Hidehiko Sano

In vivo Degradation of Resin-Dentin Bonds in Humans Over 1 to 3 Years

The longevity of resin restorations is currently an area of great interest in adhesive dentistry. However, no work has been conducted to investigate the durability of resin-dentin bond structures using human substrate in vivo. The purpose of this study was to investigate the degradation of the resin-dentin bond structures aged in an oral environment for 1, 2, or 3 years. Cavities were prepared in primary molars, and an adhesive resin system (Scotchbond Multi-Purpose) was applied to the cavity. After I to 3 years, following the eruption of the succedaneous permanent teeth, the resin-restored teeth were extracted. Immediately after extraction, those teeth were sectioned perpendicular to the adhesive interface and trimmed to produce an hourglass-shaped specimen. Then, a micro-tensile test was performed at a crosshead speed of 1.0 mm/min. The mean bond strengths were statistically compared with one-way ANOVA and Fishers PLSD test (p < 0.05). Further, all fractured surfaces were observed by SEM, and the area fraction of failure mode was calculated by means of a digital analyzer on SEM photomicrographs. There were significant differences in tensile-bond strength among all 3 groups (p < 0.05), with mean values ranging from 28.3 ± 11.3 MPa (control), to 15.2 ± 4.4 MPa (1 to 2 years), to 9.1 ± 5.1 MPa (2 to 3 years). Moreover, under fractographic analysis, the proportion of demineralized dentin at the fractured surface in specimens aged in an oral environment was greater than that in control specimens. Furthermore, degradation of resin composite and the depletion of collagen fibrils was observed among the specimens aged in an oral environment. Analysis of the results of this study indicated that the degradation of resin-dentin bond structures occurs after aging in the oral cavity.

Long-term Durability of Dentin Bonds Made with a Self-etching Primer, in vivo

The long-term durability of bonds between adhesive resins and dentin is of significant importance for the longevity of bonded restorations. We carried out an in vivo one-year study to evaluate the durability of resin-dentin bonds in the oral cavity, as well as to test the hypothesis that the adhesive interface would show morphological changes in vivo over time. Very shallow saucer-shaped dentin cavities were prepared in 12 intact teeth of one Japanese monkey (Macaca fuscata) under general anesthesia. The cavities were restored with Clearfil Liner Bond II and Clearfil Photo Posterior resin composite. The teeth were extracted at three different times: immediately, and 180 and 360 days after placement of the restorations. One day after the monkey was killed, specimens of the three time periods were subjected to the micro-tensile bond test at a crosshead speed of 1 mm/min. The surfaces of the failed bonds were observed under a field emission scanning electron microscope (FE-SEM). Bond strength measurements in this study were successfully performed and were stable at approximately 19 MPa during the one-year testing. Scanning electron microscopic observations of the failed surfaces revealed, at the top of the hybrid layer and within the adhesive resin, porosity which increased over time. Long-term bonds can be assessed in vivo by the combined evaluation of the microtensile bond strength and SEM morphological examination of the adhesive interface.

Tensile bond strength to and SEM evaluation of ground and intact enamel surfaces

OBJECTIVES The aim of this study was to evaluate the bonding of four commercially available adhesive systems to ground and intact enamel surfaces. METHODS Extracted human teeth were used to measure the microtensile bond strength to enamel and a field-emission scanning electron microscopy (FE-SEM) was used to observe the bonded interface and the effect of the surface conditioning of each material. Intact buccal enamel surfaces were cleansed with tooth paste using a rotary dental brush, and the ground enamel surfaces were prepared by reducing approximately 0.5 mm from the buccal enamel surfaces using a high-speed diamond bur. One-Step (OS, Bisco), Single Bond (SB, 3M), Clearfil Liner Bond II (LBII, Kuraray), and Tokuso Mac Bond II (MBII, Tokuso) were evaluated for their ability to bond to enamel. RESULTS There was no significant difference in bond strengths between the materials when they were applied to ground enamel surfaces (p > 0.05). However, the bond strengths of the self-etching systems, LBII and MBII, had significantly lower bond strengths to intact enamel than the bonding systems OS and SB using phosphoric acid etching (p < 0.05). FE-SEM revealed that the etching pattern of self-etching primers was not deep enough to obtain good penetration of bonding resin when applied to intact enamel surfaces. CONCLUSIONS Phosphoric acid etching produced good resin adhesion to ground and intact enamel. The self-etching/self-priming systems also produced good adhesion to ground enamel, but had lower bond strengths to intact enamel.

In vitro degradation of resin–dentin bonds analyzed by microtensile bond test, scanning and transmission electron microscopy

Our knowledge of the mechanisms responsible for the degradation of resin-dentin bonds are poorly understood. This study investigated the degradation of resin-dentin bonds after 1 year immersion in water. Resin-dentin beams (adhesive area: 0.9mm(2)) were made by bonding using a resin adhesive, to extracted human teeth. The experimental beams were stored in water for 1 year. Beams that had been stored in water for 24h were used as controls. After water storage, the beams were subjected to microtensile bond testing. The dentin side of the fractured surface was observed using FE-SEM. Subsequently, these fractured beams were embedded in epoxy resin and examined by TEM. The bond strength of the control specimens (40.3+/-15.1MPa) decreased significantly (p<0.01) after 1 year of water exposure (13.3+/-5.6MPa). Loss of resin was observed within fractured hybrid layers in the 1 year specimens but not in the controls. Transmission electron microscopic examination revealed the presence of micromorphological alterations in the collagen fibrils after 1 year of water storage. These micromorphological changes (resin elution and alteration of the collagen fibrils) seem to be responsible for the bond degradation leading to bond strength reduction.

Hydrolytic Stability of Self-etch Adhesives Bonded to Dentin

Functional monomers chemically interact with hydroxyapatite that remains within submicron hybrid layers produced by mild self-etch adhesives. The functional monomer 10-MDP interacts most intensively with hydroxyapatite, and its calcium salt appeared most hydrolytically stable, as compared with 4-MET and phenyl-P. We investigated the hypothesis that additional chemical interaction of self-etch adhesives improves bond stability. The micro-tensile bond strength (μTBS) of the 10-MDP-based adhesive did not decrease significantly after 100,000 cycles, but did after 50,000 and 30,000 cycles, respectively, for the 4-MET-based and the phenyl-P-based adhesives. Likewise, the interfacial ultrastructure was unchanged after 100,000 thermocycles for the 10-MDP-based adhesive, while that of both the 4-MET- and phenyl-P-based adhesives contained voids and less-defined collagen. The findings of this study support the concept that long-term durability of adhesive-dentin bonds depends on the chemical bonding potential of the functional monomer.

Fluid Movement across the Resin-Dentin Interface during and after Bonding

This study evaluated the extent of water penetration through resin-dentin interfaces before and after being sealed with adhesives. Four adhesive resin systems (2 total-etch adhesives and 2 self-etching primer adhesives) were used in this study. Dentin disks were placed in a split-chamber device, and in situ fluid movement across dentin was measured, with and without physiological pressure, during bonding procedures or 24 hrs after bonding. The fluid movement across dentin occurs via dentin tubules after acid-etching. Large outward or inward fluid shifts across dentin were observed during air-drying and light-curing for resin application. The amount of fluid movement across resin-bonded dentin when total-etch adhesives were used was significantly greater than that with self-etching adhesives. The milder acid-etching effects of self-etching primers may retain hybridized smear plugs within the tubules that reduce outward fluid flow, resulting in superior dentin sealing.

Effects of mechanical properties of adhesive resins on bond strength to dentin.

OBJECTIVES The purpose of this study was to evaluate the relationship between the micro-tensile bond strength to dentin and mechanical properties of the cured adhesive resins. METHODS Coronal dentin surfaces of extracted human teeth were treated with four commercial self-etching priming systems (Clearfil SE Bond; UniFil Bond; Tokuso Mac-Bond II; and Imperva Fluoro Bond) and bonded with a resin composite. After 24h storage in water at 37 degrees C, the bonded specimens were trimmed and subjected to micro-tensile bond strength testing at a cross-head speed of 1mm/min. Debonded surfaces were observed under a FE-SEM. For testing mechanical properties, 0.7-mm thick slabs of each adhesive resin were prepared, light-cured, and stored dry at the room temperature for 24h. After trimming, ultimate micro-tensile strength was measured. The nano-hardness and Youngs modulus were also evaluated using cured adhesives that were prepared in the same manner as described above. RESULTS The micro-tensile bond strengths to dentin and ultimate micro-tensile strengths of the resins were not significantly different among all systems (P>0.05). However, the nano-hardness and Youngs modulus of Clearfil SE Bond and Imperva Fluoro Bond adhesive resins were significantly higher than those of UniFil Bond and Tokuso Mac-Bond II resins (P<0.05). The micro-tensile bond strength significantly correlated with the ultimate micro-tensile strength of the resins (r(2)=0.77; P<0.05), but was not correlated with the nano-hardness or Youngs modulus (P>0.05). SEM observation of the debonded surfaces revealed a mixed type of fracture with a combination of interfacial and cohesive failure within the adhesive resin. SIGNIFICANCE The four self-etching priming systems exhibited similar dentin bond strengths, which also correlates with the ultimate strength of the adhesive resins.

The effect of hybrid layer thickness on bond strength: demineralized dentin zone of the hybrid layer.

OBJECTIVES The purpose of this study was to evaluate the correlation between hybrid layer thickness and bond strength using specimens acid-conditioned for varying lengths of time. METHODS The dentin surfaces of human premolars, sectioned to remove the enamel from the labial surface, were conditioned with 35.0% phosphoric acid of an adhesive resin system (Scotchbond Multi-Purpose; 3M) for 15 (as directed by the manufacturer), 60, 120, or 180 s (experimental acid-conditioning times). The bonded specimens were then sectioned perpendicular to the adhesive interface to measure the hybrid layer thickness by SEM. The specimens for the micro-tensile test were sectioned perpendicular to the adhesive interface and trimmed to an hourglass-shape. Then, the micro-tensile test was performed at a crosshead speed of 1.0 mm/min. The bond strengths and hybrid layer thickness were statistically compared with Students t-test (p < 0.05). All fractured surfaces were also observed by SEM. RESULTS Significant differences between the groups exposed to acid for 15 and 60 s, and those exposed for 120 and 180 s were observed in hybrid layer thickness and bond strength (p < 0.05). SEM observation of the fractured surfaces revealed that a demineralized dentin zone without resin impregnation remained within the hybrid layer. SIGNIFICANCE A demineralized dentin zone was formed in the bond structures after prolonged acid-conditioning, resulting in low bond strength. The shrinkage of the hybrid layer due to desiccation during the SEM examination process provided evidence of the presence of the demineralized dentin zone within the hybrid layer.

CD14 directly binds to triacylated lipopeptides and facilitates recognition of the lipopeptides by the receptor complex of Toll-like receptors 2 and 1 without binding to the complex

It has demonstrated that the recognition of triacylated lipopeptides by Toll‐like receptor (TLR) 2 requires TLR1 as a coreceptor. In the NF‐κB reporter assay system in which human embryonic kidney 293 cells were transfected with TLR2 and TLR1 together with an NF‐κB luciferase reporter gene, S‐(2,3‐bispalmitoyloxypropyl)‐N‐palmitoyl‐Cys‐Lys‐Lys‐Lys‐Lys (Pam3CSK4) and Pam3CSSNA were recognized by TLR2/TLR1, but the recognition level was unexpectedly very low. However, cotransfection of CD14 drastically enhanced the recognition of triacylated lipopeptides by TLR2/TLR1. The CD14‐induced enhancement did not occur without cotransfection of TLR1. Both CD14dS39‐A48, a mutant with deletion of the part of possible N‐terminal ligand‐binding pocket, and anti‐CD14 monoclonal antibody reduced the CD14‐induced enhancement. Transfection of a TIR domain‐deficient mutant of TLR2 (TLR2dE772‐S784) or TLR1 (TLR1dQ636‐K779) completely abrogated the CD14‐induced enhancement. Soluble recombinant CD14 added extracellularly enhanced the recognition of Pam3CSSNA by TLR2/TLR1. Immunoprecipitation analysis demonstrated that CD14 was not associated with TLR2 but that TLR1 was associated with TLR2. In addition, surface plasmon resonance‐based assay demonstrated that CD14 binds to Pam3CSK4 at a dissociation constant of 5.7 µM. This study suggests that CD14 directly binds to triacylated lipopeptides and facilitates recognition of the lipopeptides by the TLR2/TLR1 complex without binding to the receptor complex.

Components of Dentinal Adhesives Modulate Heat Shock Protein 72 Expression in Heat-stressed THP-1 Human Monocytes at Sublethal Concentrations

Few studies have investigated the ability of dental resins to induce cellular stress at sublethal concentrations. Cellular stress, especially in immune cells such as monocytes, may modulate the biological response to materials or the hosts ability to respond to bacterially mediated inflammation. The current study examined the ability of sublethal concentrations of 2-hydroxylethylmethacrylate (HEMA) and triethyleneglycol dimethacrylate (TEGDMA) to induce heat shock protein 72 (HSP72) in human monocytes. HEMA and TEGDMA significantly suppressed heat-induced HSP72 expression, even at sublethal levels, but did not induce HSP72 by themselves. The results of the current study suggest that components released from dental resin could modulate the HSP stress response without altering cellular metabolic activity.