A Comparison of Shear Bond Strength of New Nanofilled Composite and Nano-Ionomer Restorative Materials with Traditional Adhesive Material for Orthodontic Bracket Bonding: An In Vitro Study

Background: New materials that are introduced in operative dentistry can potentially have orthodontic applications. One of the most important advances in the dental material field is the application of nanotechnology to resin composites. Recently, new fillers with size ranging from approximately 5-100 nm have been developed. These materials could thus be considered as precursors of nanocomposites. Due to the reduced dimension of the particles and to a wide size distribution, an increased filler load can be achieved that reduces polymerization shrinkage and increases mechanical properties such as tensile and compressive strength and resistance to fracture. As new adhesives, composite resins, and bonding techniques are introduced, orthodontists adopt some of these innovations and add them to their armamentarium. The purpose of this study was to compare the shear bond strength (SBS) of the brackets bonded with the newly introduced nanocomposites with that of conventional orthodontic adhesive and to compare amount of remnant adhesive on tooth surface after debonding. Materials and Methods: This in vitro study was carried out on 100 premolar teeth using conventional orthodontic bracket bonding adhesive and the newly introduced nanocomposites for bonding the 0.022 MBT Brackets (Gemini series, 3M Unitek). Teeth were randomly divided into five groups. The bonding was done according to the manufacturer’s instructions followed by the curing using halogen curing unit (3M ESPE, ELIPAR, 2500). Instron Universal Testing Machine was used in this study to record the SBSs of the brackets in MPa. Following the debonding procedure, the Adhesive Remnant Index (ARI) was used to determine failure site locations. Results: The analysis of variance test to compare SBS values of the five groups indicated statistically significant difference (P < 0.0001) among the groups except for Tetric N Ceram (Group III). The bond strength (MPa) is highest in Group I, followed by Group III > Group IV > Group II > Group V. Chi-square test showed no significant association between the ARI scores and the different groups. Weibull analysis showed maximum failure probability of 85% with Group V (KetacTM N100 Group) and minimum failure probability of 25% with Group I (Transbond XT Group). Conclusions: SBSs of restorative nanocomposites groups recorded are significantly lower when compared with conventional bonding adhesive group. Although, the bond strength values of the brackets bonded with nanocomposites is within the range of 5.9-7.8 MPa and beyond, which is clinically acceptable for effective orthodontic bonding, the probability of failure is higher for Groups II, IV, and V, which may be due to high viscosity and inability of these materials to penetrate the mesh of the bracket base. In all groups most of the material remained adhered to the tooth surface after the debonding of the brackets which is desirable. The nanocomposites and nanoionomers can be used for bonding orthodontic brackets. However, reformulation of the composition of nanocomposites to produce better flow is desired. Key Words: Nanofilled, nanohybrid, nano-ionomer, shear bond strength Introduction Since the introduction of the acid etch bonding technique by Buonocore1 in 1955, the concept of bonding various resins to enamel is being used widely in dentistry, including the bonding of orthodontic brackets, either direct or indirect. The adhesive systems have undergone considerable evolution after 1966, when the earliest orthodontic brackets were bonded directly onto the enamel. One of the most dramatic changes in the orthodontic specialty in the 1970s was the use of composite resin as a bonding material.2 The chemically cured resins were the first systems developed for bracket bonding.3,4 Ultraviolet light-cured materials were introduced as an alternative to self-curing resins. Due to safety problems5 and limited depth of cure,6,7 ultraviolet light curing has been replaced by visible light curing and this has become the most popular method of polymerizing orthodontic adhesives. When bonding an orthodontic bracket, the bond strength should be sufficient to withstand the forces of mastication and stresses exerted by arch wires. However, there are many factors