Solubility parameters, fractional polarities, and bond strengths of some intermediary resins used in dentin bonding

In vitro evaluation of enamel surface roughness and morphology after orthodontic debonding: Traditional cleanup systems versus polymer bur

INTRODUCTION

The primary aim of this randomized in vitro study was to compare the effectiveness of carbide, fibreglass and polymer burs on resinous remnant removal after bracket debonding, by the evaluation of enamel surface roughness and morphology. The secondary objective was to compare the time dispended on the procedures.

METHODS

The buccal surfaces of 28 bovine incisors were analysed by a profilometer to initial roughness measurement (Ra-T1). Brackets were bonded with a light-cured resin and debonded with a debonding plier. The samples were randomly divided into four groups, according to the bur used (n=7): A-Tungsten carbide; B-Fibreglass; C-Polymer; D-Polymer with 75% ethanol pre-treatment. The second roughness measurements were made after resin removal (Ra-T2). Time for removal procedures was also recorded. The third measurements were made after polishing (Ra-T3). Scanning Electronic Microscopy was performed in two samples of each group: after resin removal and after polishing. Results of roughness and time measurements were statically analysed by analysis of variance with post-hoc Bonferroni.

RESULTS

After polishing, tungsten carbide (P=0.1555) and fibreglass burs provided final surface roughness statistically similar to the baseline condition (P=1.0000). Yet, polymer burs, associated (P<0.0001) or not to alcohol (P<0.0001), provided surface roughness significantly higher when compared to baseline values. Polymer burs were more time-consuming on resinous remnant removal than tungsten carbide and fibreglass burs (P<0.05).

CONCLUSION

Polymer burs were less effective and more time-consuming to remove the remaining resin than tungsten carbide and fibreglass burs. The polishing step created smoother surfaces regardless of the burs used for resin removal.

The solubility parameter for biomedical polymers-Application of inverse gas chromatography

The solubility parameter seems to be a useful tool for thermodynamic characterisation of different materials. The solubility parameter concept can be used to predict sufficient miscibility or solubility between a solvent and a polymer, as well as components of co-polymer matrix in composite biomaterials. The values of solubility parameter were determined for polycaprolactone (PCL), polylactic acid (PLA) and polyethylene glycol (PEG) by using different procedures and experimental data, collected by means of inverse gas chromatography.

Influence of intermediate resin on the bond strength of light-curing composite resin to polymer substrate

OBJECTIVES

The aim of this study was to examine the effect of intermediate resin (IMR) of different monomer compositions and viscosities on the shear bond strength between polymer substrate and light-curing composite.

METHODS

The substrate used in the study was an autopolymerizing polymethyl methacrylate (PMMA) based polymer. The substrate was treated with the IMR for 3 min before application of light polymerizable particulate filler composite resin. The monomers of the IMR were either bisphenol-A-glycidyl dimethacrylate (BisGMA) and triethyleneglycol dimethacrylate (TEGDMA) or BisGMA and methyl methacrylate (MMA). The shear bond strength of the IMR treated substrate to the particulate filler composite was evaluated after storing the specimens dry and after thermocycling the specimens in water. Light microscope examination was accomplished to determine the swelled layer of the substrate.

RESULTS

Significant differences were found between the shear bond strength values of the IMRs (p < 0.001). The bond strengths were generally higher in the BisGMA-MMA groups than in the BisGMA-TEGDMA groups. Two-way ANOVA revealed significant effects of type of IMR (p < 0.001) and thermocycling (p = 0.017) on the shear bond strength. No interaction was found between these variables (p > 0.05).

CONCLUSIONS

The results suggest that the monomer composition and ratio of the IMRs used in the study influence the shear bond strength of the polymer substrate to the new resin.

Surface treatment with methyl formate-methyl acetate increased the shear bond strength between reline resins and denture base resin

BACKGROUND

Chemical surface treatment increases the shear bond strength (SBS) between hard reline resins (HRRs) and denture base resin.

OBJECTIVE

To evaluate the effect of methyl formate-methyl acetate (MF-MA), when used as a surface treatment agent, on the SBS between denture base resin and different HRRs.

MATERIALS AND METHODS

One hundred and twenty specimens of heat-polymerised acrylic resin denture base (Meliodent(®) ) were divided into 12 groups. These groups comprised denture base relined with three self-polymerised HRRs [Unifast trad(®) (UT), Tokuyama(®) RebaseII Fast (TR), Ufi gel hard(®) (UG)], and treated with their respective Bonding Agent (BA) or by MF:MA solutions at ratios of 35:65, 25:75, and 15:85 for 15 s. The SBS was measured using a Universal Testing Machine. The data were analysed using two-way anova and post hoc Tukey's analysis at p < 0.05.

RESULTS

The highest SBS was in the UT treated with MF:MA at a ratio of 25:75 group, followed by UT treated with MF:MA at ratios of 15:85, 35:65, UT treated with BA, and all UG treated with MF:MA groups. The SBS of the UT treated with MF:MA at a ratio of 25:75 group was significantly higher than those of the groups treated with BA. The SBS of the UG treated with MF:MA groups was significantly higher than control. The TR groups treated with BA or MF:MA groups showed no significant difference in SBS.

CONCLUSION

Surface treatment with MF-MA significantly enhanced the SBS of denture base resin and UT and UG compared to that of the groups treated with BA.

Bioadhesion: a review of concepts and applications

Bioadhesion refers to the phenomenon where natural and synthetic materials adhere to biological surfaces. An understanding of the fundamental mechanisms that govern bioadhesion is of great interest for various researchers who aim to develop new biomaterials, therapies and technological applications such as biosensors. This review paper will first describe various examples of the manifestation of bioadhesion along with the underlying mechanisms. This will be followed by a discussion of some of the methods for the optimization of bioadhesion. Finally, nanoscale and macroscale characterization techniques for the efficacy of bioadhesion and the analysis of failure surfaces are described.

Durability of resin-dentin bonds to water- vs. ethanol-saturated dentin

Higher 24-hour resin-dentin bond strengths are created when ethanol is used to replace water during wet bonding. This in vitro study examined if ethanol-wet-bonding can increase the durability of resin-dentin bonds over longer times. Five increasingly hydrophilic experimental resin blends were bonded to acid-etched dentin saturated with water or ethanol. Following composite build-ups, the teeth were reduced into beams for 24-hour microtensile bond strength evaluation, and for water-aging at 37 degrees C for 3, 6, or 12 months before additional bond strength measurements. Although most bonds made to water-saturated dentin did not change over time, those made to ethanol-saturated dentin exhibited higher bond strengths, and none of them fell over time. Decreased collagen fibrillar diameter and increased interfibrillar spacing were seen in hybrid layers created with ethanol-wet-bonding. Increases in bond strength and durability in ethanol-wet-bonding may be due to higher resin uptake and better resin sealing of the collagen matrix, thereby minimizing endogenous collagenolytic activities.

Compressive Fracture Resistance of Porcelain Laminates Bonded to Enamel or Dentin with Four Adhesive Systems

PURPOSE

To measure the compressive strength of porcelain laminates of 0.5 or 1.0 mm thickness when bonded to enamel or dentin using these resin cements: All-Bond 2 + Choice, Panavia 21, Scotchbond + Opal, and Super-Bond C&B.

MATERIALS AND METHODS

The buccal and lingual aspects of human molars were sectioned to prepare specimens at least 3 x 3 x 3 mm in size. Thirty horizontally flat enamel surfaces were prepared with a diamond disc for each group using a milling machine. Ten enamel specimens were randomly selected to test the fracture strength of 0.5-mm thick porcelain laminates without resin cement, and the data were recorded for a control group. The enamel specimens of each group were divided into two subgroups of 15 specimens to bond with either 0.5- or 1.0-mm thick porcelain laminates. Four resin cements were used for bonding of individual groups. All bonded specimens were stored in 37 degrees C for 24 hours before fracture testing. The horizontally flat dentin surfaces were prepared on the fractured bonded specimens using a diamond disc for each group. Ten 0.5 mm porcelain laminates were randomly selected to test the fracture strength on dentin (control group). The bonded laminates to dentin were prepared using the same procedure as for enamel. The fracture strengths were statistically analyzed at alpha= 0.05.

RESULTS

Statistically significant differences in mean fracture strengths between groups were revealed. No significant difference in fracture strengths of control specimens between enamel and dentin was found. Super-Bond C&B provided a higher fracture resistance of porcelain than the other resin cements. Increasing the thickness of porcelain laminate increased the fracture strength. The 0.5-mm thick porcelain bonded to enamel had higher fracture strength than that of 1.0-mm thick porcelain bonded to dentin when using Super-Bond C&B and Scotchbond + Opal cements.

CONCLUSIONS

Bonding techniques and curing systems of resin cements influenced the fracture resistance of porcelain laminates. Dry bonding with auto-polymerization of Super-Bond C&B resin provided the highest fracture resistance of porcelain. Porcelain bonded to enamel with this resin had a much higher fracture strength than when bonded to dentin.

Effect of curing mode on microtensile bond strength to dentin of two dual-cured adhesive systems in combination with resin luting cements for indirect restorations

This study evaluated the microtensile bond strength (microTBS) of dual-cured adhesive systems when the different components were either light activated or left in the uncured state prior to cementation of an indirect composite restoration. Occlusal dentin surfaces of 40 human third molars were flattened. The teeth were randomly assigned to 8 groups (n = 5) according to the dual-cured systems (bonding agents/resin cements) and curing modes: All Bond 2/Duolink (AB2-BISCO Inc) and Optibond Solo Plus Dual Cure/Nexus 2 (SOLO-Kerr). Resin cements were applied to pre-cured resin composite discs (2 mm thick/Z-250/3M ESPE), which were fixed to dentin surfaces containing adhesive resin in either cured (LP) or uncured states (SP). The restored teeth were light activated according to the manufacturers' instructions (LRC-XL3000/3M ESPE) or allowed to self-cure (SRC). The restored teeth were water-stored at 37 degrees C for 24 hours. They were then both mesial-distally and buccal-lingually sectioned to obtain bonded specimens (1.2 mm2). Each specimen was tested in tension at a crosshead speed of 0.6 mm/minute until failure. Data (MPa (SD)) were analyzed by two-way ANOVA and Tukey's post-hoc test (p < .05). AB2/SP exhibited higher microTBS than AB2/LP (p = .00001); however, no significant differences were noted between SOLO/LP and SOLO/SP. Results suggested that dual-cured adhesive systems were as strong or even stronger when they were left in the uncured state prior to indirect resin composite cementation.

Interfacial characteristics of adhesive luting resins and composites with dentine

PURPOSE

To evaluate the interfacial characteristics of five adhesive resin luting agents with dentine including tensile bond strength, failure mode, extent of demineralisation, morphological changes and hybrid layer formation.

MATERIALS AND METHODS

The products tested were Bistite II DC (BDC), C&B Super-Bond (CBM), M-Bond (MBD), Panavia-F (PAF) and Rely-X Unicem (RXU). For tensile bond strength measurements (TBS), metallic rods were bonded to standardised dentine surface areas (n=10), thermal-cycled (3000x 5-55 degrees C, 4 cycles/min) and debonded at 1mm/min crosshead speed. The failure mode was examined on dentine surfaces by LV-SEM (n=10), whereas the thickness of the hybrid layer by HV-SEM (n=4). FT-IR microscopy (n=4) and ESEM (n=4) were used to assess the extent of demineralisation and the morphological changes induced on dentine by the conditioning and priming treatments.

RESULTS

TBS (MPa) values were BDC (13.01), MBD (9.19) and PAF (7.07) significantly different from CBM (4.79) and RXU (4.47). The percentage of debonded dentine area covered with resin showed the highest values in BDC (47.80) and MBD (38.12) significantly different from CBM (17.20), PAF (16.47), and RXU (16.50). The extent of demineralisation for CBM was 100%. No statistical differences were found among BDC (60.86%), MBD (60.22%) and PAF (51.99%). RUX (45.03%) showed the lowest value. CBM induced the most pronounced tubule funneling and intertubular dissolution, followed by PAF, BDC and MDB. RXU partially removed the smear layer without opening tubule orifices. The thickest hybrid layer was found in CBM (4.17microm) followed by MBD (2.39microm). No statistically significant differences were found between PAF (0.95microm) and BDC (1.12microm), whereas RXU showed no detectable hybrid layer.

SIGNIFICANCE

Significant differences were found in the interfacial properties among the materials tested, that may lead to differences in their clinical performance.

Effect of wet vs. dry testing on the mechanical properties of hydrophilic self-etching primer polymers

Self-etching primers and adhesives contain very hydrophilic methacrylate monomers that result in high water sorptions by their polymers. Water sorption plasticizes the polymers and lowers their mechanical properties. The purpose of this work was to rank the hydrophilicity of a series of acidic primers by their Hoy's solubility parameters (delta) to determine if there was a significant relationship between the delta of polymers and their mechanical properties. A series of six acidic primer blends containing a fixed concentration of phenyl-P but variable amounts of 2-hydroxyethyl methacrylate (HEMA), 2,2 bi[4-(2-hydroxy-3-methacryloyloxy)propane (BisGMA), and triethylene-glycol dimethacrylate (TEGDMA) was formulated and their Hoy's solubility parameters calculated. The polymers were cast into small 'I' beams and light-cured. The modulus of elasticity (E) and ultimate tensile strength (UTS) were measured in dry polymers and after immersion in water for 24 h. The results showed significant correlations between E and UTS under dry or wet conditions. Both E and UTS fell significantly when the specimens were immersed in water. After water immersion, the E and UTS showed significant correlations with Hoy's delta(p) values. Both E and UTS correlated significantly with the BisGMA concentration of the polymers, either wet or dry. The percentage changes in E or UTS were significantly correlated with the water sorption of the polymers.

Net expansion of dried demineralized dentin matrix produced by monomer/alcohol saturation and solvent evaporation

The purpose of this work was to determine if nonaqueous methacrylate monomer/alcohol mixtures could expand dried collapsed demineralized dentin matrix. Thin disks (ca. 200 microm) of human dentin were demineralized and placed in wells beneath contact probes of linear variable differential transformers. The probes were placed on water-saturated expanded matrices to record the shrinkage associated with drying. Monomer mixtures containing hydroxyethyl methacrylate, 2,2-bis[4-(2-hydroxy-3 methacryloyloxy)propoxyphenyl] propane, or triethyleneglycol dimethacrylate were mixed with methanol or ethanol at alcohol/monomer mass fraction % of 90/10, 70/30, 50/50, or 30/70. They were randomly applied to the dried matrices to determine the rate and magnitude of expansion; then shrinkage was recorded during evaporation of the alcohols. The results indicated that matrix expansion was positively correlated with the Hoy's solubility parameters for hydrogen bonding forces (delta(h)) of the monomer/solvent mixtures (p < 0.001). Expansions were more rapid with methanol-containing than with ethanol-containing monomer mixtures. For the test solutions, triethyleneglycol dimethacrylate-containing mixtures produced the slowest rate of matrix expansion and hydroxyethyl methacrylate-containing mixtures the most rapid expansion. When the solvents were evaporated, the matrix shrank in proportion to the solvent content and the delta(h) of the monomer-solvent mixtures. The results indicate that expansion of dried, collapsed dentin matrices requires that the delta(h) of the mixtures be larger than 17 (J/cm(3))(1/2). The greater the delta(h) of the monomer solutions, the greater the rate and extent of expansion.

Effect of resin hydrophilicity on water-vapour permeability of dental adhesive films

This study examined the water-vapour permeability of thin polymerized resin films fabricated from five co-monomer blends of increasing degrees of hydrophilicity, as measured by their Hoy's solubility parameters. Neat resin films were prepared from five experimental light-curable resins (n = 10). Each film was mounted in a Fisher permeability cup with 8 g of water placed inside the cup. The experiments were conducted in a modified twin-outlet desiccator connected to a vacuum pump in one outlet to permit a continuous airflow to encourage water evaporation. Weight losses by water evaporation were measured at 3, 6, 9, 24, 30, and 48 h by using an analytical balance. Additional resin films were examined by using transmission electron microscopy (TEM) after immersion in ammoniacal silver nitrate. A significant correlation was observed between the cumulative water loss at 48 h and the Hoy's total cohesive energy density (delta(t)). Transmission electron microscopy revealed silver-filled channels along film peripheries and silver grains of decreasing dimensions toward the film centres in co-monomer blends 3, 4, and 5 of increasing hydrophilicity. Hydrophilic dentin adhesives polymerized in thin films are prone to water loss by evaporation. This probably accounts for the water droplets seen on the surface of vital-bonded dentin after the application of simplified dentin adhesives.

Effects of polar solvents on the fracture resistance of dentin: role of water hydration

Although healthy dentin is invariably hydrated in vivo, from a perspective of examining the mechanisms of fracture in dentin, it is interesting to consider the role of water hydration. Furthermore, it is feasible that exposure to certain polar solvents, e.g., those found in clinical adhesives, can induce dehydration. In the present study, in vitro deformation and fracture experiments, the latter involving a resistance-curve (R-curve) approach (i.e., toughness evolution with crack extension), were conducted in order to assess changes in the constitutive and fracture behavior induced by three common solvents-acetone, ethanol and methanol. In addition, nanoindentation-based experiments were performed to evaluate the deformation behavior at the level of individual collagen fibers and ultraviolet Raman spectroscopy to evaluate changes in bonding. The results indicate a reversible effect of chemical dehydration, with increased fracture resistance, strength, and stiffness associated with lower hydrogen bonding ability of the solvent. These results are analyzed both in terms of intrinsic and extrinsic toughening phenomena to further understand the micromechanisms of fracture in dentin and the specific role of water hydration.

Effect of resin hydrophilicity and water storage on resin strength

This study evaluated the change in the ultimate tensile strength (UTS) of five polymerised resin blends of increasing hydrophilicity, after ageing in distilled water or silicon oil. Resin blocks were prepared from each resin blend by dispensing the uncured resin into a flexible, embedding mould, containing multiple cavities. The resins were polymerised in the moulds under nitrogen at 551.6 kPa and light-activated at 125 degrees C for 10 min. After dry ageing for 24 h at 37 degrees C, the middle third of each resin specimen was trimmed into an 'I' shape. Fifteen control specimens were randomly selected from each resin blend for baseline UTS evaluation. The UTS of the experimental specimens were determined after 1, 3, 6 and 12 months of ageing in water or oil. The UTS of each group of resins at different storage periods in water or oil were analysed using the Friedman multiple ANOVA on ranks and Dunn's multiple comparison tests at 95% confidence level. Significant reduction (p < 0.01) in UTS was observed in Groups II-V resins after 12-month storage in water, while the most hydrophobic Group I resin showed no significant change (p > 0.05) in the same period. The percentage reduction in UTS increased with the hydrophilicity of the resin blends. Long-term water storage of hydrophilic resin blends such as those employed in dentine adhesives, resulted in a marked reduction in their mechanical strength that may compromise the durability of resin-dentine bonds.

Importance of mini-dumbbell specimen to access tensile strength of restored dentine: historical background and the future perspective in dentistry

OBJECTIVES

The development of adhesive resins to dentine enables resin restorations to be more durable. Several bond strength measurement techniques for measuring adhesion have been proposed. A standardised method is needed which produced higher and more consistent bond strength values which allow bond stability and the bonding mechanism to be studied. Our aim was to investigate mainly the adhesives and not the substrate.

DATA SOURCES

This review is based on the literature on an adhesive, 4-META/MMA-TBB resin and conditioners to modify dentine substrates. The latter is a very important topic for developing our understanding of the bonding to dentine. The objective of the review is to explain the efficacy of the mini-dumbbell specimen in measuring the tensile strength of resin to dentine and to analyze the resin to dentine interface. Both the adhesive and the substrate control the quality of hybridized dentine.

CONCLUSIONS

By creating an impermeable acid resistant barrier to both biological and chemical stimuli between the exposed dentine and the restored tooth surface we are able to protect exposed dentine from caries (infection) at the same time as protecting the pulp and preventing toothache. This barrier also helps maintain tooth vitality and for the purposes of this article is termed 'artificial enamel'. Microleakage free restorations are possible through the introduction of this 'impermeable artificial enamel' barrier.

AFM observation of collapse and expansion of phosphoric acid-demineralized dentin

The objective of this study is to provide additional data regarding morphological changes that occur to dentin matrices following demineralization with etchants. Our understanding of the mechanism of diffusion of comonomers into the demineralized substrate is very limited. It has been hypothesized that certain water-soluble polyelectrolytes (acidic proteins) and neutral proteins in dentin can influence the collapse of demineralized dentin when it is air dried. Some of these solubilized substances are thought to aggregate by the action of Ca cations, which become dissolved during H(3)PO(4) etching, ultimately resulting in some degree of collapse. In the current study, dentin surfaces were examined by atomic force microscopy (AFM) before and after treatment by 10% H(3)PO(4)containing increasing concentrations of CaHPO(4). Reversal of matrix collapse by aqueous 30% 2-hydroxyethyl methacrylate (HEMA) was evaluated by AMF for 60 min. The results demonstrate two forms of matrix collapse; we speculate that one form is induced by acidic noncollagenous polyelectrolytes and the other by neutral peptides. Our data indicate that further evaluation of the influence of endogenous noncollagenous proteins must be studied to understand the mechanism of the collapse and reexpansion dynamics of demineralized dentin networks.

Influence of dentinal polyelectrolytes on wet demineralized dentin, a bonding substrate

The objective of this study was to show the influence of dissolved dentinal polyelectrolytes on the characteristics of dentin (bonding substrate) demineralized by citric acid in the absence or presence of ferric chloride. The demineralizing agent was an aqueous mixture of 0, 1, 3, or 10% ferric chloride in 10% citric acid (10-0, 10-1, 10-3, 10-10, respectively). The hypothesis was that the concentration of dissolved dentinal noncollagenous substances, mainly polyelectrolytes soluble in water, must be decreased by their aggregation with ferric ions, which changes the characteristics of demineralized dentin, the rates of demineralization, and dehydration. Cervical bovine dentin was prepared in 3 x 2 x 2-mm blocks, each weighing 20.0 +/- 0.5 mg. The rate of demineralization was investigated by measuring the weight loss resulting from demineralization by immersion in 10 mL of conditioner at 2-h intervals. The dehydration rate of wet demineralized dentin was determined using two methods: (1) weight loss in a desiccator under 263 Pa pressure and (2) differential scanning calorimetry (DSC). Twenty, 12, 8, and 4 h were required to complete demineralization of the blocks with the 10-0, 10-1, 10-3, and 10-10 solutions, respectively. The 10-10 wet demineralized dentin showed the highest rate of dehydration, followed in descending order by the 10-3, 10-1, and 10-0 specimens. Ferric chloride in dentin conditioners provided both a higher rate of dentin demineralization and a higher dehydration rate of wet demineralized dentin. These results suggest that in the presence of ferric chloride, a decreasing amount of dissolved polyelectrolytes aggregated with ferric ions in the substrates may increase the permeability of dentin to water and citric acid. Improvement of monomer permeability is essential to the preparation of good hybridized dentin, providing a more stable and reliable bonding and also protecting the dentin and pulp from infection. A further study of bonding substrates is required in order to understand the role of hybridized dentin in improved dental treatment.

Effects of water and water-free polar solvents on the tensile properties of demineralized dentin

OBJECTIVES

The aim of this study was to test the null hypothesis that the tensile properties of demineralized dentin are not influenced by the hydrogen bonding ability of anhydrous polar solvents.

METHODS

Dentin disks 0.5mm thick were prepared from mid-coronal dentin of extracted, unerupted, human third molars. 'I' beam and hour-glass shaped specimens were prepared from the disks, the ends protected with nail varnish and the central regions completely demineralized in 0.5M EDTA for 5 days. Ultimate tensile stress (UTS) and low-strain apparent modulus of elasticity (E) were determined with the specimens immersed for 60 min in water, methanol, HEMA, acetone or air prior to testing in those same media. Apparent moduli of elasticity were measured on the same specimens in a repeated measures experimental design. The results were analyzed with a one-way ANOVA on ranks, followed by Dunn's test at alpha=0.05. Regression analysis examined the relationship between UTS or E and Hansen's solubility parameter for hydrogen bonding (delta(h)) of each solvent.

RESULTS

The UTS of demineralized dentin in water, methanol, HEMA, acetone and air was 18(7), 29(7), 31(6), 41(13) and 146(27)MPa, x(SD), n=10. Low-strain E for the same media were 11(7), 43(12), 79(21), 132(31) and 253(115)MPa. Regression analysis of UTS vs delta(h) revealed a significant (p<0.0005, r=-0.69, R(2)=0.48) inverse, exponential relationship. A similar inverse relationship was obtained between low-strain E vs delta(h) (p<0.0001, r=-0.93, R(2)=0.86).

SIGNIFICANCE

The tensile properties of demineralized dentin are dependent on the hydrogen bonding ability of polar solvents (delta(h)). Solvents with low delta(h) values may permit new interpeptide H-bonding in collagen that increases its tensile properties. Solvents with high delta(h) values prevent the development of these new interpeptide H-bonds.

Determination of residual double bonds in resin-dentin interface by Raman spectroscopy

OBJECTIVES

The quality of the hybrid layer is believed to be more important than the thickness of this layer. The purpose of this study was to investigate a method to analyze the percentage of adhesive resin residual double bonds in the dentin-resin interface using laser Raman spectroscopy.

METHODS

Bovine dentin was treated with dentin adhesives and resin composite was bonded according to the manufacturers' instructions. The specimens were sectioned parallel to dentinal tubules and the surfaces were then polished to 1 microm diamond pastes. Raman spectra were recorded along a line perpendicular to the dentin-resin interface in steps of 0.2 microm. The measurement of residual C=C bond was made on a relative basis by comparing the C=C unpolymerized methacrylate stretching vibration (1638 cm(-1)) against the C=O stretching mode of the ester group (1719 cm(-1)). The percentage of residual double bonds including pendant and monomeric double bonds was calculated by comparing the obtained ratio with that of uncured adhesive resin.

RESULTS

The amount of residual double bonds in the hybrid layer varied from 10 to 25% compared to the uncured adhesives, a relatively higher percentage was detected for Fluoro Bond (12.3-23.6%) and Single Bond (9.5-21.8%), and lower for Mac Bond II (10.6-18.0%) and Mega Bond (10.7-16.3%). No relationship was seen between the percentage of remaining double bonds and the location within the resin-dentin interface.

SIGNIFICANCE

Laser Raman microscopy used was a useful tool for measuring the residual double bonds in the dentin-resin interface.

Dimensional changes in acid-demineralized dentin matrices following the use of HEMA-water versus HEMA-alcohol primers

Although dried acid-etched dentin can be reexpanded by hydroxyethyl methacrylate (HEMA)/water primers, the primed dentin collapses when the water is evaporated. Experimental HEMA/alcohol primers should stiffen the matrix and permit less shrinkage when the solvent is evaporated. The purpose of this study was to test the hypotheses that matrix shrinkage induced by solvent evaporation from HEMA primers is inversely related to solvent-induced matrix stiffness. Dentine discs were prepared from midcoronal dentine of unerupted human third molars. After demineralization in 37% phosphoric acid, the specimens were placed in the well of a linear variable differential transformer instrument, which measures changes in the matrix height and stiffness by load displacement after the application of weights. This was done in their hydrated state after water had been applied, after drying with dry nitrogen gas, and after the application of 35 vol % HEMA-water, HEMA-methanol, HEMA-ethanol, or HEMA-propanol primers. The degree of reexpansion after the application of the primers to the dentine in the dried state was found to be highest using the HEMA-water primer, followed by HEMA-methanol and HEMA-ethanol, with the HEMA-propanol primer producing no expansion. However, when the solvents were evaporated the HEMA-water-treated specimens shrank the most, the HEMA-ethanol-treated specimens shrank an intermediate amount, and the HEMA-methanol-treated specimens shrank the least. The net result of expansion minus shrinkage produced by evaporation determines how much HEMA remains in the hybrid layer just prior to polymerization.

Influence of composition on rate of polymerization contraction of light-curing resin composites

A slow contraction may result in reduced gap formation when a restorative resin polymerizes in a dental cavity. It was the aim in the present work to investigate the rate of contraction in relation to composition of experimental light-curing resin composites. The monomer of the resin composites consisted of mixtures of BisGMA, TEGDMA, and in one series HEMA. The resins contained varying amounts of initiators, co-initiators, and inhibitor, and were made composite by adding a silanized glass filler to a content of 74% by weight of the composite paste. The polymerization contraction up to 120 sec was determined by means of the bonded-disk method. Within the ranges studied, the concentration of initiator and co-initiator in the monomer mixture had only an insignificant influence on rate of polymerization. In comparison to camphorquinone, the initiators 1-phenyl-1,2-propanedione and benzil reduced the rate of polymerization without affecting the final contraction. In comparison to N,N-dimethyl-p-aminobenzoic acid ethyl ester, N,N-cyanoethyl methylaniline was as effective, while N,N-diethanol-p-toluidine was less effective as co-initiator. A relatively high content of the inhibitor methoxyhydroquinone reduced the initial rate but not the final polymerization contraction. The rate of polymerization increased with the level of HEMA and TEGDMA in the monomer mixture. It was concluded that intrinsic slow cure may be obtained with certain compositions of resin composites without impairing the final extent of polymerization.

Wet or dry, normal or deproteinized dentin surfaces as substrate for dentin adhesives

Bond strength between resin composite and dentin mediated by several dentin adhesives applied to dry or wet acid-etched dentin or to dry or wet acid-etched and deproteinized dentin were measured and analyzed. Human dentin were A) acid-etched and blot-dried for 1 sec (= wet), B) as A but dried with air for 10 sec (= dry), C) acid-etched and treated with hypochlorite and then dried for 1 sec, and D) as C but dried with air for 10 sec. Eight dentin adhesives were used in each group for bond strength measurements. The results were compared by means of ANOVA and the Tukey test. Collagen removal increased significantly the strength of the bond by 10-18 MPa for five adhesives when tested dry (D versus A). When tested wet, collagen removal increased the strength by 10-12 MPa for three adhesives (C versus A). Normal etched dentin showed a reduction in strength of 14-15 MPa for three or the adhesives when tested dry instead of wet (B versus A). For one dentin adhesive no significant change in bond strength due to collagen removal and/or drying conditions was observed. It was hypothesized that low technique sensitivity of an adhesive may be linked to its ability to wet and adhere to collapsed collagen fibers and to the surface of the underlying mineralized tissue. Comparisons of bond strengths obtained by using dried or wet acid-etched dentin and dried or wet acid-etched and deproteinized dentin may be useful for evaluating the efficiencies or dentin adhesives.

Solvent-induced dimensional changes in EDTA-demineralized dentin matrix

The purpose of this study was to test the null hypothesis that the re-expansion of dried matrix and the shrinkage of moist, demineralized dentin is not influenced by polar solvents. Dentin disks were prepared from midcoronal dentin of extracted human third molars. After complete demineralization in 0.5M of EDTA (pH 7), the specimens were placed in the well of a device that measures changes in matrix height in real time. Dry, collapsed matrices were created by blowing dry N(2) on the specimens until they shrank to a stable plateau. Polar solvents [water, methanol, ethanol, n-propanol, n-butanol, formamide, ethylene glycol, hydroxyethyl methacrylate (HEMA), or mixtures of water-HEMA] as model primers then were added and the degree of re-expansion measured. These same solvents also were applied to moist, expanded matrices and the solvent-induced shrinkages measured. Regression analysis was used to test the correlations between matrix height and Hansen's dispersive, polar, hydrogen bonding, and total solubility parameters (delta(d), delta(p), delta(h), delta(t)). The results indicate that water-free polar solvents of low hydrogen bonding (H-bond) ability (e.g., neat HEMA) do not re-expand dried matrices and that they shrink moist matrices. When HEMA was mixed with progressively higher water concentrations, the model water-HEMA primers expanded the dried matrix in proportion to their water concentrations and they produced less shrinkage of moist matrices. Solvents with higher H-bonding capacities (methanol, ethanol, ethylene glycol, formamide, and water) re-expanded the dried matrix in proportion to their solubility parameters for H-bonding (delta(h)). They also induced small transient shrinkages of moist matrices, which slowly re-expanded. The results require rejection of the null hypothesis.

Effect of dentin primers on the morphology, molecular composition and collagen conformation of acid-demineralized dentin in situ

OBJECTIVES

The purpose of this study was to assess the effect of two commercial primers and distilled water on the morphology, molecular composition and collagen conformation of acid-demineralized dentin in situ.

METHODS

Dentin specimens etched with Scotchbond Etchant were imaged by tapping mode AFM and analyzed by MIR-FTIR spectroscopy. They were then primed with Scotchbond Multi-Purpose Plus Primer, Scotchbond 1 Adhesive or distilled water and imaged and analyzed again. The chemical modifications induced on the uppermost 2 microns of primed dentin were studied after water and original primer subtraction. The conformational changes of type I collagen at this region were evaluated by deconvoluting the amide I band components. The absorbance ratio An(1655/1627) was used to semiquantitatively assess, on a relative basis, the extent of collagen denaturation.

RESULTS

All the priming treatments swelled the collapsed dentin collagen left after etching. No evidence of primary bonding was found after priming treatments, while approximately 50% of the conditioned dentin collagen appeared denatured. Treatment with distilled water did not change the status of denatured collagen, however, application of the commercial primers refolded the alpha-helix to approximately 95% of the extent found on the native reference dentin.

SIGNIFICANCE

The dynamic response of dentin collagen to demineralization and priming treatments reveals the critical role of some primers in rapidly restoring the conformational status of acid-denatured collagen. Implementation of reactive adhesive groups in alpha-helix recovery may provide an associative means of modifying the mechanical properties of the demineralized collagen based on the extent of their intermolecular bonding.

Design, formulation, and evaluation of isocyanatoacrylate copolymer dental adhesives

Experiments have recently been completed to explore the development of isocyanatoacrylate copolymers as new dental adhesives. A main goal of this work was to test the utility of solubility parameter differences between the candidate adhesives and etched dentin as a predictor of relative bond strength. All candidate adhesive mixtures contained 2-isocyanatoethyl methacrylate (IEM), a selected amount of tri-n-butylborane oxide (TBBO) initiator, and one of 13 methacrylate comonomers. Reactivity ratios were computed for comonomer pairs as indicators of relative reactivity. The concentration of TBBO was optimized for each comonomer mixture to obtain working times of 2-6 min and setting times of 6-10 min. The solubility parameter difference Deltadelta (J/cm(3))(1/2) was calculated for each test mixture with respect to an etched dentin substrate, as an approximation of wetting ability. Using standard techniques for shear bond strength evaluation, mean shear bond strength values ranging between 7-15.5 MPa were obtained for comonomer adhesives in bonding Z-100 composite to treated dentin. Shear bond strength values showed a good correlation (r = -0.612, P </= 0.05) with solubility parameter differences. This study illustrates the usefulness of reactivity ratios and solubility parameters in the design and development of effective dentin bonding agents.

Application of solubility parameter theory to dentin-bonding systems and adhesive strength correlations

The principal aim of this study was to investigate the relationships between the solubility parameters of ectched dentin, and adhesive primer solutions and adhesive bond strength. Solubility parameters characterize the molecular interactions which determine physical properties such as wetting, and thus can serve as tools to aid development of polymeric adhesives and interpenetrating polymer networks. If an adhesive monomer has a solubility parameter close to that of a polymer substrate, then the monomer may act as a solvent for the polymer and penetrate below the surface. Subsequent polymerization of the monomer may then produce an interpenetrating network, thus adhering without necessarily forming primary chemical bonds to the substrate. The dentin substrate considered in this study was abraded dentin treated with ethylenediaminetetraaceitc acid. Solubility parameters delta pr, delta h, and delta d calculated for the etched dentin substrate were 20.3, 23.6, and 16.0 (J/cm3)1/2, respectively. Solubility parameters of the primers were expressed using Hansen's three-dimensional scheme. The data indicate a correlation between the calculated solubility parameters of the etched dentin, and dentin primers and the resulting bond strengths. The results corroborate the significance of solubility parameter considerations for adhesive bonding to dentin.

Can chemical softening agents minimize cavity enlargement during removal of failed anterior resin composite restorations?

This in-vitro investigation sought to identify the most effective softening agents for three commercial anterior resin composites (Prisma APH, Brilliant and Blend-a-Lux) from four potential agents (75% ethanol, polyacrylic acid, acetone and acetic acid), selected as having close solubility parameter values to BIS-GMA. The Vickers microhardness test was applied to samples before and following a 180 s application of each agent. In addition, the degree of cavity enlargement following removal of resin composite restorations, aged by thermocycling, was assessed by image analysis. Using the same method the influence of both colour match and the 180 s application of softening agents, in the optimum chemical/material combinations, upon this was determined. Analyses of variance revealed that significant (P < 0-01) reductions in surface microhardness occurred when 75% ethanol was applied to Prisma-APH and acetic acid to Blend-a-Lux. Removal of restorations of all materials resulted in highly significant (P < 0.001) increases in cavity size. Application of either 75% ethanol to Prisma-APH restorations or acetic acid to Blend-a-Lux did not alter this result. Thermocycling only produced a significant (P < 0.01) (McNemar Test) deterioration in shade match of the Prisma APH restorations but analyses of variance, for all materials, revealed that the observed changes in colour match had no significant (P < 0.05) effect upon the degree of cavity size enlargement. It is concluded that the application of softening agents to reduce cavity enlargement is not a viable clinical option.

Surface energy characteristics of adhesive monomers

OBJECTIVES

The aim of the investigation was to determine the surface free-energy components of potentially adhesive monomer mixtures.

METHODS

Four liquids with known components of surface free-energy were used as reference. Small drops of the liquids were placed on the polished surfaces of four types of solid (metal, porcelain, resin composite and hydrocarbon), and the contact angles were measured. By means of the fundamental equations for wetting, the three components of the surface free-energy of the four solids were calculated. Small drops of various monomeric mixtures were then placed on the four solid surfaces, and on the basis of the previously calculated components of surface free-energy of the solids, the surface energy characteristics of the monomeric mixtures were determined. The relationships between contact angles and composition, and between surface tension and composition, were studied by regression analyses. Comparisons between values were carried out by means of Neuman-Keuls' multiple range test at a level of statistical significance of p = 0.05.

RESULTS

Statistically significant differences between the monomeric mixtures as regards the wetting of the four solids were observed. These differences reflected differences in the acid or base component of the surface free-energy of the monomers. In particular, monomeric mixtures containing HEMA, MAN or 4-META exhibited a significant acid component of the surface free-energy.

SIGNIFICANCE

Knowledge of the surface free-energy components of monomers throws light on the mechanisms associated with the adhesion of resin composites, including resin cements. A better understanding of the interfacial interactions may act as guide in a research aimed at developing resin materials of increased adhesion to metal, porcelain or resin composite.

The overwet phenomenon in two-component acetone-based primers containing aryl amine and carboxylic acid monomers

OBJECTIVES

The overwet phenomenon was first reported when a moist bonding technique was used with an earlier commercial version of All-Bond 2 (Bisco) that contained BPDM in primer B. This study investigated whether ultrastructural features of the overwet phenomenon could also be detected in other commercially available two-component acetone-based primers containing BPDM, PMDM and PMGDM, as well as an experimental two-component primer containing DSDM.

METHODS

Thirty 1 mm dentin discs prepared from third molars were each conditioned with 10% H3PO4 for 20 s and rinsed for 20 s. They were randomly divided into 5 groups: Group I (Bond-It, Jeneric/Pentron:PMGDM); Group II (Wet Bond, Chameleon Dental Products:PMGDM); Group III (Tenure S, Den-Mat:PMDM); Group IV (present commercial version of All-Bond 2, Bisco:BPDM) and Group V (Experimental two-component primer system containing DSDM in primer B). Following a moist bonding technique using the respective system, discs from each group were further bonded together to form three disc pairs using a chemical-cured resin. Bonded disc pairs were demineralized in EDTA and processed for TEM examination. For this ultramicroscopical study, results such as the features of the overwet phenomenon were analyzed by visual inspection of the specimens in each group (n = 12).

RESULTS

Isolated blister-like spaces of variable dimensions were observed within the primer layer in all groups and possessed the following characteristics: 1) a layer of resin-impregnated dentin was always present along the base of the primary blister; 2) surface primer globules, sometimes containing secondary blisters, were identified within these primary blisters; 3) dentinal tubules within the blister-like spaces were not completely sealed; 4) primer globules were circumscribed by a halo of fine kinked strands of material.

SIGNIFICANCE

Although the technique of moist bonding is based on valid biological principles, incorporation of resin monomers that are immiscible with water rendered the application of current two-component, acetone-based primers very technique-sensitive in terms of tubular seal, when used on moist, acid-conditioned dentin. Further studies should be directed at elimination of this type of oil-in-water (O/W) "macroemulsion" formation through optimal micellar solubilization of these resin monomers in water.

Current concepts on adhesion to dentin

This review examines fundamental concepts in bonding to dentin. Emphasis is placed on the structure and permeability characteristics of dentin and how they may influence its interaction with adhesive resin. Several new techniques to examine the interfaces between resin and dentin are reviewed along with some of their limitations. The advantages and disadvantages of acid etchants/conditioners vs. self-etching conditioners/primers are discussed. The problems of matching the surface tension of resin-bonding systems to the surface energy of the substrate are reviewed in terms of wetting the various components of dentin. The problems associated with matching the permeability of intertubular dentin to the diffusibility of bonding reagents are explored. Speculation is advanced on how to ensure polymerization and wetting of dentinal collagen. Theoretical problems associated with dentin bonding and with bond testing are reviewed to encourage future research in this rapidly developing area.

Short-term contamination of luting cements by water and saliva

OBJECTIVES

The aim of this study was to determine the effect of water, artificial saliva and natural saliva on the hardening process of three dental luting cements.

METHODS

Cement samples, 1 mm thick and 5 mm in diameter, were subjected to various storage conditions in an oven maintained at 37 degrees C. Some samples were aged in 100% humidity or water for up to 1 wk. The other samples were covered with water, artificial saliva or natural saliva at various times after mixing. The Knoop hardness values of the cement surfaces were measured. Differences between groups were evaluated with an ANOVA followed by a Tukey multiple comparison at the 5% level of significance. The hardness ratio of the contaminated samples was calculated using the 30 min mean KHN of the samples aged in 100% humidity as the divisor.

RESULTS

The glass ionomer samples were significantly harder (48.3 +/- 3.8) than the zinc phosphate (38.9 +/- 7.5) or composite cements (35.4 +/- 10.2) after 1 wk in 100% humidity storage condition. When immersed in water, the hardness of both the glass ionomer and the zinc phosphate decreased to almost half that of the specimens stored in 100% humidity (26.2 +/- 2.7 and 16.9 +/- 2.5, respectively). Contamination decreased the hardness of zinc phosphate and glass ionomer (hardness ratio, water contamination at 5 min: 0.39 +/- 0.10 and 0.52 +/- 0.12, respectively) but had very little effect on the composite. Overall, water had a greater softening effect than artificial or natural saliva on the cements.

SIGNIFICANCE

In light of these results, glass ionomer cement should be protected from water and saliva for the first 15 min after mixing.

Factors affecting the adherence energy of experimental resin cements bonded to a nickel-chromium alloy

Reliable adherence of resin-based cements is of prime importance for the longevity of cemented restorations. The present study investigated whether a relationship exists between adherence energy to a metal substrate and the degree of cross-linking and wetting characteristics of resin-based luting agents. The adherence energies between a sand-blasted metal surface and a series of experimental resin cements were measured by means of the wedge test. The degree of cross-linking was calculated from the monomer composition of the resin cements. The measured wetting characteristics were work of adhesion and surface tension, and their dispersive and polar components. Adherence energy varied between 22 and 81 J/m2 and was influenced by the nature of the resin cements: Those with a low degree of cross-linking resulted in high adherence values. Furthermore, resin cements whose monomers were relatively polar gave rise to high adherence values. Although other metals may not behave in exactly the same way, these results may help in the formulation of new, more retentive resin cements.

New adhesives in Class V restorations under combined load and simulated dentinal fluid

OBJECTIVES

This study compared the efficacy of three dentinal adhesives using the "all etch" technique (All-Bond 2, Bisco; Scotchbond MP,3M Dental Products Co.; OptiBond, Kerr) with a dentinal adhesive which still uses phosphoric acid to condition enamel and a self-etching primer for dentin (A.R.T.-Bond, Coltene/Whaledent).

METHODS

Eight V-shaped mixed Class V restorations were placed per group in extracted human premolars. The restorations were subjected to 1,200,000 mechanical occlusal cycles (max. force 49 N; frequency 1.7 Hz) and 3,000 simultaneous thermal cycles (5-50-5 degrees C). Dentinal fluid was simulated using 1:3 diluted horse serum and fed into the pulp chamber both during restoration and loading. Percentages of "continuous margin" were assessed on SEM replicas of enamel and dentinal margins at 200x magnification immediately before and after stressing, respectively.

RESULTS

No significant differences were observed before stress between the materials either in enamel or in dentin. After stress, however, OptiBond and A.R.T.-Bond performed significantly better in dentin than the two other adhesives (Kruskal-Wallis, Mann-Whitney; p < 0.05). Although high initial values were observed, All-Bond 2 and Scotchbond MP were not stress-resistant under simulated physiological conditions.

SIGNIFICANCE

The predicted clinical potential of All-Bond 2 and Scotchbond MP is inferior to that of OptiBond and A.R.T.-Bond.

Clinical relevance of laboratory studies on dental materials: strength determination--a personal view

The measurement of strength of materials is undertaken in a laboratory primarily to determine its relationship to the microstructural features of the material, as the understanding it brings will allow the production of stronger materials. Problems arise when attempts are made to derive some sort of clinical significance from these data without due regard to the functional aspects of the device from which the material is made. Three examples are presented which seek to highlight that: (1) strength, however desirable, is not necessarily the most important consideration; (2) great care has to be exercised in interpreting strength data; and (3) the most appropriate properties are considered for each individual dental application of the material.