Overestimating hybrid layer quality in polished adhesive/dentin interfaces

Multifunctional monomer acts as co-initiator and crosslinker to provide autonomous strengthening with enhanced hydrolytic stability in dental adhesives

OBJECTIVE

The purpose of this study was to evaluate a new synthesized multifunctional monomer, aminosilane functionalized methacrylate (ASMA), containing polymerizable methacrylate, tertiary amine, and methoxysilane functionalities in dental adhesive formulations, and to investigate the polymerization kinetics, leachates, thermal and mechanical properties of copolymers.

METHODS

Adhesive contained HEMA/BisGMA (45/55, w/w) was used as a control, and mixtures based on HEMA/BisGMA/ASMA at the mass ratio of 45/(55-x)/x were used as experimental adhesive. Adhesives were characterized with regard to water miscibility, photo-polymerization behavior (Fourier transform infrared spectroscopy, FTIR), leached co-monomers (high performance liquid chromatography, HPLC), thermal properties (modulated differential scanning calorimeter, MDSC), and mechanical properties (dynamic mechanical analyzer, DMA). Stress relaxation times and the corresponding moduli, obtained from stress relaxation tests, are used in a simulated linear loading case.

RESULTS

As compared to the control, ASMA-containing adhesives showed higher water miscibility, lower viscosity, improved monomer-to-polymer conversion, significantly greater T_g and rubbery modulus. HPLC results indicated a substantial reduction of leached HEMA (up to 85wt%) and BisGMA (up to 55wt%) in ethanol. The simulation reveals that the ASMA-containing adhesive becomes substantially stiffer than the control.

SIGNIFICANCE

ASMA monomer plays multiple roles, i.e. it serves as both a co-initiator and crosslinker while also providing autonomous strengthening and enhanced hydrolytic stability in the adhesive formulations. This multifunctional monomer offers significant promise for improving the durability of the adhesive at the composite/tooth interface.

Threats to adhesive/dentin interfacial integrity and next generation bio-enabled multifunctional adhesives

Nearly 100 million of the 170 million composite and amalgam restorations placed annually in the United States are replacements for failed restorations. The primary reason both composite and amalgam restorations fail is recurrent decay, for which composite restorations experience a 2.0-3.5-fold increase compared to amalgam. Recurrent decay is a pernicious problem-the standard treatment is replacement of defective composites with larger restorations that will also fail, initiating a cycle of ever-larger restorations that can lead to root canals, and eventually, to tooth loss. Unlike amalgam, composite lacks the inherent capability to seal discrepancies at the restorative material/tooth interface. The low-viscosity adhesive that bonds the composite to the tooth is intended to seal the interface, but the adhesive degrades, which can breach the composite/tooth margin. Bacteria and bacterial by-products such as acids and enzymes infiltrate the marginal gaps and the composite's inability to increase the interfacial pH facilitates cariogenic and aciduric bacterial outgrowth. Together, these characteristics encourage recurrent decay, pulpal damage, and composite failure. This review article examines key biological and physicochemical interactions involved in the failure of composite restorations and discusses innovative strategies to mitigate the negative effects of pathogens at the adhesive/dentin interface. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B:2466-2475, 2019.

Non-thermal atmospheric plasmas in dental restoration: improved resin adhesive penetration

OBJECTIVE

To investigate the influence of non-thermal plasma treatment on the penetration of a model dental adhesive into the demineralized dentine.

METHODS

Prepared dentine surfaces were conditioned with Scotchbond Universal etchant for 15s and sectioned equally perpendicular to the etched surfaces. The separated halves were randomly selected for treatment with an argon plasma brush (input current 6mA, treatment time 30s) or gentle argon air blowing (treatment time 30s, as control). The plasma-treated specimens and control specimens were applied with a model adhesive containing 2,2-bis[4-(2-hydroxy-3-methacryloxypropoxy) phenyl]-propane (BisGMA) and 2-hydroxyethyl methacrylate (HEMA) (mass ratio of 30/70), gently air-dried for 5s, and light-cured for 20s. Cross-sectional specimens were characterized using micro-Raman spectral mapping across the dentine, adhesive/dentine interface, and adhesive layer at 1-μm spatial resolution. SEM was also employed to examine the adhesive/dentine interfacial morphology.

RESULTS

The micro-Raman result disclosed that plasma treatment significantly improved the penetration of the adhesive, evidenced by the apparently higher content of the adhesive at the adhesive/dentine interface as compared to the control. Specifically, the improvement of the adhesive penetration using plasma technique was achieved by dramatically enhancing the penetration of hydrophilic monomer (HEMA), while maintaining the penetration of hydrophobic monomer (BisGMA). Morphological observation at the adhesive/dentine interface using SEM also confirmed the improved adhesive penetration. The results further suggested that plasma treatment could benefit polymerization of the adhesive, especially in the interface region.

CONCLUSION

The significant role of the non-thermal plasma brush in improving the adhesive penetration into demineralized dentine has been demonstrated. The results obtained may offer a better prospect of using plasma in dental restoration to optimize adhesion between tooth substrate and restorative materials.

Synthesis and evaluation of novel dental monomer with branched carboxyl acid group

To enhance the water miscibility and increase the mechanical properties of dentin adhesives, a new glycerol-based monomer with vinyl and carboxylic acid, 4-((1,3-bis(methacryloyloxy)propan-2-yl)oxy)-2-methylene-4-oxobutanoic acid (BMPMOB), was synthesized and characterized. Dentin adhesive formulations containing 2-hydroxyethyl methacrylate (HEMA), 2,2-bis[4-(2-hydroxy-3-methacryloxypropoxy) phenyl]propane (BisGMA), and BMPMOB were characterized with regard to real-time photopolymerization behavior, water sorption, dynamic mechanical analysis, and microscale three-dimensional internal morphologies and compared with HEMA/BisGMA controls. The experimental adhesive copolymers showed higher glass transition temperature and rubbery moduli, as well as improved water miscibility compared to the controls. The enhanced properties of the adhesive copolymers indicated that BMPMOB is a promising comonomer for dental restorative materials.

Effect of application mode on interfacial morphology and chemistry between dentine and self-etch adhesives

OBJECTIVE

To investigate the influence of application mode on the interfacial morphology and chemistry between dentine and self-etch adhesives with different aggressiveness.

METHODS

The occlusal one-third of the crown was removed from un-erupted human third molars, followed by abrading with 600 grit SiC under water. Rectangular dentine slabs were prepared by sectioning the tooth specimens perpendicular to the abraded surfaces. The obtained dentine slabs were treated with one of the two one-step self-etch adhesives: Adper Easy Bond (AEB, pH∼2.5) and Adper Prompt L-Pop (APLP, pH∼0.8) with (15s, active application) or without (15s, inactive application) agitation. The dentine slabs were fractured and the exposed adhesive/dentine (A/D) interfaces were examined with micro-Raman spectroscopy and scanning electron microscopy (SEM).

RESULTS

The interfacial morphology, degree of dentine demineralization (DD) and degree of conversion (DC) of the strong self-etch adhesive APLP showed more significant dependence on the application mode than the mild AEB. APLP exhibited inferior bonding at the A/D interface if applied without agitation, evidenced by debonding from the dentine substrate. The DDs and DCs of the APLP with agitation were higher than those of without agitation in the interface, in contrast to the comparable DD and DC values of two AEB specimen groups with different application modes. Raman spectral analysis revealed the important role of chemical interaction between acid monomers of self-etch adhesives and dentine in the above observations.

CONCLUSION

The chemical interaction with dentine is especially important for improving the DC of the strong self-etching adhesive at the A/D interface. Agitation could benefit polymerization efficacy of the strong self-etch adhesive through enhancing the chemical interaction with tooth substrate.

Collagen cross linking increases its biodegradation resistance in wet dentin bonding

PURPOSE

The biodegradation of exposed dentin collagen within the adhesive/dentin (a/d) interface is one of the main reasons for composite restoration failures and seriously affects the durability of dental restorations. In the present study, the objective was to investigate whether the inclusion of the cross-linking reagent (glutaraldehyde, GA) in the adhesive would increase collagen biodegradation resistance within the a/d interface.

MATERIALS AND METHODS

The model adhesive consisted of ~60 % monomers (HEMA/bis-GMA, 45/55 wt/wt) and ~ 40 % ethanol as a solvent. 5% GA was added to the above formulation. After the dentin surfaces were etched for 15 s with 35% phosphoric acid, rinsed with water and blotted dry, adhesives both with and without GA were applied and polymerized by visible light for 20 s. These a/d specimens were immersed in the biodegradation solution (prepared by adding 160 mg collagenase in 1 liter of TESCA buffer solution) for up to 30 days after proceeding with the sectioning/fracture to expose the a/d interfaces. The specimens were analyzed using SEM and micro-Raman spectroscopy.

RESULTS

SEM results indicated that for the adhesive without GA, there were many voids and a loss of collagen fibrils in the a/d interface after being challenged by the biodegradation solution. The Raman spectra collected from the interface showed that the amide I of collagen at 1667 cm-1 obviously decreased, indicating a removal of collagen fibrils during the degradation process. For the adhesive containing GA, the collagen fibrils within the interface did not degrade at all, which was also confirmed by the Raman results.

CONCLUSION

The results corroborate the previous findings that by using the current adhesive system and wet bonding, the collagen fibrils in the a/d interface are largely unprotected and easily undergo biodegradation. Directly including cross-linking agents in the adhesive could protect collagen fibrils from degradation in situ within the a/d interface.

Fatigue life prediction of dentin-adhesive interface using micromechanical stress analysis

OBJECTIVES

The objective of this work was to develop a methodology for the prediction of fatigue life of the dentin-adhesive (d-a) interface.

METHODS

At the micro-scale, the d-a interface is composed of dissimilar material components. Under global loading, these components experience different local stress amplitudes. The overall fatigue life of the d-a interface is, therefore, determined by the material component that has the shortest fatigue life under local stresses. Multiple 3d finite element (FE) models were developed to determine the stress distribution within the d-a interface by considering variations in micro-scale geometry, material composition and boundary conditions. The results from these models were analyzed to obtain the local stress concentrations within each d-a interface component. By combining the local stress concentrations and experimentally determined stress versus number of cycle to failure (S-N) curves for the different material components, the overall fatigue life of the d-a interface was predicted.

RESULTS

The fatigue life was found to be a function of the applied loading amplitude, boundary conditions, microstructure and the mechanical properties of the material components of the d-a interface. In addition, it was found that the overall fatigue life of the d-a interface is not determined by the weakest material component. In many cases, the overall fatigue life was determined by the adhesive although exposed collagen was the weakest material component. Comparison of the predicted results with experimental data from the literature showed both qualitative and quantitative agreement.

SIGNIFICANCE

The methodology developed for fatigue life prediction can provide insight into the mechanisms that control degradation of the bond formed at the d-a interface.

In vitro enzymatic biodegradation of adhesive resin in the hybrid layer

Penetration of adhesives into the demineralized dentin surface and their subsequent conversion are critically important to longevity of the adhesive resin (AR)-dentin bond. The durability of the resin-dentin bond is investigated by monitoring the change of adhesive concentration within the hybrid layer (HL) of aged specimens using Raman spectroscopy. Absolute molar concentrations of Bis-GMA and HEMA were measured across the HL of resin-dentin specimens 24 h after photopolymerization and after 24-week storage in one of three media: artificial saliva (SAL), SAL containing cholesterol esterase to attack resin (EST), and SAL containing bacterial collagenase to attack collagen (COL). No significant difference among these groups for both Bis-GMA and HEMA molar concentrations at 24-h storage was found; however, concentrations decreased from the AR to the middle of the HL. Concentrations remained unchanged at any resin-dentin position after aging in SAL. In the HL, concentrations significantly decreased with aging in COL and tended to decrease in EST. While showing potential enzymatic biodegradative effects of endogenous matrix metalloproteinases and salivary esterases, this methodology may also prove to be a valuable assessment of new chemistries and future approaches to improve resin-dentin bond performance. (c) 2010 Wiley Periodicals, Inc. J Biomed Mater Res, 2010.

Adhesive/Dentin interface: the weak link in the composite restoration

Results from clinical studies suggest that more than half of the 166 million dental restorations that were placed in the United States in 2005 were replacements for failed restorations. This emphasis on replacement therapy is expected to grow as dentists use composite as opposed to dental amalgam to restore moderate to large posterior lesions. Composite restorations have higher failure rates, more recurrent caries, and increased frequency of replacement as compared to amalgam. Penetration of bacterial enzymes, oral fluids, and bacteria into the crevices between the tooth and composite undermines the restoration and leads to recurrent decay and premature failure. Under in vivo conditions the bond formed at the adhesive/dentin interface can be the first defense against these noxious, damaging substances. The intent of this article is to review structural aspects of the clinical substrate that impact bond formation at the adhesive/dentin interface; to examine physico-chemical factors that affect the integrity and durability of the adhesive/dentin interfacial bond; and to explore how these factors act synergistically with mechanical forces to undermine the composite restoration. The article will examine the various avenues that have been pursued to address these problems and it will explore how alterations in material chemistry could address the detrimental impact of physico-chemical stresses on the bond formed at the adhesive/dentin interface.

Enzymatic biodegradation of HEMA/bisGMA adhesives formulated with different water content

Dentin adhesives may undergo phase separation when bonding to wet demineralized dentin. We hypothesized that adhesives exhibiting phase separation will experience enhanced biodegradation of methacrylate ester groups. The objective of this project was to study the effect of enzyme-exposure on the release of methacrylic acid (MAA) and 2-hydroxyethyl methacrylate (HEMA) from adhesives formulated under conditions simulating wet bonding. HEMA/bisGMA(2,2-bis[4(2-hydroxy-3-methacryloyloxy-propyloxy)-phenyl] propane), 45/55 w/w ratio, was formulated with different water content: 0 Wt % (A00), 8 wt % (A08), and 16 wt % (A16). After a three day prewash, adhesive discs were incubated with/without porcine liver esterase (PLE) in phosphate buffer (PB, pH 7.4) at 37 degrees C for 8 days. Supernatants were collected daily and analyzed for MAA and HEMA by HPLC. For all formulations, daily MAA release in the presence of PLE was increased compared to MAA release in PB. HEMA release in the presence of PLE was not detected while HEMA release was consistently measured in PB. A08 and A16 released significantly larger amounts of HEMA compared to A00. Analysis of the cumulative release of analytes showed that the leachables in PLE was significantly increased (p < 0.05) as compared with that released in PB indicating that MAA release was not only formed from unreacted monomers but from pendant groups in the polymer network. However, the levels of analytes HEMA in PB or MAA in PLE were increased in A08 and A16 as compared with A00, which suggests that there could be a greater loss of material in HEMA/bisGMA adhesives that experience phase separation under wet bonding conditions.

Preparation and Properties of Novel Dentin Adhesives with Esterase Resistance

A new methacrylate monomer, trimethylolpropane mono allyl ether dimethacrylate (TMPEDMA), was synthesized and evaluated. This branched methacrylate was designed to increase esterase-resistance when incorporated into conventional HEMA (2-hydroxyethyl methacrylate)/BisGMA (2,2-bis[4(2-hydroxy-3-methacryloyloxy-propyloxy)-phenyl] propane) dental adhesives. The new adhesives, HEMA/BisGMA/TMPEDMA in a 45/30/25 (w/w) ratio were formulated with H(2)O at 0 (A0T) and 8 wt % water (A8T) and compared with control adhesives (HEMA/BisGMA, 45/55 (w/w), at 0 (A0) and 8 wt % (A8) water). Camphoroquinone (CQ), 2-(dimethylamino) ethyl methacrylate and diphenyliodonium hexafluorophosphate were used as photoinitiators. The new adhesives showed a degree of conversion comparable with the control and improved modulus and glass transition temperature (T(g)). Exposure of photopolymerized discs to porcine liver esterase for up to eight days showed that the net cumulative methacrylic acid (MAA) release in adhesives formulated with the new monomer and 8% water (A8T: 182 μg/mL) was dramatically (P < 0.05) decreased in comparison to the control (A8: 361.6 μg/mL). The results demonstrate that adhesives made with the new monomer and cured in water to simulate wet bonding are more resistant to esterase than conventional HEMA/BisGMA adhesive.

Parametric study of the effect of phase anisotropy on the micromechanical behaviour of dentin-adhesive interfaces

A finite element (FE) model has been developed based upon the recently measured micro-scale morphological, chemical and mechanical properties of dentin-adhesive (d-a) interfaces using confocal Raman microspectroscopy and scanning acoustic microscopy (SAM). The results computed from this FE model indicated that the stress distributions and concentrations are affected by the micro-scale elastic properties of various phases composing the d-a interface. However, these computations were performed assuming isotropic material properties for the d-a interface. The d-a interface components, such as the peritubular and intertubular dentin, the partially demineralized dentin and the so-called "hybrid layer" adhesive-collagen composite, are probably anisotropic. In this paper, the FE model is extended to account for the probable anisotropic properties of these d-a interface phases. A parametric study is performed to study the effect of anisotropy on the micromechanical stress distributions in the hybrid layer and the peritubular dentin phases of the d-a interface. It is found that the anisotropy of the phases affects the region and extent of stress concentration as well as the location of the maximum stress concentrations. Thus, the anisotropy of the phases could effect the probable location of failure initiation, whether in the peritubular region or in the hybrid layer.

Comparison of interfacial characteristics of adhesive bonding to superficial versus deep dentine using SEM and staining techniques

OBJECTIVES

A primary reason for premature failure of Class II composite restorations is recurrent decay at the gingival margin. The integrity of the gingival margin depends on the bond formed with dentine. Using scanning electron microscopy (SEM) and histomorphologic technique, the purpose of this study was to characterise the adhesive/dentine (a/d) bond in dentine sections from the dentinoenamel junction (DEJ) and the cementoenamel junction (CEJ) with commercial adhesives of varying hydrophobic/hydrophilic composition.

MATERIALS AND METHODS

The occlusal 1/3 of the crown was removed from 20 human third molars, this exposed the surface used for the DEJ sections. The teeth were sectioned occluso-gingivally into equal halves: one-half representing the DEJ; the remaining half was sectioned 4mm deep to the exposed surface (CEJ). Each half was treated with the same adhesive. The teeth were randomly selected for treatment with Single Bond (SB, 3M) or Dentastic UNO (UNO, Pulpdent) using wet bonding technique as per manufacturers' instructions. Thin (3-5mum) sections of native a/d interfaces were cut and stained with Goldner's trichrome for light microscopic (LM) examination. Companion slabs were polished and acid-bleach treated, then analysed with SEM.

RESULTS

DEJ sections: SB/dentine specimens had approximately 4mum exposed protein at the interface, UNO infiltrated the depth of the demineralised dentine and encapsulated exposed protein. CEJ sections: SB/dentine specimens had highly variable interface structure with localized exposed protein, UNO/dentine exhibited 3-5mum exposed protein at the interface. The quality of the interface was exaggerated and the above differences were not revealed based on SEM characterisation.

CONCLUSIONS

As a result of adhesive phase separation, SB does not form structurally integrated a/d bonds at the DEJ or CEJ. The increased hydrophilic composition of UNO contributes to the formation of an integrated a/d bond at the DEJ, but structural differences and increased moisture leads to unprotected protein at the a/d interface in the CEJ sections. The inability of the SEM technique to reveal the complexity of the interface could be related to the modifications of the a/d interface caused by polishing and acid-bleach treatment.

Micro-Raman imaging analysis of monomer/mineral distribution in intertubular region of adhesive/dentin interfaces

It is generally proposed that bonding of resins to dentin results from infiltration of the adhesive monomers into the superficially demineralized dentin. However, it is still not clear how well the mineral phase of dentin is removed and how far each monomer penetrates into the thin zone of "wet" demineralized dentin. The quality and molecular structure of adhesive/dentin interfaces formed under "wet" bonding conditions are studied using 2-D Raman microspectroscopic mapping/imaging techniques. Micro-Raman imaging analysis of the adhesive/dentin interface provides a reliable and powerful means of identifying the degree and depth of dentin demineralization, adhesive monomer distribution, and flaws or defects in the pattern of adhesive penetration. The image of mineral reveals a partially demineralized layer on the top of dentin substrate. Adhesive monomers readily penetrate into dentin tubules and spread into intertubular region through open tubules. The extent of adhesive monomer penetration is higher in the intertubular regions close to tubules as compared to the middle regions between the tubules. The diffusion of resin monomers differs substantially. In a comparison with a hydrophilic monomer, the hydrophobic monomer resists diffusion into the demineralized intertubular dentin area.

Interfacial chemistry of class II composite restoration: structure analysis

The gingival margins of class II composite restorations are particularly vulnerable to marginal leakage and secondary caries. In identifying the factors contributing to caries development, the molecular structure and differences in the structure at the proximal and gingival margins have been largely overlooked. The purpose of this study was to compare the molecular structure at the adhesive/dentin interface of the proximal and gingival walls of class II composite restorations. Class II preparations were cut in 12 unerupted third molars with a water-cooled high-speed dental handpiece. The prepared teeth were randomly selected for treatment with Single Bond (SB) + Z100 (3M). Teeth were restored, per manufacturer's directions, under humidity and temperature characteristic of the oral cavity. Restored teeth were kept in sterile Delbecco's phosphate saline for 48 h. The samples were sectioned occluso-gingivally and micro-Raman spectra were acquired at approximately 1.5-microm spatial resolution across the composite/adhesive/dentin interfaces. Samples were wet throughout spectral acquisition. Raman spectral characteristics at the proximal and gingival margins were distinctly different; the depth of demineralized dentin was 6-7 microm at proximal margin, 12-13 microm at gingival margin. SB adhesive penetrated the depth of demineralized dentin in a gradient at the proximal margin. The "single bottle" adhesive used in this study, gradually penetrated the depth of the demineralized dentin at the proximal margin but failed to infiltrate the depth at the gingival margin, leaving a thick exposed collagen layer.

Physicochemical interactions at the dentin/adhesive interface using FTIR chemical imaging

To date, much of our understanding of dentin bonding has been based on investigations performed on sound, healthy dentin. This is not the substrate generally encountered in clinical practice, rather dentists must frequently bond to caries-affected dentin. Because of the extreme complexity and variability of the caries-affected dentin substrate, conventional characterization techniques do not provide adequate information for defining those factors that impact bond formation. Using Fourier-transform infrared imaging, we characterized the inhomogeneities and compositional differences across the length and breadth of the caries-affected dentin/adhesive interface. Differences in mineral/matrix ratio, crystallinity, and collagen organization were noted in the comparison of caries-affected and healthy dentin. As compared to healthy dentin, there were striking differences in depth of demineralization, adhesive infiltration, and degree of conversion at the interface with caries-affected dentin.

Nanoleakage within the hybrid layer: A correlative FEISEM/TEM investigation

The aim of this study was to compare the nanoleakage patterns of the resin-dentin interfaces of three dentin bonding systems at both TEM and field emission in lens SEM (FEI-SEM) levels. A standardized smear layer was created with 180-grit silicon carbide paper (SiC) on dentin disks obtained from 18 noncarious human third molars. Specimens were randomly divided into three groups and bonded with a two-step total etching adhesive (Single Bond, SB), a two-step, self-etching adhesive (Clearfil SE BOND, SEB), and a one-step, self-etching adhesive (XENO III, XEIII). Nanoleakage was evaluated by using an ammoniacal silver-nitrate solution. Specimens were processed for TEM and FEI-SEM observation. The TEM of SB revealed silver deposits in adhesive and hybrid layers (HL). High-magnification FEI-SEM micrographs clearly identified these deposits as spherical clusters mainly associated with nonembedded collagen fibrils. TEM and FEI-SEM examination of SEB revealed some clusters of silver deposits within porosities and small channels of the HL. Additional silver deposits were observed between the peritubular dentin walls and the resin tags. XEIII revealed very fine and diffuse silver grains throughout the entire HL. SEM visualization of nanoleakage at a high level of resolution has not been previously described. FEI-SEM technology supported the TEM visualization with three-dimensional morphological data of the relations between the HL constituents and nanoleakage. The results of the present study confirm the hypothesis that both total- and self-etch adhesives are not able to fully infiltrate the dentin substrate.