To study the shear bond strength (SBS), sites of failure, and micromorphology of bonded molar tubes used on teeth affected by dental fluorosis.
This in vitro study included 140 first molars classified according to Dean's index for dental fluorosis. Samples were divided into seven groups: (1) healthy teeth etched for 15 seconds, (2) teeth with moderate fluorosis (MOF) etched for 15 seconds, (3) teeth with MOF etched for 150 seconds, (4) teeth with MOF microabrasion etched for 15 seconds, (5) teeth with severe fluorosis (SEF) etched for 15 seconds, (6) teeth with SEF etched for 150 seconds, and (7) teeth with SEF microabrasion etched for 15 seconds. All samples were incubated and were then submitted to the SBS test and evaluated with the modified adhesive remnant index (ARI) and analyzed by using a scanning electronic microscope.
The SBS mean value for healthy enamel was 20 ± 10.2 MPa. For the group with MOF, the etched 150-second mean value was the highest (19 ± 7.6 MPa); for the group with SEF treated with microabrasion and etched for 15 seconds, the mean value was (13 ± 4.1 MPa). Significant differences (P ≤ .05) were found in the ARI between healthy and fluorosed groups.
Fluorotic enamel affects the adhesion of bonded molar tubes. The use of overetching in cases of MOF and the combination of microabrasion and etching in SEF provides a suitable adhesion for fixed appliance therapy.
Dental fluorosis (DF) is an alteration on enamel surfaces of tooth caused by chronic ingestion of fluoride during development, leading to enamel with a lower mineral content because of changes on its external structure surface.1,2 In Mexico, the prevalence of dental fluorosis ranges from 30%–100% in areas where water is naturally fluoridated to 52%–82% in areas where fluoridated salt is used.3
Actually, populations demand not only restorative treatment but also treatments that improve esthetics, such as orthodontic treatments, and the clinical success of this treatment depends on adequate bond strength of orthodontic attachments to enamel.4 Hypoplastic enamel such as in dental fluorosis could influence the shear bond strength (SBS) of orthodontic fixed appliances. A significant decrease in SBS was reported when orthodontic brackets were bonded to fluorosed teeth.5 Bond strength between composite materials and fluorosed enamel and different techniques have been evaluated; extended enamel conditioning with phosphoric acid to remove the acid-resistant hypermineralized surface layer to increase SBS has been recommended.6,7 On the other hand, microabrasion with aluminum oxide particles followed by etching is another technique proposed for fluorosed enamel.8,9
Bonding of attachments to molars, rather than banding, is a less frequently adopted practice despite the periodontal advantages it confers over molar bands10 and the need for separators. Antibiotic prophylaxis in patients at risk from bacteremia is eliminated, and oral hygiene is also facilitated as bonded attachments attract less plaque.11 To date, some studies have evaluated the SBS of brackets to fluorosed enamel,4,5,12 but no one has studied this union to bonded molar tubes, and although from a bonding point of view it could be concluded that molar tubes are not that different from brackets, we believe that this study is important as different studies have concluded that the severity of fluorosis increased from anterior to posterior teeth13 and that first molars are the most affected teeth.14
Therefore, the aim of this in vitro study was to evaluate the effect of enamel fluorosis on the SBS of bonded orthodontic molar tubes and to determine which treatment would increase their bond strengths.
MATERIALS AND METHODS
Subjects and Sample Preparation
Patients undergoing extraction of first molars at the hospital and private clinics were asked to donate their extracted teeth, and written consent was obtained prior to clinical examination according to the ethical principles of the Declaration of Helsinki (version 2008) and approval by San Luis Potosi University, Mexico Institutional Review Board. Erupted first molars were collected from three different locations of San Luis Potosi, which have a water fluoride level between 0.1 and 5 ppm F. All samples were cleaned and disinfected in an ultrasonic bath (Biosonic UC-300-115B, Coltène-Whaledent) with a Suprasson P5 Booster (Satelec, ZI du Phare 33700, Merignac, France) washed in running water, dried, and analyzed by visual observation for fluorosis severity according to the Dean's Index15 and then were classified. All molars were stored in 0.9% phosphate-buffered saline and 0.002% sodium azide at 4°C until the experimental procedures were performed.
The study was blinded for the diagnosis of dental fluorosis, and SBS evaluation was carried out by a second observer. Twenty healthy first molars, 60 with moderate fluorosis (MOF), and 60 with severe fluorosis (SEF) were included. The selected molars were divided into seven groups of 20 samples (10 maxillary and 10 mandibular molars): group 1 (control), healthy enamel etched for 15 seconds; group 2, MOF enamel etched for 15 seconds; group 3, MOF enamel etched for 150 seconds; group 4, MOF enamel microabraded with aluminum oxide particles and etched for 15 seconds; group 5, SEF enamel etched for 15 seconds; group 6, SEF fluourosis enamel etched for 150 seconds; group 7, SEF enamel treated with microabrasion and etched for 15 seconds. The roots of all teeth were embedded in acrylic cubes and were placed in a second ultrasonic bath.
Molar Bonded Tubes and Bonding Procedure
In this study, orthodontic molar buccal tubes (0.018, standard edgewise, nonconvertible, Dentsply GAC International, Bohemia, NY, USA) were used. The average surface area of the tube base was determined by randomly measuring 10 bases of the tubes. For superior tubes, the average was 24.11 mm2, and for mandibular tubes, it was 16.99 mm2. The bonding procedures were as follows: buccal surface samples were cleaned with a rubber cup (Crescent, Dentsply, Mexico City, DF, Mexico) and prophylaxis paste (Nupro, Dentsply, Mexico City, DF, Mexico) for 10 seconds with a low speed. Samples were thoroughly washed for 30 seconds and air dried. For groups 1, 2, and 5, the buccal surface was etched for 15 seconds with 37% H3PO4 (Total Etch, Ivoclar Vivadent, Schaan, Liechtenstein), washed with deionized water for 15 seconds, and dried with oil-free air to an opaque white appearance. For groups 3 and 6, the buccal surface was etched with 37% phosphoric acid for 150 seconds, washed with deionized water for 15 seconds, and equally dried. For groups 4 and 7, the buccal surface was treated with microabrasion using aluminum oxide particles of 50 µm for 5 seconds with a 40-lb pressure, a distance of 10 mm, etched with 37% phosphoric acid for 15 seconds, washed, and dried.
In all groups, an adhesive primer (Transbond XT, 3M Unitek, Monrovia, Calif, USA) was applied on the etched surface and light cured for 10 seconds. The light-cured composite (Transbond XT, 3M Unitek) was applied on the base of the tube and pressed firmly onto the tooth, and the excess adhesive was removed. The tubes were light cured for a total of 30 seconds: 10 seconds at each of the mesial, distal, and occlusal sides. After the bonding procedure, all samples were stored in 0.9% phosphate-buffered saline and 0.002% sodium azide at 37°C for 24 hours.
A knife-edge blade was used to produce a direct force to the buccal-orthodontic tube interface parallel to the axis of the tooth. A universal testing machine (Mecmesin, South Australia) was used for the SBS at a crosshead speed of 1 mm/min, and the failure load was recorded in N and converted to MPa, dividing the force in N by the mean area of the orthodontic tube base.
Modified Adhesive Remnant Index
After debonding the molar tubes, the buccal surface of each molar was examined at 10× magnification by a stereomicroscope (SZ-PT Olympus, Tokyo, Japan). The adhesive that remained after debonding was assessed and scored according to the modified adhesive remnant index (ARI).16
Examiners were calibrated by an expert in DF using the intraclass correlation coefficient and were also calibrated for the ARI using a kappa test. All data were expressed as a mean ± standard deviation. Shapiro-Wilks and Brown Forsythe tests were used to assess the normality of the data distribution. One-way analysis of variance (ANOVA) and Tukey's multiple comparison tests were used to compare SBS among groups. The chi-square test was used to determine significant differences in the ARI scores among groups. JMP program version 9.0 (SAS Institute, Cary, NC) and Stata version 11.0 (Stata Corp LP, College Station, Tex) were used for statistical analysis. Statistical significance was identified at P < .05.
The distribution of the database was parametric, obtaining a value >.05 with the Shapiro-Wilks test and >.01 with the Brown Forsythe test. The SBS values (in MPa) and descriptive statistics are shown in Table 1. An ANOVA test showed a significant difference among the groups; in enamel with MOF, a significant difference was observed when microabrasion and etching was used compared with the control group, and in enamel with SEF, we found a significant difference in all SEF groups compared with the control group, and this can be seen in Figure 1. The ARI scores for adhesive remaining after debonding are shown in Table 2. There was a significant difference among the groups. Only group 1 showed bond failure primarily at the composite-tube interface, whereas in all the others groups, the failures were at the enamel-composite interface.
Figure 2a shows a representative scanning electronic microscope (SEM) image of the enamel surface after debonding. The images of enamel affected with MOF etched for 15 and 150 seconds (Figure 2b and 2c) showed a surface without irregularities sufficient to provide retention, while the image of MOF treated with microabrasion followed by etching (Figure 2d) showed porosities. The images of enamel affected with SEF etched for 15 seconds (Figure 2e) showed an intact surface, while the overetched enamel (150 seconds; Figure 2f) showed a rough surface. The enamel treated with microabrasion followed by etching (Figure 2g) displayed not only a rough surface but also a large number of pores probably formed by the aluminum oxide.
This study was designed to evaluate different treatments to improve the adhesion of bonded molar tubes to fluorotic enamel, because the clinical success of orthodontic fixed appliance therapy depends on adequate bond strength to the enamel. All groups displayed clinically acceptable mean bond strengths (greater than 8 MPa).17 It is important to emphasize that although there are studies that have evaluated the adhesion of brackets to fluorosed teeth, adherence should be studied in first molars because bonded molar tubes are used as anchorage in orthodontic treatment, and different studies have concluded that the severity of fluorosis is increased in this kind of teeth.13,14
The bond strength values of this study ranged from 5.9 to 7.8 MPa, which has been suggested as adequate for most clinical orthodontic needs.17 The findings of this study demonstrated that SBS was decreased in fluorosed teeth compared with the control group (20 ± 10.2 MPa). In this group, similar results were obtained by Scougall et al.18 However, there is a controversy in this regard. Some reports support our findings that dental fluorosis severity affects SBS,5,13 and therefore Opinya and Pameijer7 recommended extending the etching time to 150 seconds in this kind of enamel. On the other hand, others stated that there were no statistically significant differences between bond strength values when fluorosed and nonfluorosed teeth were compared.19
In the MOF group, the treatments of acid etching for 15 seconds (group 2) and overetching for 150 seconds (group 3) showed no statistically significant difference in relation to the control group, although overetching in MOF showed higher values for bond strength (19 ± 7.6 MPa). In the SEF cases, all treatments showed a statistically significant difference when compared with the control group, with microabrasion followed by etching showing higher values (13 ± 4.1 MPa). These findings were supported with the microphotographs by SEM. Figure 2a shows a healthy enamel etched for 15 seconds with enlargement of micropores and a rough surface. These findings are consistent with those reported by Loyola-Rodríguez et al.,20 who used atomic force microscopy to compare the enamel surface roughness and absolute depth profile before and after using phosphoric acid. The microphotographs of MOF teeth treated for 15 seconds (Figure 2b) showed a surface with less microporosity, which could affect the retention of composites. In the microphotograph of MOF enamel under overetching (Figure 2c), acid penetrated the fluorosed enamel, creating irregularities similar to those achieved with etching the healthy enamel for 15 seconds. Finally, the microphotograph of MOF enamel treated with microabrasion followed by etching (Figure 2d) revealed excessive tissue damage after abrasion, so we believe that this treatment is not an option to obtain optimal conditions for a successful adhesion; this is due to enamel destruction, which could not allow remineralization of this tissue.21
With respect to the micrographs taken from molars with SEF, after etching for 15 seconds, the surface appeared almost intact; when performing etching for 150 seconds, a rough surface was observed, but the SBS was less than that obtained with etching for 15 seconds. This was explained by Torres-Gallegos et al.,22 who used AFM to evaluate the surface roughness and depth profile in healthy and fluorosed enamel. They concluded that increasing the etching time in SEF cases can decrease the surface roughness and depth profile, which may produce a less effective micromechanical enamel surface for effective adhesive bonding. With the microabrasion followed of etching by 15 seconds, we observed roughness and pores probably formed by the aluminum oxide powder, and we believe that these pores increase the adhesion in this kind of enamel.
An orthodontic molar tube bonded to a tooth surface is a necessary procedure, but it is a temporary procedure. After debonding the tube, the goal of the clinician is to return the enamel surface to its original state after completing treatment.23 With improvements in the physical and mechanical properties of composite resins, removing the adhesive remnants after debonding has become a clinical problem.24 Tooth cleaning is easier and faster, and iatrogenic damage during cleaning is less likely to occur when brackets fail at the enamel-resin interface.25,26 However, bond failure at the bracket-adhesive interface or within the adhesive is considered to be safer than failure at the enamel-adhesive interface.24 There was a significant difference in the ARI scores; most samples of the healthy enamel group were within score 3, contrary to all groups with fluorosis, in which most of the samples were at score of 5. These findings suggest that in fluorosed teeth, the enamel-adhesive union is not strong enough compared with that produced in healthy enamel.
Additional studies should be performed in a variety of bonding procedures in fluorosed teeth. In conclusion, the following would improve the use of bonded orthodontic molar tubes in fluorosed enamel.
Overetching the enamel with MOF provided better adherence, and the organic components of the enamel surface were preserved intact, which will help in the remineralization process. This was not observed using microabrasion, as there was a profound surface degradation.
In enamel with SEF, the microabrasion procedure followed by acid etching for 15 seconds provides better adherence versus etching and overetching results in the aluminum oxide powder, creating pores that enhance the retention to the resin.
This study was supported by CONACYT (grant numbers 157329 and 169020) and FAI-UASLP.