In this investigation, we determined the shear bond strength (SBS) of metallic and ceramic orthodontic brackets with new self-adhesive cements.
One hundred extracted premolars were used. They were sterilized and their roots embedded in stone bases, with the facial surfaces perpendicular to the bottom of the bases. The teeth were divided into 2 main groups, to receive metallic or ceramic brackets (Victory series 3M Unitek, Monrovia, Calif). In each group, the specimens were further divided into 5 subgroups according to the cement used: an etch-and-rinse control, Transbond-XT (3M Unitek); a resin cement with self-etch primer, Esthetic Cement system NC-100, (Kuraray, Okayama, Japan); and 3 self-adhesive resin cements: Rely-X Unicem (3M ESPE, Seefeld, Germany), Biscem DC (Bisco, Schaumburg, Ill), and Breeze (Pentron, Wallingford, Conn). Ten brackets were cemented in each subgroup. The specimens were stored in distilled water at 37°C for 7 days and subjected to 3000 thermocycles between 5°C and 55°C. The brackets were then debonded in shear with a testing machine.
Mean values for the metallic brackets cemented with Transbond XT, Esthetic Cement system, Rely-X Unicem, Biscem DC, and Breeze were 18.6, 6.0, 6.0, 2.2, and 8.4 MPa, respectively. For the ceramic brackets, the values were 22.7, 17, 7.7, 1.6, and 9.5 MPa, respectively. Analysis of variance (ANOVA) showed significant differences among the subgroups ( P <0.05) for both bracket types. For the ceramic brackets, the Tukey test showed no statistical difference in mean SBS between Transbond XT and Esthetic Cement system.
The SBS values of brackets cemented with etch-and-rinse cement were significantly higher than thosse of the 3 self-adhesive cements. However, when the self-etch adhesive, Esthetic Cement system, was used with ceramic brackets, no significant difference was found in the SBS compared with Transbond XT ( P = 0.052).
Enamel bonding for orthodontic applications was introduced in 1965 and is considered a significant milestone in orthodontic treatment. As reported by Owens and Miller, direct bonding of orthodontic brackets to enamel was made a reality by Buonocore, Bowen and Tavas. Adhesion occurs primarily through microscopic interlocking between the adherent and the brackets with the adhesive.
Traditional adhesive systems used for bonding orthodontic brackets require a clean enamel surface to be etched with phosphoric acid for 30 to 60 seconds, rinsed thoroughly with water, dried, and coated with resin adhesive before bracket bonding with resin cement. Although this is a relatively simple and effective technique, the need to etch, rinse, and dry the enamel while maintaining an uncontaminated environment can be challenging in some clinical situations, especially when patient compliance is an issue. Contamination of the etched enamel before bracket bonding will negatively affect bond strength of the bracket, and this might compromise the stability of the appliance. Another disadvantage of traditional adhesive systems is that some of the etched enamel surface might not have adhesive protection, leading to demineralized enamel white spots. This will undermine the clinician’s primary concern to maintain a sound, unblemished enamel surface after removal of the bracket.
Studies showed that cements with high bond strength to enamel can result in enamel fracture during debonding. Alternatively, considerable amounts of adhesive can remain on the tooth surfaces after debonding and require more chair time for removal. In addition, the process of removing the residual adhesive might result in enamel loss.
New self-adhesive cements have been introduced recently to orthodontics. These could simplify the bonding process by reducing the bonding steps and eliminating the need for etching and priming, thus lessening the risk of contamination. Although these materials are manufactured primarily for cementation of crowns, fixed partial dentures, and inlays or onlays, their potential use with orthodontic brackets has not yet been fully explored. Since they are directly applied to tooth surfaces, they might reduce the risk of unnecessary enamel etching with subsequent demineralization. Also during bracket debonding, the new self-etching cements can reduce the risk of enamel fracture if their bond strength to enamel is lower than that of conventional cement. Vicente et al found that Rely-X Unicem, a self-adhesive resin cement for bonding orthodontic brackets, produced a shear bond strength (SBS) that was significantly lower than that of a conventional etch-and-rinse cement after storing the specimens in water for 24 hours at 37°C. However, they speculated that the lower SBS of Rely-X Unicem would be clinically sufficient for the normal time that the bracket is to be attached. To closely simulate the clinical situation, Bishara et al determined the SBS of metallic brackets bonded with Rely-X Unicem 30 minutes after bonding. This is typically the time between bonding the brackets to the teeth and ligation of the archwires. They concluded that the SBS of this self-adhesive resin cement was insufficient for successfully bonding orthodontic brackets.
Other new self-adhesive resin cements are also available. The purpose of this study was to determine the SBS of 2 types of brackets, metallic and ceramic, with 3 self-adhesive resin cements. The null hypothesis was that there is no significant difference in the SBS values of the 2 types of cements. The effects of water storage and thermocycling on SBS were also investigated. A conventional etch-and-rinse cement and a cement with a self-etch primer were used as controls for comparison.
Material and methods
One hundred extracted premolars with intact buccal enamel surfaces were used. The teeth were cleaned and sterilized with gamma irradiation and stored in distilled water. A split mold was used to mount the roots in dental stone bases with the buccal surfaces aligned perpendicular to the bottom of the mold. The buccal surfaces were cleaned with coarse pumice and rubber prophylactic cups for 10 seconds and then rinsed and dried with an air-water syringe.
One hundred maxillary premolar brackets with 7° of torque, no angulations, and 0.022-in archwire slots were used; 50 were metallic, and the other 50 were ceramic (Clarity, Victory series, 3M Unitek, Monrovia, Calif). The surface areas of the bracket bases were determined with a digital caliper (10.6 mm 2 for the metallic brackets and 11.9 mm 2 for the ceramic brackets). The teeth were divided into 2 equal groups according to the type of brackets (metallic and ceramic). In each group, the specimens were divided into 5 subgroups according to the resin cement used (n = 10). These were an etch-and-rinse control, Transbond-XT LC adhesive system (3M Unitek) (TBXT), a resin cement with self-etch primer, Esthetic Cement system NC-100 (Kuraray, Okayama, Japan) (ECS), and 3 self-adhesive resin cements: Rely-X Unicem (3M ESPE, Seefeld, Germany) (RXU), Biscem DC (Bisco, Schaumburg, Ill) (BCM), and Breeze (Pentron, Wallingford, Conn) (BRZ). The brackets were then bonded to the mounted teeth according to the manufacturers’ instructions.
In the TBXT subgroups, the teeth of 1 subgroup of the metallic and 1 subgroup of the ceramic brackets were etched with 35% phosphoric acid gel for 15 seconds. The etchant was applied at the center of the middle third of the buccal surface. The teeth were then thoroughly rinsed with water and dried, and a layer of TBXT primer/sealant was applied to the etched area. TBXT adhesive paste was then applied to the bracket base and placed on the tooth. The bracket was pressed firmly for 10 seconds to ensure uniform adhesive thickness. Excess adhesive was removed with a microbrush, and the cement was cured with a halogen light (Spectrum 800 Curing Unit, Dentsply, York, Pa) for 20 seconds (10 seconds from each proximal side).
In the ECS subgroups, the brackets of 2 subgroups similar to the above were bonded with ECS. Equal amounts of ECS bond liquids A and B were mixed and applied to tooth surfaces and left for 20 seconds. The tooth surfaces were then dried and light-cured for 20 seconds. ECS was then applied to the bracket, placed on the tooth, and light cured as above.
In the RXU subgroups, the brackets of 2 subgroups similar to the above were bonded with RXU. An RXU capsule was activated in a Maxicap activator (3M ESPE) and mixed for 15 seconds in a high frequency mixing unit (Caulk, Dentsply). The capsule was placed in the Maxicap gun and applied to the bracket base. The bracket was bonded and light cured as above.
In the BCM subgroups, the brackets of 2 subgroups similar to the above were bonded with BCM. Cement was mixed and applied to the bracket base, which was placed on the tooth and cured as above.
In the BRZ subgroups, the brackets of 2 subgroups similar to the above were bonded with Breeze. Mixed cement was applied to the bracket and placed on the tooth and cured as above.
All bracket placements were carried out by the principal author (M.A.). The specimens were stored in distilled water at 37°C for 7 days. They were then subjected to 3000 thermocycles between 5°C and 55°C. After thermocycling, some brackets separated: 4 metallic brackets cemented with BCM and 2 with BRZ, and 6 ceramic brackets cemented with BCM and 1 with RXU. The separated brackets were recorded, and the remaining specimens were subjected to a shear test ( Tables I and II ).
|RXU||9||7.7 B, C||2.5||4.4-12.7|
The thermocycled brackets were debonded with a shear load applied in a universal testing machine (Instron, Canton, Mass) with a metal chisel at a crosshead speed of 1 mm per minute. The specimens were mounted on the machine so that the end of the chisel applied a compressive load directly to the incisal aspect of bracket-tooth interface parallel to the long axis of the bond interface. The maximum loads to debond the brackets were recorded.
All tests were conducted by the principal author. Each tooth and its corresponding bracket were viewed with a stereomicroscope (SMZ800, Nikon Instruments Inc, Melville, NY) and scored according to the adhesive remnant index (ARI). The values for the ARI are as follows: 0, no adhesive left on the tooth; 1, less than half of the adhesive left on the tooth; 2, more than half of the adhesive left on the tooth; and 3, all adhesive left on the tooth with an impression of the bracket mesh.
Data analyses were carried out with SPSS software (version 15.0, SPSS, Chicago, Ill). The data were statistically analyzed with 1-way analysis of variance (ANOVA) at P <0.05. Furthermore, the Tukey test was used to show subgroups with significant differences in SBS. Two-way ANOVA was used to determine whether there was an interaction between the 2 variables—bracket material and cement type.
The chi-square test was calculated for the cross-tabulation of cement type and ARI, which was initially dichotomized into 2 categories (1 = 0 + 1; 2 = 2 + 3). P = 0.05 was considered statistically significant.