2.1

Sample preparation

The root portions from approximately 30 bovine incisors were removed and cut into two halves with a diamond saw under water cooling. The cut was made perpendicularly to the long root axis, with each half having an approximate length of 6–7 mm. From these, the halves that had a root canal larger than the intended diameter were rejected. Next, the canal of each selected root segment was enlarged with a 1.9-mm diameter fiber post drill using water as coolant (3M ESPE, Dental products, St. Paul, MN, USA) to obtain a circular hole of ∼2-mm diameter. Afterwards, the root segments were trimmed down using a lathe to remove the cementum and the external layer of dentin to produce hollow dentin cylinders of 5 ± 0.05-mm outer diameter and 2.05 ± 0.05-mm inner diameter ( Fig. 1 ).

Schematic representation of the technical steps to obtain the modified Brazilian disk.

The machined dentin cylinders were randomly assigned to three bonding systems: Total-etch Adper™ Single Bond Plus (SB), Self-etch Adper™ Easy Bond Adhesive (EB) and total-etch Adper™ Scotchbond™ Multi-Purpose Adhesive (MP); see Table 1 . Each bonding system was applied to the inner surface of the hollow dentin cylinders with a disposable brush applicator provided by the manufacturer and subsequently cured according to the manufacturer’s instructions. After this, the canal of each cylinder was restored with either Filtek™ Z250 Universal Restorative or Z100™ Restorative; see also Table 1 . The composites were applied incrementally to minimize shrinkage stress, with each layer being less than 2-mm thick. The first increment was placed in the middle section of the cylinder with the aid of a composite condenser and the subsequent increments placed at its ends. The first increment was cured from both ends for 20 s each. The end increments were cured from the respective ends of the cylinder for 40s. Light curing was done with an Elipar Trilight (3M ESPE, Dental Products, St. Paul, MN, USA) curing light operated at 800 mW/cm ² .

The filled dentin cylinders were cut using an Isomet™ (Buehler, Lake Bluff, IL, USA) diamond saw into 2-mm thick dentin-resin composite disks under cooling water; see Fig. 1 and inset in Fig. 2 . Two or three disk specimens could be obtained from each root segment. The disks were examined for defects under a microscope (Olympus MVX10, Olympus America Inc.) with a magnification of 3.2×. Defective samples with pores on the surface, air bubbles that could be clearly seen in the composite, irregularities or cracks were removed from the study. A total of 15–17 disks were used for each group and they were stored in 0.1% thymol solution at 4 °C overnight before testing.

Experimental setup for diametral compression, digital image correlation and acoustic emission measurement. Inset: Modified Brazilian disk for measuring interfacial bond strength between root dentin and direct composite restoration.

2.2

Diametral compression

The diametral compression test was carried out using a Universal Test Machine (858 Mini Bionix, MTS, USA), with the specimen located between two flat and parallel steel components ( Fig. 2 ). A loading rate of 0.5 mm/min was applied to fracture the samples. The load and displacement time histories were recorded during the loading process.

2.3

Acoustic emission (AE) measurement

An AE system (Physical Acoustics Corporation, NJ, USA) was used to monitor microcracking of the specimens during loading. The AE sensor was attached to the lower support plate ( Fig. 2 ). Signals detected by the sensor were passed through a preamplifier of 10-dB gain with a band pass of 100 kHz–2 MHz and a threshold set at 35 dB. The AE results were used together with the load time histories and DIC data (see below) to identify the point at which interfacial debonding first occurred.

2.4

Digital image correlation (DIC)

Surface deformation of the disks was measured by using DIC to identify interfacial debonding. This technique utilized a non-contact optical method for tracking movements of surface features. The system consisted of a high-speed CCD camera (Point Grey Grasshopper GRAS-20S4C-C) and propriety software (DaVis 7.2, LaVision-GmbH, Goettingen, Germany) for displacement and strain calculation ( Fig. 2 b). To allow deformations to be determined, the disk surface facing the CCD camera was first sprayed with a white fixation paint (Krylon products group, Cleveland, OH, USA) followed by a thin layer of charcoal particles (Sigma Aldrich, St. Louis, MO, USA). These created irregular speckles on the surface for local displacement tracking. A reference image was taken before the specimen was loaded. During loading, further images were taken at 10–20 frames per second (fps) for comparison with the reference image. The proprietary software was then used to calculate the full-field strain maps, and interfacial debonding could be identified from the strain concentration developed.

2.5

Finite element (FE) simulation

There is no simple analytical solution for the stress distribution within the dentin-composite disk. The FE method was therefore used to calculate the interfacial bond strength based on the load that caused debonding. Due to symmetry, a 2-D model representing a quadrant of the disc specimen ( Fig. 3 a) was constructed using Hypermesh 11.0 (HyperWorks, Altair Engineering, Troy, MI, USA). Appropriate boundary conditions were assigned to the vertical and horizontal planes of symmetry of the quadrant model ( Fig. 3 a). A point load was applied downward at the topmost node of the model to simulate diametral compression. To capture the stress distribution at the dentin-composite interface accurately, the regions around the interface were meshed more finely than other regions. This numerical model was then exported to ABAQUS (version 6.10-EF1; Dassault Systèmes Simulia, Waltham, MA, USA) to solve for the stresses. The model was meshed with the plane-stress elements CPS4I and CPS3 . All materials were assumed to be isotropic, homogeneous and behave linear elastically. The material properties for each component are listed in Table 2 . Fig. 3 b shows the radial stress distribution within the disk and the node at the interface (indicated by the arrow) where debonding was observed to initiate.

a) 2-D quadrant FE model for the dentin-composite disk specimen. Load was applied at the topmost node, indicated by the arrow. b) Radial stress distribution within the disk specimen. The arrow indicates the node where stress was sampled for bond strength calculation.

2.6

Scanning electron microscopy (SEM)

To evaluate the mode of interfacial debonding, SEM images of the fracture surfaces of the specimens were obtained using an environmental scanning electron microscope (TM-3000, Hitachi, High-Technologies Corporation, Tokyo, Japan) with the following settings: X50 magnification, compositional mode and 15 kV.

2.7

Statistical analysis

A two-way analysis of variance (ANOVA) was used to explore the effect of the composite and adhesive systems on the bond strength and the interaction between them. To determine whether a significant difference in bond strength existed among the different combinations of composites and adhesives, a one-way ANOVA was carried out using Tukey’s HSD test as the post hoc test. Fisher’s exact test was used to analyze the failure modes and their association with the composite and adhesive systems used. SAS ^® 9.3 Software (SAS Institute Inc., Cary, NC, USA) was used to perform the statistical analysis.

2.1

Sample preparation

The root portions from approximately 30 bovine incisors were removed and cut into two halves with a diamond saw under water cooling. The cut was made perpendicularly to the long root axis, with each half having an approximate length of 6–7 mm. From these, the halves that had a root canal larger than the intended diameter were rejected. Next, the canal of each selected root segment was enlarged with a 1.9-mm diameter fiber post drill using water as coolant (3M ESPE, Dental products, St. Paul, MN, USA) to obtain a circular hole of ∼2-mm diameter. Afterwards, the root segments were trimmed down using a lathe to remove the cementum and the external layer of dentin to produce hollow dentin cylinders of 5 ± 0.05-mm outer diameter and 2.05 ± 0.05-mm inner diameter ( Fig. 1 ).

Schematic representation of the technical steps to obtain the modified Brazilian disk.

The machined dentin cylinders were randomly assigned to three bonding systems: Total-etch Adper™ Single Bond Plus (SB), Self-etch Adper™ Easy Bond Adhesive (EB) and total-etch Adper™ Scotchbond™ Multi-Purpose Adhesive (MP); see Table 1 . Each bonding system was applied to the inner surface of the hollow dentin cylinders with a disposable brush applicator provided by the manufacturer and subsequently cured according to the manufacturer’s instructions. After this, the canal of each cylinder was restored with either Filtek™ Z250 Universal Restorative or Z100™ Restorative; see also Table 1 . The composites were applied incrementally to minimize shrinkage stress, with each layer being less than 2-mm thick. The first increment was placed in the middle section of the cylinder with the aid of a composite condenser and the subsequent increments placed at its ends. The first increment was cured from both ends for 20 s each. The end increments were cured from the respective ends of the cylinder for 40s. Light curing was done with an Elipar Trilight (3M ESPE, Dental Products, St. Paul, MN, USA) curing light operated at 800 mW/cm ² .

Table 1

Compositions of composites and bonding systems used in this study.

Product	Composition	Batch number
Z100™ Restorative	Silane treated ceramic, triethylene glycol dimethacrylate (TEGDMA), bisphenol a diglycidyl ether dimethacrylate (BISGMA), 2-benzotriazolyl-4-methylphenol a .	N362970
Filtek™ Z250 Universal Restorative	Silane treated ceramic, bisphenol a polyethylene glycol diether dimethacrylate (BISEMA6), diurethane dimethacrylate (UDMA), bisphenol a diglycidyl ether dimethacrylate (BISGMA), triethylene glycol dimethacrylate (TEGMA), benzotriazol, ethyl 4-dimethyl aminobenzoate (EDMAB) a .	N326080
Adper Single Bond Plus	Ethyl alcohol, silane treated silica (nanofiller), bisphenol a diglycidyl ether dimethacrylate (BISGMA), 2-hydroxyethyl methacrylate(HEMA), glycerol 1,3-dimethacrylate, copolymer of acrylic and itaconic acids, water, diurethane dimethacrylate (UDMA) diphenyliodonium hexafluorophosphate, ethyl 4-dimethyl aminobenzoate (EDMAB) a .	N509492
Adper™ Easy Bond Self-Etch Adhesive	Bisphenol a diglycidyl ether dimethacrylate (BISGMA), 2-hydroxyethyl methacrylate, ethanol, water, phosphoric acid−6-methacryloxy-hexylesters, silane treated silica, 1,6-hexanediol dimethacrylate, copolymer of acrylic and itaconic acid, (dimethylamino)ethyl methacrylate camphorquinone, 2,4,6-trimethylbenzoyldiphenylphosphine oxide a .	N497589
Adper™ Scotchbond™ Multi-Purpose Adhesive	Primer: Water, 2-hydroxyethyl methacrylate (HEMA), copolymer of acrylic and itaconic acids Bond: Bisphenol a diglycidyl ether dimethacrylate (BISGMA), 2-hydroxyethyl methacrylate (hema), triphenylantimony a .	N491015 N494505

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