| Abstract|| |
Introduction: The purpose of this study was to evaluate the effect of a highly filled light-cured sealant (HFLCS) on the shear bond strength of metal, ceramic and titanium brackets bonded with resin-modified glass ionomer cement (RMGIC).
Materials and Methods: A total of 60 freshly extracted maxillary premolars were randomly divided into six groups (10 in each group). In all groups, the teeth were etched with 37% phosphoric acid for 20 s and RMGIC (Fuji Ortho LC, GC Europe) was used for bracket bonding. Group 1: Titanium brackets were bonded directly to etched enamel surfaces. Group 2: Titanium brackets were bonded to etched enamel surfaces covered with HFLCS (Pro Seal, Reliance Orthodontic Products, Itasca, IL, USA). Group 3: Metal brackets were bonded directly to etched enamel surfaces. Group 4: Metal brackets were bonded to etched enamel surfaces covered with HFLCS. Group 5: Ceramic brackets were bonded directly to etched enamel surfaces. Group 6: Ceramic brackets were bonded to etched enamel surfaces covered with HFLCS. The specimens were tested in shear mode with a universal testing machine. After debonding, the teeth and the brackets were examined under a scanning electron microscope. Univariate analysis of variance (analysis) was performed to test the main effects of bracket type and HFLCS.
Result and Conclusion: The effect of HFLCS on etched enamel surfaces did not affect the bond strength values and bond failure modes of metal, ceramic and Titanium brackets bonded with RMGIC
Keywords: Highly filled light-cured sealant, resin-modified glass ionomer cement, titanium brackets
|How to cite this article:|
Mahajan V. Effect of light-cured filled sealant on the shear bond strength of metal, ceramic and titanium brackets bonded with resin-modified glass ionomer cement. Indian J Dent Res 2013;24:745-9
Light-cured sealants have been proven to cure completely on smooth enamel surfaces and prevent enamel demineralization effectively in vitro.  Recently, A new product, Pro Seal (Reliance Orthodontic Products, Itasca, IL, USA), claims to protect enamel against demineralization during the orthodontic treatment with fixed appliances. This sealant is a highly filled light-cured resin (HFLCS). The manufacturer claims that it stands up to toothbrush abrasion and erosion by oral fluids. Our aim in this study was to evaluate the effects of a HFLCS on the shear bond strength and bond failure site of metal, ceramic and titanium brackets bonded with resin-modified glass ionomer cement (RMGIC).
|How to cite this URL:|
Mahajan V. Effect of light-cured filled sealant on the shear bond strength of metal, ceramic and titanium brackets bonded with resin-modified glass ionomer cement. Indian J Dent Res [serial online] 2013 [cited 2019 Oct 23];24:745-9. Available from: http://www.ijdr.in/text.asp?2013/24/6/745/127625
| Materials and Methods|| |
A total of 60 human premolars were collected and stored in 1% thymol and they were cleaned with water, pumice and a rubber cup and then rinsed with water. The criteria for tooth selection included intact buccal enamel with no pre-treatment with chemical agents and any visible cracks or caries. The samples were mounted in acrylic resin cylinders and were randomly divided into six groups of 10 each, to be bonded with the following premolar brackets:
Group 1: Titanium brackets were bonded directly to etched enamel surfaces (Eqilibrium ti-Dentaurum Corporation Germany)
Group 2: Titanium brackets were bonded to etched enamel surfaces covered with HFLCS (Pro Seal, Reliance Orthodontic Products, Itasca, IL, USA)
Group 3: Metal brackets were bonded directly to etched enamel surfaces (Gemini-3M Unitek Corporation, Monrovia, California, USA)
Group 4: Metal brackets were bonded to etched enamel surfaces covered with HFLCS
Group 5: Ceramic brackets were bonded directly to etched enamel surfaces (Clarity-3M Unitek Corporation, Monrovia, California, USA)
Group 6: Ceramic brackets were bonded to etched enamel surfaces covered with HFLCS.
The average base surface area of the brackets was calculated by using a digital caliper. The mean base surface area was 14.09 mm 2 for the metal brackets 14.13 mm 2 for the ceramic brackets and 14.96 mm 2 for titanium brackets.
Conditioned (etched) bonding protocols were followed for Fuji Ortho LC cement. When the enamel surface was conditioned, it was conditioned by brushing 37% phosphoric acid on to the labial surface. The conditioner was left undisturbed for 30 s and then rinsed thoroughly with distilled water for 30 s and dried with an oil-free air source until the etched surfaces appeared chalky White. In groups 2, 4 and 6 HFLCS (Pro Seal, Reliance Orthodontic Products, Itasca, IL, USA) was applied before bracket bonding in a thin uniform layer on the etched enamel with a brush and light-cured with a visible light-curing unit at close range for 20 s. In groups1, 3and 5, HFLCS was not applied. The brackets were bonded with Fuji Ortho LC (GC Europe) in all groups. The Fuji Ortho LC cement was mixed following manufacturers guidelines. One level spoonful of powder and two drops of liquid were dispensed on to a mixing pad. The powder was incorporated into liquid in two equal portions. The first portion was mixed into the liquid for 10 s. The second part was then added and mixed for another 10-15 s. The total mixing time did not exceed 20-25 s. Each bracket was bonded to the buccal surface so that slot lies parallel to the incisal edge of the premolar. Each mix was used to bond two brackets only. Then the cement was light cured, using a dentsply visible-light-curing unit for 20 s on the mesial and distal sides of the bracket base, for a total of 40 s. Upon completion of bonding and curing procedures, each specimen was stored for 24 h in deionized distilled water at 37°C in container. The shear bond strength of brackets was found out by using the Instron universal testing machine (3654). Each specimen was clamped in a holding ring so that the bracket base was parallel to the direction of the force. The sharpened chisel blade suspended from the upper arm of the testing machine was placed at the bracket enamel interface just short of contact in an incisogingival direction. Using a crosshead speed of 0.5 mm/min the bracket bases were shear tested to failure. The maximum force was recorded in Newtons (N) and converted to megapascals (MPa). Mean shear bond strength was calculated for each group. The debonded enamel surfaces were examined under a scanning electron microscope (SEM) at 25-times magnification to assess the residual adhesive on the tooth surface.
The adhesive remnant index (ARI) was used to quantify the amount of cement left on the tooth following debonding of the bracket. 
0 = No cement left on the tooth
1 = Less than half of the cement left on the tooth
2 = More than half of the cement left on the tooth
3 = Al1 of the cement left on the tooth plus a distinct impression of the bracket base.
Descriptive statistics including mean, standard deviation, standard error and 95% confidence interval for mean were calculated for each group. Univariate analysis of variance (analysis) was performed to test the main effects of bracket type and HFLCS application [Table 1]. The Chi-square test was used to determine the significant differences in the ARI scores among the groups [Table 2]. Post-hoc test was performed to find the multiple comparisons between the groups. The significance was predetermined at P < 0.05.
| Results|| |
Interaction between HFLCS and bracket type was not statistically significant (P = 1). Pre-treatment with HFLCS did not cause a statistically significant change in the shear bond values of either titanium, metal and ceramic brackets (P = 0.596). Shear bond values of the titanium brackets were higher than those of ceramic brackets and bond strength of ceramic brackets were higher than those of the metal brackets independent of HFLCS application (P < 0.001).
The Chi-square test results indicated no significant differences among the ARI scores of the groups. In all groups, a significant percentage of the teeth had little (lesser than 10%) or no adhesive on the enamel surface [Table 2].
| Discussion|| |
Because of the increased difficulty in adequately removing bacterial plaque around orthodontic appliances, adjunctive fluoride therapy is commonly used to prevent demineralization. A dose response relationship has been found between the frequency of fluoride application and the degree of enamel protection. , However, effective protection with fluoride requires appropriate patient compliance. Various methods of decreasing demineralization have been examined that do not require patient compliance. Fluoride varnishes are an option that allows the orthodontist to control the timing and amount of fluoride used. However, varnishes require several office applications and are generally applied only after lesions are found to prevent their progression. Another method of decreasing demineralization without patient compliance is through fluoride-releasing bonding systems. , Caries inhibition has been demonstrated by both fluoride-releasing composites and glass ionomer cements, but the bond strengths of these materials are low. Recently, adhesive systems have been modified from acrylic and epoxies to epoxy-acrylates and from glass ionomer, fluoride-releasing cements, to the current RMGICs. In 1972, Wilson and Kent formulated new translucent cement for dentistry, the glass ionomer cement (GIC). However, several investigators reported that GICs showed significantly decreased shear bond strengths as compared with the traditional composite resin adhesives. Because of their weaker shear bond strengths, the light cured second generation GICs were not advocated for direct bonding of orthodontic brackets. Recently, a light-cured resin reinforced glass ionomer, Fuji Ortho LC (GC, Tokyo, Japan) was introduced as an alternative direct bonding agent. The manufacturer suggests that Fuji Ortho LC may be used without etching and in the presence of water or saliva and still maintain clinically useful shear bond strengths. RMGICs possess desirable fluoride releasing properties and clinically acceptable bond strengths.  The bonding of RMGIC to resin based cement could be the result of the curing process, which occurs by three reactions, when the powder and liquid are mixed; an acid base reaction similar to that of conventional GIC is initiated. In addition, this material can be cured quickly by light activation from the visible light-curing device. The light activates free radical polymerization of hydroxyethyl-methacrylate (HEMA) and other two monomers to form a poly HEMA matrix that hardens the material. The third reaction is a self-cure of resin monomers. It is believed that poly HEMA and polyacrylic metal salt ultimately forms a homogenous matrix that surrounds the glass particles. As a result, light activated polymerization reaction is well-harmonized with acids base reaction in this formation.  Both RMGIC and the resin based material (Pro Seal) are cured by a free radical initiator system, which provides a potential for the chemical bonding between these two materials. Moreover, HEMA incorporated into the GIC, forms a chemical bond with the Pro Seal. Several mechanisms are thought to be involved in the chemical adhesive bond between resin-modified glass ionomers and resin based cement. Increased availability of unsaturated double bonds in the air inhibited layer of the RMGICs may assist in the chemical bonding to the resin based cement. Unpolymerized HEMA on the surfaces of RMGIC increases the surface wetting capability of the bonding agent when polymerized. Unsaturated methacrylate pendants, which are available on the polyacid chain within the polymerized RMGIC, may also form covalent bonds with the resin. RMGIC contain modified polyacrylic acids (PAA), which polymerize to form cross linked PAA that could increase the strength of cement and ultimately the adhesive bond strength to resin.  However; their clinical handling properties are less than ideal. Previous investigations have examined the effectiveness of resin sealants to protect the enamel surface. ,,,,, Both chemical and light cure products have been examined with only mediocre results. Due to oxygen inhibition at the surface, chemical cure systems fail to reach complete polymerization. This results in a thin or often non-existent, layer remaining. Light-cure resins, although still susceptible to some oxygen inhibition at the surface, reach a higher degree of polymerization and offer more complete coverage than chemical cure products.
Unfortunately, the unfilled or lightly filled resins with the desired low viscosity and high flowability lack the strength to resist abrasion over an extended period of time. Reliance Orthodontic Products (Pro Seal, Itasca, IL, USA) recently released an enamel sealant specifically for orthodontic use. The application of resin sealant on the enamel surface around and beneath the orthodontic bracket is a prophylactic method to prevent demineralization. According to the manufacturer, the HFLCS Pro Seal offers maximum protection against decalcification and white spot formation and it can be used with light-cure, chemical-cure or dual-cure paste systems. It is different from unfilled or lightly filled sealants, because it achieves 100% polymerization without incorporating a residual oxygen-inhibiting layer.
Pro Seal treated enamel samples are typical to that of normal human enamel. The enamel surface showed perikymata grooves as wave-like parallel rings and obvious perikymata ridges. The enamel surface exhibited localized depressions (focal holes) with no evidence of rod ends. The smooth enamel surface with intact surface details, (perikymata ridges and focal holes), denoted that Pro Seal could have provided protection to the enamel from acid penetration and preserve crystal organization, which is in agreement with that described by Farina  in normal condition. Hence, the effect of HFLCS on etched enamel surfaces did not affect the bond strength values and bond failure modes of metal, ceramic and titanium brackets bonded with RMGIC. This is in accordance with the study conducted by Bishara  and Paschos. 
Shear bond values of titanium brackets were higher than those of ceramic brackets and ceramic brackets were higher than those of the metal brackets independent from the HFLCS application (P < 0.001). The mean shear bond strength values of metal brackets were at the lower limit of the bond strength range independent of sealant application was considered adequate for most clinical orthodontic needs. In previous studies, bond strength values well below the accepted standards were reported for metal brackets bonded with RMGIC. The low shear bond strength with Metal brackets could be due to a number of variables as explained by Maijer and Smith (1981): 
- Weld spots in the integral woven foil mesh base could reduce the retentive area
- Corrosion of the mesh bases following leakage at the resin mesh interface
- Lack of resin penetration into the woven foil mesh bonding pad
- Stress concentration at the resin mesh interface.
Ceramic brackets showed significantly higher shear bond strength values than metal brackets independent of sealant application. This is in accordance with the study conducted by Viazis, et al.  Odegaard and Segner  also evaluated the bond strength of ceramic and metal brackets and found that shear bond strength of ceramic brackets were more as compared with the metal brackets. Hitmi et al.,  who evaluated the clinical performances of RMGIC, reported similar findings.
Titanium brackets showed significantly higher shear bond strength values than metal and ceramic brackets independent of sealant application. Titanium brackets with laser structured bases showed the highest mean shear bond strength values [Table 1]. This is in accordance with the study conducted by Corey et al.  that the laser structured bracket base retention mechanism provided the strongest bond strength and the use of discovery retention mechanism with resin reinforced GIC is sufficient to obtain clinically optimal bond strength. Titanium brackets have an additional advantage in having miniaturization over stainless steel because of greater strength and low elastic modulus. This low bracket profile makes the appliance less conspicuous and can be helpful in assessing lip balance during treatment. The retention of the adhesive to the base of titanium attachments is refined by utilizing computer aided laser cutting process that generate a myriad of micro and macro undercuts making it possible to design an adhesive pattern for each tooth. Laser undercuts also eliminates the additional chemical treatment of the base such as sialinization.
In this study, the ARI scores had no significant difference among the groups; HFLCS application did not change the mode of bond failure. Six groups had a higher prevalence of ARI scores of 0 and 1 [Table 2], [Figure 1] and [Figure 2], meaning that the failure occurred at the enamel-adhesive inter-face. The site of failure also provides useful information about the bond. Ideally, an adequate bond that fails at the enamel-cement interface is desirable because it makes cleaning and polishing of the teeth much easier.  Moreover, the cleaning procedures to remove adhesive remnants can be accompanied with less enamel loss.  This is in accordance with the study conducted by Larmour and Stirrups,  Rix et al.  and Coups-Smith et al.  In other studies, different modes of bond failure (at the adhesive bracket interface) were reported by Godoy-Bezerra et al.  and Cacciafesta et al.  These results might be attributed to several factors such as variations in bonding protocol or testing procedures such as crosshead speed of the testing machine, bracket designs, tooth type, storage conditions before testing and the type and concentration of the acid. 
|Figure 1: SEM photograph showing tooth surface after debonding (ARI score 0)|
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|Figure 2: SEM photograph showing tooth surface after debonding (ARI score1)|
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| Conclusion|| |
The effect of HFLCS on enamel surfaces etched with 37% phosphoric acid did not affect the bond strength values and bond failure modes of metal, ceramic and titanium brackets bonded with RMGIC. These results suggest that HFLCS can be used in combination with RMGIC as a preventive measure at initial bonding as it results in a significant reduction of enamel demineralization in vitro. This method might be useful in orthodontics, but further clinical investigations are warranted, based on the positive results of our study.
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Departments of Orthodontics and Dentofacial Orthopaedics, Himachal Dental College, Sundernagar, District Mandi, Himachal Pradesh
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2]
[Table 1], [Table 2]