2.1

Adhesive and restorative materials

The adhesives and restorative systems used in this study are listed in Table 1.1 . Roles of the components and rendered tests for the 2 hypotheses are listed in Table 1.2 .

For hypothesis (1), two combinations of self-etching restorative systems were compared. Combination #1 (BF), the modified 4-META/MMA-TBB (BF) consisted of a self-etching primer and MMA based restorative with adhesive characteristics. Restorative material is self-curing and comes with a PMMA powder and liquid. The recommended method for placing is the dip-brush method as for CBM, which is not very commonplace for most western clinicians. Combination #2 (EBZ), consisted of Adper EasyBond (3 M ESPE, St. Paul, MN) (EB) self-etching single bottle adhesive used with Z100 (Z) (3 M ESPE) restorative.

For hypothesis (2), EB was used for Combination #2 (EBZ) and Combination #3 (EBSF) in which, Z was replaced with bulk-fill material, SureFil SDR flow (SF) (Dentsply Caulk Milford, DE). BF is not radio-opaque, which would be clinically necessary for the bulk-fill use. Thus in Combination #4 (BFO): BF’s powder (BF-P) was replaced with experimental radio-opaque powder (BF-PO) provided by the manufacturer. In preliminary study in our lab, we confirmed BF and BFO had similar characteristics in shear bond strength and elastic modulus as determined by nanoindentation (results not shown).

2.2

Shear bond strength test

Extracted teeth ( n = 72) were sectioned with a slow speed water cooled saw with a diamond blade (Buehler, Lake Bluff, IL) to yield proximal surfaces, then finished to #320 grit roughness with silicon carbide paper (Buehler) to create a smear layer with similar roughness to the carbide bur . In part one, two operators of different levels of bonding experience prepared specimens to evaluate the difficulty of learning the dip-brush technique: veteran (V) who was a clinical dentist, with experience in the lab for more than 10 years, and novice (N), student volunteer who had no previous bonding experience. Combinations BF and EBZ were used to make bonded specimens after studying manufacturers’ instructions (written and video). The novice received hands-on demonstration by the veteran once for each combination. Demetron Optilux VC-401 (Kerr, Orange, CA) with a light output intensity 400–500 mW/cm ² , was used to light cure the materials. EB was cured for 40 s (double of recommended cure time). Z was placed in small increments, cured 80 s at each increment (double of recommended cure time). Each specimen was mounted for single plane shear test ( n = 36/combination) as described previously . Briefly, mylar tape with a 3 mm diameter hole was applied to the dentin surface to obtain standardized bonded areas. Bonded specimens were stored 24 h in 100% humidity before bond strength testing. The test specimens were loaded parallel to the bonded interface, using a universal testing machine (Instron Model 1122, Instron Corp., Canton, MA, USA), with cross head speed 5 mm/min. Failure patterns (interfacial or cohesive within dentin or composite) were examined with a stereomicroscope at 30× magnification.

2.3

Bonded interface characteristics

2.3.1

Preparation of the specimens

AFM : An occlusal dentin disk was prepared from each of 6 extracted molars by cutting with a slow speed water cooled saw with a diamond blade. The distance from the dentin enamel junction (DEJ) and orientation of the dentin tubules of the dentin disk used for this part of the studies were equivalent to the shear bond test specimen. The surface was finished to #320 grit roughness with silicon carbide paper then bonded by V, using combination BF and EBZ as shear bond test specimens except thickness of the Z and BF were about 1 mm. After storing in HBSS for 24 h in 100% humidity, bonded specimens were cross sectioned and polished to 0.25 μm with diamond paste suspension (Buehler) for AFM (Contact mode) imaging and AFM-based nanomechanical testing to determine reduced elastic modulus ( E ).

SEM : Bonded specimens for scanning electron microscopy (SEM) were prepared in the same way as for AFM ( n = 6 for each combination). One half of each bonded specimen was cross sectioned and polished, then the hybrid layer was revealed with 0.1 M HCl and 10% NaOCl . The other half was fractured. All specimens were fixed as described elsewhere , then the bonded specimens were sputter-coated with a 10–20 nm thick Au thin film using a sputter coater (Denton Vacuum Inc., Model # Desk II, Moorestown, NJ).

2.3.2

AFM (Contact mode—wet) and SEM images of bonded interfaces

Conventional atomic force microscopy (AFM) (Nanoscope III, Bruker, Santa Barbara, CA) was used to obtain topographic images of the cross sectioned bonded specimens in wet condition, using contact mode with silicon nitride tip (NP-10, Bruker, Camarillo, CA), which yields sharper, better quality images than the nano-indentation tip.

SEM images were obtained using a Hitachi S-4300 field emission gun scanning electron microscope (Hitachi High Technologies America, Pleasanton, CA) at an accelerating voltage of 20 kV. SEM images at 60–1000× were taken to evaluate bonded interfaces of cross-sectioned segments.

2.3.3

Nano mechanical properties of bonded interfaces

Nanoindentation was used in this study to assess the quality of the bonded interface. Nanoindentations on bonded and cross sectioned specimens were made in a liquid cell containing distilled and filtered water (“wet” condition) with the standard head replaced by a Triboscope indenter system (Hysitron Inc., Minneapolis, MN), nPoint closed loop scanner (4715120-MM, nPoint, Madison, WI) and AFM, as described elsewhere . A cube corner diamond indenter with a tip radius of about 20 nm was used for indentations and imaging. Fused silica was used for calibration of the machine compliance, the elastic modulus, and to define the tip area function for indentation depths over a range of 50–600 nm with the cube corner tip. Indentation loads of 400 μN on sound dentin resulted in indentation depths between 180 and 260 nm.

A linear series of indentations, spaced at approximately 1–2 μm intervals across the bonded interfaces at 2 randomly selected areas per specimen were evaluated ( n = 3 specimens/combination, 2 lines/specimen). Preliminary studies revealed the width of the region of interest was about 5 μm under the interface, requiring a narrower distance between indentations to capture the differences. Preliminary study using indentation intervals of 1, 2 and 5 μm with 150 μN load on demineralized dentin showed no significant difference in indentation depth at the maximum load (hmax) between any of the indentation intervals used, thus no edge effects were evident with 1 μm intervals (data not shown). Target indentation depth was 200 nm on the affected dentin. Means of E for Z and BF were computed from the 2 μm separated indentations. AFM imaging ensured positioning of the indenter on the intertubular dentin. Indents on tubules and any peritubular dentin were excluded from further analysis. The indentation load–displacement data were analyzed to determine the reduced elastic modulus according to the method of Oliver and Pharr .

The reduced elastic modulus E is calculated from the contact stiffness, S , defined as the slope of the linear portion of the force/displacement curve during unloading near the maximum load and “ a ” is the contact area of the indentation.

2.4

Contraction gap evaluation for bulk-fill specimens

Extracted and sterilized human third molars ( n = 12/combination) were used to evaluate combinations #2, 3 and 4 ( Table 1 ). 3 × 3 × 3 mm class I preparations were made with a high speed hand piece with a water cooled cylindrical diamond bur. The cusp tips were ground to obtain flat occlusal surfaces perpendicular to the long axis of the teeth using a belt grinder (Buehler), then polished with #320 grit silicon carbide paper before the class I cavity preparation. After preparing class I preparations with primer or adhesive, restorative materials were bulk-filled and cured. Six of the bonded specimens of each combination were immediately cross sectioned with a slow speed saw and polished as described previously for AFM and light microscopy evaluation. The other six were stored for 24 h in DI water before sectioning. The average of 3 gap width measurements at randomly selected positions along 50 μm interface width at the center of the pulpal floor and 0.5 mm from the pulpal-axial line-angle on the axial wall were recorded as the gap measurement. Low magnification light microscopy was used in addition to AFM to confirm the selected 50 μm width of region of interest was representative of the overall trend of the specimen. The location of separation along 50 μm was also classified and recorded as (1): interfacial, if the separation occurred at more than 50% of the observed interface between EB/Z, EB/dentin, EB/SF or BFO/dentin. Due to the composition design of BFO ( Table 1.2 ), there is no interfacial failure of BFO on the composite resin side. (2): cohesive, if it was in EB, Z, SF, BFO or dentin, and (3): “mixed” if the fracture was both interfacial and cohesive; none of these failure modes occupied more than 50% length of the observed fracture surface. Contact mode AFM in dry condition or light microscopy was used if the gap was very wide.

2.5

Statistics

Shear bond strength : Pre-test failures were imputed as SBS = 1 in order to include these outcomes in all analyses. Analysis of variance was used to estimate mean and standard deviation (S.D.) per group. Weibull failure analysis was used to estimate maximum likelihood means (95% confidence intervals (C.I.)) of modulus and characteristic strength, by restorative material and clinical experience, and to test for differences between materials and operators.

Elastic modulus across the interface : Line profiles were obtained for ±30 μm from the adhesive/dentin interface. To focus on the most relevant aspect of affected dentin depth under the adhesive, the plots were constrained to distances of 0–14 μm. Values below 0.1 (tubules) and above 24 GPa (peritubular dentin) were excluded from the analysis, assuming these did not represent intertubular dentin. Mixed-effects linear regression was used to estimate mean (95% C.I.) elastic modulus as a function of categorical distance from the adhesive/dentin interface (location identified visually using AFM images) into the restorative material (negative distances) and into the dentin, by material (BF or EBZ). The model accounted for correlations among measurements within indentation lines and specimens.

Contraction gap width comparisons : We used a stratified linear regression model to estimate mean (95% C.I.) gap width (μm) as a function of material (EBZ = control; BFO = test 1; EBSF = test 2), time (0 vs 24 h), and their interaction, stratified by location (axial, pulpal). Within location, p -values with 2 degrees of freedom were used to assess statistical significance of differences between materials at baseline, changes over time within material, and differential changes over time among materials.

2.1

Adhesive and restorative materials

The adhesives and restorative systems used in this study are listed in Table 1.1 . Roles of the components and rendered tests for the 2 hypotheses are listed in Table 1.2 .

Table 1

Adhesive and restorative materials.

1.1—Materials, composition and lot
Material	Code	Composition	Lot #	Function
Bondfill SB (Sun Medical)	BF		KT1
Teeth Primer	BF-T	Water, acetone, 4-MET, reductant	ES1	Self-etching primer
Bondfill liquid	BF-L	MMA, 4META, hydrophobic dimethacrylate	ER2	Restorative material with adhesive function
Catalyst V		TBB, solvent	ER2
Bondfill Powder	BF-P	PMMA, TMPT reactive organic filler	ER11
Experimental Radio-opaque Powder RO3	BF-PO	PMMA, TMPT reactive organic filler, inorganic additives	N/A	Modified powder with radio-opacity
3M ESPE, Adper EasyBond (3M ESPE)	EB	HEMA, Bis-GMA, methacrylated phosphoric esters, 1,6 hexanediol dimethacrylate, methacrylate functionalized polyalkenoic acid, dispersed bonded silica filler, ethanol, water. Initiators based on camphorquinone	480148	Self-etching primer and adhesive system
3M ESPE, Z100 (3M ESPE)	Z	Bis-GMA, TEGDMA, silanated zirconium silica, synthetic mineral	394568	Restorative material
SureFil ® SDR ® flow (Dentsply Caulk, Milford, DE)	SF	Strontium alumino-fluoro-silicate glass, barium-alumino-fluoro-borosilicate glass, modified urethane dimethacrylate resin, EBPADMA, TEGDMA, camphorquinone photonitiator, photoaccelerator, BHT, UV stabilizer, titanium dioxide, iron oxide pigments, fluorescing agent	1204162	Restorative material. Posterior bulk fill flowable base

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