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Polymer Prepr. Author manuscript; available in PMC 2015 October 08. Published in final edited form as: Polymer Prepr. 2010 ; 51(2): 50–51.
BONDING OF EXPERIMENTAL POLYMERIC ACP ADHESIVES TO HUMAN DENTIN J. M. Antonucci1 and D. Skrtic2 1
Polymers Division, National Institute of Standards and Technology (NIST), Gaithersburg, MD
2
Author Manuscript
Paffenbarger Research Center (PRC), American Dental Association Foundation (ADAF), Gaithersburg, MD
Introduction
Author Manuscript
Amorphous calcium phosphate (ACP)-based dental composites represent a new generation of bioactive, remineralizing dental materials. They are capable of providing an extended supply of Ca and PO4 ions needed to re-form damaged mineral structures, thus counteracting the recurrent decay that frequently develops at tooth/conventional filling interfaces. When embedded in polymerized methacrylate matrices and exposed to an aqueous environment, ACP releases sufficient levels of remineralizing Ca and PO4 ions in a sustained manner to promote re-deposition of thermodynamically stable, apatitic tooth mineral [1]. Towards this end, extensive physicochemical studies have been and continue to be performed in our group on ACP polymeric composites intended for a variety of clinical applications.
Author Manuscript
In this study, we assess the possible correlation between the ACP filler particle size and the composite adhesion to tooth structures. We assume that: 1) bioactive, Zr-hybridized ACP filler will not adversely affect the strength of the composite-adhesive-dentin bond in comparison with similar glass-filled composites, and 2) a more homogeneous particle size of ACP filler achieved by milling will aid in maintaining desired bond strengths over longer periods of aqueous exposure. To test the above hypotheses, two types of experimental composite materials intended for base-lining and orthodontic applications were formulated with as-prepared and milled ACP and Sr-glass. In addition to the shear bond strength (SBS) test, the analysis of failure modality of de-bonded composites was performed to assess the fracture behavior of these composites.
Disclaimer Certain commercial materials and equipment are identified in this article to specify the experimental procedure. In no instance does such identification imply recommendation or endorsement by NIST or ADAF or that the material or equipment identified is necessarily the best available for the purpose. “Official contribution of the National Institute of Standards and Technology; not subject to copyrights in USA”.
Antonucci and Skrtic
Page 2
Author Manuscript
Experimental ACP synthesis and characterization; resin formulation and composite preparation ACP fillers were synthesized following a modified preparation protocol proposed by Eanes et al. [2]. ZrOCl2 was used to introduce zirconia into precipitating ACP at a mole fraction of 10 % based on Ca content. The resulting zirconia-hybridized filler is designated as asprepared ACP. A fraction of as prepared-ACP was milled using a previously described wetmilling protocol [3]. Both types of ACP filler were characterized by X-ray diffraction (XRD; Rigaku X-ray diffractometer; Rigaku-USA Inc., Danvers, MA, USA), Fouriertransform Infrared (FTIR; Nicolet Magna-IR FTIR System 550, Nicolet Instrument Corporation, Madison, WI, USA) spectroscopy and particle size analysis (PSA; CIS-100 Particle Size Analyzer; Ankersmid Ltd., Yokneam, Israel) as detailed in [1]. Sr-glass was used to formulate the control composite.
Author Manuscript Author Manuscript
The two experimental resins (Table 1) were formulated from 2,2-bis[p-(2′-hydroxy-3′methacryloxypropoxy)phenyl] propane (Bis-GMA) or ethoxylated bisphenol A dimethacrylate (EBPADMA) as base monomers, triethylene glycol dimethacrylate (TEGDMA) as diluent monomer, and 2-hydroxyethyl methacrylate (HEMA), methacryloxyethyl phthalate (MEP) or zirconyl dimethacrylate (ZrDMA) as surface active monomers. They were photo-activated by inclusion of camphorquinone (CQ) and ethyl-4N,N-dimethylaminobenzoate (4EDMAB). Resin acronyms (BTHZ and ETHM) are obtained by combining the initial letter of the acronym for each monomeric constituent of the matrix. All resin components were used as received from the manufacturer without additional purification. After combining all the monomers, BTHZ or ETHM mixtures were magnetically stirred (38 rad/s) in the absence of blue light. The homogenized monomer mixtures were used to formulate ACP (40 mass % filler) or Sr-glass composites (75 mass % was needed to achieve handling properties comparable to ACP-composites). Bonding protocols, shear bond strength (SBS) measurements and fracture analysis
Author Manuscript
To test the shear bond strength (SBS) of the experimental composites to dentin through an adhesive system, the occlusal surfaces of extracted, caries-free, human molars were removed and their roots embedded in polycarbonate holders with chemical curing poly(methyl methacrylate) tray resin (Bosworth Fastray Powder and Liquid, Bosworth Company, Skokie, IL, USA). The exposed dentin surfaces were ground flat perpendicular to the longitudinal axis of the teeth with 320 grit silicon carbide paper. The bonding protocol included the following steps: ground dentin surfaces were first dried, then etched for 15 s with phosphoric acid gel (mass fraction H3PO4 38 %; Etch-Rite, Pulpdent Corporation, Watertown, MA, USA). The acid was rinsed away with distilled water for 10 s, and a moistened paper towel (Kimwipes; Kimberly-Clark Global Sales, Inc., Roswell, GA, USA) was used to blot the surface to a near-dry condition. In the BTHZ composite series, dentin surfaces were sequentially primed first with N-phenylglycine (NPG; 5 % mass fraction) solution in acetone for 30 s, and then with five consecutive coats of pyromellitic glycerol dimethacrylate (PMGDMA; mass fraction 20 %) in acetone solution with CQ (mass fraction 0.028 %) as photo-activator. In the ETHM composite series, a single coating of PMGDMAacetone primer (DenTASTIC UNO, Pulpdent Corporation, Watertown, MA, USA) was
Polymer Prepr. Author manuscript; available in PMC 2015 October 08.
Antonucci and Skrtic
Page 3
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applied. Following the application of NPG and PMGDMA, or PMGDMA alone, the surfaces were air-dried for 10 s to remove acetone and visible-light cured for 10 s (Spectrum Curing Light, Dentsply Caulk Limited, Milford, DE USA). A poly(tetrafluoroethylene) (PTFE)-coated iris (4 mm in diameter and 1.5 mm thick) that defined the bonding area was positioned on the tooth surface, filled by the experimental composite and light-cured for 20 s (BTHZ composites) or 60 s (ETHM composites). BTHZ specimens were completed by applying a commercial resin-based composite (TPH, Dentsply Caulk, Milford, DE, USA), which was then cured for additional 60 s. The assemblies were then exposed to air for 5 min to allow further dry-curing at room temperature and finally immersed at 37 °C in either distilled water (BTHZ series) or a saliva-like solution [4] (ETHM series) for up to 4 months.
Author Manuscript
De-bonding was performed by using the shear test method [5] with a computer-controlled universal testing machine (Instron 5500R, Instron Corp., Canton, MA, USA). Specimens were placed in the test apparatus and fixed such that their iris surface was flush against the PTFE support block. The SBS was determined normal to the longitudinal axis of the tooth, at a crosshead speed of 0.5 mm/min. It was calculated according to the following equation:
where P = applied load at failure and D = diameter of the cured composite as defined by the inner diameter of the iris.
Author Manuscript
The fractured surfaces of the specimens (tooth and iris) were evaluated immediately after testing using a stereomicroscope (Leica StereoZoom 6, Leica Microsystems, Wetzlar, Germany), and observations regarding the surface condition (i.e., irregularities, general mode of failure) were recorded. Surfaces were then digitally photographed (Leica MZ16 Optical Stereomicroscope, Wetzlar, Germany; QCapture Pro, QImaging Corporation, Surrey, British Columbia, Canada) and imaging software was used to measure the surface areas of different failure modes (ImageJ, National Institutes of Health, Bethesda, MD, USA) [6]. Statistical data analysis Experimental data were analyzed by the analysis of variance (ANOVA; α= 0.05). Significant differences between the groups were determined by all pair-wise multiple comparisons (Tukey-test). One standard deviation (SD) is identified in this paper for comparative purposes as the estimated uncertainty of the measurements.
Author Manuscript
Results and Discussion The XRD patterns of both as-prepared and milled ACP fillers consisted of two diffuse broad bands in 2θ = (4 to 60)° region, while their FTIR spectra showed wide absorbance bands typical for non-crystalline phosphates in the (1200 to 900) cm−1 and (630 to 500) cm−1 regions. Median particle diameter (dm) was reduced by almost 95 % in going from asprepared ACP [(58.23 ± 14.44) μm] to milled ACP [(3.10 ± 0.88) μm]. The Sr-glass had a dm of (2.64 ± 0.86) μm and its volume distribution was close to that of the milled ACP.
Polymer Prepr. Author manuscript; available in PMC 2015 October 08.
Antonucci and Skrtic
Page 4
Author Manuscript Author Manuscript
The changes in the SBS values as a function of time of aqueous exposure for the experimental BTHZ (Fig. 1) and ETHM composites (Fig. 2) were practically identical. Indepth statistical analysis revealed stronger bonds for Sr-glass composite groups compared with as-prepared ACP composite only for the 24 h immersion data. At longer time intervals, the differences in mean values between the filler groups were not statistically significant. We have previously shown [7] that highly agglomerated as-prepared ACP typically disperses unevenly throughout matrices regardless of the compositional makeup of the resin, possibly causing inadequate wetting of the filler and composite surface irregularities. The ACP filler-matrix interface is susceptible to random spatial changes during water sorption and mineral ion release from composites. Generally, composites formulated with the asprepared ACP exhibit lower strength and are more affected by water exposure than the corresponding unfilled copolymer resins [1]. Recently [8], we have demonstrated that employing the more flexible, low viscosity, EBPADMA instead of the rigid, high viscosity, and somewhat more hydrophilic Bis-GMA along with milled ACP instead of as-prepared ACP, yields composites with improved mechanical properties under prolonged aqueous challenge. However, SBS results reported here indicated no improvement with the use of milled ACP filler.
Author Manuscript
The failure analysis by optical microscopy of de-bonded specimens in BTHZ composite group revealed failures occurring predominantly at the dentin-adhesive resin interface regardless of the type of the filler. A breakdown of the observed failure modes for the ETHM composites was as follows: failure at the dentin-primer interface occurred in 43 % of all specimens. The corresponding complete interfacial failures accounted for 22 % of the observed fracture surfaces. Complete failure at the primer/composite interface accounted for less than 7 % of failures, and did not occur at every immersion time. The complete and dominant cohesive failures within the composites accounted for 5 % of the observed failures. However, cohesive failures occurred only for Sr-glass composites, with the SBS of complete cohesive failures consistently in the top 15 % of all treatments at corresponding immersion times.
Author Manuscript
Besides the chemical and physical bonding to the substrate, gradients in fracture toughness and elastic modulus across the bonded interface most likely control the strength and durability of the composite/dentin bond. An additional factor influencing the results of bond strength values may be the test methodology itself. The chisel-on-iris shear method used in this study may not be sensitive enough to detect subtle differences in the adhesiveness of the tested materials. Alternatively, shear bond tests which would involve much higher stress concentrations, and uniaxial tensile test methods such as the micro-tensile measurements that should have greater sensitivity, might be more appropriate tests to discriminate the adhesive properties of the composites. One certainly should not ignore other factors, such as tooth-intrinsic flaws and the method of dentin pre-treatment, which may have a crucial role in determining the overall strength of the composite-adhesive-dentin bond. Therefore, although unproven by the test method used in this study, the possibly decisive role of the filler type cannot yet be disregarded. The SBS tests of strong adhesives commonly produce cohesive failures in dentin [8], a failure mode not typically seen in clinical settings. In this study, majority of failures
Polymer Prepr. Author manuscript; available in PMC 2015 October 08.
Antonucci and Skrtic
Page 5
Author Manuscript
occurred at the primer/dentin interface. Failures were rarely observed within the composite itself (indeed, never for milled-ACP composites). This suggests strong discrepancies between the test method and the clinical situations it is intended to mimic and accentuates a need for either altering the existing test method or applying different tests to make the results more clinically relevant. Additionally, the high degree of scatter inherent in SBS tests is a problem that continues to require attention. The biological variability of the human species and anisotropic nature of the tooth itself both present significant obstacles to accurate, reliable measurements, and though widespread test standardization (loading mechanism, crosshead speed, etc.) may achieve some progress in this direction, altering the test method in the interest of clinical relevance might prove more beneficial in the long run. Therefore, modifying the SBS test method to include a fatigue component or combined fatigue and modified-bond-geometry (e.g., micro-shear and micro-tensile) approach may be prudent for future studies [6].
Author Manuscript
Conclusions Both as-prepared and milled ACP, despite the flaws that as-prepared ACP filler may introduce into the microstructure of composites, did not adversely affect their short and midterm bonding behavior to adhesive resins. Composites formulated with either type of ACP filler performed at least as well as Sr-glass-reinforced composites, while providing an additional bioactive component that may arrest demineralization and possibly promote remineralization of enamel and/or dentin. However, additional studies that would involve alternative testing methods may be needed to detect subtle differences in the adhesive bonding of these experimental materials.
Author Manuscript
Acknowledgments This study was supported by the National Institute of Dental and Craniofacial Research (NIDCR: grant DE 13169). It is a part of the dental material research program conducted by NIST in cooperation with ADAF, supported by both NIST and ADAF. Generous contributions of the monomers from Esstech, Essington, PA, USA and Sr-glass from Caulk Dentsply, Milford, DE, USA are gratefully acknowledged.
References
Author Manuscript
1. Skrtic D, et al. J Res Natl Inst Stands Technol. 2003; 108(3):167. 2. Eanes ED, et al. Nature. 1965; 208:365. [PubMed: 5885449] 3. O’Donnell JNR, et al. J Comp Mater. 2008; 42:2231. 4. tenCate JM, Duijsters PPE. J Caries Res. 1982; 16:201. 5. Venz S, Dickens B. J Dent Res. 1993; 72:582. [PubMed: 8383709] 6. Antonucci JM, et al. J Adhes Sci Technol. 2009; 23:1133. [PubMed: 19696914] 7. Skrtic D, et al. Biomaterials. 2004; 25:1141. [PubMed: 14643587] 8. Skrtic D, Antonucci JM. J Biomater Appl. 2007; 21:375. [PubMed: 16684798]
Polymer Prepr. Author manuscript; available in PMC 2015 October 08.
Antonucci and Skrtic
Page 6
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Fig. 1.
Shear bond strength (SBS; mean+standard deviation) of BTHZ composites as a function of aging time. Number of specimens n ≥ 7.
Author Manuscript Author Manuscript Polymer Prepr. Author manuscript; available in PMC 2015 October 08.
Antonucci and Skrtic
Page 7
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Fig. 2.
SBS of ETHM composites as a function of aging time. n ≥ 5.
Author Manuscript Author Manuscript Polymer Prepr. Author manuscript; available in PMC 2015 October 08.
Antonucci and Skrtic
Page 8
Table 1
Author Manuscript
The composition (mass %) of the resins used in the study. Monomers and components of the initiator system
BTHZ resin
ETHM resin
Bis-GMA
35.5
-
EBPADMA
-
62.8
TEGDMA
35.5
23.2
HEMA
27.0
10.4
MEP
-
2.6
ZrDMA
1.0
-
CQ
0.2
0.2
4EDMAB
0.8
0.8
Author Manuscript Author Manuscript Author Manuscript Polymer Prepr. Author manuscript; available in PMC 2015 October 08.
Polymer Prepr. Author manuscript; available in PMC 2015 October 08. Published in final edited form as: Polymer Prepr. 2010 ; 51(2): 50–51.
BONDING OF EXPERIMENTAL POLYMERIC ACP ADHESIVES TO HUMAN DENTIN J. M. Antonucci1 and D. Skrtic2 1
Polymers Division, National Institute of Standards and Technology (NIST), Gaithersburg, MD
2
Author Manuscript
Paffenbarger Research Center (PRC), American Dental Association Foundation (ADAF), Gaithersburg, MD
Introduction
Author Manuscript
Amorphous calcium phosphate (ACP)-based dental composites represent a new generation of bioactive, remineralizing dental materials. They are capable of providing an extended supply of Ca and PO4 ions needed to re-form damaged mineral structures, thus counteracting the recurrent decay that frequently develops at tooth/conventional filling interfaces. When embedded in polymerized methacrylate matrices and exposed to an aqueous environment, ACP releases sufficient levels of remineralizing Ca and PO4 ions in a sustained manner to promote re-deposition of thermodynamically stable, apatitic tooth mineral [1]. Towards this end, extensive physicochemical studies have been and continue to be performed in our group on ACP polymeric composites intended for a variety of clinical applications.
Author Manuscript
In this study, we assess the possible correlation between the ACP filler particle size and the composite adhesion to tooth structures. We assume that: 1) bioactive, Zr-hybridized ACP filler will not adversely affect the strength of the composite-adhesive-dentin bond in comparison with similar glass-filled composites, and 2) a more homogeneous particle size of ACP filler achieved by milling will aid in maintaining desired bond strengths over longer periods of aqueous exposure. To test the above hypotheses, two types of experimental composite materials intended for base-lining and orthodontic applications were formulated with as-prepared and milled ACP and Sr-glass. In addition to the shear bond strength (SBS) test, the analysis of failure modality of de-bonded composites was performed to assess the fracture behavior of these composites.
Disclaimer Certain commercial materials and equipment are identified in this article to specify the experimental procedure. In no instance does such identification imply recommendation or endorsement by NIST or ADAF or that the material or equipment identified is necessarily the best available for the purpose. “Official contribution of the National Institute of Standards and Technology; not subject to copyrights in USA”.
Antonucci and Skrtic
Page 2
Author Manuscript
Experimental ACP synthesis and characterization; resin formulation and composite preparation ACP fillers were synthesized following a modified preparation protocol proposed by Eanes et al. [2]. ZrOCl2 was used to introduce zirconia into precipitating ACP at a mole fraction of 10 % based on Ca content. The resulting zirconia-hybridized filler is designated as asprepared ACP. A fraction of as prepared-ACP was milled using a previously described wetmilling protocol [3]. Both types of ACP filler were characterized by X-ray diffraction (XRD; Rigaku X-ray diffractometer; Rigaku-USA Inc., Danvers, MA, USA), Fouriertransform Infrared (FTIR; Nicolet Magna-IR FTIR System 550, Nicolet Instrument Corporation, Madison, WI, USA) spectroscopy and particle size analysis (PSA; CIS-100 Particle Size Analyzer; Ankersmid Ltd., Yokneam, Israel) as detailed in [1]. Sr-glass was used to formulate the control composite.
Author Manuscript Author Manuscript
The two experimental resins (Table 1) were formulated from 2,2-bis[p-(2′-hydroxy-3′methacryloxypropoxy)phenyl] propane (Bis-GMA) or ethoxylated bisphenol A dimethacrylate (EBPADMA) as base monomers, triethylene glycol dimethacrylate (TEGDMA) as diluent monomer, and 2-hydroxyethyl methacrylate (HEMA), methacryloxyethyl phthalate (MEP) or zirconyl dimethacrylate (ZrDMA) as surface active monomers. They were photo-activated by inclusion of camphorquinone (CQ) and ethyl-4N,N-dimethylaminobenzoate (4EDMAB). Resin acronyms (BTHZ and ETHM) are obtained by combining the initial letter of the acronym for each monomeric constituent of the matrix. All resin components were used as received from the manufacturer without additional purification. After combining all the monomers, BTHZ or ETHM mixtures were magnetically stirred (38 rad/s) in the absence of blue light. The homogenized monomer mixtures were used to formulate ACP (40 mass % filler) or Sr-glass composites (75 mass % was needed to achieve handling properties comparable to ACP-composites). Bonding protocols, shear bond strength (SBS) measurements and fracture analysis
Author Manuscript
To test the shear bond strength (SBS) of the experimental composites to dentin through an adhesive system, the occlusal surfaces of extracted, caries-free, human molars were removed and their roots embedded in polycarbonate holders with chemical curing poly(methyl methacrylate) tray resin (Bosworth Fastray Powder and Liquid, Bosworth Company, Skokie, IL, USA). The exposed dentin surfaces were ground flat perpendicular to the longitudinal axis of the teeth with 320 grit silicon carbide paper. The bonding protocol included the following steps: ground dentin surfaces were first dried, then etched for 15 s with phosphoric acid gel (mass fraction H3PO4 38 %; Etch-Rite, Pulpdent Corporation, Watertown, MA, USA). The acid was rinsed away with distilled water for 10 s, and a moistened paper towel (Kimwipes; Kimberly-Clark Global Sales, Inc., Roswell, GA, USA) was used to blot the surface to a near-dry condition. In the BTHZ composite series, dentin surfaces were sequentially primed first with N-phenylglycine (NPG; 5 % mass fraction) solution in acetone for 30 s, and then with five consecutive coats of pyromellitic glycerol dimethacrylate (PMGDMA; mass fraction 20 %) in acetone solution with CQ (mass fraction 0.028 %) as photo-activator. In the ETHM composite series, a single coating of PMGDMAacetone primer (DenTASTIC UNO, Pulpdent Corporation, Watertown, MA, USA) was
Polymer Prepr. Author manuscript; available in PMC 2015 October 08.
Antonucci and Skrtic
Page 3
Author Manuscript
applied. Following the application of NPG and PMGDMA, or PMGDMA alone, the surfaces were air-dried for 10 s to remove acetone and visible-light cured for 10 s (Spectrum Curing Light, Dentsply Caulk Limited, Milford, DE USA). A poly(tetrafluoroethylene) (PTFE)-coated iris (4 mm in diameter and 1.5 mm thick) that defined the bonding area was positioned on the tooth surface, filled by the experimental composite and light-cured for 20 s (BTHZ composites) or 60 s (ETHM composites). BTHZ specimens were completed by applying a commercial resin-based composite (TPH, Dentsply Caulk, Milford, DE, USA), which was then cured for additional 60 s. The assemblies were then exposed to air for 5 min to allow further dry-curing at room temperature and finally immersed at 37 °C in either distilled water (BTHZ series) or a saliva-like solution [4] (ETHM series) for up to 4 months.
Author Manuscript
De-bonding was performed by using the shear test method [5] with a computer-controlled universal testing machine (Instron 5500R, Instron Corp., Canton, MA, USA). Specimens were placed in the test apparatus and fixed such that their iris surface was flush against the PTFE support block. The SBS was determined normal to the longitudinal axis of the tooth, at a crosshead speed of 0.5 mm/min. It was calculated according to the following equation:
where P = applied load at failure and D = diameter of the cured composite as defined by the inner diameter of the iris.
Author Manuscript
The fractured surfaces of the specimens (tooth and iris) were evaluated immediately after testing using a stereomicroscope (Leica StereoZoom 6, Leica Microsystems, Wetzlar, Germany), and observations regarding the surface condition (i.e., irregularities, general mode of failure) were recorded. Surfaces were then digitally photographed (Leica MZ16 Optical Stereomicroscope, Wetzlar, Germany; QCapture Pro, QImaging Corporation, Surrey, British Columbia, Canada) and imaging software was used to measure the surface areas of different failure modes (ImageJ, National Institutes of Health, Bethesda, MD, USA) [6]. Statistical data analysis Experimental data were analyzed by the analysis of variance (ANOVA; α= 0.05). Significant differences between the groups were determined by all pair-wise multiple comparisons (Tukey-test). One standard deviation (SD) is identified in this paper for comparative purposes as the estimated uncertainty of the measurements.
Author Manuscript
Results and Discussion The XRD patterns of both as-prepared and milled ACP fillers consisted of two diffuse broad bands in 2θ = (4 to 60)° region, while their FTIR spectra showed wide absorbance bands typical for non-crystalline phosphates in the (1200 to 900) cm−1 and (630 to 500) cm−1 regions. Median particle diameter (dm) was reduced by almost 95 % in going from asprepared ACP [(58.23 ± 14.44) μm] to milled ACP [(3.10 ± 0.88) μm]. The Sr-glass had a dm of (2.64 ± 0.86) μm and its volume distribution was close to that of the milled ACP.
Polymer Prepr. Author manuscript; available in PMC 2015 October 08.
Antonucci and Skrtic
Page 4
Author Manuscript Author Manuscript
The changes in the SBS values as a function of time of aqueous exposure for the experimental BTHZ (Fig. 1) and ETHM composites (Fig. 2) were practically identical. Indepth statistical analysis revealed stronger bonds for Sr-glass composite groups compared with as-prepared ACP composite only for the 24 h immersion data. At longer time intervals, the differences in mean values between the filler groups were not statistically significant. We have previously shown [7] that highly agglomerated as-prepared ACP typically disperses unevenly throughout matrices regardless of the compositional makeup of the resin, possibly causing inadequate wetting of the filler and composite surface irregularities. The ACP filler-matrix interface is susceptible to random spatial changes during water sorption and mineral ion release from composites. Generally, composites formulated with the asprepared ACP exhibit lower strength and are more affected by water exposure than the corresponding unfilled copolymer resins [1]. Recently [8], we have demonstrated that employing the more flexible, low viscosity, EBPADMA instead of the rigid, high viscosity, and somewhat more hydrophilic Bis-GMA along with milled ACP instead of as-prepared ACP, yields composites with improved mechanical properties under prolonged aqueous challenge. However, SBS results reported here indicated no improvement with the use of milled ACP filler.
Author Manuscript
The failure analysis by optical microscopy of de-bonded specimens in BTHZ composite group revealed failures occurring predominantly at the dentin-adhesive resin interface regardless of the type of the filler. A breakdown of the observed failure modes for the ETHM composites was as follows: failure at the dentin-primer interface occurred in 43 % of all specimens. The corresponding complete interfacial failures accounted for 22 % of the observed fracture surfaces. Complete failure at the primer/composite interface accounted for less than 7 % of failures, and did not occur at every immersion time. The complete and dominant cohesive failures within the composites accounted for 5 % of the observed failures. However, cohesive failures occurred only for Sr-glass composites, with the SBS of complete cohesive failures consistently in the top 15 % of all treatments at corresponding immersion times.
Author Manuscript
Besides the chemical and physical bonding to the substrate, gradients in fracture toughness and elastic modulus across the bonded interface most likely control the strength and durability of the composite/dentin bond. An additional factor influencing the results of bond strength values may be the test methodology itself. The chisel-on-iris shear method used in this study may not be sensitive enough to detect subtle differences in the adhesiveness of the tested materials. Alternatively, shear bond tests which would involve much higher stress concentrations, and uniaxial tensile test methods such as the micro-tensile measurements that should have greater sensitivity, might be more appropriate tests to discriminate the adhesive properties of the composites. One certainly should not ignore other factors, such as tooth-intrinsic flaws and the method of dentin pre-treatment, which may have a crucial role in determining the overall strength of the composite-adhesive-dentin bond. Therefore, although unproven by the test method used in this study, the possibly decisive role of the filler type cannot yet be disregarded. The SBS tests of strong adhesives commonly produce cohesive failures in dentin [8], a failure mode not typically seen in clinical settings. In this study, majority of failures
Polymer Prepr. Author manuscript; available in PMC 2015 October 08.
Antonucci and Skrtic
Page 5
Author Manuscript
occurred at the primer/dentin interface. Failures were rarely observed within the composite itself (indeed, never for milled-ACP composites). This suggests strong discrepancies between the test method and the clinical situations it is intended to mimic and accentuates a need for either altering the existing test method or applying different tests to make the results more clinically relevant. Additionally, the high degree of scatter inherent in SBS tests is a problem that continues to require attention. The biological variability of the human species and anisotropic nature of the tooth itself both present significant obstacles to accurate, reliable measurements, and though widespread test standardization (loading mechanism, crosshead speed, etc.) may achieve some progress in this direction, altering the test method in the interest of clinical relevance might prove more beneficial in the long run. Therefore, modifying the SBS test method to include a fatigue component or combined fatigue and modified-bond-geometry (e.g., micro-shear and micro-tensile) approach may be prudent for future studies [6].
Author Manuscript
Conclusions Both as-prepared and milled ACP, despite the flaws that as-prepared ACP filler may introduce into the microstructure of composites, did not adversely affect their short and midterm bonding behavior to adhesive resins. Composites formulated with either type of ACP filler performed at least as well as Sr-glass-reinforced composites, while providing an additional bioactive component that may arrest demineralization and possibly promote remineralization of enamel and/or dentin. However, additional studies that would involve alternative testing methods may be needed to detect subtle differences in the adhesive bonding of these experimental materials.
Author Manuscript
Acknowledgments This study was supported by the National Institute of Dental and Craniofacial Research (NIDCR: grant DE 13169). It is a part of the dental material research program conducted by NIST in cooperation with ADAF, supported by both NIST and ADAF. Generous contributions of the monomers from Esstech, Essington, PA, USA and Sr-glass from Caulk Dentsply, Milford, DE, USA are gratefully acknowledged.
References
Author Manuscript
1. Skrtic D, et al. J Res Natl Inst Stands Technol. 2003; 108(3):167. 2. Eanes ED, et al. Nature. 1965; 208:365. [PubMed: 5885449] 3. O’Donnell JNR, et al. J Comp Mater. 2008; 42:2231. 4. tenCate JM, Duijsters PPE. J Caries Res. 1982; 16:201. 5. Venz S, Dickens B. J Dent Res. 1993; 72:582. [PubMed: 8383709] 6. Antonucci JM, et al. J Adhes Sci Technol. 2009; 23:1133. [PubMed: 19696914] 7. Skrtic D, et al. Biomaterials. 2004; 25:1141. [PubMed: 14643587] 8. Skrtic D, Antonucci JM. J Biomater Appl. 2007; 21:375. [PubMed: 16684798]
Polymer Prepr. Author manuscript; available in PMC 2015 October 08.
Antonucci and Skrtic
Page 6
Author Manuscript Author Manuscript
Fig. 1.
Shear bond strength (SBS; mean+standard deviation) of BTHZ composites as a function of aging time. Number of specimens n ≥ 7.
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Antonucci and Skrtic
Page 7
Author Manuscript Author Manuscript
Fig. 2.
SBS of ETHM composites as a function of aging time. n ≥ 5.
Author Manuscript Author Manuscript Polymer Prepr. Author manuscript; available in PMC 2015 October 08.
Antonucci and Skrtic
Page 8
Table 1
Author Manuscript
The composition (mass %) of the resins used in the study. Monomers and components of the initiator system
BTHZ resin
ETHM resin
Bis-GMA
35.5
-
EBPADMA
-
62.8
TEGDMA
35.5
23.2
HEMA
27.0
10.4
MEP
-
2.6
ZrDMA
1.0
-
CQ
0.2
0.2
4EDMAB
0.8
0.8
Author Manuscript Author Manuscript Author Manuscript Polymer Prepr. Author manuscript; available in PMC 2015 October 08.
