Micro tensile bond Strength of resin composite to artificially demineralized dentin after remineralization by different remineralizing agents : In vitro study /

Abstract

The purpose of this In vitro study was to investigate the micro-tensile bond strength of resin composite restoration to proximal artificially demineralized dentin, and sound dentin compared to demineralized dentin after being remineralized with two remineralizing agents (NaF solution and eggshell nHAp) separately and combined using the universal bonding system. As well as evaluating the effect of the two modes of application (Etch-and-Rinse / Self-Etch) on the applied remineralizing agent and the micro-tensile bond strength of resin composite restoration. A total of 30 sound molars’ proximal surfaces were used in this study. The teeth’s occlusal enamel was removed till the DEJ to allow exposing the proximal DEJ, then the proximal surfaces were cut 3mm from the outer proximal enamel surface to allow working on the deep proximal dentin. The teeth were split into two halves parallel to the long axis (mesial half and distal half) to allow each tooth to be its own comparator regarding the bonding mode, all done using a water-cooled Sectioning IsoCut Wafering Blade mounted on IsoMet 4000 linear sawing (Buehler). Finally, the teeth were decoronated perpendicular to the long axis, the roots were discarded and the coronal samples were used in the study. Each deep dentin sample (mesial and distal halves) was fixed in pink self-cured acrylic resin cylinders; their proximal deep dentin surface facing upward and their pulpal surface in the acrylic, the samples were placed slightly above the acrylic resin surface by approximately 1 mm. Then each half was coded to make sure that each tooth was its own comparator regarding the bonding mode. The samples were stored in distilled water till the day of the study (2 days after samples preparation). The samples were randomly divided into five groups according to the remineralization protocols (G), positive control group (G1): the dentin was left sound without demineralization process or any surface treatment, negative control group (G2): the dentin surfaces were demineralized and left without any surface treatment, (G3) the dentin surfaces were demineralized then treated by 2% NaF solution as a remineralizing protocol, (G4) the dentin surfaces were demineralized then treated by eggshell nHAp as a remineralizing protocol, and (G5) the dentin surfaces were demineralized then treated by a combination of eggshell nHAp with 2% NaF solution as a remineralizing protocol. Each group was then further divided into two subgroups using the previous coding to make sure that each tooth was its own comparator regarding the mode of application of the bonding agent (B), where (B1) represents the Etch-and-Rinse mode, while (B2) represents the Self-Etch mode. Dentin artificial demineralization was done to all the groups except G1, using demineralizing gel {5 mL of 6% carboxymethylcellulose acid gel (0.1 M lactic acid titrated to pH 5.0 in a KOH solution) at pH 5.0 and 37 ◦C}. The deep proximal dentin surfaces were covered in gel for 48 hours without renewal. Where this model has been reported to supposedly provide a demineralized dentin similar to that of caries-affected dentin. After 48 hours the samples were rinsed with distilled water and air dried. Surface treatments were done to only G3, G4 and G5 as follow: G3: The surfaces were treated by a chemically prepared 2% NaF solution which was applied by a 1mm polyethylene plastic pipette and left on the surfaces for 1 min then blot dried. G4: The surfaces were treated by chemically prepared eggshell nHAp dissolved in distilled water (1.8 g of nHAp was mixed with 0.3 mL of distilled water), applied by a spatula, and left on the surfaces for 1 min then rinsed with distilled water and blot dried. G5: The surfaces were treated by a combination of eggshell nHAp dissolved in a 2% NaF solution (1.8 g of nHAp was mixed with 0.3 mL of 2% NaF solution), applied by a spatula, and left on the surfaces for 1 min then rinsed with distilled water and blot dried. In Etch-and-Rinse groups the pretreated dentin surfaces were etched with Meta Etchant 37% phosphoric acid semi-gel for 15 seconds, rinsed for 10 seconds, and blot dried. Then for both groups (Etch-and-Rinse and Self-Etch) 3M™ Single Bond Universal Adhesive was applied following the manufacturer instructions. Then 3M™ Filtek™ Z350 XT Universal Restorative (A3 Body Shade nanocomposite) was used to build a 3-4 mm composite block on the deep dentin surfaces and it was measured using a periodontal probe. The samples were then kept in artificial saliva for 24 hours before sectioning. The samples were transferred to the IsoMet 4000 Linear Precision Saw (BUEHLER, Ltd., Lake Bluff, IL, USA) to allow sectioning of each surface longitudinally in horizontal cuts, then vertical cuts using a Sectioning IsoCut Wafering Blade to create rods, of similar cross sectional area (1mm x 1mm). The rods with dentin thickness 2-3 mm were chosen for each group, specimens that showed pretest failures were not included (n = 9). A total of 72 rods for 8 subgroups were collected and stored in distilled water. Each beam was then mounted to the Gerald Eli’s jig to allow micro tensile testing. The results of the current study revealed that the bond strength of resin composite restoration to negative control group (G2) proximal artificially demineralized dentin without any surface treatment showed the worst results with pretest failure, and that all surface treatments done regardless of the type of the remineralizing agent (G3, G4, G5) showed an improvement in the bond strength to be similar to positive control (G1) sound dentin. The NaF solution group (G3) and the eggshell nHAP group (G4) when used separately showed the highest results and weren’t affected by the different bonding modes. While the combined group nHAP and NaF (G5) showed less results, yet it showed improvement in comparison to the untreated dentin surfaces. As well as it was affected by the etching step in the Etch-and-Rinse mode, therefore it showed better results when used with universal adhesive in Self-Etch mode.