Bond strength and marginal adaptation of resin composites and correlations with clinical results.

OBJECTIVES: Due to innumerable confounding factors and a high number of types and brands of dental restorative materials, the clinical performance of restorative materials are sought predicted by various in vitro tests. However, only few such tests have been found to correlate well with clinical findings. Thus, the present study determined the in vitro dentin bond strength and marginal adaptation of Class II restorations and correlated the results to their clinical outcomes. METHODS: Dentin bond strength (µTBS and µSBS) and marginal gap formation of Class II restorations (replica technique and SEM) were measured after 24 h and 6 m water storage using eight combinations of adhesive and resin composite. Clinical outcomes (mean survival time, Hazard Ratio, annual failure rate; n = 10.695) were gained from a data set of a retrospective multicenter study of direct restorations. RESULTS: Significant differences were found for dentin bond strength and marginal gap formation between the restorative material groups, and negative effects of long-term storage were observed. µTBS correlated significantly with certain clinical outcomes of Class I restorations, while µSBS correlated with certain clinical outcomes of Class II, III, IV and V restorations. Marginal gap formation in enamel and number of paramarginal fractures correlated with certain clinical outcomes of Class II restorations. SIGNIFICANCE: Using the same restorative materials in vitro as in vivo, gave significant, but weak correlations between in vitro bond strength or marginal adaptation and clinical outcomes, lending support to the use of in vitro tests in early stages of material selection.

Bonding of restorative materials to dentin with various luting agents.

The aim was to compare eight types of luting agents when used to bond six indirect, laboratory restorative materials to dentin. Cylinders of the six restorative materials (Esteticor Avenir [gold alloy], Tritan [titanium], NobelRondo [feldspathic porcelain], Finesse All-Ceramic [leucite-glass ceramic], Lava [zirconia], and Sinfony [resin composite]) were ground and air-abraded. Cylinders of feldspathic porcelain and glass ceramic were additionally etched with hydrofluoric acid and were silane-treated. The cylinders were luted to ground human dentin with eight luting agents (DeTrey Zinc [zinc phosphate cement], Fuji I [conventional glass ionomer cement], Fuji Plus [resin-modified glass ionomer cement], Variolink II [conventional etch-and-rinse resin cement], Panavia F2.0 and Multilink [self-etch resin cements], and RelyX Unicem Aplicap and Maxcem [self-adhesive resin cements]). After water storage at 37°C for one week, the shear bond strength of the specimens (n=8/group) was measured, and the fracture mode was stereomicroscopically examined. Bond strength data were analyzed with two-factorial analysis of variance (ANOVA) followed by Newman-Keuls' Multiple Range Test (α=0.05). Both the restorative material and the luting agent had a significant effect on bond strength, and significant interaction was noted between the two variables. Zinc phosphate cement and glass ionomer cements produced the lowest bond strengths, whereas the highest bond strengths were found with the two self-etch and one of the self-adhesive resin cements. Generally, the fracture mode varied markedly with the restorative material. The luting agents had a bigger influence on bond strength between restorative materials and dentin than was seen with the restorative material.

Marginal Gap Formation in Approximal "Bulk Fill" Resin Composite Restorations After Artificial Ageing.

The aim of this in vitro study was to investigate the marginal gap formation of a packable "regular" resin composite (Filtek Supreme XTE [3M ESPE]) and two flowable "bulk fill" resin composites (Filtek Bulk Fill [3M ESPE] and SDR [DENTSPLY DeTrey]) along the approximal margins of Class II restorations. In each of 39 extracted human molars (n=13 per resin composite), mesial and distal Class II cavities were prepared, placing the gingival margins below the cemento-enamel junction. The cavities were restored with the adhesive system OptiBond FL (Kerr) and one of the three resin composites. After restoration, each molar was cut in half in the oro-vestibular direction between the two restorations, resulting in two specimens per molar. Polyvinylsiloxane impressions were taken and "baseline" replicas were produced. The specimens were then divided into two groups: At the beginning of each month over the course of six months' tap water storage (37°C), one specimen per molar was subjected to mechanical toothbrushing, whereas the other was subjected to thermocycling. After artificial ageing, "final" replicas were produced. Baseline and final replicas were examined under the scanning electron microscope (SEM), and the SEM micrographs were used to determine the percentage of marginal gap formation in enamel or dentin. Paramarginal gaps were registered. The percentages of marginal gap formation were statistically analyzed with a nonparametric analysis of variance followed by Wilcoxon-Mann-Whitney tests and Wilcoxon signed rank tests, and all p-values were corrected with the Bonferroni-Holm adjustment for multiple testing (significance level: α=0.05). Paramarginal gaps were analyzed descriptively. In enamel, significantly lower marginal gap formation was found for Filtek Supreme XTE compared to Filtek Bulk Fill ( p=0.0052) and SDR ( p=0.0289), with no significant difference between Filtek Bulk Fill and SDR ( p=0.4072). In dentin, significantly lower marginal gap formation was found for SDR compared to Filtek Supreme XTE ( p<0.0001) and Filtek Bulk Fill ( p=0.0015), with no significant difference between Filtek Supreme XTE and Filtek Bulk Fill ( p=0.4919). Marginal gap formation in dentin was significantly lower than in enamel ( p<0.0001). The percentage of restorations with paramarginal gaps varied between 0% and 85%, and for all three resin composites the percentages were markedly higher after artificial ageing. The results from this study suggest that in terms of marginal gap formation in enamel, packable resin composites may be superior to flowable "bulk fill" resin composites, while in dentin some flowable "bulk fill" resin composites may be superior to packable ones.

Influence of curing rate of resin composite on the bond strength to dentin.

This study determined whether the strength with which resin composite bonds to dentin is influenced by variations in the curing rate of resin composites. Resin composites were bonded to the dentin of extracted human molars. Adhesive (AdheSE, Ivoclar Vivadent) was applied and cured (10 seconds @ 1000 mW/cm2) for all groups. A split Teflon mold was clamped to the treated dentin surface and filled with resin composite. The rate of cure was varied, using one of four LED-curing units of different power densities. The rate of cure was also varied using the continuous or pulse-delay mode. In continuous curing mode, in order to give an energy density totaling 16 J/cm2, the power densities (1000, 720, 550, 200 mW/cm2) emitted by the various curing units were compensated for by the light curing period (16, 22, 29 or 80 seconds). In the pulse-delay curing mode, two seconds of light curing at one of the four power densities was followed by a one-minute interval, after which light cure was completed (14, 29, 27 or 78 seconds), likewise, giving a total energy density of 16 J/cm2. The specimens produced for each of the eight curing protocols and two resin composites (Tetric EvoCeram, Ivoclar Vivadent; Filtek Supreme XT, 3M ESPE) were stored in water at 37 degrees C for seven days. The specimens were then either immediately subjected to shear bond strength testing or subjected to artificial aging (6,000 cycles between 5 degrees C and 55 degrees C baths) prior to testing. Failure modes were also assessed. The shear bond strengths were submitted to factorial analysis of variance, and the failure modes were submitted to a Chi-square test (alpha = 0.05). All but power density (curing mode, resin composite material and mode of aging) significantly affected shear bond strength. The curing mode and resin composite material also influenced the failure mode. At the selected constant energy density, pulse-delay curing reduced bonding of the resin composite to dentin.

Resin composite properties and energy density of light cure.

According to the "total energy concept", properties of light-cured resin composites are determined only by energy density because of reciprocity between power density and exposure duration. The kinetics of polymerization is complex, and it was hypothesized that degree of cure, flexural strength, and flexural modulus were influenced not only by energy density, but also by power density per se. A conventional resin composite was cured at 3 energy densities (4, 8, and 16 J/cm(2)) by 6 combinations of power density (50, 100, 200, 400, 800, and 1000 mW/cm(2)) and exposure durations. Degree of cure, flexural strength, and flexural modulus increased with increasing energy density. For each energy density, degree of cure decreased with increasing power density. Flexural strength and modulus showed a maximum at intermediate power density. Within clinically relevant power densities, not only energy density but also power density per se had significant influence on resin composite properties.

Retention of Root Canal Posts: Effect of Cement Film Thickness, Luting Cement, and Post Pretreatment.

The aim of this study was to investigate the effect of the cement film thickness of a zinc phosphate or a resin cement on retention of untreated and pretreated root canal posts. Prefabricated zirconia posts (CosmoPost: 1.4 mm) and two types of luting cements (a zinc phosphate cement [DeTrey Zinc] and a self-etch adhesive resin cement [Panavia F2.0]) were used. After removal of the crowns of 360 extracted premolars, canines, or incisors, the root canals were prepared with a parallel-sided drill system to three different final diameters. Half the posts did not receive any pretreatment. The other half received tribochemical silicate coating according to the manufacturer's instructions. Posts were then luted in the prepared root canals (n=30 per group). Following water storage at 37°C for seven days, retention of the posts was determined by the pull-out method. Irrespective of the luting cement, pretreatment with tribochemical silicate coating significantly increased retention of the posts. Increased cement film thickness resulted in decreased retention of untreated posts and of pretreated posts luted with zinc phosphate cement. Increased cement film thickness had no influence on retention of pretreated posts luted with resin cement. Thus, retention of the posts was influenced by the type of luting cement, by the cement film thickness, and by the post pretreatment.

Quantity of remaining double bonds of propanal-containing resins.

Improved mechanical properties of resin composites have been reported to be a consequence of the addition of aldehyde to the resins. The objective of the present study was to examine whether this improvement could be attributed to an increased degree of conversion of double bonds. For this purpose, propanal was added to monomer mixtures, which were then made light-curing. The monomer mixtures were varied with respect to monomer composition and content of propanal. The quantity of remaining double bonds was determined by means of transmission infrared spectra of the resins recorded before and after wet or dry storage for one week. The addition of propanal resulted in a decrease in the quantity of remaining double bonds by as much as 89% as compared with the mixtures without propanal. Negative correlations of statistical significance were found between the content of propanal and quantity of remaining double bonds in BISGMA:TEGDMA-based as well as in UEDMA:HEMA-based resins for both modes of storage. Significant negative correlations were found between the quantity of remaining double bonds and the previously determined mechanical properties.

Quantity of remaining double bonds of diacetyl-containing resins.

The present study determined the effect of diacetyl addition on the quantity of remaining double bonds of dental resins. Diacetyl was added to monomer mixtures, which were then made light-curing. The monomer mixtures were varied with respect to monomer composition and content of diacetyl. The quantity of remaining double bonds was determined by means of transmission infrared spectra of the resins recorded before and after wet or dry storage for one wk. Addition of diacetyl resulted in a decrease in the quantity of remaining double bonds by as much as 78% as compared with the mixtures without diacetyl. Negative correlations of statistical significance were found between content of diacetyl and quantity of remaining double bonds in BISGMA: TEGDMA-based as well as in UEDMA: HEMA-based resins for both modes of storage. Significant negative correlations were found between quantity of remaining double bonds and the previously determined mechanical properties.

Influence of carboxylic anhydrides on selected mechanical properties of heat-cured resin composites.

Resin composites are still in need of improved abrasion resistance for them to be ideal restorative materials for use in large occlusal cavities. The present study proposes a concept for additional cross-linking of dental monomers, by which mechanical properties and possibly the resistance to abrasion of the resin composites are increased. Cyclic acid anhydrides were added as cross-linking agents to different monomer mixtures, which were then loaded with filler. The monomer mixtures were varied with respect to type and ratio of monomer and anhydride. For measurement of diametral tensile strength, flexural strength, modulus of elasticity, and modulus of resilience, specimens were initially cured by light and then post-cured for one h at 150 degrees C. Resin composites based on UEDMA and HEMA were found to be superior to BISGMA- and TEGDMA-based composites. Increases in mechanical properties were highest when unsaturated anhydrides were used. An optimal effect of anhydride addition was found in resin composites also containing methacrylamide. Such materials resulted in a 20% increase in the mechanical properties investigated.

Influence of pulse-delay curing on softening of polymer structures.

Resin composites may be polymerized according to one of several light-curing modes. These modes include variations in intensity of the curing lights and time delay before final cure. The so-called pulse-delay method has earlier been found to reduce the formation of gaps due to polymerization contraction, without mechanical properties of the resin composite being compromised. It was hypothesized that the slow pre-cure of this method would give rise to a different polymer structure than results when the polymer is cured in one step at high intensity. It was found that although the quantity of remaining double bonds was unaffected, the pulse-delay technique led to polymers of increased susceptibility to softening in ethanol. The softness increased with the intensity of the pre-cure and with the waiting time before final cure. The increased softening may be interpreted as the manifestation of a polymer structure having fewer crosslinks.

Influence of aldehydes on selected mechanical properties of resin composites.

The present study investigated whether propanol, a monofunctional aldehyde, was able to improve the mechanical properties of dental polymers. The underlying hypothesis was that a cross-linking reaction is possible between various functional groups of different polymers. Propanol was added to monomer mixtures, which were then made light-curing and loaded with filler. The monomer mixtures were varied with respect to monomer composition and content of aldehyde. Four mechanical properties of the experimental resin composites were determined. Addition of propanol gave rise to significant improvements in mechanical properties, which may be indicative of a cross-linking ability of monofunctional aldehydes. With the exception of modulus of elasticity, the mechanical properties of resin composites based on UEDMA/HEMA were superior to those of BISGMA/TEGDMA-based materials, even though the improvements in flexural strength and modulus of resilience were most pronounced for the BISGMA/TEGDMA-based resin composites.

Influence of ketones on selected mechanical properties of resin composites.

The present study investigated a concept for additional cross-linking of dental polymers, by which resistance to wear of resin composites might be increased. Bifunctional ketones were added to monomer mixtures, which were then made light-curing and loaded with filler. The monomer mixtures were varied with respect to type and ratio of monomer and ketone. For measurement of possible effects of the cross-linking agents added, four mechanical properties of the experimental resin composites were determined. Addition of the bifunctional ketone diacetyl resulted in the following increases in mechanical properties: diametral tensile strength, 11%; flexural strength, 29%; modulus of elasticity, 19%; and modulus of resilience, 50%.

Influence of the solubility parameter of intermediary resin on the effectiveness of the gluma bonding system.

The aim of the present study was to investigate the effect of the solubility parameter of the intermediary resin in the Gluma system on the bonding to dentin. The solubility parameter of the resins was varied between 18.8 x 10(3) and 21.1 x 10(3) J(1/2)/m3/2 by varying the composition of the resin. The efficacy of the bonding system was determined by measurements of marginal gaps formed by polymerization contraction of a restorative resin in dentin cavities treated with the bonding system. The bonding system had maximum efficacy at a solubility parameter of the intermediary resin of delta = 20.0 x 10(3) J(1/2)/m3/2. This finding corroborates a concept of bonding to dentin that involves a mechanical interlocking by interpenetrating resins.

Retention of composite inlays in enamel dentin cavities.

The retention of composite inlays depends on acid-etching of marginal enamel of the preparation. In many cases, only little marginal enamel is available, making loss of retention a liability. The present study evaluated the retention of three brands of composite inlays under various conditions. Inlays were fabricated and cemented in standardized enamel/dentin cavities prepared in extracted human teeth. The force necessary to extract a cemented inlay was used to express the retention of the inlay. The effects of thermocycling and choice of dentin-bonding agent on inlay retention were also determined. Inlays made of Estilux posterior C VS were more retentive than inlays of either Brilliant or SR-Isosit. The latter two products were found to provide similar retentive strengths. The retention of Estilux posterior C VS and SR-Isosit inlays declined when samples were thermocycled. Treatment with Gluma increased retention of inlays, resulting in retentive strengths of the same magnitude for all three inlay systems. The choice of dentin-bonding agent was found to affect composite inlay retention to a greater extent than the choice of either composite brand, mode of inlay curing, or effect of thermocycling.

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.

Influence of UEDMA BisGMA and TEGDMA on selected mechanical properties of experimental resin composites.

OBJECTIVES: This study was conducted to determine the effect of UEDMA, BisGMA and TEGDMA on selected mechanical properties of experimental resin composites. METHODS: Thirty monomer mixtures of TEGDMA and BisGMA and/or UEDMA were produced. Five base monomer mixtures had the following molar relationships between TEGDMA and BisGMA: 30:70, 40:60, 50:50, 60:40 and 70:30. Monomer mixtures were then produced in which BisGMA was successively substituted by UEDMA, 10 mol% at a time. The resins were made light-curing and loaded with filler. Diametral tensile strength, flexural strength and modulus of elasticity were determined on 1 week old specimens. The results were analyzed by ANOVA and by response surface methodology. RESULTS: The diametral tensile strength of the resin composites varied between 52 and 59 MPa, the flexural strength between 137 and 167 MPa, and the modulus of elasticity between 8.0 and 11.1 GPa. The statistical analyses showed that substitution of BisGMA or TEGDMA by UEDMA resulted in an increase in tensile and flexural strength, and that substitution of BisGMA by TEGDMA increased tensile, but reduced flexural strength. Further, it was found that, for a given content of UEDMA, variations in the ratio BisGMA/TEGDMA gave rise to a maximum in modulus of elasticity. The size of this maximum in modulus decreased with increasing content of UEDMA. SIGNIFICANCE: Varying the relative amounts of UEDMA, BisGMA and TEGDMA has a significant effect on the mechanical properties of the resin composition. Thus, by selecting specific combinations of these components, it may be possible to design composites with properties that are tailor made to specific applications.

A survey of the use of dentin-bonding systems in Denmark.

OBJECTIVES: Dentin-bonding systems, which usually involve multistep procedures, are relatively new in dentistry. This study was conducted to survey dental practitioners in Denmark concerning their use of dentin-bonding systems. METHODS: A questionnaire regarding dentin-bonding systems was distributed to dentists at an annual dental conference, and 462 dentists were included in the survey. RESULTS: Dentin-bonding systems were used by 99% of the dentists. Twenty-one different dentin-bonding systems were being used, with six of the systems being used by 78% of the dentists. Of the 456 dentists who used a dentin-bonding system, 77% recalled a clinical procedure for their dentin-bonding system that was in accordance with the written instructions of the manufacturer. The degree to which the dentists complied with the instructions for use, was influenced by the number of operating steps involved for a given dentin-bonding system, by the frequency with which the dentin-bonding system was used, and by the degree to which the dentist was satisfied with the instructions for use provided by the manufacturer. The frequency with which dentin-bonding systems were used, was influenced by year of graduation, place of work, and gender of the dentist. SIGNIFICANCE: Further studies, which focus on the adherence of dental practitioners to instruction manuals are warranted as are investigations of the potential clinical consequences of incorrect use of dentin-bonding systems.

In vitro wear, hardness, and conversion of diacetyl-containing and propanal-containing resin materials.

OBJECTIVES: This study was conducted to determine the effect of diacetyl or propanal activities: 1) on the in vitro wear of 22 experimental resin composites; and 2) on the Wallace indentation hardness of the unfilled resins. The objective was to examine the correlation between wear, hardness, and quantity of remaining double bonds (determined previously). METHODS: Diacetyl or propanal agents were added in varying concentrations to monomer mixtures. The resins were made light-curing and those used for measurement of wear were loaded with filter. The results were analyzed by ANOVA and Newman-Keuls' multiple range tests. RESULTS: In the composite systems, adding diacetyl or propanal resulted in decreased in vitro wear. The quantity of remaining double bonds in the dental polymers with these additives was also reduced. Their effect on Wallace indentation depth was less clear-cut. Low concentrations of additives decreased indentation depth of the unfilled materials, whereas high concentrations increased indentation depth. If it assumed that differences in the polymer have a major influence on the wear when the filter content and particle matrix interface are kept constant, then the hardness data on the unfilled resin can be used to correlate property changes of the polymer in the composite. A three-dimensional regression analysis found that in vitro wear decreased with decreasing Wallace indentation depth and decreasing quantity of remaining double bonds. SIGNIFICANCE: Addition of diketone or monoaldehyde to resin monomers may provide a means of increasing wear resistance of composites and allow their use in stress-bearing areas.

Modulus of resilience as predictor for clinical wear of restorative resins.

Eight posterior restorative resins were tested with respect to flexural strength, modulus of elasticity, and modulus of resilience. The mechanical properties were correlated to the two-year results of clinical wear tests. Linear relationships were found between flexural strength and clinical wear and between modulus of resilience and clinical wear. It was concluded that modulus of resilience be used in research and quality control for the prediction of clinical wear.

Characterization of resin composites polymerized with plasma arc curing units.

OBJECTIVES: Newly developed curing units (plasma arc curing units) operate at relatively high intensity and are claimed to result in optimum properties of resin composites in a short cure time. This study was conducted to determine a number of characteristics of resin composites polymerized by plasma arc curing units. METHODS: The investigated polymerization characteristics were quantity of remaining double bonds, depth of polymerization, flexural strength and modulus, and wall-to-wall polymerization contraction. The investigated plasma arc curing units were Apollo 95E and 1000 PAC. The conventional curing unit XL 3000 was used as baseline. RESULTS: Irradiation with Apollo 95E resulted in a higher quantity of remaining double bonds than did XL 3000, whereas the results obtained with 1000 PAC depended on the resin composite. The depth of cure with the plasma arc units was equal to or less than that obtained with the conventional unit, depending on the resin composite. The flexural strength did not depend on the curing unit. The flexural modulus resulting from curing with Apollo 95E was less than that resulting from curing with XL 3000 in 3 out of 4 comparisons. The wall-to-wall polymerization contraction was equal to or less with the plasma arc units than with the conventional unit. SIGNIFICANCE: Plasma arc curing units make it possible to polymerize resin composite in much shorter times than conventional curing units. However, the polymerization characteristics associated with the units may be less than optimal.