Fixed orthodontic appliances are used in modern orthodontic treatment in about 70-80% of cases. Fixed orthodontics is unthinkable without adhesive composite resin materials and glass ionomer cements (GIC). In the early 1950s, a pioneering introduction of adhesive materials in dental medicine took place. This was followed by further development of dental adhesives, enabling their use also in fixed orthodontics (1). Unfilled Bowen’s ethers, or ethers with small filler content, are mainly used in orthodontics as adhesives. Primers are most often the chemical agents applied in a monomolecular layer on the surface of the material to be bonded, changing its properties in order to provide better adhesion. The bond between enamel and adhesive is achieved by mechanical bonding based on geometric surface roughness and creation of micro pores, as well as by rheological effect that occurs when adhesive changes from fluid to solid (2).
Orthodontic composite resin adhesives
Two-component composite resins are mainly used as adhesives in orthodontics. A contact of two components causes polymerization reaction. Nowadays, the adhesive MIP (Moisture Insensitive Primer) materials are being developed. They contain hydrophilic primer that dissolves in acetone, and the recommendation is to use it on a slightly moist conditioned enamel (3).
Glass ionomer cements (GIC)
First GIC were described by Smith and Wilson (4). They are prepared by mixing of powder and water solution of acid (Ca-fluoroaluminosilicate powder and polyacrylic acid). The basic chemical reaction during cement polymerization is neutralization between fluid (acid) and powder (base), which results in creation of salt (polymerized GIC) and water (1, 5).
In an attempt to improve chemical and mechanical properties of cements, various additives were added to them, such as amalgam or silver. The most important additives to GIC from the aspect of their evolution are resins. Resins were added in an attempt to improve mechanical and aesthetic qualities, along with increasing adhesion and preserving fluoride release capacity. These materials were able to solve most of the problems in brackets bonding during fixed orthodontics treatment (6).
The most common clinical problems are still bracket debonding, emergence of early carious lesions (white spots) and low resistance of adhesive materials to moisture prior to the polymerization (7-11). Orthodontic adhesive should be appropriate for allowing bracket to stay bonded to the enamel surface during the orthodontic therapy and also, to allow simple bracket removal when it is needed, but with no signs of damaging to enamel and without discomfort for patients (12). Research-based findings have constantly led to the development of new materials and usage of new techniques that are aimed at simplifying the clinical procedures (13). Different studies have already been conducted concerning the usage of almost all these materials but always with different procedures, therefore the evaluation and the comparison are limited (14). The purpose of the present study was to examine the shear bond strength (SBS) of orthodontic brackets to enamel surface with respect to the type of adhesive material used and enamel pretreatment. After brackets debonding, we also wanted to analyze the amount of remaining adhesive material on tooth surface. The null hypothesis was that the type of adhesive material has impact on SBS of brackets to enamel surface, while methods of enamel pretreatment have no impact on SBS of brackets to enamel surface.
Materials and methods
Experimental groups consisted of 80 human first premolars of both jaws that had been extracted due to orthodontic reasons. The buccal enamel surface of all teeth was intact, without caries lesions and macroscopic visible fracture lines after extraction. Following extraction, the residue on the teeth was removed and washed away with tap water. The buccal surface was cleaned with rotating synthetic brush on slow hand piece and pumice. The teeth were disinfected in 0.1% (weight/volume) thymol solution for 24 hours. All samples were transferred to distilled water for a maximum of 4 months before testing, while the distilled water was changed every week. A written consent was obtained from the teeth donors. Two orthodontic adhesives were used:
Light cure resin-reinforced GIC, Fuji Ortho LC (GC Corporation, Tokyo, Japan)
Light cure adhesive composite resin paste, Transbond XT (3M Unitek, Monrovia, California. USA).
With regard to the enamel pretreatment and type of orthodontic adhesive, the teeth were divided into four equal examination groups (20 teeth each):
ENAMEL PRETREATMENT ORTHODONTIC ADHESIVE
Group A: 10% polyacrylic acid, 20 sec.; Fuji Ortho LC
Group B: 37% phosphoric acid, 15 sec.; Fuji Ortho LC
Group C: self-etching primer- Transbond Plus SEP, 3 sec.; Transbond XT
Group D: 37% phosphoric acid, 15sec.; primer-Transbond MIP; Transbond XT
Metallic brackets Discovery for premolars were used (Dentaurum, Germany). All enamel surfaces were prepared according to the above mentioned protocols and manufacturer’s instructions. LED polymerization lamp Bluephase (Ivoclar Vivadent, Schaan, Liechtenstein) was used for light curing. After brackets bonding and adhesive material polymerization, all teeth were stored for 24 hours in saline at 370C. Subsequently they were inserted into plaster molds and examination started as follows: the debonding force values for every specimen were recorded in a digital shredding machine (Zwick nr: 112627, Ulm, Germany). To calculate SBS, the debonding force values (N) were converted to SBS (MPa) by taking into account the surface area of the bracket base, which was 10.3 mm2 (obtained from the manufacturer- Dentaurum, Germany).
The second test was performed by light microscopy (Richter Optica U2B Binocular Lab Microscope, China). The Adhesive Remnant Index (ARI) was determined (15). The ARI was ranked from 0 to 3 as follows:
0 = no adhesive on the enamel;
1 = less than 50% adhesive on the enamel;
2 = more than 50% adhesive on the enamel;
3 = 100% adhesive on the enamel.
Statistical analysis was carried out with SPSS 17.0 for Windows statistical software package. Among descriptive statistical parameters, the arithmetic means, standard deviations and median and interquartile range were calculated. For testing the difference between parametric variables, one-way analysis of variance (ANOVA) was used. Afterwards, the ANOVA Scheffe’s test was used for multiple comparisons between individual groups. Chi- square test was used to analyze the ARI data. The data distribution was tested by Kolmogorov-Smirnov test. Statistical difference in all tests of p < 0.05 was considered statistically significant.
The results of ANOVA test are presented in Table 1 and a statistically significant difference between the groups was found (p=0.007).
After the difference between the groups was established based on ANOVA, Scheffe’s post-hoc test was used for multiple comparisons between individual groups. According to Scheffe’s test, a significantly lower SBS of the group B was found in relation to the group C (p=0.031). Also, a significantly lower SBS of the group B was found in relation to the group D (p=0.026). There were no statistically significant differences between group A and group B (p= 0.091), between group A and group C (p=0.975) and between group A and group D (p=0.961). Also, there were no statistically significant differences between group C and group D (p=1). Chi- square test was used to analyze the ARI data. The difference of ARI scores is presented in Table 2. Results of ARI were almost similar in all testing groups and it was not possible to determine any statistically significant difference of the ARI between all four experimental groups.
SBS of bracket - adhesive - enamel system in orthodontic bonding varies and depends on factors such as the adhesive types, design of the bracket base, morphology of the enamel, appliance force systems and the clinician's technique (16). The orthodontic profession is constantly searching improvements and optimization of the technique of bonding brackets to enamel (17). In this study, SBS of metallic orthodontic brackets was analyzed regarding to applied enamel pretreatment and the type of used adhesive material (resin-reinforced GIC or composite resin). The aim was also to determine the influence of various adhesive materials and enamel pretreatment on the ARI. Therefore, it can be said that the aims of the present study were directed toward clarification of the facts to what extent enamel pretreatment methods influenced SBS and to what extent SBS depended on the applied adhesive material. After statistical analysis of the collected data, statistically significant lower SBS of the group B was found in relation to the groups C (p=0.031) and D (p=0.026).
The results of previous studies were uneven. The results of Shinya et al. revealed that SBS was neither significantly influenced by etching pattern on the enamel surface nor by the adhesive system (18). Our results showed that SBS was significantly influenced by the adhesive system. Scougall Vilchis et al. showed that when enamel was conditioned with SEP, SBS was statistically lower than when it was etched with 37% phosphoric acid (19). Our results were not consistent with their conclusion. Our results were in agreement with the report by Cal-Neto et al. who did not find any significant difference SBS between usage of Transbond MIP and Transbond SEP (20). Scribante at al. compared SBS with Trans-bond XT, Fuji Ortho LC and Tetric Flow adhesive systems and reported that the highest SBS was found with Trans-bond XT adhesive system (21). Our results were consistent with that report. Yassaei et al. compared SBS with Trans-bond XT and Fuji Ortho LC for bonding metal and ceramic brackets and reported that SBS when using Trans-bond XT was statistically better than with Fuji Ortho LC (22). We used metal brackets only and our results were identical. The null hypothesis was supported and confirmed by our results. Our study showed that the great amount of ARI scores in all examined groups were 0 or 1, referring to the fact that adhesives had a better chance to stay on the bracket as opposed to the enamel after debonding procedures. From the clinical perspective, that would be desirable as it would take less time for enamel clean-up and less discomfort for patients. Less adhesive remaining on the tooth surface would result in a reduction of the damage of enamel during the debonding procedures (23). No significant difference between groups according to the ARI was found in our study and the established variation of the ARI within testing groups could be declared as accidental, which is in accordance with previous study of Rix et al. and Movahhed et al. (24, 25). ARI scores are used to define the site of bond failure between the enamel, adhesive, and bracket base. In our study, bond failure site was unaffected by the type of adhesive material and methods of enamel pretreatment.
Composite resin showed a higher shear bonding strength than GIC. From our results it can be concluded that the use of composite resin material with appropriate enamel pretreatment according to manufactures recommendation is the “gold standard” for brackets bonding for fixed orthodontic appliances.