation on an Aropol 2V-PU® metallographic sander/polisher (Arotec, Cotia, SP, Brazil).
After planning, enamel prophylaxis was performed for 10 seconds with a low speed micromotor, rubber cup, pumice stone and water. The crowns were then washed with water until the pumice stone paste was completely removed and air dried for 10 seconds. The exposed vestibular surface area was delimited by an insulating tape (Scotch®, 3M, Sumaré, SP, Brazil) with a 4.5 mm diameter hole.
Next, conditioning with phosphoric acid 37% (Condac 37®, FGM, Joinville, SC, Brazil) was performed for 15 seconds. The acid was removed with water (30 seconds), and the enamel was blast air-dried for 5 seconds. The Transbond XT® Primer was applied with a microbrush (Brushfine®, KG Sorensen, Cotia, SP, Brazil), and excess adhesive was removed for 2 seconds.
The 40 crowns were randomly distributed for bracket bonding according to 4 photoactivation protocols (Table 1), which were subdivided into: “photoactivated faces” (center, mesial/distal/cervical/incisal, mesial/distal or cervical/incisal) and “photoactivation time” (6 or 3 seconds). The description of the bonding material used is shown in Table 2.
Table 1
– Photoactivation protocols performed during bracket bonding according to photoactivated faces and photoactivation time on each face.
PHOTOACTIVATION PROTOCOLS |
VALO CORDLESS® (N = 40/n = 10) |
• V3C = 3 seconds on center • VC6 = 6 seconds on center • V3M3D = 3 seconds on mesial + 3 seconds on distal • V3C3I = 3 seconds on cervical + 3 seconds on incisal |
Table 2
– Bonding materials used in this study.
MATERIAL/LOT | MANUFACTURER | COMPOSITION | % BY WEIGHT |
Transbond XT® Primer Lot N668081 | 3M Unitek (Monrovia, CA, USA) | - BisGMA - TEDMA - Triphenyl antimony − (4-dimethylamine) -benzenethanol - DL-Caforquinone - Hydroquinone | 45–55* 45–55* < 1* < 0,5* < 0,3* < 0,03* |
Transbond XT® Adhesive Paste Lots N660371 N763291 | 3M Unitek (Monrovia, CA, USA) | - Treated silane quartz - BisGMA - Bisphenol A Dimethacrylate Bis (2-hydroxyethyl ether) - Treated silane silica - Diphenyliodonium hexafluorophosphate * Photoinitiating component: Camphorquinone (400-500nm) | 70–80* 10–20* 5–10* < 2* < 0,2* |
BisGMA: Bisphenol A Dimethacrylate A diglycidyl ether; TEGDMA: Dimethylacrylate triethylene glycol; HEMA: 2-Hydroxyethyl methacrylate. |
* The specific chemical identity and/or exact percentage of this composition is a trade secret. |
An Edgewise Slim® prescription lower incisor bracket (Morelli, Sorocaba, SP, Brazil) was positioned perpendicular to the long axis of the tooth with Transbond XT® Adhesive Paste orthodontic resin at its base. Excess resin was removed with an explorer probe no.5 (SSWhite Duflex, Rio de Janeiro, RJ, Brazil). Light exposure was directed so that the tip of the photo activator gently touched the bracket surface, ensuring that its position was not altered. The insulating tape was removed at the end.
The samples were stored in an incubator (37°C) in distilled water containers (one/each group) for 4 months to achieve the required aging prior to the SBS test, this was performed to keep them in an environment similar to the oral cavity and lasted 4 months aimed not to exceed 6 months before the shear bond test, as recommended by ISO/TS 11405 (2003) [16].
The shear test was performed on a Zwick Roell® universal testing machine (Zwick Roell, Ulm, Germany) with a 100 Kgf load cell coupled to a device that vertically focused on the bracket/enamel interface (Fig. 1D) at a constant speed of 1 mm/minute until fracture occurred. The force required to remove the bracket was generated in Newton (N) and converted to Megapascal (MPa) according to the formula: [SBS(MPa) = F(N)⁄A(mm2)]. Where “A” is the bracket base area of 9.24 mm2 as measured with a digital caliper (Mitutoyo, Suzano, SP, Brazil. Accuracy: ± 0.02 mm).
The types of union failure were evaluated in a stereomicroscope (Nikon SMZ800, Tokyo, Japan) and analyzed according to the Adhesive Remnant Index (ARI) proposed by Bishara and Trulove (1990) [17]: Score 1) All the resin was in the enamel; Score 2) More than 90% of the resin was in the enamel; Score 3) More than 10% and less than 90% of the resin was in the enamel; Score 4) Less than 10% of the resin was in the enamel; Score 5) No resin was left in the enamel. The Kappa Index (K) was also to evaluate the agreement with a reevaluation of 13% of the sample after 15 days. For illustrative purposes, representative samples of each of the scores found were observed in Scanning Electron Microscopy/SEM® (TM3000, HITACHI, Tokyo, Japan) at 30x magnification.
Next, 40 resin discs were made with the same photoactivation protocols used in the SBS analysis for the DC analysis. Using an insertion spatula no.1 (SSWhite Duflex, Rio de Janeiro, RJ, Brazil), a segment of approximately 2 mm of the Transbond XT® adhesive paste was placed on a glass blade for microscopy. Then, it was fixed to the glass blade a polyester strip (Preven, Guapirama, PR, Brazil) with a metal bracket, which had been previously fixed to the strip with cyanoacrylate-based adhesive (Super Bonder®, Loctite, Diadema, SP, Brazil), so that no movement occurred during the photoactivation phase (Fig. 2A).
A force was gradually applied with the spatula inserted into the bracket slot to form a resin disc of 0.1 mm thick and an average of 5 mm in diameter (Fig. 2B). Then, photoactivation was performed according to the 4 protocols (Table 1), simulating the bonding on the tooth enamel. The polyester strip was subsequently removed, cleaned with 70% alcohol (Quality Vic Pharma, Taquaritinga, SP, Brazil) and reserved for making the next disc.
Each disc was carefully removed with a 15C scalpel (Lamedid Solidor, Osasco, SP, Brazil) attached to a scalpel handle nº 3 (Golgran, São Caetano do Sul, SP, Brazil) and stored for 24 hours in a black opaque container (Figs. 2C-2E). After disc removal, the glass blade was cleaned with sterile cotton (Cremer, São Paulo, SP, Brazil) and 70% alcohol and then dried with sterile cotton.
Measurements to determine DC were performed by a Fourier Transform Infrared Spectrophotometer - FT-IR (IRAffinity-1®, Shimadzu, Tokyo, Japan) equipped with attenuated total reflectance device (ZATS prism HIR MIRacle module, PIKE Technologies, Madison, WI, USA). All specimens were positioned in the center of the spectrophotometer window (Figs. 2F-2G).
The spectra were measured using a Shimadzu IRsolution 1.60® (Shimadzu Corporation, 2011). The results of each sample were initially normalized, so that the baseline was adjusted. Spectra were obtained under the following conditions: Mode of absorbance; Number of scans: 32; Range: 700–4000 cm− 1; Resolution: 4 cm− 1.
With the peak absorption height data at 1637 and 1608 cm− 1 for polymerized and unpolymerized samples [18], the values were entered into the following equation: [DC(%) = 1–R(1637/1608) polymerized sample/R(1637/1608) unpolymerized sample)x100]. For unpolymerized samples, 10 segments of approximately 2 mm of the Transbond XT® were individually placed in the center of the spectrophotometer window for analysis. Finally, data from these samples were averaged and the standard values were defined for the 1637 and 1608 cm− 1 peaks of the “unpolymerized sample”.
The research database was built in Microsoft Excel® software version 2016 (Microsoft Corporation, Redmond, USA), and the Statistical Package for Social Sciences® (version 20.0 – IBM SPSS Statistics 20) was used for statistical analysis. The results of the shear test and of the degree of conversion were analyzed descriptively and by One-way Analysis of Variance (ANOVA) and Tukey’s test. The bonding failure scores analyzed according to the ARI were evaluated descriptively and using the Kruskal-Wallis test. A significance level of 5% was considered in all cases.
The methodology was meticulously carried out and followed what is scientifically established in the literature to avoid possible errors regarding the evaluation of shear bond strength and the degree of conversion using brackets and orthodontic resins. All machines and equipment used in this research were evaluated, tested, and calibrated before each methodological step of this study.