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Evaluation and comparison of the effect of incorporating zinc oxide and titanium dioxide nanoparticles on the bond strength and microleakage of two orthodontic fixed retainer adhesives
Adding zinc oxide and titanium dioxide nanoparticles at 1% weight preserves the bond strength.
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Adhesive remnant index was not significantly different among.
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Titanium dioxide significantly increased the microleakage of Transbond XT adhesive.
ABSTRACT
Background
An adhesive with both proper mechanical and antimicrobial properties seems to be beneficial. We aimed to investigate the effect of zinc oxide (ZnO) and titanium dioxide (TiO2) nanoparticles (NPs) on bond strength and microleakage of two different fixed retainer adhesives.
Methods
In this in vitro experimental study, 168 extracted human incisors were randomly divided into six groups of 28 (eight double-tooth specimens for the bond strength test and 12 specimens for the microleakage test). In three groups: Transbond XT (3M Unitek, Monrovia, CA) without NPs, with 1% ZnO NPs and with 1% TiO2 NPs were applied. The other three groups included Ortho Connect Flow (GC orthodontics, Tokyo, Japan) composite with the same order to bond a 0.175-inch multistrand wire to the lingual surfaces of the teeth. The bond strength was measured using the Universal Testing Machine, and the adhesive remnant index was reported using a stereomicroscope (Nikon, SMZ800, Tokyo, Japan). The dye-penetration method was used to determine the microleakage.
Results
For bond strength, there was no significant difference among groups. For microleakage, there was no significant difference between GC and Transbond XT groups. However, in subgroups of Transbond XT, the addition of TiO2 NPs increased the microleakage significantly in comparison with ZnO and control groups (P = 0.011). There was no significant statistical difference between the groups in terms of residual adhesives (P = 0.166).
Conclusions
Through the incorporation of 1% TiO2 and ZnO NPs into the fixed retainer adhesive, the bond strength was maintained within the clinically acceptable range. The addition of TiO2 NPs to Transbond XT significantly increased the percentage of microleakage.
However, fixed retainers make proper oral hygiene more difficult and time consuming, and their long-term use is associated with greater accumulation of plaque and increased probing depth [
]. Because of the rough surface of the orthodontic adhesive, it act as a strong predisposing factor for enamel demineralization and a desirable place for rapid attachment and growth of oral microorganisms [
]. More specifically, the lingual retainer adhesives exposed to the oral environment requires certain physical and chemical properties for their optimal clinical performance and the durability of the bonded appliance. Bond failure is said to be the only possible factor in increasing the irregularity of lower incisors 5 years after debonding in cases with canine-to-canine lingual retainer [
]. Microleakage of adhesive resins can affect the durability of bonding and cause treatment failure, especially in lingual retainers that remain in the oral environment for a long time [
]. The combination of TiO2 nanoparticles (NPs) with dental composites has been reported to improve mechanical properties, such as the modulus of elasticity, microhardness, and flexural strength, and has also shown bond strength values similar to or even higher than control groups without NPs [
] demonstrated that by increasing concentrations of adhesive filler, shear and torsional bond strengths increase. The favorable antibacterial effects of TiO2 NPs also have been proven in a number of other studies [
Antimicrobial effect, frictional resistance, and surface roughness of stainless steel orthodontic brackets coated with nanofilms of silver and titanium oxide: a preliminary study.
Comparative evaluation and influence on shear bond strength of incorporating silver, zinc oxide, and titanium dioxide nanoparticles in orthodontic adhesive.
Similarly, zinc oxide (ZnO) has intrinsic antibacterial properties and, in the form of NPs, shows much higher antibacterial activity because of the increase in the surface-to-volume ratio [
Regardless of the antimicrobial effects of the NPs added to the composites, the physical and mechanical properties of the adhesive must remain within the acceptable range so as not to affect the standard orthodontic treatment adversely. Therefore, if the SBS of adhesive does not change or changes in the acceptable range, we can confidently take advantage of NPs antimicrobial property [
The aim of the present study was to investigate the effects of ZnO and TiO2 NPs on the bond strength and microleakage of two orthodontic fixed retainer adhesive systems (Transbond XT [3M Unitek, Monrovia, CA] and Ortho Connect Flow [GC orthodontics, Tokyo, Japan]). The null hypothesis was that these concentrations would make no difference in altering the SBS and microleakage and no adverse effect would be observed.
2. Methods and materials
2.1 Sample size calculation
For bond strength, based on a similar study, considering test power = 80%, α = 0.05, mean SD = 16, and effect size = 0.57, 8 samples for each group (total: six groups) would be needed [
Antibacterial activity and debonding force of different lingual retainers bonded with conventional composite and nanoparticle containing composite: an in vitro study.
]. For microleakage with the same α error and test power and considering the mean SD = 0.44 and effect size = 0.46, the required sample size for each of the six study groups would be 12 [
To prepare the nanocomposite, 990 mg of Transbond XT composite was separately mixed once with 10 mg of TiO2 and once with 10 mg of ZnO NPs to obtain 1000 mg of nanocomposite with 1% concentration each time. This process was also repeated for the Ortho Connect Flow composite.
2.3 Evaluation of bond strength
2.3.1 Specimen preparation
To prepare the samples, 96 intact human incisors free of caries, enamel cracks, abrasions, and fractures, which were extracted for periodontal or other therapeutic reasons, were collected. After decontamination with a scaler, they were immersed for 1 week in 0.5% chloramine solvent at 4°C.
For the bond strength test, the incisors which had similar morphology and size were mounted in pairs (giving a total of 48 blocks) up to the level of the cementoenamel junction in cylindrical metal molds, using self-cure acrylic resin.
The samples were divided into six groups (each containing eight blocks) including:
1.
Transbond XT with 1% ZnO NPs,
2.
Transbond XT with 1% TiO2 NPs,
3.
Ortho Connect Flow with 1% ZnO NPs,
4.
Ortho Connect Flow with 1% TiO2 NPs,
5.
One control group (Transbond XT and Ortho Connect Flow, without NPs), and
6.
One control group (Transbond XT and Ortho Connect Flow, without NPs).
After cleaning the teeth with a prophylactic brush without pumice, they were rinsed and dried. Then, the lingual surfaces of the incisors were etched with 37% phosphoric acid gel (Ultra-Etch, Ultradent Product, Inc., South Jordan, UT, USA) for 30 seconds, followed by washing and drying for 15 seconds. For Transbond XT groups, a thin layer of Transbond XT primer was applied on the etched surface and cured for 10 seconds with a LED D light cure device (Woodpecker, Guilin, Guangxi,China) [
Then, the 0.175-inch multistranded stainless steel wire (American Orthodontics, Sheboygan, WI) was cut to the required length to be bonded on top of the cingulum, between the incisal third and the middle third. To standardize and equalize the amount of adhesive, a mini-mold wire bonder (Dynaflex, Hauge, The Netherlands) (Fig. 1A and B ) was used to place the adhesive on the teeth. Finally, the adhesive was exposed to 40 seconds of curing.
Fig. 1(A) Mini-mold wire bonder for equalizing the amount of adhesive, (B) applying the adhesive on samples with mini-mold wire bonder.
Then, the samples were placed in a thermocycling device (Vafaei Industrial, Tehran, Iran) for 1500 cycles to simulate the conditions of the oral environment. In each cycle, the blocks were thermocycled between 5°C and 55°C water (each for 15 seconds), with a dwell time of 20 seconds outside the water.
2.3.2 Bond strength measurement
The values of bond strength were measured and registered using the Universal Testing Machine (Zwick/Roell, Ulm, Germany). In pilot tests, we observed that the thick blade of the device touched the incisors while applying the force and caused errors in the accurate recording of the debonding force. Thus, a cement spatula was used instead because of its suitable thin and firm head (Fig. 2). Then, the compressive force was applied to the middle of the retainer wire between the two teeth at a speed of 1 mm/min and continued until debonding occurred on at least one of the teeth in each block. The force causing this bond failure (in Newtons) was recorded as bond strength.
Fig. 2Modified Universal Testing Machine with cement spatula.
In this step, each incisor that showed bond failure in the bond strength test was examined under a stereomicroscope (Nikon, SMZ800, Tokyo, Japan) with 10 × magnification (Fig. 3). Based on the amount of remaining adhesive on the enamel, the score of adhesive remnant index (ARI) was determined with respect to the following classification:
0: No adhesive on the enamel.
1: <50% of adhesive on the enamel.
2: >50%, but <100% of adhesive on the enamel.
3: All adhesive (with wire impression) on the enamel.
Fig. 3Examination of ARI with 10x magnification under stereomicroscope.
Two examiners registered the ARI scores. In case of inconsistency, the third examiner assessed separately and determined which score to be included in the final analysis.
2.5 Evaluation of microleakage
To evaluate the microleakage, 12 teeth were dedicated to each of the explained six groups (a total of 72 teeth). Cleaning, etching, bonding, and thermocycling of the samples were performed as described previously. Then, the root apices of the teeth were sealed with sticky wax, and nail polish was applied to the entire surface of the samples, with the exception of almost 1 mm surrounding the retainer composite.
The samples were then immersed in a 0.5% solution of fuchsine dye at room temperature for 24 hours. After that, the teeth were washed with water to remove the superficial dye and dried. The specimens were then cut in a transverse plane (tangent to the retainer wire) with a water-cooled low-speed diamond bur.
Dye-penetration method was used to evaluate the microleakage. In this test, the samples were examined under a stereomicroscope (EZ4D, Leica Microsystem, Wetzlar, Germany), and the length of dye penetration was measured in millimeters along the composite/enamel interface from both mesial and distal margins. The sum of the length of penetrated dye from mesial and distal surfaces, divided by the total interface length was reported in percent and defined as microleakage percent (Fig. 4). All measurements were done three times, and the mean value was used in the final analysis.
The data analyses were performed with SPSS (version 25, IBM Corp., Armonk, NY). A two-way ANOVA test was used to analyze the bond strength values for determining the effect of two adhesives and two types of NPs. To analyze the data of microleakage values and ARI scores, the Mann–Whitney and Kruskal–Wallis tests were applied. P < 0.05 was considered statistically significant.
The study was approved by the ethics committee of Tehran University of Medical Sciences (project number: IR.TUMS.DENTISTRY.REC.1400.031). All experimental procedures were done by the same researcher (LJ), and the registration of measurements were done by two examiners (LJ and SSK).
3. Results
3.1 Bond strength
Table 1 shows the results of the bond strength test. According to the two-way ANOVA test, there was no statistically significant difference in the values of the bond strength between the study groups. In other words, no significant difference was found between the groups considering either the material (adhesive type) or the element (NP type) (Table 2).
Table 1Descriptive data of bond strength in six study groups given in Newton.
Table 3 shows the microleakage values at the adhesive-enamel interface in the study groups. Pairwise comparisons of groups showed that considering the type of adhesive, there was no statistically significant difference in the microleakage percent between GC groups and Transbond XT groups. Regarding the effect of NP type, in GC groups, there was no statistically significant difference between the control, TiO2, and ZnO groups. However, in the Transbond XT groups, the microleakage percent in the group with TiO2 NPs was significantly higher than in the control and ZnO groups (P = 0.011).
Table 3Mean and SD of microleakage percent in adhesive-enamel interface of six studied groups.
The scores indicating the amount of remained adhesive on samples of the study groups are shown in Table 4. According to the results of the Kruskal–Wallis test, there were no statistically significant differences in ARI scores between the study groups (P = 0.166).
]. The addition of NPs to composite resins is one of the most effective ways to cope with the side effects of orthodontic treatment, such as enamel demineralization [
]. Considering the concerns about the effects of these particles on the bond strength of composites; in the present study, we investigated the effect of the incorporation of ZnO and TiO2 NPs on the bond strength of two fixed retainer adhesive systems (Transbond XT and Ortho Connect Flow).
4.1 Bond strength
According to the results of this study, although the highest bond strength values belonged to the control groups (without NPs), the difference between control groups and experimental groups (with 1% ZnO or TiO2 NPs) was not statistically significant. These results were in line with the results revealed by a previous study, which investigated the effect of four different concentrations of ZnO + chitosan NPs in equal proportions on the SBS of Transbond XT composite [
]. There was only a statistically significant difference between 10% concentration and the control group. The 1% and 5% concentrations did not make a significant difference.
Kotta et al. evaluated the effect of adding 1% TiO2 NPs on the bond strength of a type of retainer adhesive in combination with three types of fixed retainers. They reported that the highest bond strength was from the group with braided stainless steel wire and NP-free adhesive, and the lowest value belonged to the fiber-reinforced retainer bonded with TiO2-containing adhesive [
Antibacterial activity and debonding force of different lingual retainers bonded with conventional composite and nanoparticle containing composite: an in vitro study.
]. Therefore, similar to the present study, it can be concluded that we can take advantage of antibacterial effects of the NP incorporation without being worried about diminished bond strength.
] examined the SBS of the Transbond XT composite containing 1%, 5%, and 10% TiO2 NPs and stated that NP incorporation of up to 5% concentration maintained the SBS in the acceptable range, whereas the 10% concentration showed a clinically adverse effect on SBS.
], who evaluated the SBS of an experimental resin composite with 1% and 3% TiO2 NPs, reported the highest mean value of SBS for 1% TiO2 and the lowest value for the control group. Dissimilar results may be due to their use of an experimental resin composite versus the prefabricated gold standard composites in our study, different sizes of NPs (30–50 nm vs. 20 nm), different mixing methods, different types of experimented teeth (premolar vs. incisor), different thermocycling procedures (5000 cycles vs. 1500 cycles), different materials (brackets against fixed retainers), and difference in the area of force application by the Universal Testing Machine. Similarly, the results of the study by Felemban et al. [
] on zirconium oxide + TiO2 NPs in 0%, 0.5%, and 1% concentrations showed the highest shear, tensile, and compressive bond strength for 1% concentration.
Comparative evaluation and influence on shear bond strength of incorporating silver, zinc oxide, and titanium dioxide nanoparticles in orthodontic adhesive.
] who compared the effects of TiO2, silver, and ZnO NPs with 1% concentration on bond strength of orthodontic adhesives, the mean SBS of the control group was significantly higher than that of the other three groups. The SBS values of silver, ZnO, and TiO2 groups reduced respectively. They concluded that the addition of NPs could reduce the SBS of orthodontic adhesives.
] on the evaluation of antibacterial, physical, and mechanical properties of flowable resin composites with 1% ZnO NPs showed that these composites had a significantly higher compressive strength, flexural modulus, and bond strength than the control group. They attributed this increase to the filler properties of ZnO NPs. However, the difference in the results of our study may be due to the different methods, including the type of used composites (Heliomolar flux composite vs. Transbond XT and GC), the type of mounted teeth (premolar vs. incisor), and the removal of the prismless layer in their study.
] reported a slight increase in tensile bond strength after the addition of TiO2 NPs to glass ionomer at concentrations of 3%, 5%, and 7% in comparison with the control group. However, it was not statistically significant.
] investigated the effect of adding ZnO NPs in concentrations of 1% and 3% on the microleakage of two types of restorative flowable composites (Clearfill AP-X and Filtek Z-350) and reported a decrease in microleakage as the concentration is increased in Z-350 composite group, whereas the Clearfill group did not show significant changes. These results were contrary to the present study, which showed an increase in the mean percentage of microleakage after the addition of ZnO NPs to both Transbond XT and GC adhesives. However, this increase was not statistically significant. Dissimilar results may be attributed to different types of composites and their indications (restorative vs. orthodontic adhesive), different thermocycling methods, and different concentrations of fuchsine solution.
] evaluated the microleakage with flexible spiral wire retainers and Transbond LR adhesive in the interfaces of the composite/wire and composite/enamel with direct and indirect bonding methods. Their results showed no statistically significant differences at these interfaces for direct versus indirect application methods [
] comparing the microleakage of three composites (Transbond LR, Transbond XT, and an orthodontic flowable composite [Venus Flow]) with 0.0215 multistranded wire indicated no significant differences in enamel/composite interface. In the present study, the difference between Transbond XT and GC was not statistically significant, but in Transbond XT groups, the microleakage with TiO2 NPs was significantly higher than with ZnO NPs.
4.3 ARI
Our results on ARI scores showed no statistically significant difference between the study groups. This was in agreement with the results of the study by Mirhashemi et al. [
] who reported no significant difference in the ARI index between the groups containing ZnO/nanochitosan NPs and the control group. Similar results were obtained by Sodagar et al. [
] also observed no significant difference in this index after the incorporation of silver/hydroxyapatite NPs with 1% concentration into the composite primer.
5. Conclusions
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The addition of TiO2 and ZnO NPs maintained the bond strength of retainer adhesives within the clinically acceptable range.
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The ZnO NPs did not affect the microleakage significantly.
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A significant increase in the microleakage of Transbond XT occurred with the addition of TiO2.
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No significant difference was observed in the ARI scores by the addition of NPs.
Antimicrobial effect, frictional resistance, and surface roughness of stainless steel orthodontic brackets coated with nanofilms of silver and titanium oxide: a preliminary study.
Comparative evaluation and influence on shear bond strength of incorporating silver, zinc oxide, and titanium dioxide nanoparticles in orthodontic adhesive.
Antibacterial activity and debonding force of different lingual retainers bonded with conventional composite and nanoparticle containing composite: an in vitro study.
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