Stiffness increase during the setting of dental composite resins

Effect of butylated hydroxytoluene (BHT) concentrations on polymerization shrinkage stress and other physicochemical properties of experimental resin composites

The present study examined different concentrations of the butylated hydroxytoluene (BHT) inhibitor on the kinetics of conversion, polymerization shrinkage stress, and other correlated physicochemical properties of experimental resin composites (ERC). A model composite was formulated with 75 wt% filler containing 0.5 wt% camphorquinone and 1 wt% amine with BHT concentrations of 0.01 wt% (BHT-0.01); 0.1 wt% (BHT-0.1); 0.25 wt% (BHT-0.25); 0.5 wt% (BHT-0.5); 1 wt% (BHT-1), and control (no BHT). They were tested on polymerization shrinkage stress (PSS; n = 5), degree of conversion (DC; n = 3), maximum polymerization rate (RpMAX; n = 5), water sorption (Wsp; n = 0), and solubility (Wsl; n = 10), flexural strength (FS; n = 10), flexural modulus (FM; n = 10), Knoop microhardness (KH; n = 10), and microhardness reduction (HR; n = 10). Data concerning these tests were submitted to one-way ANOVA and Tukey's test (α = 0.05; β = 0.2). BHT-0.25, BHT-0.5, and BHT-1 showed a gradually significant decrease in PSS (p = 0.037); however, BHT-1 demonstrated a decrease in the physicochemical properties tested. Thus, within the limitations of this study, it was possible to conclude that BHT concentrations between 0.25 and 0.5 wt% are optimal for reducing shrinkage stress without affecting other physicochemical properties of ERCs.

Effect of layer thickness on the elution of monomers from two high viscosity bulk-fill composites: A high-performance liquid chromatography analysis

Aim:

The aim is to evaluate the effect of different layer thickness on the amount of elution of monomers from two high viscosity bulk-fill composites after 24 h and 1 month storage in ethanol using high-performance liquid chromatography (HPLC) analysis.

Materials and Methods:

Forty-eight samples prepared from two high viscosity bulk-fill composite resins; Tetric EvoCeram Bulk Fill and x-tra fil were divided into three groups (n = 8) based on their layer thickness, i.e., Group 1 (2 mm), Group 2 (4 mm), and Group 3 (6 mm) and were then subdivided based on their storage period. The analysis of the eluates was performed using HPLC unit. Statistical analysis was performed using One-way ANOVA, independent sample t-test, and paired “t” test at a significance level of 0.05 (P < 0.05).

Results:

Increase in layer thickness resulted in increased amount of bisphenol A-glycidyl methacrylate (Bis-GMA) and urethane dimethacrylate (UDMA) elution for both composites, i.e., Group 3 > Group 2 > Group 1 and there was statistically significant difference among all the groups. Tetric EvoCeram Bulk Fill showed a statistically significant increase in amount of Bis-GMA and UDMA elution for all groups compared to x-tra fil. Furthermore, increase in storage period resulted in statistically significant increase in amount of Bis-GMA and UDMA elution for both composites, i.e., Group 1B > Group 1A, Group 2B > Group 2A, and Group 3B > Group 3A. The mean value of UDMA elution was higher when compared to that of Bis-GMA elution for both composites and there was statistically significant difference for all groups.

Conclusion:

Within the limitations of this in vitro study, it was concluded that increase in layer thickness resulted in increased amount of Bis-GMA and UDMA elution for both Tetric EvoCeram Bulk Fill and x-tra fil.

Vickers Hardness and Shrinkage Stress Evaluation of Low and High Viscosity Bulk-Fill Resin Composite

The aim of this in vitro study was to evaluate the hardness and shrinkage stress (SS) of six bulk-fill resin composites. To evaluate microhardness (MH), ten 6 mm specimens were prepared using a metal mold for each selected bulk-fill resin composite and irradiated from the top side for 40 s using an LED light. After 24 h of storage, Vickers MH was evaluated on the upper, lower and lateral sides of the specimens. SS evaluation was then performed with a universal machine, which evaluated the contraction force generated by a bulk-fill composite specimen placed between two metal cylinders during and after light curing. The results were evaluated with a one-way ANOVA test with a post-hoc Bonferroni test and linear regression analysis (p < 0.05). All materials showed a significant MH decrease between the top and bottom surfaces. However, the bulk-fill materials tested performed differently when considering lateral depth progression. ANOVA tests for SS evaluation showed that both SDR and Venus Bulk Fill had significantly lower stress during irradiation than other tested materials. Further, MH decrease became significantly lower from the top surface at different depths in each tested group. Among the different resins, Venus Bulk Fill and SDR showed not only inferior hardness, but also a significant reduction in SS.

Polymerization Behavior and Mechanical Properties of High-Viscosity Bulk Fill and Low Shrinkage Resin Composites

The present study determined the mechanical properties and volumetric polymerization shrinkage of different categories of resin composite. Three high viscosity bulk fill resin composites were tested: Tetric EvoCeram Bulk Fill (TB, Ivoclar Vivadent), Filtek Bulk Fill posterior restorative (FB, 3M ESPE), and Sonic Fill (SF, Kerr Corp). Two low-shrinkage resin composites, Kalore (KL, GC Corp) and Filtek LS Posterior (LS, 3M ESPE), were used. Three conventional resin composites, Herculite Ultra (HU, Kerr Corp), Estelite ∑ Quick (EQ, Tokuyama Dental), and Filtek Supreme Ultra (SU, 3M ESPE), were used as comparison materials. Following ISO Specification 4049, six specimens for each resin composite were used to determine flexural strength, elastic modulus, and resilience. Volumetric polymerization shrinkage was determined using a water-filled dilatometer. Data were evaluated using analysis of variance followed by Tukey's honestly significant difference test (α=0.05). The flexural strength of the resin composites ranged from 115.4 to 148.1 MPa, the elastic modulus ranged from 5.6 to 13.4 GPa, and the resilience ranged from 0.70 to 1.0 MJ/m3. There were significant differences in flexural properties between the materials but no clear outliers. Volumetric changes as a function of time over a duration of 180 seconds depended on the type of resin composite. However, for all the resin composites, apart from LS, volumetric shrinkage began soon after the start of light irradiation, and a rapid decrease in volume during light irradiation followed by a slower decrease was observed. The low shrinkage resin composites KL and LS showed significantly lower volumetric shrinkage than the other tested materials at the measuring point of 180 seconds. In contrast, the three bulk fill resin composites showed higher volumetric change than the other resin composites. The findings from this study provide clinicians with valuable information regarding the mechanical properties and polymerization kinetics of these categories of current resin composite.

Microshear bond strength of Nano-Bond adhesive containing nanosized aluminum trioxide particles

OBJECTIVES

The present study was conducted to evaluate the effect of nanosized aluminum trioxide (Al2O3) particles when added to the Nano-Bond adhesive system and its effect on the microshear bond strength of nanocomposite resin to dentin.

MATERIALS AND METHODS

A newly developed adhesive (Nano-Bond) and one type of light-cured resin restorative material (nanocomposite resin) were used in this study. The occlusal surfaces of extracted human molar teeth were ground perpendicular to the long axis of each tooth to expose a flat dentin surface. The adhesives were applied to the dentin surfaces according to manufacturers' instructions. The nanocomposite resin was then placed and light cured for 40 s. After immersion in water at 37°C for 24 h, the specimens were subjected to thermocycling before testing, and a microshear bond test was carried out. The recorded bond strengths (MPa) were collected, tabulated, and statistically analyzed. A one-way analysis of variance and Tukey's tests were used to test for significance between the means of the groups; statistical significance was assumed when the P ≤ 0.05.

RESULTS

The mean microshear bond strength of the Nano-Bond adhesive system containing nanosized Al2O3 at a concentration of 2% was 23.15 MPa (Group B), which was significantly greater than that of the Nano-Bond adhesive system without additives (15.03 MPa, Group A).

CONCLUSIONS

These results indicate that nanosized Al2O3 added to the Nano-Bond adhesive system at a concentration of 2% increases the microshear bond strength.

Comparison of shear bond strength and adhesive remnant index between precoated and conventionally bonded orthodontic brackets

BACKGROUND

The purpose of this study was to compare the shear bond strength and adhesive remnant index (ARI) at the enamel-bonding interface of precoated and conventionally bonded brackets, utilizing standardized procedures.

METHODS

The test sample consisted of 90 recently extracted bovine permanent mandibular incisors. The teeth were bonded using the same protocol and were tested in three different situations. A material testing systems machine was utilized for debonding, and the remaining adhesive on the tooth was recorded.

RESULTS

Immediately after bonding, we found that the shear bond strength of the precoated brackets (6.27 MPa) was significantly higher than that of conventional brackets (5.37 MPa) (p<0.05). However, no significant differences in bond strength were found between the two bracket systems after 24 h of bonding or after thermocycling. The conventional brackets had higher ARI scores than the precoated bracket systems immediately after bonding and after 24 h.

CONCLUSIONS

Since there were no significant differences in the bonding strength after 24 h, the immediate bonding strength of the precoated brackets during the first day does not appear to be a major advantage over the conventional bracket systems. However, less adhesive on the tooth after debonding is an advantage of precoated brackets.

Alternatives in polymerization contraction stress management

Polymerization contraction stress of dental composites is often associated with marginal and interfacial failure of bonded restorations. The magnitude of the stress depends on the composite's composition (filler content and matrix composition) and its ability to flow before gelation, which is related to the cavity configuration and curing characteristics of the composite. This article reviews the variations found among studies regarding the contraction stress testing method, contraction stress values of current composites, and discusses the validity of contraction stress studies in relation to results from microleakage tests. The effect of lower curing rates and alternative curing routines on contraction stress values is also discussed, as well as the use of low elastic modulus liners. Moreover, studies with experimental Bis-GMA-based composites and recent developments in low-shrinkage monomers are described.

Comparative analysis of the shrinkage stress of composite resins

The aim of this study was to compare the shrinkage stress of composite resins by three methods. In the first method, composites were inserted between two stainless steel plates. One of the plates was connected to a 20 kgf load cell of a universal testing machine (EMIC-DL-500). In the second method, disk-shaped cavities were prepared in 2-mm-thick Teflon molds and filled with the different composites. Gaps between the composites and molds formed after polymerization were evaluated microscopically. In the third method, the wall-to-wall shrinkage stress of the resins that were placed in bovine dentin cavities was evaluated. The gaps were measured microscopically. Data were analyzed by one-way ANOVA and Tukey's test (alpha=0.05). The obtained contraction forces were: Grandio = 12.18 +/- 0.428N; Filtek Z 250 = 11.80 +/- 0.760N; Filtek Supreme = 11.80 +/- 0.707 N; and Admira = 11.89 +/- 0.647 N. The gaps obtained between composites and Teflon molds were: Filtek Z 250 = 0.51 +/- 0.0357%; Filtek Supreme = 0.36 +/- 0.0438%; Admira = 0.25 +/- 0.0346% and Grandio = 0.16 +/- 0.008%. The gaps obtained in wall-to-wall contraction were: Filtek Z 250 = 11.33 +/- 2.160 microm; Filtek Supreme = 10.66 +/- 1.211 microm; Admira = 11.16 +/- 2.041 microm and Grandio = 10.50 +/- 1.224 microm. There were no significant differences among the composite resins obtained with the first (shrinkage stress generated during polymerization) and third method (wall-to-wall shrinkage). The composite resins obtained with the second method (Teflon method) differed significantly regarding gap formation.

Class II restorations: influence of a liner with rubbery qualities on the occurrence and size of cervical gaps

The aim of this study was to evaluate the ability of new rubbery liners, used as a cervical increment, to relieve contraction stress and thereby reduce the formation of cervical gaps in class II composite restorations. The investigated liners were made of polyester-acrylate (PE(1), PE(2) or PE(3)) or silicone-acrylate (S), mixed with UDMA, without (A, B, C, D) or with HEMA (AH, BH, CH, DH). A silanized filler was added to the mixture, DH, to give composites with 20, 40, 60, and 70% (w/w) of filler (DHF20, DHF40, DHF60, DHF70, respectively). The presence and width of cervical gaps were determined using a light microscope. Statistical analysis showed that six of the 12 rubbery liners (AH-DH, DHF20-DHF40) significantly decreased gap formation in comparison with the control group. In addition, the polymerization shrinkage, flow, and strain capacity of these liners were measured and the influence of these factors on gap formation was examined. Two- and three-dimensional regression analyses showed significantly negative linear correlations between gap formation and strain capacity, and between gap formation and flow, and a significantly positive linear correlation between gap formation and shrinkage.

The influence of curing times and light curing methods on the polymerization shrinkage stress of a shrinkage-optimized composite with hybrid-type prepolymer fillers

OBJECTIVES

The aim of the study was to determine the influence of different light curing units (LCU) and regimes on the polymerization shrinkage stress (PSS) and the mechanical properties of a nano-hybrid composite.

MATERIAL AND METHODS

The polymerization shrinkage force (PSF) was measured continuously with compliance compensation for 300s after photo-initiating the composite, Tetric EvoCeram (Ivoclar Vivadent, Schaan, FL, Shade A3) in a Stress-Strain Analyser. Astralis 10, Bluephase and MiniL.E.D LCU with exposure times 10, 20 and 40s were used (C-factor=0.33, n=8 per group). Immediately after the PSF measurements, mechanical properties of the samples were measured at the top and the bottom using a Fischerscope H100C (Helmut Fischer GmbH, Sindelfingen, Germany). Statistical analyses were done using one-way ANOVA (p<0.05) and Tukey post hoc test.

RESULTS

Significant differences in the PSS for 10, 20 and 40s polymerization using Astralis 10 were found. The MiniL.E.D recorded low stress values. Modulus of elasticity is high after curing the composite with Astralis 10 at 10, 20 and 40s and for Bluephase 40s. Low moduli of elasticity were recorded for the MiniL.E.D and for the Bluephase 20 and 10s. The hardness values (HV) followed the same pattern as the modulus of elasticity. The Ramping mode of the MiniL.E.D had prolonged gel point.

CONCLUSIONS

High intensity LCU produce not just high HV but also high shrinkage, making it important to balance both the effects by choosing the appropriate curing time. Soft-start regimes have no paramount benefit in a LED regarding stresses in the clinical situation.

Abrasion resistance of composites polymerized by light-emitting diodes (LED) and halogen light-curing units

This study compared the abrasion resistance of direct composite resins cured by light-emitting diodes (LED) and halogen light-curing units. Twenty specimens (12 mm in diameter; 1.0 mm thick) of each composite resin [TPH (Dentsply); Definite (Degussa); Charisma (Heraus Kulzer)] were prepared using a polytetrafluoroethylene matrix. Ten specimens per material were cured with the LED source and 10 with the halogen lamp for 40 s. The resin discs were polished, submitted to initial surface roughness reading (Ra initial - mum) in a roughness tester and stored in water at 37°C for 15 days. The specimens were weighed (M1) and submitted to simulated toothbrushing using slurry of water and dentifrice with high abrasiveness. After 100 minutes in the toothbrushing simulator, the specimens were cleaned, submitted to a new surface roughness reading (Ra final - mum) and reweighed (M2). Mass loss was determined as the difference between M1 and M2. Data were recorded and analyzed statistically by one-way ANOVA and Tukey Test at 5% significance level. The composite resin with greater size of inorganic fillers (TPH) showed the lowest mass loss and surface roughness means, indicating a higher resistance to toothbrush abrasion (p<0.05). Definite cured with LED presented the least resistance to toothbrush abrasion, showing the highest means of surface roughness and mass loss (p<0.05). The LED source did not show the same effectiveness as the halogen lamp for polymerizing this specific composite resin. When the composite resins were cured a halogen LCU, no statistically significant difference was observed among the materials (p>0.05). It may be concluded that the type of light-curing unit and the resin composition seemed to interfere with the materials' resistance to abrasion.

Alternatives in polymerization contraction stress management

Polymerization contraction stress of dental composites is often associated with marginal and interfacial failures of bonded restorations. The magnitude of stress depends on composite composition (filler content and matrix composition) and its ability to flow before gelation, which is related to the cavity configuration and curing characteristics of the composite. This article reviews variations among studies regarding contraction-stress-testing methods and contraction stress values of current composites, and discusses the validity of contraction stress studies in relation to results from microleakage tests. The effects of lower curing rates and alternative curing routines on contraction stress values are also discussed, as well as the use of low-elastic-modulus liners. Moreover, studies with experimental dimethacrylate-based composites and recent developments in low-shrinkage monomers are described.

The apparent increase of the Young's modulus in thin cement layers

OBJECTIVES

The bond of adhesive luting cements to the tooth tissues and restorative materials is expected to hinder their transverse contraction for the layer thickness applied in dental restorations. It was hypothesized that the hindering of the transverse deformation will influence the relation between stress and strain (the stiffness) in the direction perpendicular to the substrate surface. The aim of this study was to investigate the relation between cement layers with different ratio between bonded and free surface (C-factor) and the stiffness of these layers, i.e. an apparent increase of the Young's modulus of the dental luting cement.

METHODS

A commercial luting cement RelyX ARC (3M, St Paul, MN, USA) was used in this study. The 'real" Young's modulus and the Poisson ratio were determined and these values were used in models with layers with different C-factors (0.5, 1.0, 2.0, 3.0, 4.0, 5.0 and 6.0) in a three-dimensional Finite Element Analysis program (FEMAP, E.S.P., Maryland Height, MO, USA). The apparent Young's modulus was measured for layers with thicknesses of 0.5 mm (C = 6.0) and 6.0 mm (C = 0.5) and compared with the results of the Finite Element Analysis (FEA) analysis.

RESULTS

The apparent Young's modulus in the 0.5 mm layer was 20% higher than the apparent Young's modulus in the 6.0 mm layer. This result was confirmed by the results of the F.E. analysis. For very thin layers the stiffness will be 25% higher than the Young's modulus.

SIGNIFICANCE

The hindering of the transverse contraction has to be taken into account studying the mechanical properties of dental luting cements because it influences the behavior of these luting cements in thin layers.

Contraction stress of flowable composite materials and their efficacy as stress-relieving layers

BACKGROUND

The authors compared the polymerization contraction stress produced by flowable resin-based composites with stress values produced by nonflowable composites. They also measured the stress reduction produced by placing a precured layer of flowable composite under a nonflowable composite.

METHODS

The authors first tested four flowable and six nonflowable composite materials for contraction stress in a tensiometer. In the second part of the study, they applied a 1.4-millimeter-thick layer of flowable composite or unfilled resin and precured it in the test apparatus to assess the stress relief produced by a low-modulus material during light curing of a subsequent layer of highly filled composite. Flexural moduli of the precured materials were determined via a three-point bending test.

RESULTS

The stress values ranged between 6.04 and 9.10 megapascals. The authors found no significant differences in stress between flowable and nonflowable composites. Microfilled composites produced lower contraction stress than did hybrids. The flexural modulus of the flowable composites varied between 4.1 and 8.2 GPa. Regarding the effect of a precured layer of composite on contraction stress, the authors observed significant reductions with only one of the flowable materials and with the unfilled resin.

CONCLUSIONS

The flowable composites produced stress levels similar to those of nonflowable materials. Most of the flowable materials tested did not produce significant stress reductions when used under a nonflowable composite.

CLINICAL IMPLICATIONS

Using a flowable resin-based composite as a restorative material is not likely to reduce the effects of polymerization stress. When used in a thin layer under a nonflowable composite, the stress reduction depended on the elastic modulus of the lining material.

[Microhardness of resins as a function of color and halogen light]

The aim of this study was to evaluate the influence of light intensity and the influence of the color of a composite resin on Knoop hardness. Samples were confected utilizing polyester matrices with 6 mm of diameter and 2 mm of depth. The matrices were filled with composite resin (Fill Magic - Vigodent), colors A3, B3, C3, D3 and I, and light-cured by means of an Elipar light-curing unit in three different light intensities: 450 mW/cm2, 800 mW/cm2 and an increasing intensity setup of 100 mW/cm2 to 800 mW/cm2. Ninety test specimens were confected, with the standard curing time of 40 seconds. The specimens were stored at 37 +/- 1 degrees C and immersed in distillate water. The Knoop test was carried out in superficial and deep areas of the specimens. The results revealed that there was no statistical difference (Tukey) between the tested colors. However, there was statistical difference between different light intensities. The authors concluded that the color of the composite resin did not influence Knoop hardness and that the progressive intensity setup led to the best Knoop hardness.

Modeling of viscoelastic behavior of dental chemically activated resin composites during curing

Shrinkage stresses generated in dental resin composites during curing are among the major problems in adhesive dentistry, because they interfere with the integrity of the restored tooth. The aim of this study was to find a mechanical model to describe the viscoelastic behavior of a two-paste resin composite during curing, to aid our understanding of the process of shrinkage stress development. In this study, stress-strain data on Clearfil F2 during curing were obtained by a dynamic test method and analyzed using three mechanical models (Maxwell, Kelvin, and the Standard Linear Solid model). With a modeling procedure, the model's stress response was compared with the experimental stress data, and the material parameters were calculated. On the basis of the modeling and evaluation results, a model for describing the viscoelastic behavior of the shrinking resin composite was selected. The validation results showed that the modeling procedure is free of error, and that it was capable of finding material parameters associated with a two-parametric model with a high degree of accuracy. The viscoelastic behavior of the shrinking resin composite, as excited by the conditions of the test method, cannot be described by a single mechanical model. In the early stage of curing, the most accurate prediction was achieved by the Maxwell model, while during the remainder of the curing process the Kelvin model can be used to describe the viscoelastic behavior of the two-paste resin composite.

Visco-elastic parameters of dental restorative materials during setting

Contraction stresses generated in restoratives during setting are among the major problems in adhesive dentistry, since they often result in loss of adhesion from the cavity walls or in post-operative pain. The rate of stress development and the ultimate magnitude of the stress, which determine the seriousness of these problems, depend on the relatively unknown visco-elastic behavior of the restoratives during setting. The aim of this study was to determine the visco-elastic parameters during setting, to aid our understanding of the process of contraction stress development. A dynamic mechanical method was used in which the materials were subjected to periodic strain cycles in a universal testing machine during the first 60 min of setting. The visco-elastic parameters (viscosity eta and Young's modulus E) were calculated by analysis of the experimental stress-strain data with a simple mechanical model according to Maxwell. Two restorative materials from different classes were investigated: a two-paste resin composite and a conventional glass-ionomer cement. A comparison of the results showed significant differences in the development of viscosity and stiffness in the early stage of setting. The resultant relaxation time (eta/E) of the glass ionomer remained at a low level during the first 15 min, whereas that of the resin composite increased markedly. This is of clinical importance, since it implies that, during the early setting stage, glass ionomers are better capable of reducing the contraction stresses than resin composites, thus increasing the likelihood that the bond with the cavity walls will form and survive during setting.

The effects of adhesive thickness on polymerization contraction stress of composite

A layer of an unfilled adhesive resin placed between the tooth and composite restoration has been shown to absorb some of the stress generated in the composite during polymerization and to reduce interfacial leakage. The objectives of this study were to measure the change in polymerization contraction stress of bonded composite as the thickness of the resin adhesive was systematically varied, and to correlate the effects of the adhesive thickness and reduced stress on marginal leakage in class V cavities. The maximum contraction force of composite (Herculite XRV) was measured in a tensilometer as the thickness of the adhesive bonding agent (Scotchbond MP) was varied from 20 to 300 microm. Composite was placed in Class V cavities prepared on the labial surfaces of bovine teeth to which different thicknesses of adhesive had been applied by layering, and a marginal leakage test was performed by means of staining with silver nitrate. Contraction stress decreased significantly as the adhesive thickness was increased. This result was supported by a theoretical examination of the data. In class V cavities, additional adhesive layering in the marginal area reduced the overall degree of microleakage. The contraction stress generated during the placement of a composite restoration contributes significantly to early marginal leakage, and this stress was significantly absorbed and relieved by the application of an increasing thickness of low-stiffness adhesive.

Polymers in dentistry

There is a wide choice of materials available for restorative dentistry covering a range of requirements. Fundamental knowledge about the properties of the polymers in use in dentistry is an advantage as it provides information relevant to clinical practice. Dentistry, perhaps, has the unique distinction of using the widest variety of materials, ranging from polymers, metal and metal alloys, ceramics, inorganic salts and composite materials. In the present paper, polymers and polymer composites used directly or indirectly for restorations, prostheses or for production of appliances in dentistry is discussed.

The effect of curing light variations on bulk curing and wall-to-wall quality of two types and various shades of resin composites

OBJECTIVES

This study evaluated the influence of light intensity and irradiation time variations on the curing efficacy of two types and various shades of resin composites and the effect of reduced light intensity on the preservation of wall-to-wall continuity.

MATERIALS AND METHODS

Three microfilled composites (in three different shades) and one hybrid composite were used in this study. Polymerization shrinkage, and the hardness and adaptation of adhesive restorations in dentin cavities were determined at light intensities of 175 and 700 mW/cm2 and irradiation times of 10 and 60 s. Data were compared using in a general linear model analysis.

RESULTS

Shrinkage measurements were the indication of conversion and conversion rate. Reduced intensity slowed down the rate of polymerization but did not reduce the conversion as long as an irradiation time of 60 s was employed. High-energy irradiation caused increased separation of the composite from the tooth structure. On the basis of obtaining optimal conversion and adaption, it was demonstrated that the irradiation time to be more effective than irradiation energy.

SIGNIFICANCE

Light-cured composites require an understanding of their structure, pigmentation and irradiation parameters to obtain optimal performance. High intensity light-curing does not necessarily lead to optimal quality.

The influence of surface loading and irradiation time during curing on mechanical properties of a composite

PURPOSE

The aim of this study was to determine the influence of different surface loadings during curing with various irradiation times on hardness and diametral tensile strength of a light-cured composite.

MATERIAL AND METHODS

A mold was fabricated to allow loading during curing of cylindrical specimens of a composite. Four surface loadings of 0, 0.35, 0.87, and 1.73 MPa and four irradiation times of 20, 40, 60, and 180 seconds were used (n = 15). Each specimen was subjected to a microhardness test and to a diametral tensile strength test.

RESULTS

Surface loading during curing affected both hardness and strength properties, whereas irradiation time influenced only the hardness of the material. Both parameters gained between 15% and 20% improvement when the material was loaded with 0.87 MPa surface pressure and cured by 60-second irradiation time. Higher loading or longer irradiation times did not improve these properties.

CONCLUSION

Loading composite during curing improves its mechanical properties, probably through decreasing flaws and air voids of the material.

The effect of static loading in orthodontic bonding

The aim of this investigation was to determine the effect of static loading on the measured shear bond strength of steel attachments bonded to enamel. Sixty steel attachments were bonded to extracted and prepared premolar teeth with Orthodontic Concise (3M St Paul, Minnesota, USA) and allowed to bench cure for 2 weeks. Half the specimens had 78 g weights suspended from the attachment, close to the bondline, prior to testing. The mean shear bond strength for the statically loaded samples was 127 N and for the unloaded 85.4 N. The 95 per cent confidence intervals for the difference between the means was 26.7-56.8 N indicating that static loading prior to shear testing significantly increases shear bond strength.

Shear modulus--measurement methodology with application to light-cured resin composites

OBJECTIVES

This study was conducted to test a novel methodology for shear modulus determination of light-cured resin composites.

METHODS

Using a butterfly-shaped specimen, a special loading device was able to apply pure uniform shear stress on a significant region of the specimen. Validation of the method was proven by photoelastic experiments. Shear modulus was directly obtained through shear stress-strain curve where the strains were measured by a shear strain gauge rosette. The composite materials analyzed were one microfill (Silux Plus, 3M Dental Products) and four hybrids (Brilliant, Coltene; Herculite XRV, Kerr; Z-100, 3M Dental Products; Graft LC, GC). An analysis of variance was used to evaluate the G modulus (p<0.01).

RESULTS

Modulus values varied between 2.54-8.01 GPa. The microfilled material presented the lowest value, and Graft LC presented the highest value. A statistically significant difference was found between all materials (p<0.01).

SIGNIFICANCE

The described methodology can also be applied to other restorative materials, as well as to adhesion strength measurements.

Tooth stiffness with composite veneers: a strain gauge and finite element evaluation

OBJECTIVES

This study was conducted to determine the impact of composite veneer procedures on the functional properties of incisors.

METHODS

Ten extracted human maxillary central incisors were mounted in pairs in a nylon ring. One strain gauge was bonded along the long axis of each tooth on the center of the lingual surface. Each pair formed half of a Wheatstone bridge circuit and was wired to eliminate all but the voltage resulting from experimentally applied procedures. The teeth were ramp-loaded to 50 N near the incisal edge on the lingual surface. Loading was performed on the unaltered teeth, teeth with preparations and restored teeth. Two-dimensional finite element (FE) models were generated to evaluate each test condition. Relative stiffness, compared with the unaltered tooth, was calculated from measurements with the strain gauge steps and from the FE models.

RESULTS

A relative stiffness value of unity represents recovery of stiffness to the level of the unaltered tooth. Both methods of evaluation demonstrated a decrease in mean relative stiffness with each subsequent reduction in tooth structure. The composite restoration increased its mean relative stiffness compared to its corresponding preparation but never to the level of the unaltered tooth. Across all procedures, the two-dimensional FE model correlated well in both direction and magnitude with the experimental strain gauge method (R = 0.83).

SIGNIFICANCE

A resin composite veneer does not restore the stiffness to the level of an unaltered tooth.

Determination of polymerization shrinkage kinetics in visible-light-cured materials: methods development

An instrument for the reproducible measurement of polymerization shrinkage kinetics is described, constructed around a disc-shaped specimen sandwiched between two glass plates. Test specimens of light-sensitive dental restorative materials were irradiated through the lower, rigid plate. The upper, non-rigid plate was readily deflected by an increase of the adhesive stress from the polymerizing and shrinking sample. Deflection was measured by an LVDT transducer and computer-recorded. Dimensional changes were confined to the specimen disc-thickness dimension, such that the fractional linear shrinkage approximated the volumetric shrinkage. Shrinkage data are reported for representative materials: unfilled and resin composites, base-lining materials, and an impression material. Equilibrium shrinkage magnitudes ranged from 0.65%, for the impression material, to 7.9% for the unfilled resin. The kinetic behavior was approximately characterized by an overall time constant, ranging from 12.5 to 280 s, associated with an exponential growth curve, although the initial shrinkage was near-linear in time, for many materials, due to non-steady-state concentrations of polymer free-radicals. The test-specimen geometry facilitates rapid and essentially uniform cure and hence the determination of minimum possible time-constants at each ambient temperature and incident light-intensity. Study of hybrid glass-ionomer materials, without spurious dehydration effects, was also achieved.

Marginal integrity related to bond strength and strain capacity of composite resin restorative systems

The shear strength of composite resin restorative systems bonded to dentin was measured and marginal integrity of class V restorations was assessed. A correlation between bond strength and marginal adaptation could not be demonstrated. The application of an intermediate layer of unfilled resin or the use of low-stiffness composite resins to improve the strain capacity of the restoration significantly influenced the quality of the marginal integrity.

Mixed Class V restorations: the potential of a dentine bonding agent

The marginal micromorphology and seal of mixed Class V restorations (i.e. restorations with margins located 50 per cent in dentine and 50 per cent in enamel) using Scotchbond VLC or Scotchbond 2 bonded to dentine in conventional and saucer-shaped cavities were evaluated. The four operative systems tested included a two-step incremental filling technique using the light-curing composite Silux, two direct inlay techniques using Silar or Silux as a luting cement and an inlay technique utilizing a Ca(OH)2 liner. All restoration types showed poor marginal adaptation in dentine irrespective of cavity design and bonding agent used. The inlay technique, tested in seven different variations, did not optimize marginal adaptation. Cavity lining with the Ca(OH)2 base material adversely affected dentine bonding. In mixed Class V restorations, marginal adaptation at the dentinal cavosurface line angle continues to pose a problem.

Annealing as a mechanism of increasing wear resistance of composites

The objective of this study was to examine the influence of a short-term exposure to heat (125 degrees C) on the wear resistance of composites. Both light- and chemically initiated materials improved by 20-60%. However, the improvement was also attained in the course of time if the materials had not been exposed to heat. The heat-induced improvement could not be explained by a continuation of polymerization but rather by stress relief, which is common for annealing processes. Polymerization shrinkage stresses, initially concentrated mainly around the filler particles, became more homogeneously distributed by the heat treatment. The long-term improvement of the non-heat-treated materials was based on the same mechanism, but proceeded more gradually.

Gradual increases in marginal leakage of resin composite restorations with thermal stress

The effects of thermal stress on the marginal leakage of resin composite restorations in bovine teeth were investigated by a method that preserved the specimens. The changes in marginal leakage of specimens with increasing numbers of thermal cycles were measured by an electrical conductivity method. Four brands of posterior resin composites were used to fill cylindrical cavities (2.0 mm in diameter and 1.5 mm in depth) on the labial surfaces of bovine incisors, according to the manufacturer's instructions. Thermal cycling stress was applied to the specimens for up to about seven weeks (9000 cycles). During this time, the electrical conductance between the pulp and a drop of physiological saline solution covering the resin restoration was measured periodically by application of an electrical potential (60 Hz, 10 Vp-p). Thermal stress increased the marginal leakage gradually, rather than step-wise. Even before application of any thermal stress, wide variations of marginal leakage were found among different specimens restored with the same brand of resin. Specimens with less initial leakage showed less increase in leakage, and vice versa, throughout the experimental period.

Quantitative determination of stress reduction by flow in composite restorations

In this study, the reduction of the polymerization shrinkage stress by flow of four chemically-initiated composites was investigated in relation to the cavity configuration. In an experimental set-up simulating restorations bonded to cavity walls, the developing shrinkage stress accompanied by flow was recorded as a function of time for several configurations. For each configuration, theoretical shrinkage stress curves were also drawn, excluding stress reduction by flow. These data were obtained from Young's modulus determinations at the early setting stage and the corresponding polymerization shrinkage. By comparison of the theoretical stress with the experimentally determined stress, a measure for the ability to flow in the bonded situation could be obtained. It was found that the flow strongly depended on the type of composite and on the configuration of the cavity.

Early marginal microleakage in Class II resin composite restorations

The aim of this study was to evaluate in vitro the early marginal microleakage around conventional and sandwich Class II (MOD) restorations. Ten different combinations of dentin bonding agents, glass-ionomer cements, and posterior resin composites were used. Class II cavities were prepared in freshly extracted third molars whose root surfaces had previously been coated. All gingival margins were prepared at the cementum-enamel junction. Teeth were filled by use of a multi-incremental technique, finished with discs, and immediately immersed in 2% erythrosin B for 12 h at 37 degrees C. Three transverse sections were made at 0.5 mm, 2 mm, and 3.5 mm over the cementum-enamel margin and evaluated under a stereomicroscope. Microleakage was scored linearly along the dentin/enamel-restoration interface. Sections at the cementum-enamel level showed significantly higher microleakage than the other two. Significant differences were observed between the various combinations of materials. Early marginal microleakage in Class II restorations depends not only on filling technique and on dentin bonding systems but also on the resin composite. Glass-ionomer cements significantly reduce early marginal microleakage in Class II restorations.

A comparison between the microleakage of direct and indirect composite restorative systems

The present study compares the microleakage of direct and indirect restorations. Two dentine bonding agents were evaluated with both techniques. Class V cavities, approximately 2 mm in depth and 4 mm in diameter, were prepared in extracted human molars. Cavities were placed in either mesial or distal surfaces and were centred at the amelodentinal junction. Microleakage was rated after silver nitrate staining. With both dentine bonding agents, the indirect method resulted in significantly (P less than 0.001) reduced microleakage. Choice of adhesive for the indirect method was also significant, perhaps due to polymerization shrinkage of the composite cement used for placement. It is suggested that the indirect method may be less technique sensitive and less dependent on the early bond strengths of different adhesives.