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A comparative evaluation of bracket bonding with 1-, 2-, and 3-component adhesive systems

A comparative evaluation of bracket bonding with 1-, 2-, and 3-component adhesive systems



American Journal of Orthodontics and Dentofacial Orthopedics, 2007-08-01, Volume 132, Issue 2, Pages 144.e1-144.e5, Copyright © 2007 American Association of Orthodontists


Introduction Today, 1- and 2-component adhesives are available for bracket bonding that could diminish the possibility of contamination during the bonding procedure and save the clinician chair-side time. Our aim in this study was to compare the shear bond strengths and the adhesive remnant index (ARI) scores of 1-, 2-, and 3-component adhesives after thermocycling. Methods Fifty stainless steel brackets (10 per adhesive group) were bonded to extracted third molars with 5 adhesives. Group 1 was a 1-component adhesive, RelyX Unicem (3M Espe, Seefeld, Germany). Group 2 was a 1-component adhesive, Maxcem (Kerr, Orange, Calif). Group 3 was a self-conditioning 2-component adhesive system, Multilink (Ivoclar-Vivadent, Schaan, Liechtenstein). Group 4 was a 2-component adhesive system, Transbond Plus primer (self-etching) and Transbond XT adhesive (3M Unitek, Monrovia, Calif). Group 5 (control group) was a conventional 3-component adhesive system consisting of an etchant, Transbond XT primer, and XT adhesive (3M Unitek). All samples were thermocycled (6000 × 5°C/55°C) in a mastication device before shear bond strength testing and evaluation with the ARI. Results No significant differences of shear bond strength between the 2- and 3-component adhesive systems were found. Significant decreases of shear bond strength were observed with 1-component adhesives, RelyX Unicem and Maxcem, compared with 2- and 3-component systems. The ARI scores indicated no significant differences between the groups. Conclusions With enhanced shear bond strength, 1- component adhesives have the potential to compete successfully with 2- or 3-component adhesives.

Since the basic investigations of Buonocore in 1955, etching tooth surfaces for providing micromechanical retention has been commonly used in dentistry. Initially, only 3-component adhesive systems were available for bracket bonding. These adhesive systems have 3 agents: an enamel conditioner, a primer solution, and an adhesive resin for bonding brackets to the conditioned tooth surface. However, this procedure is time-consuming, and there can be saliva contamination between the conditioning steps. Moisture contamination is believed to be the main cause of bond failure and makes the bonding process highly technique sensitive. To simplify the technique and decrease the amount of enamel loss, efforts were made to reduce the number of conditioning steps. Therefore, self-etching primer products were developed, containing a mixture of acidic functional monomers and other constituents. Diffusion of monomers and partial dissolving of hydroxyapatite is enabled simultaneously, effecting a resin-infiltrated zone entrapped with minerals. Most self-etching primers are 2-component systems, with the etching and priming procedures combined in the first step. An adhesive must be applied on the bracket for bonding in a second step. However, some studies disagree whether self-etching primers provide similar shear bond strength (SBS) as do conventional adhesive systems.

New 1-component self-etching and self-adhesive resin systems have been introduced. It is suggested that these dual-cure systems can be used without surface preparation. These products combine etchant, primer, and adhesive resin in a single paste. This approach could diminish the possibility of contamination during the bonding procedure and save the clinician chair-side time.

Our aim in this study was to compare the SBS and the adhesive remnant index (ARI) scores of two 1-component self-etching and self-priming adhesive systems with two 2-component self-etching primer systems and a conventional 3-component adhesive system (etchant, primer, and adhesive) by using foil-mesh metal brackets. The null hypothesis was that bond strength of the various adhesive systems does not differ significantly. To simulate temperature changes and moisture in the oral cavity, all samples were exposed to thermocycling (6000 × 5°C/55°C) in a mastication device before shear bond testing.

Material and methods

Fifty recently extracted third molars were collected and stored in 0.5% chloramine solution. The roots were removed, and the crowns were embedded in auto-polymerization acrylic resin so that the facial surface of each tooth was parallel to the base of the polymer. The teeth were cleaned with a nonfluoridated pumice paste and rubber cups.

Five adhesive groups (10 teeth per group) were formed. The adhesive in group 1 was the dual-curing, self-etching, and self-priming 1-component system RelyX Unicem (3M Espe, Seefeld, Germany). Group 2 consisted of the self-etching and self-priming 1-component system Maxcem (Kerr, Orange, Calif). In these 2 groups, stainless steel brackets (Ormesh, Ormco, Glendora, Calif) were bonded to the teeth without enamel conditioning. Group 3 consisted of the self-conditioning 2-component adhesive system Multilink (Ivoclar-Vivadent, Schaan, Liechtenstein). After application for 30 seconds, the self-conditioning primer was dried with oil-free air. Then the adhesive was applied on the bracket bases and bonded to the teeth. In group 4, another 2-component adhesive system was used, Transbond Plus primer (self-etching) and Transbond XT adhesive (3M Unitek, Monrovia, Calif), with application the same as described in group 2. Group 5 was the control group; a conventional 3-component adhesive system (etchant, primer, and adhesive) was tested. The enamel of each tooth of group 5 was etched with 20% phosphoric acid (Gluma Etch 20 Gel, Heraeus Kulzer, Hanau, Germany) for 30 seconds. A frosted appearance indicated a successful etch. Transbond XT primer (3M Unitek) was applied, gently thinned with air, and light-cured for 20 seconds. After this procedure, stainless steel brackets were bonded to the teeth with Transbond XT adhesive.

In all groups, Ormesh stainless steel brackets were bonded. All brackets were placed centrally on the flat buccal surfaces of the teeth. The excess resin was carefully removed from the tooth with a dental probe. The samples were then light-cured with a light-emitting diode curing device (Ortholux LED, 3M Unitek) for 60 seconds. All brackets were bonded by the same operator.

To simulate the moisture and temperature changes in the oral environment, all samples were exposed to thermocycling 24 hours after preparation. All groups were alternatively flooded every 2 minutes with warm (55°C) and cold (5°C) distilled water for 6000 cycles in a mastication device.

A universal testing machine (model 1446, Zwick, Ulm, Germany) was used at a cross-head speed of 1 mm per minute for SBS testing. The embedded tooth and the adhesively fixed bracket were positioned in the machine so that the bracket slot was horizontal. A knife-edge shearing rod was used to deliver the shear force at the bracket base-enamel interface. All brackets were shear tested to failure. SBS was determined according to the formula σ shear = F max /A bracket base surface (in megapascals). The surface area of the bracket bases was determined by measuring length and width and computing the mean area. Furthermore, the ARI scores were assessed. ARI scores are 0 to 3 (0, no adhesive on enamel; 1, <50% adhesive on enamel; 2, >50% adhesive on the enamel; 3, 100% adhesive on enamel).

To determine statistical differences, the Mann-Whitney U test was performed. Medians and 25% and 75% percentiles were calculated. The level of significance was set at α = 0.05.

Results

After thermocycling, the 1-component adhesive systems, RelyX Unicem and Maxcem, had the lowest median SBS values ( Table I ). The statistical analysis of these data showed no significant difference between the 2- and 3-component adhesive systems ( P >.05). A significant decrease of SBS was observed with the 1-component adhesives, RelyX Unicem and Maxcem ( P = .005), which had similar low SBS values ( Fig ). The highest median values of SBS (9.84 ± 1.43 MPa) were measured for the conventional 3-component adhesive, Transbond XT primer and Transbond XT adhesive with 20% phosphoric acid before bonding ( Table I ).

Table I
Median values and standard deviations of SBS
SBS (MPa)
RelyX Unicem 7.12 ± 0.69
Maxcem 7.06 ± 0.74
Multilink primer and adhesive 9.40 ± 0.74
Transbond Plus primer and Transbond XT adhesive 8.67 ± 1.21
Acid-etching (20% phosphoric acid), Transbond XT primer, and Transbond XT adhesive 9.84 ± 1.43

Shear bond strength (in MPa) of brackets bonded with 1-, 2-, and 3-component adhesives.
Fig
Shear bond strength (in MPa) of brackets bonded with 1-, 2-, and 3-component adhesives.

The ARI scores for the 5 test groups are shown in Table II . The ARI scores indicated no significant differences between the groups. However, the 1-component systems (RelyX Unicem and Maxcem) tended more to failures on the enamel-adhesive interface, ie, with most of the adhesive remaining on the bracket ( Table II ).

Table II
ARI scores of adhesive groups
ARI score
0 1 2 3
RelyX Unicem 0 3 7 0
Maxcem 1 2 6 1
Multilink primer and adhesive 0 1 7 2
Transbond Plus primer and Transbond XT adhesive 0 1 6 3
Acid-etching (20% phosphoric acid), Transbond XT primer, and Transbond XT adhesive 0 1 7 2

Discussion

To permit sufficient penetration of the primer into the enamel surface, the use of acid etchants followed by primer was a fundamental element of the bonding process of adhesives. Nowadays, 1- and 2-component adhesives save time and reduce the probability of contamination during bracket bonding, although these advantages only have real value if the adhesive bond strengths are comparable with those of conventional 3-component systems.

The 1-component composite RelyX Unicem is a dual-curing, self-etching, and self-priming adhesive resin cement. The resin contains bifunctional (meth) acrylate and releases fluoride ions. The proportion of inorganic fillers is approximately 72% by weight, and the grain size is 9.5 μm. Maxcem, another self-etching, self-priming, dual-curing adhesive resin cement, contains 66% filler by weight with an average filler particle size of 3.6 μm. The underlying principle of self-etching primer systems is the formation of a continuum between the etched enamel and the adhesive by simultaneous demineralization and penetration of the tooth surface with acidic monomers. Calcium and phosphorous ions released from the dissolution of the hydroxyapatite crystals are suspended in the primer solution because the acidic primer is not rinsed off during application. According to Dunn and Soderholm, the high concentration of calcium and phosphorous ions limits further dissolution of the apatite and thus restricts the depth of enamel demineralization.

The results of our study indicate that the tested self-etching (2-step) primer systems have sufficient bond strength compared with the conventional 3-component system, but the 1-component adhesive systems showed significantly less bond strengths than the 2- and 3-component systems we tested. Furthermore, more adhesive fractures on the enamel-adhesive interface were found with the 1-component adhesives. The quantity of residual adhesive on the enamel strongly affects the cleanup procedure after debonding. Therefore, any time savings during the bonding procedure with 1- or 2-component adhesive systems would be lost during cleanup if too much adhesive remained on the enamel after debonding. However, the ARI scores of this study indicate that almost the same or even less adhesive remained on the enamel after debonding with 1- and 2-component adhesives. Therefore, no time loss can be expected with self-conditioning adhesive systems. Additionally, formation of white spot lesions on enamel or decalcification might be reduced by eliminating the acid-etching procedure. Many in-vitro studies are available about bond strength of adhesives with 2-component self-etching primers. Most of these studies describe sufficient bond strength of self-etching primers (2-component systems) compared with conventional 3-component systems. Their findings agree with our results.

Nevertheless, only a few studies have been published about the efficiency of 1-component adhesive systems. House and Sheriff performed an in-vitro study in which bond strength of a 1-component adhesive system was comparable with that of a conventional 3-step system. Bishara et al investigated the influence of a 1-component self-etching and self-priming adhesive on SBS. Their results indicated that the 1-step bonding system provided lower SBS than the tested 2-step self-etching primer system. However, they did not compare their results with a conventional 3-component adhesive system, and SBS was tested without water storage and thermocycling. Hence, they suggested a thermocycling procedure for further investigations of the durability of 1- and 2-component adhesive systems. Buonocore recommended thermocycling of specimens to consider the durability of the bond. It was reported that bond strengths of some adhesive resins are lower after thermocycling. Two reasons for this phenomenon can be suggested: water uptake and the effect of various coefficients of thermal expansion. Increased water sorption is most likely the main factor to affect bond strength. Water can penetrate the polymer. As a result, the secondary chemical bonding forces (van der Waals forces) between the polymer chains are reduced, and the mechanical properties of the resin decrease. This plasticizing effect affects mainly the properties of the adhesive. Another reason for the decrease of bond strength of thermocycled samples could be differences in the coefficient of thermal expansion between the bracket, the adhesive, and the enamel. Consequently, we decided to use thermocycling (6000 × 5°C/55°C) in a mastication device before shear bond testing to simulate cyclic stress at 2 temperature extremes and to reproduce the water sorption expected in the oral environment.

Nevertheless, the difficulty of orthodontic bracket bonding is its semipermanent nature. Bond strength should be high enough to resist accidental debonding during treatment but low enough to remove the bracket from the tooth without generating excessive force that might damage its periodontium. The maximum bond strength of an orthodontic bracket should be less than the breaking strength of enamel, which is about 14 MPa. All tested adhesive systems in this study had lower SBS values. It is a common belief that clinically adequate bond strength for a stainless steel bracket to enamel should be more than 6 MPa. In this investigation, the bond strength values of the adhesives were approximately within this range. Therefore, it is questionable whether the lower SBS values of the 1-component adhesive systems have clinical relevance.

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