Karl-Johan M. Söderholm

Some effects of water on dentin bonding

OBJECTIVES The goals of this study were to determine how: 1) the bond strengths of hydroxyethylmethacrylate (HEMA)-based dentin primers are affected by different solvents (water or acetone), 2) the application time of these primers affect the bond strength, and 3) the conversion of water contaminated bonding resins. METHODS HEMA (35%), mixed with water or acetone, was placed on moist dentin for 30 or 120 s, dried, covered with a bonding resin, and light-cured. Composite cylinders were bonded to these surfaces, and the shear bond strength was determined after 30 d of water storage at 37 degrees C. The conversion of bonding resins containing 0.05, 0.1, 0.2, 0.4 and 0.8 mL water per mL of bonding resin was determined with FTIR. RESULTS The two acetone groups gave significantly (p<0.5) higher bond strength values (sigma 30s=22.2 +/- 2.1 MPa and sigma 120s=21.5 +/- 3.2 MPa) than the two water groups (sigma 30s=7.0 +/- 3.3 MPa and sigma 120s=16.2 +/- 4.8 MPa). In contrast to the acetone-based primer, the water-based primer improved its bond strength with increased priming time without reaching that of the two acetone groups. The conversion of the bonding resin was 53.5%, which decreased to approximately 25% when 0.2 mL or more water was added per mL resin. SIGNIFICANCE Compared to acetone, water is inferior as a solvent for HEMA-based dentin primers and gives both lower bond strength and requires longer priming time than acetone. A possible explanation of these results is the ability of water to interfere with the polymerization of the resin systems.

Effects of Treatment and Storage Conditions on Ceramic/Composite Bond Strength

During the past few years, the interest in using ceramic inlays and veneers has increased. New materials and methods have been introduced to bond these restorations to resinous materials. Since our knowledge of how to optimize such bonding is limited, the objective of this study was to test the hypothesis that various surface treatment variables and combinations of these variables affect the strength of the ceramic/composite interphase of ceramic inlays differently. The influences of material composition, surface-roughening method, silane treatment, silane heat treatment, and storage condition on bond strength were investigated. Three ceramics (Dicor®, Mirage®, Vitabloc®), three surface-roughening methods (etching, sandblasting, grinding), three silane treatments (γ-methacryloxypropyltrimethoxysilane [MPS], MPS + paratoluidine, vinyltrichlorosilane), two heat treatments (20°C for 60 s, 100°C for 60 s), and two storage conditions (24-hour dry, one yr in water at 37°C) were studied. For each of the 108 combinations, five specimens were tested. Ceramic cylinders were treated according to group assignment and bonded to blocks of the same ceramic material with a dual-cured resin. The shear bond strength was determined, and the experimental factors were evaluated by analysis of variance. The results showed that surface-roughening method had the strongest effect on bond strength, while ceramic selection had the least significant effect. Of the surface-roughening methods, etching was associated with higher bond strength values than either sandblasting or grinding. Bond strength to etched ceramics remained constant after water storage, while the bond strength to both sandblasted and ground specimens decreased by from 50 to 75% compared with dry storage. Heating the MPS-coated specimens to 100°C resulted in bond strength twice as high than if no heating was used. Mechanical interlocking appears to be the key factor influencing the ceramic-composite bond strength.

Molecular Orientation of Silane at the Surface of Colloidal Silica

The objective of this study was to investigate the silica-silane bond formation present at the filler interface of dental composites. Diffuse reflectance infrared Fourier transform spectroscopy was used, and the spectra of pyrogenic silica (Cab-O-Sil) treated with different concentrations of γ-methacryloxypropyltrimethoxy-silane (MPS) were analyzed. The outcome of the study suggested that the γ-methacryloxypropyltrimethoxysilane (MPS) molecules oriented parallel to the colloidal silica surface (Cab-O-Sil) and formed two types of bonds. One of these bonds was a siloxane bridge formed by a condensation reaction between the silanol groups of both the silica surface and the hydrolyzed silane. Water formed during this reaction and soon became recaptured by the silanol groups of the silica surface. These water molecules were not available for additional hydrolyzation reactions ofthe unhydrolyzed silane under the experimental conditions. The intensity of the isolated OH-groups decreased because of this reaction. Simultaneous with the condensation reaction, the carbonyl group of the MPS molecule formed hydrogen bonds. This hydrogen bond formation resulted in a peak shift of the carbonyl band from 1718-1720 cm-1 to 1700-1702 cm-1. This hydrogen bond formation also occurred with the isolated OH-groups. After consumption of the isolated OH-groups, no additional surface reaction occurred because no further OH-groups were available for additional condensation reactions or hydrogen bond formation. The findings suggest that the amount of silane needed for filler treatment depends on the number of isolated OH-groups available on the filler surface.

Correlation of in vivo and in vitro performance of adhesive restorative materials: A report of the ASC MD156 task group on test methods for the adhesion of restorative materials☆

Before a standard designed to test the adhesion of adhesive restorative materials can be accepted, it is necessary to identify those parameters which are of clinical relevance and which can be studied under in vitro testing conditions. The goal of this report has been to identify information related to how dentinal fluid, intra-tubular pressure, polymerization shrinkage, cavity preparation, cavity shape, placement technique, curing method, tooth age, environmental conditions, masticatory forces, thermal cycling, and microleakage influence the bond strength++ of adhesive restorative materials. This review is also intended to function both as an information base of past experimental data as well as a source of guidance for further, needed research.

Leaking of Fillers in Dental Composites

The leakage of filler elements from four composites after storage in water was investigated. The results showed that all fillers leaked Si, and that the micro-filled composite and the Ba- and Sr-containing glass composites leaked more Si than did the quartz-containing material. The leakage from the different fillers was explained by hydroxy-ion and stress-corrosion attacks.The leakage of filler elements from four composites after storage in water was investigated. The results showed that all fillers leaked Si, and that the micro-filled composite and the Ba- and Sr-containing glass composites leaked more Si than did the quartz-containing material. The leakage from the different fillers was explained by hydroxy-ion and stress-corrosion attacks.

Influence of Water Exposure on the Tensile Strength of Composites

The objective of this study was to investigate whether water storage causes permanent damage to composites by determining how the tensile strength of nine different composite materials changes with both water storage and water storage followed by dehydration. Eighteen samples (ASTM-D Specification 1708-66) of each of the nine materials were prepared and divided into three groups of six samples each. Group I was stored dry at 60°C, while Groups II and III were stored in distilled water at 60°C. After six months, Groups I and II were subjected to tensile testing, while Group III was transferred to a desiccator and dehydrated for two weeks at 60°C before this group was tested in tension. Mean values, pooled by storage group independent of material, revealed a significant (p<0.05) reduction in strength for both Groups II and III relative to Group I. These findings prove that water has an irreversible effect on most dental composites. A comparison of Group II with Group III data revealed that the samples which were aged in water and tested (Group II) were significantly (p<0.05) weaker than the dehydrated samples (Group III). However, some products within Group III did not show any tendency to recover their strength after dehydration.

Influence of Filler Type and Water Exposure on Flexural Strength of Experimental Composite Resins

The objective of this investigation was to determine the influence of water exposure on the flexural strength of three experimental composite materials. The composites consisted of an experimental resin system which contained silane-treated filler particles of quartz, barium glass, and porous silica. The amorphous silica particles were spheres approximately 5 μm in diameter. In addition to the different composites, pure resin samples were investigated as a control group. The results of this investigation did not support the hypothesis that use of porous amorphous silica filler particles reduced the hydrolytic degradation effect of composites containing such silica particles. Instead, the results indicated that water may have a more detrimental effect on the strength of the matrix than on the filler-matrix interface. However, the relatively high frequency of fracture lines in the porous silica particles after storage in water indicate that water had a weakening effect on this type of filler.

Effect of light power density variations on bulk curing properties of dental composites

OBJECTIVE The hypothesis that low light intensity and long but sufficient curing time can produce composites with volumetric shrinkage, degree of conversion (DC%) and Youngs modulus (E-modulus) comparable to those of high light intensity cured composite was tested, when the contraction strain and heat generation were lower with low light intensity curing. METHODS Dental composites (Z100 and Z250, 3M ESPE) were investigated. Specimens were cured with light intensities of 200, 450 and 800 mW/cm(2) for 140, 60 and 35 s from a distance of 7 mm. Strain-gages were used for contraction strain measurements. DC% was measured at the top and the bottom of 4 mm thick samples using FT-Raman spectroscopy. Volumetric polymerization shrinkage was determined using a water displacement method. E-modulus was determined in tension on composite specimens. RESULTS The results were analyzed using ANOVA and Duncans multiple range tests and regular t-test. Polymerization stress level decreased significantly (p<0.05) when cured with 200 mW/cm(2) rather than with 800 mW/cm(2). Temperature rises were significantly different (p<0.05) for different composites and light intensity values. Reduction in light intensity did not decrease the DC% values significantly at the top surfaces. The most dramatic differences existed between top and bottom surfaces (p<0.05) rather than among curing groups. Measured E-modulus and volumetric shrinkage values were not significantly different (p>0.05) between different light intensity groups. CONCLUSION DC%, E-modulus and the volumetric shrinkage values in cured composites were not affected by low light intensity, however, the contraction strain and polymerizations exotherm were decreased. Thus our results support the proposed hypothesis.

Influence of Silane Treatment and Filler Fraction on Thermal Expansion of Composite Resins

The coefficient of thermal expansion of experimental composite materials containing either silane-treated or untreated fillers in a triethylene glycol dimethacrylate (TEGDMA) matrix was investigated. The results show that an inverse linear relationship existed between volume fraction filler and coefficient of thermal expansion. No differences were seen between silane-treated and untreated composites, while it was found that repeated heating (aging) caused the thermal expansion to decrease for all material combinations. Reduction in the coefficient of thermal expansion with increased filler fraction of unbonded filler indicates that the polymerization shrinkage of the matrix induces hoop stresses around the fillers. By use of a simplified theoretical model (Appendix), these stresses could be estimated. These estimates revealed that the induced stresses were remarkably high, and that increased filler fraction increased the tensile stress level surrounding the filler particles. Since these tensile stresses could facilitate crazing and crack growth in the matrix, these estimates may explain why filled resins containing low fractions of microfilled particles seem to possess remarkably good clinical wear resistance when compared with composites containing higher filler concentrations, at least during the first years in service.The coefficient of thermal expansion of experimental composite materials containing either silane-treated or untreated fillers in a triethylene glycol dimethacrylate (TEGDMA) matrix was investigated. The results show that an inverse linear relationship existed between volume fraction filler and coefficient of thermal expansion. No differences were seen between silane-treated and untreated composites, while it was found that repeated heating (aging) caused the thermal expansion to decrease for all material combinations. Reduction in the coefficient of thermal expansion with increased filler fraction of unbonded filler indicates that the polymerization shrinkage of the matrix induces hoop stresses around the fillers. By use of a simplified theoretical model (Appendix), these stresses could be estimated. These estimates revealed that the induced stresses were remarkably high, and that increased filler fraction increased the tensile stress level surrounding the filler particles. Since these tensile stresses could facilitate crazing and crack growth in the matrix, these estimates may explain why filled resins containing low fractions of microfilled particles seem to possess remarkably good clinical wear resistance when compared with composites containing higher filler concentrations, at least during the first years in service.

Effects of filler size, water, and alcohol on hardness and laboratory wear of dental composites

Three UEDMA/TEGDMA (50:50 by weight) based dental composites were made, each with filler loadings of 53 vol.%. The three composites contained silane‐treated filler particles with average particle diameters of 1.5, 3.0, or 10.0 μm. Twelve specimens per composite were mounted on wear wheels and run through 200,000 cycles in an ACTA wear machine. Six of these specimens per material were worn in slurries consisting of 30 g ground Millet seed shells and 120 g ground rice mixed with 275 mL water. The remaining six specimens were worn in similar 25% ethanol‐water slurries. The composite wear profiles were recorded with a profilometer and used to calculate the wear. Hardness values of the composites were also measured both before and after storage for 2 weeks in either water or in a 25% ethanol–water solution. The wear and hardness values from the measurements were analyzed using ANOVA. The wear analysis showed that the finer composites (1.5 μm filler diameter) wore the least and the coarsest composites (10 μm filler diameter) the most. The wear was significantly higher in the ethanol–water slurry than in the water slurry. The hardness value of the coarsest composite decreased more than the finest composite during storage in water or 25% ethanol–water. The hardness decrease was most pronounced in the alcohol solution.