Introduction
The purpose of this study was to evaluate the shear bond strength of orthodontic brackets and the effect of an enamel bonding agent (EBA) on enamel surface after debonding.
Methods
Seventy-five extracted maxillary premolars were collected and randomly divided into 3 groups of 25. The enamel cracks were surveyed with a stereomicroscope. In the first group, 1 layer of EBA was applied before the adhesive resin. In the second group, 2 layers of EBA were used; in the third group, no EBA was used. Bonding was followed by incubation for 48 hours at 37°C and thermocycling for 1000 cycles. Debonding was performed with a shear force. The surfaces of the teeth were evaluated, and the length, depth, direction of cracks, and adhesive remnants on the enamel surface were recorded. Data were analyzed by using analysis of variance and paired samples t tests.
Results
There were no significant differences between the shear bond strengths ( P = 0.341) of the 3 groups, or in the direction, length, and depth of the cracks before and after debonding ( P > 0.05). There was a significant increase in the number of cracks after debonding in the 3 groups ( P < 0.05). The evaluation of adhesive remaining on the surface of the teeth indicated that most bond failures occurred at the resin-bracket interface.
Conclusions
The use of EBA does not enhance bond strength or provide greater protection to the enamel surface during debonding.
The enamel-etching technique of Buonocore is commonly used to attach orthodontic brackets to the enamel surface. During debonding, clean separation of the bracket, with no adhesive remaining on the teeth, is the desirable outcome because removal of the residual resin is time-consuming and can damage the enamel surface. On the other hand, debonding at the adhesive-enamel interface can produce enamel surface cracks, also are an undesirable outcome. Maintaining a smooth and intact enamel surface after debonding is an important consideration.
Since the 1970s, new dental adhesives have been developed in an attempt to achieve high bond strength when bonding to etched enamel or dentin. It is suggested that, when a thin layer of resin (also called primer, bonding agent, or sealant) is applied and penetrates the etched enamel, it can increase bond strength between the tooth and composite resin. Application of an enamel bonding agent (EBA) was suggested to enhance bond strength, increase resistance against marginal microleakage, reduce decay susceptibility, increase etched enamel damping, protect enamel during debonding, protect against demineralization after etching, and increase etched enamel retention. On the other hand, it was also reported that bonding without a resin can diminish the incidence of dermatitis.
Our aims in this study were (1) to evaluate the effect of an EBA on the shear bond strength (SBS) of orthodontic brackets, (2) to assess the protective effect of EBA on enamel during debonding, and (3) to compare the residual adhesive on the tooth surfaces.
Material and methods
Seventy-five noncarious, recently extracted premolars were used. The teeth were extracted for orthodontic purposes and stored for a maximum of 3 months in saline solution until use. No teeth had hypoplasia or caries on their buccal surfaces. The teeth were mounted in self-curing acrylic.
A stereomicroscope (Blue Light Industry, Waltham, Mass) equipped with a camera was used to assess cracks before and after the experiment. The buccal surfaces of the teeth were divided into 9 areas, and each area was assessed at 20-times magnification. The cracks were categorized as shallow or deep, according to whether they could be seen by the naked eye or by the microscope. Photographs were obtained and analyzed by using Photoshop software (version 6.0, Adobe, San Jose, Calif).
The teeth were divided randomly into 3 groups (A, B, and C) of 25 teeth. The teeth were etched with 37% phosphoric acid for 30 seconds and rinsed and dried for another 20 seconds. Bonding was done as follows. In group A, 1 layer of the EBA (Margine Bond-Coltene, Konstanz, Germany) was applied. Standard edgewise stainless steel brackets (Dentaurum, Inspringen, Germany) were loaded using a bonding agent (3M Unitek, Monrovia, Calif), which is a no-mix, self-cured polymerizing composite resin. In group B, 2 layers of EBA were applied; in group C, no EBA was used. After bonding, all teeth were incubated in distilled water at 37°C for 48 hours (Precision Scientific THELCO 31483, Mandel Scientific Company, Minneapolis, Minn). The teeth were thermocycled between 5°C and 55°C for 1000 cycles to simulate accelerated aging from thermally induced stress.
The brackets were debonded by using shear loading with a universal testing machine (model Z250, Zwick, Ulm, Germany). The crosshead speed was set at 5 mm per minute, and the force was recorded in megapascals at bond failure. After debonding, the enamel surface was examined with Photoshop to determine the amount of residual adhesive with the adhesive remnant index (ARI). The ARI scores are 0, no adhesive remaining; 1, ≤50% of the adhesive remaining; 2, >50% of theadhesive remaining; and 3, 100% of the adhesive remaining.
Results
SBS, enamel cracks before and after debonding, and ARI scores after debonding were recorded and compared among the 3 different groups. The Kolmogorov-Smirnov test showed normal distribution of the data.
The basic statistics and results of the comparisons among the 3 groups are shown in Table I . Analysis of variance (ANOVA) (F = 1.091) indicated no significant differences ( P = 0.341) among the groups. The SBS values were 9.1 ± 1.9 MPa for group B, 8.8 ± 1.7 MPa for group C, and 8.3 ± 1.7 MPa for group A.
| Group | (n) Teeth | Mean | Maximum | Minimum | SD |
|---|---|---|---|---|---|
| A | 25 | 8.3 | 12.3 | 4.4 | 1.7 |
| B | 25 | 9.1 | 13.5 | 4.3 | 1.9 |
| C | 25 | 8.8 | 14.5 | 5.3 | 1.7 |
| F | P | ||||
| 1.091 | 0.341 | ||||
There were no significant differences in the direction, length, and depth of the cracks after debonding in the groups ( P >0.5). However, the paired-samples t test indicated that the numbers of cracks in all groups increased significantly after debonding ( Table II ).
| Paired differences | |||||
|---|---|---|---|---|---|
| Group | Mean | SD | t | df | P |
| A. Number of cracks before vs after debonding | –0.28 | 0.61 | –2.281 | 24 | 0.032 |
| Length of cracks before vs after debonding | –0.39 | 0.45 | –0.438 | 24 | 0.665 |
| B. Number of cracks before vs after debonding | –0.24 | 0.53 | –2.295 | 24 | 0.031 |
| Length of cracks before vs after debonding | –0.12 | 0.61 | –0.988 | 24 | 0.333 |
| C. Number of cracks before vs after debonding | –0.27 | 0.55 | –2.324 | 24 | 0.030 |
| Length of cracks before vs after debonding | 0.01 | 0.38 | –0.177 | 24 | 0.861 |
The ARI scores evaluated the mode of bond failure at the bracket-adhesive-enamel interfaces ( Table III ). The results of the chi-square comparisons indicated no significant differences among the groups. Most bond failures were at the bracket-adhesive interface in all groups.
