In vitroevaluation of adhesive characteristics of 4-META/MMA-TBB resin with organic filler

Highlights

  • Self-etching Bondfill SB showed higher shear bond strength than EasyBond.

  • Operator’s experience level did not affect bond strength of Bondfill SB nor EasyBond.

  • Bondfill SB had narrower zone of affected dentin at bonded interface than Easybond.

  • Experimental opaque powder had as small a contraction gap as Easybond + SureFil SDR.

Abstract

Objectives

A commercial restorative material, BondfillSB (BF), is a modification of 4-META/MMA-TBB resin cement. BF uses a self-etching primer and added pre-polymerized organic fillers. We compared BF with another self-etching system, EasyBond (EB), in shear bond strength, bonded interface characteristics to human dentin and contraction gap when used in bulk-filling.

Methods

Shear bond strength of BF and EB + Z100 (Z), bonded by different experience-level operators, was evaluated. Bonded interfaces were characterized by SEM, AFM, and AFM based nano-indentation. Contraction gaps (CG) at 0 h and 24 h after polymerization were evaluated for BF or EB bulk filled class I cavities. To meet the clinical recommendation, BF’s powder was replaced by experimental radioopaque powder (BFO) for the CG study. EB was used with Z (EBZ) or with a resin marketed for bulk-fill base (SureFil-SDR-flow (EBSF)).

Results

Shear bond strengths (Mean ± Standard Deviation (S.D.)) of BF (37.4 ± 2.6 MPa; n = 36) were higher and less variable than EBZ (18.2 ± 7.6 MPa; n = 36) ( p < 0.0001, One-way ANOVA). Weibull characteristic strength ( η ) differed significantly between materials ( p < 0.0001) but not between operators ( p = 0.90). EBZ often had non-uniform interfaces and a wider band of reduced elastic modulus ( E ) of greater than 20 μm across the interface. BF had uniform interfaces and a smaller width of affected dentin under the interface (∼1 μm). There was a difference in dentin- E between EBZ and BF up to 9 μm from the interface (mixed-effects model; P = 0.03). A stratified linear regression model used for CG. EBSF and BFO showed significantly smaller CG than EBZ at time 0. None of three combinations showed any significant change between 0h-CG and 24h-CG.

Significance

BF possessed bonding characteristics required to serve as a restorative.

Introduction

There is increasing need to restore worn dentition since people live longer and retain more teeth than ever , and non-carious lesions have become more prominent in the adult population . Historically non-carious cervical lesions (NCCL) and occlusal dentin defects due to attrition have been harder to restore than class 1 and 2 stress-bearing lesions , because flex and compression can occur simultaneously on a tooth with eccentric occlusal loading . Although, with the improvements in adhesive technology, NCCL restored with 2 step self-etching adhesive were recently reported to have a 96% retention rate after 13 years . Restoring those lesions with materials with lower moduli of elasticity could be effective since when the failure rate was higher with older bonding systems, lower modulus materials showed better clinical results .

Recently a new restorative material was introduced, 4-META/MMA-TBB (4-methacryloxyethyl trimellitate anhydride/methyl methacrylate/tri-n-butylborane) resin restorative material, Bondfill SB (BF) (Sun Medical Co., Ltd, Moriyama, Japan) that is a modification of 4-META/MMA-TBB resin cement, C&B Metabond (Super-Bond) (Sun Medical) (CBM). It has been modified by adding trimethylol-propane-trimethacrylate (TMPT) organic filler and uses a self-etching primer instead of 10% citric acid with ferric chloride (10-3) etch and rinse system.

The original resin cement CBM is unfilled monomethacrylate base resin. It was suggested monomethacrylate polymerizes with less cross linking due to molecular structure than dimethacrylate based resins , which results in lower flexural modulus . CBM is reported to have lower elastic modulus and lower or similar flexural strengths than other dimethacrylate based resin cements or composite resins (CR) . CBM has flexural modulus of 1.7 GPa , flexural strength reports varied from 88 to 130 MPa . The flexural modulus is lower than dimethacrylate base resin cements, such as Panavia F (Kuraray, Okayama, Japan), Unicem (3 M ESPE)) ∼7 GPa or various CRs (3.45–11.3 GPa) . Flexural strengths of other resin based cements range from 94 to 176 MPa , CRs ranged from 66 to 147 MPa . All such values reported above were obtained from 3 point bending tests.

Most resin cements contain about 50% inorganic fillers by weight . Theoretically, filler content of a resin composite material is related to various properties of the material. Thus modifying resin composition by adding fillers could affect properties such as compressive strength, wettability , wear resistance and polymerization shrinkage . Adding 2% of untreated poly methyl methacrylate (PMMA) fillers to MMA improved physical properties, but more (23.1%) caused debonding between matrix and fillers .

Adding prepolymerized TMPT to the original CBM did not contribute significantly to improved wear resistance of BF whereas TMPT used in CR had higher wear resistance than CBM , but the flexural strength and tensile bond strength to enamel was unchanged between BF and CBM .

In the first part of this study we investigated if BF has comparable bond strength and interface characteristics so the material can be suitable to restore NCCL and attrition lesions based on the standard approach to restore them with lower moduli materials. It has been used clinically based on that assumption but no long term reports have been made.

We chose a one bottle adhesive system and light-cure conventional resin restorative as controls. BF system consists of 2 steps from etching to placement of the restoration. The number of steps is equivalent to using a one bottle adhesive combined with traditional light-cure CR.

Special attention and techniques are required for large and deep CR restorations. If the contraction force is greater than the bond strength between dentin and resin, it will accumulate stress to cause micro cracks or debonding . Bulk-filling using contemporary resin materials results in larger contraction gaps compared to the incremental filling method and will leave uncured resin if the depth is greater than the depth of cure limit. Newly introduced resin restoratives designed for bulk-fill have new polymer formulations with improved polymerization with less stress and deeper depth of cure than conventional resin materials but the gap formation result is unknown. An alternate approach is initiating polymerization at the dentin–resin interface to alter the direction of polymerization shrinkage and location of the accumulated stress .

In the second part of this paper, we sought to determine if BF can be used as a bulk-fill material without the introduction of significant contraction gaps. It is based on the result of the first part and a previous report of 4-META/MMA-TBB indicating it provides interfacial polymerization . BF is a chemical cure resin so that there is no depth of cure limit of curing as with light curing CR. It has been reported that the reductant metal ions in the primer are the promotors of interfacial polymerization in MMA-TBB system, resulting in contraction towards the dentin wall instead of the center of the mass . BF is not radio-opaque and to be useful as a bulk-fill material in the clinical setting, this is an important characteristic. Thus we used an experimental radio-opaque powder provided by the manufacturer to replace the powder in BF commercial kit. Bulk-fill flowable CR was used with all-in-one adhesive used in part one of this study to determine if it has effects on gap formation.

The hypotheses tested were: (1) self-etching primer with 4-META/MMA-TBB restorative system has no significant difference in shear bond strength (SBS) and interface characteristics, as compared to a traditional restorative material used with a self-etching adhesive system; and (2) use of this system results in significantly lower contraction gaps when used as a bulk-fill material.

Materials and methods

The tooth specimens used for this study were prepared from sound human molars, collected from subjects requiring extractions following a protocol approved by the UCSF Committee on Human Research. Teeth were sterilized by gamma radiation and stored in Hank’s balanced salt solution (HBSS) without glucose until used.

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
1.2—Combination used and steps in resin-based restoration for shear bond strength and contraction gap test
Code Etchant (1) Primer (2) Adhesive (3) Restorative (4) Shear bond strength Contraction gap
Combination #1 BF BF-T BF-L + BF-P X
Combination #2 EBZ EB Z X X
Combination #3 EBSF EB SF X
Combination #4 BFO BF-T BF-L + BF-PO X
4-META: 4-methacryloxyethyl trimellitate anhydride, BIS-GMA: Bisphenol A diglycidyl ether dimethacrylate, BHT: Butylated hydroxytoluene, EBPADMA: Ethoxylated Bisphenol A dimethacrylate, HEMA: 2-hydroxyethyl methacrylate, MMA: methyl methacrylate, PMMA: poly methyl methacrylate, TBB: tri-n-butylborane, TEGDMA: triethylene glycol dimethacrylate, TMTP: trimethylol-propane-trimethacrylate.

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).

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.

Bonded interface characteristics

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).

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.

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 .

<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='E=π2aS’>E=π2aSE=π2aS
E = π 2 a S

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.

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.

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.

Materials and methods

The tooth specimens used for this study were prepared from sound human molars, collected from subjects requiring extractions following a protocol approved by the UCSF Committee on Human Research. Teeth were sterilized by gamma radiation and stored in Hank’s balanced salt solution (HBSS) without glucose until used.

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
Only gold members can continue reading. Log In or Register to continue

Nov 23, 2017 | Posted by in Dental Materials | Comments Off on In vitroevaluation of adhesive characteristics of 4-META/MMA-TBB resin with organic filler
Premium Wordpress Themes by UFO Themes