In Vitro Bond Strengths of Resin Modified Glass Ionomer Cements and Composite Resin Self-cure Adhesives: Introduction of an Adhesive System with Increased Bond Strength and Inhibition of Decalcification
Abstract
This study evaluated the shear bond strength and adhesive remnant index of 5 self-cure adhesives to comparatively evaluate a new adhesive system. Extracted human incisors were randomly divided into 7 test groups of 20 each. Incisor mesh-backed brackets were bonded to the tooth specimens in each group with their respective adhesive according to the manufacturer's instructions. The specimens were thermocycled for 2 weeks at temperatures from 5° to 55°C to simulate oral conditions and debonded using an Instron machine. Acceptable bond strength parameters were present with the Contacto No-Mix (composite resin containing glass ionomer 8.75 MPa) and Fuji Ortho SC with acid conditioning (6.98 MPa). Contacto No-Mix had a higher bond strength (11.2 MPa) when microetching and Megabond were employed than when these adjuncts were not employed. When FUJI Ortho SC specimens were conditioned with polyacrylic acid, they showed a higher bond strength (6.98 MPa) than when bonded to unetched teeth (5.41 MPa). In test 3, EXPT-fluoride adhesive (AF) demonstrated a higher bond strength (13.44 MPa) than both resin composite Contacto No-Mix (8.8 MPa, GAC 7.4 MPa) and FUJI Ortho SC (5.41 MPa). Expt AF (Test 3) and Concise had equal bond strengths, however, the former can potentially release fluoride from the glass ionomer. Although the Expt AF had the highest bond strength, its adhesive remnant index was the same as FUJI Ortho SC acid etched. This may be attributable to the layer of Megabond used with the Expt AF. No correlation was evident between bond strengths and adhesive remnant on tooth surfaces after debonding. It was noted that a self-cure resin modified glass ionomer adhesive system that releases fluoride and has high bond strength can be employed. Microetching and adhesion promoters increase the bond strength.
INTRODUCTION
Since the advent of bonding brackets,1 clinicians and researchers have worked to improve the qualities of bonding agents. The qualities that have been of most interest include bond strength, adequate working time, shorter cure time, improved ease of use and the introduction of anticariogenic agents. Historically, bracket bonding has evolved from the original use of plastic brackets2,3 to metal brackets and more recently to the use of ceramic brackets. Adhesives have been altered from acrylics and epoxies to epoxy-acrylates4,5 to glass ionomer fluoride releasing cements6,7 to the current resin modified glass ionomer cements (compomers).
The time involved in enamel etching with phosphoric acid has been reduced from 60 or 30 seconds to 10 seconds. In addition, 10% polyacrylic acid can be used to foster bonding to moist, unetched dry surfaces when using resin modified glass ionomer cements. Chemical (self) curing adhesives have been reformulated to serve as light curing adhesives.8–10 Recent research and clinical trials have shown that microetching bracket bases can increase bond strength.11 Adhesion promoters for tooth structure have also recently been introduced.12 Advances in improving the properties of adhesive systems are constantly being developed and clinically employed.
Previous research formulations of glass ionomer cements revealed insufficient clinical bond strength. The authors have recently been involved in the formulation of an adhesive system that can release cariostatic fluoride and increase bond strength, thereby decreasing unfavorable and untimely debonding incidents and decalcifications.7–10
The purpose of this investigation was to compare the bond strength of a new self-curing resin glass ionomer adhesive system (AF) with other representative commercial adhesives. Previous investigations have noted that microetching (sandblasting) of the bracket and the use of adhesion promoters to the enamel (eg, pyromellitic glycerol dimethacrylate) increased bond strengths of the adhesives studied.13,14 These additional bonding methods have been incorporated into this new adhesive system. In addition, this investigation is an in vitro study of the possible correlation of bond strength and adhesive remnant on tooth enamel of composite resin adhesives and resin modified glass ionomer cements.15
MATERIALS AND METHODS
Teeth
A total of 140 extracted human incisors were divided into 7 groups of 20 teeth each. The teeth were stored in 0.1% thymol solution and then placed in 1% saline solution. The roots of the incisors were notched and bonded into lugs with fast-setting self-curing acrylic. These specimens were then stored in 1% saline solution until shear testing. The specimens were maintained in as wet a condition as possible to simulate the intraoral environment.
The labial surfaces of the teeth were cleaned with flour of pumice and a rubber cup, rinsed with distilled water, and dried with an air syringe free of oil and water. The tooth specimens in Tests 1, 2, 3, 6, and 7 were all acid etched for 30 seconds with a 37% phosphoric acid containing fluoride, rinsed with water and dried with an air syringe for 15 seconds. The teeth in test 4 were etched with 10% polyacrylic acid for 30 seconds. The specimens in test 5 were not etched, but were slightly moistened with a moist cotton roll before bonding. Twenty teeth were bonded for each test. Bonding was performed in accordance with the manufacturer's instructions for bonding in vivo. After bonding, excess flash was removed with an explorer with care taken to not disturb the bond.
Test condition
Each shear test specimen was inserted into the vice of the Instron machine (Instron Corp, Canton, Mass) so that the brackets were placed parallel to the direction of the load application. Stainless steel 0.012-inch ligatures were placed under the gingival wings of the twin brackets and moved gingivoincisally in a shear force. The load was applied at a crosshead speed of 1 mm per minute with a load range of 50 kg. A recording chart was used to record the force applied until bond failure occurred.
The brackets
Straight arch wire 80-gauge metal mesh braised incisor brackets (Dentaurum Inc, Newtown, Penn) were used. The cross-sectional area of all the mesh bases was 0.126 cm.2 The bond forces were recorded in kilograms and the shear stress in megapascals (MPa).
All brackets were microetched with 90-micron aluminum oxide (Danville Engineering Inc, Danville, Calif) before bonding except for 2 of the test groups. The brackets were microetched for 5 seconds at a 5-mm distance from the nozzle to the brackets. Previous in vitro and in vivo testing of microetched brackets has revealed an increase in bond strength when compared to brackets used as received from the manufacturer.11,12
Thermocycling
The specimens were bench cured for 15 minutes to ensure completed curing before being placed in a thermal cycling water bath. The test samples were exposed for 1500 temperature cyclic changes (5° to 55°C) for 14 days before testing to mimic oral, thermally stressed, accelerated aging. The dwell time was 1 minute for each group. All bonding was accomplished by one of the orthodontists.
Fracture modes were determined by examination of the debonded enamel surfaces and the bracket bases with a 10× illuminated magnifying glass. An Adhesive Remnant Index (ARI)16 was divided into 4 areas: (0) no adhesive on tooth, (1) less than half of the adhesive on tooth, (2) more than half of the adhesive on tooth, and (3) all of the adhesive on tooth. The data was analyzed using a one-way analysis of variance (ANOVA) and Tukey's method.
Adhesive systems tested
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Contacto No Mix—No Promoter (contains glass ionomer fluoride; General Orthodontic Supply Inc, West Orange, NJ). All tooth surfaces were pumiced, etched for 30 seconds, washed, and dried. The primer was applied to the tooth surfaces and brackets, the paste was placed on the bracket (primer) surface, and the brackets were bonded. Excess flash was removed in all tested brackets. As in all the bonded brackets in this study, bench curing was 15 minutes. The brackets were used as presented by the manufacturer (not microetched).
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Contacto No-Mix-Promoter (contains glass ionomer fluoride; General Orthodontic Supply). Each tooth was pumiced, etched for 30 seconds, washed, and dried. Three coats of Megabond were applied to the tooth surfaces and the teeth were bonded as described above.
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Hybrid glass ionomer cement Expt GIC AF (General Orthodontic Supply). Each tooth was pumiced, etched for 30 seconds and left slightly moist. Three coats of Megabond (adhesion promoter) were applied on the tooth surface. Equal parts of Paste A (catalyst) and B (base) were mixed on a cold glass slab for 10 to 15 seconds, the adhesive was applied to the back of the brackets, and the teeth were bonded. Expt AF contains hydroxy ethyl methacrylate (HEMA) and 50% glass ionomer fluoride filler.
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Hybrid GIC FUJI Ortho SC (self-cure; GC America Inc, Chicago, Ill). Each tooth was pumiced, etched with ortho conditioner (10% polyacrylic acid) for 30 seconds, washed, and left slightly moist. The powder and liquid were mixed on a cold glass slab according to the manufacturer's instructions and the brackets were bonded.
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Hybrid GIC FUJI Ortho SC (GC America Inc). The tooth surfaces were pumiced, washed, and left slightly moist before bonding. The liquid and powder were mixed according to the manufacturer's instructions. These tooth specimens were not etched.
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Concise Self-Cure Paste-Paste Sealant (3M Unitek, Monrovia, Calif). All teeth were pumiced, etched for 30 seconds, washed, and dried. The unfilled sealant was applied and equal parts of catalyst and base pastes were mixed on a cold glass slab. The purpose of testing Concise was to compare a sealant paste-paste composite system having high bond strength to a highly filled glass ionomer AF. Concise does not contain or release fluoride. Concise has been used as a standard in bonding studies.
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GAC No Mix (GAC International, Central Islip, NY). The teeth were etched, however, the brackets were not microetched. This self-curing no mix did not contain glass ionomer fluoride. GAC No Mix was tested as a representative sample of the customary no-mix adhesives.
Shear bond strength results
Table 1 shows the summary statistics of the bond strength values in terms of mean, median, standard deviation, and range. Figure 1 visually compares the different mean bonding strengths and their corresponding standard deviations for the different bonding agents. A 1-way ANOVA test revealed significant differences between the mean MPa values (F6,133F =23.99, P < .0001). To determine the statistically significant (α = 5%) difference between the mean bond strengths from different treatments, a post-hoc analysis was conducted (Table 2). The pertinent observations from the post-hoc analysis are as follows:
Citation: The Angle Orthodontist 71, 4; 10.1043/0003-3219(2001)071<0312:IVBSOR>2.0.CO;2
Test 3 (13.4 MPa) and Test 6 (12.3 MPa) showed significantly higher bond strengths than Tests 1 (8.75 MPa) and 7 (7.36 MPa). The average bond strength of Tests 3 and 6 were 54% and 41% higher than Test 1 and 7, respectively. The average bond strength of Test 2 (11.15 MPa) was 27% higher than Test 1 (8.75 MPa), but the difference was not statistically significant. Test 5 (5.41 MPa) showed a statistically significant reduction (41%) in the mean, median, standard deviation, and range of bond strength compared to Test 1. The remaining 2 tests, Tests 4 (6.98 MPa) and 7 (7.36 MPa) showed no significant difference from Test 1.
The mean bond strength of Test 3 (Expt AF) was significantly larger than Test 5. (FUJI SC). The mean bond strength of Test 3 (Expt AF) and Test 6 (Concise) were not statistically different. Similarly, mean MPa values of Tests 4 and 5 showed no significant difference.
Adhesive remnant index
The mean Adhesive Remnant Index (ARI) scores,16 standard deviations and ranges for the 7 groups are listed in Table 3. The ARI means were statistically different among the 7 bonding systems tested (Kruskal-Wallis nonparametric test, X2kw = 68.51, P < .001). The mean ARI values of Test 1 and Test 2 were not significantly different. Similarly, Tests 3 and 5 did not show a statistically different mean ARI, although they showed considerably lesser values compared to the previous 2 tests. Test 7 showed considerably higher ARI than Test 5 (2.8 vs 0.00) (P < .05).
DISCUSSION
Direct bonding of brackets caused a technological breakthrough in the practice of orthodontics by replacing most band fabrications and cementations over the past 3 decades. This investigation revealed that an adhesive can be formulated to release fluoride, thereby potentially decreasing decalcification while concomitantly increasing bond strength. The former facts were not corroborated in this study, but have been reported previously.7–9,15,17–20 Increased bond strength is essential when treating noncompliant patients or when bonded brackets are subjected to unexpected impact forces from hard or sticky foodstuffs or sports activities. Although glass ionomer adhesives tend to prevent decalcification, the clinician must make the patient aware of good oral hygiene to eliminate this noncompliance factor.
Surface treatment (etching) with 37% phosphoric acid can readily be accomplished by application of etchant for 15 to 30 seconds. The tests revealed that when using an adhesion promoter and microetching the mesh brackets, there was an increase in bond strength (Test 1, 8.75 MPa; Test 2, 11.15 MPa).
When the FUJI self cure adhesives were used, “conditioning” with 10% polyacrylic acid increased the bond strength even when the teeth were under slightly moist conditions. The powder-liquid mixing of the Self-Cure Fuji is operator-sensitive and may affect bonding results.
Etching of tooth enamel contributes to increased bond strength by increasing bonding surface area, and removing bacterial plaque and remnants of organic pellicle (compare Tests 4 and 5).
This in vitro investigation evaluated the mean shear debonding strengths of stainless steel (nonmicroetched and microetched) brackets bonded with composite resin and hybrid glass ionomer adhesives. A recently introduced hybrid resin modified glass ionomer adhesive (Expt AF) was also evaluated. The clinical use of Expt AF over a 2.5-year period indicates minimal premature debonding (<4%) and decalcifications. Megabond forms a protective sealant and appears to minimize any ill effect upon tooth structure after debonding (see Table 3). The effects of adhesion promoters on bond strength of the tested samples were analyzed. The amount of adhesive remnant on tooth enamel was also noted. The study of the adhesive remnant index points to a lack of correlation between bond strength and adhesive remnant on the tooth surfaces after debonding21 (compare Tests 3 and 4).
The highest bond strengths were obtained with hybrid GIC (AF Expt) and Concise. Test 3 confirms that it is possible to formulate a self-curing adhesive system that releases glass ionomer fluoride and has high bond strength. When the FUJI Self-Cure is used to bond to slightly moist unetched teeth, the bond strength is 5.41 MPa. However, when the brackets are microetched and the teeth are conditioned (10% polyacrylic acid) and left slightly moist, the bond strength is increased to 6.98 MPa.
Examination for adhesive remnant index indicates that there was more adhesive remnant left on the teeth when the brackets were not microetched and the teeth were etched (Test 7). Minimal adhesive remnants were noted on teeth not etched (Test 5).
Contacto No Mix (Test 1) (8.75 MPa) contains glass ionomer fluoride while GAC No Mix (Test 7) (7.36 MPa) does not contain glass ionomer. Neither of these self-cure adhesives was tested with microetched brackets. However, they both have adequate bond strengths.
It is difficult to extrapolate differences in bracket-adhesive fracture modes from in vitro and in vivo tests. Neither no-mix adhesive presents any difficulty in removing adhesive remnants from the enamel clinically, however, these adhesives and Concise show significant adhesive remnant indices in vitro. The shear bond strengths of the adhesives investigated in this study were within the range of clinical adequacy as reported in the literature (5 to 9 MPa).22–25 We have noted (in vitro and in vivo) that Megabond and Expt AF promote bonding under slightly moist conditions. They contain hydroxyethylene-methacrylate (HEMA) and are hydrophilic.
CONCLUSIONS
Our results show that microetching the mesh backs of brackets tends to increase bond strength. Adhesion promoters (eg, Megabond) containing pyromellitic glycerol dimethacrylate (PMGDM) and HEMA and other acrylates can enhance bond strength. An increase in bond strength is essential when evidence of patient noncompliance, such as excessive bond failure, requires the above components to form an adhesive system. Etching of enamel fosters increased bond strength whether conventional composite resin adhesives or resin modified glass ionomer cements are used. A self-cure (chemical cure) hybrid modified resin glass ionomer adhesive system can be formulated and employed to release fluoride and possess a high enough bond strength to minimize untimely bracket debonding.
The means and standard deviations of shear bond strength (MPa) obtained from different adhesive treatments. AE indicates Acid Etch; MB, Megabond; ME, Microetch
Contributor Notes
Corresponding author: George V. Newman, DDS, 395 Pleasant Valley Way, West Orange, NJ 07052.