Teeth

Sixty freshly extracted human molars were collected and stored in a solution of 0.1% (weight/volume) thymol. The criteria for tooth selection included intact buccal enamel, not subjected to any pretreatment chemical agents, eg, hydrogen peroxide, with no cracks due to the pressure of the extraction forceps, and no caries. The teeth were cleaned and then polished with nonfluoridated pumice and rubber prophylactic cups for 10 seconds.

The teeth were embedded in acrylic placed in phenolic rings (Buchler Ltd, Lake Bluff, Ill). A mounting jig was used to align the facial surface of the tooth to be perpendicular with the bottom of the mold, ie, each tooth was oriented so its labial surface would be parallel to the force during the shear strength test.

Adhesive

Two orthodontic adhesive systems were used:

• —GC Fuji Triage (GC America Inc, Alsip, Ill) which is a light-cured glass ionomer adhesive that chemically bonds to tooth structures. Triage is a new radiopaque glass ionomer and is suggested to have exceptional fluoride release properties and is used to seal noncavitated lesions as well as pits and fissures.19 The continuous fluoride release from glass ionomers over the course of orthodontic treatment is thought to help protect teeth from decalcification and prevent caries.1819 According to the manufacturer, Triage is moisture-friendly and also provides a strong lasting bond to the tooth structure.19

• —Transbond XT bonding system (3M Unitek, Monrovia, Calif) is a composite adhesive that contains Bis GMA, Bis EMA, and quartz/silica fillers.

Brackets

Maxillary central incisor brackets were used to bond all teeth (Victory Series, 3M Unitek). The average surface area of the bracket base was determined to be 11.7 mm².

Groups Tested

The teeth were randomly divided according to the enamel conditioner/etchant and adhesive used.

• Group I: In 20 teeth, the enamel was conditioned with a 10% polyacrylic acid for 20 seconds and washed for 20 seconds. Excess water was blotted away with a moist cotton roll. The teeth were bonded with the Triage glass ionomer following the manufacturer's instructions. The GC Triage capsule was triturated for 10 seconds. The bracket with the adhesive was placed on the tooth and light cured for 40 seconds, 10 seconds at a time from the mesial, distal, occlusal, and gingival.

• Group II: In 20 teeth, the enamel was conditioned with a 37% phosphoric acid gel for 15 seconds. The teeth were washed, dried, and then bonded with Triage as in Group I.

• Group III (Control): In 20 teeth, the enamel was etched with a 37% phosphoric acid gel for 15 seconds followed by thorough washing and drying. The sealant was placed on the tooth and the brackets were bonded with the Transbond XT adhesive and light cured for 20 seconds.

In all groups, before light curing the adhesive, the brackets were pressed on the tooth with 300 g of force using a force gauge (Correx Co, Bern, Switzerland), and excess adhesive was removed with a sharp scaler.

Debonding Procedure

A steel rod with one flattened end was attached to the crosshead of a Zwick test machine (Zwick GmbH & Co, Ulm, Germany). An occlusogingival load was applied to the bracket, producing a shear force at the bracket-tooth interface. A computer electronically connected with the Zwick test machine recorded the results of each test. Shear bond strengths were measured at a crosshead speed of 5 mm/minute. Bracket removal was performed within a half hour from the time the teeth were bonded to simulate the time at which the initial archwires are ligated.

Evaluation of the Residual Adhesive

After bond failure, the teeth and brackets were examined under 10× magnification. Any adhesive remaining after bracket removal was assessed using a modified adhesive remnant index (ARI) and scored with respect to the amount of resin material adhering to the enamel surface.20 The modified ARI scale has a range between 5 and 1, with 5 indicating that no adhesive remained on the enamel; 4, less than 10% of adhesive remained on the surface; 3, more than 10% but less than 90% of the adhesive remained; 2, more than 90% of the adhesive remained; and 1, all of the adhesive remained on the tooth, along with the impression of the bracket base. The ARI scores were also used as a more complex means of defining the site of bond failure between the enamel, the adhesive, and the bracket base.

Statistical Analyses

Descriptive statistics including the mean, standard deviation, and minimum and maximum values were calculated for each of the three groups tested. The analysis of variance was used to determine whether significant differences existed between the various groups. If a significant difference was present, a Duncan's multiple range test was used to identify which groups were different. The chi-square test was used to determine significant differences in the ARI scores between the different groups. For the purpose of the statistical analysis, ARI scores 1 and 2 were combined, as were ARI scores 4 and 5. Significance for all statistical tests was predetermined at P ≤ .05.

Shear Bond Strength

The descriptive statistics for the shear bond strength are presented in Table 1. The results of the analysis of variance comparing the three experimental groups (F = 10.294) indicated the presence of significant differences between the groups (P = .0001). The mean shear bond strength for the composite Transbond XT group was significantly larger 5.2 ± 2.9 MPa than for the two glass ionomer groups. The mean shear bond strength of the glass ionomer adhesive conditioned with 10% polyacrylic acid was 3.2 ± 1.8 MPa and that for the group etched with a 37% phosphoric acid was 2.3 ± 1.1 MPa.

Table 1.  Descriptive Statistics^a (in MPa) and the Results of the Analysis of Variance Comparing the Shear Bond Strengths of the Three Groups Tested

View Fullscreen

Adhesive Residual Index (ARI)

The ARI scores for the three groups tested are presented in Table 2. The χ² test results (χ² = 2.63) indicated that there were no significant differences between the three groups (P = .662) regarding the site of bond failure.

Table 2.  Frequency Distribution of the Adhesive Residual Index (ARI) Scores of the Three Groups Tested^a

View Fullscreen