Gaëtane Leloup

Meta-analytical review of factors involved in dentin adherence.

Literature data on adherence tests of dentin-bonding systems (DBS) may differ widely, even for the same DBS. The problem of bond testing is that materials are seldom compared with a standard, and experimental conditions often vary. We sought to identify the parameters that influence this variability. Using inclusion and exclusion criteria, we conducted a meta-analytical review of 75 articles, published between 1992 and 1996 in SCI reviews, that give bond strength data for 15 dentin-bonding agents of the so-called third and fourth generations. Seventeen selected parameters were classified into four groups: Group A includes factors related to the dentin substrate (i.e., nature of teeth); group B, composite and bonding area (i.e., composite stiffness); group C, storage conditions of the bonded samples (i.e., thermocycling); and group D, test design (i.e., crosshead speed). For each report, the experimental features, the bond strength means and standard deviations, and the failure mode were extracted and tabulated. Statistical Analysis System software was used to perform Pearson correlation analysis and analysis of variance, with bond strength as the dependent variable and experimental conditions as the independent variables. The meta-analytical review highlighted the significant influence of various parameters in the different groups: origin of dentin, types of teeth, pulpal pressure, tooth storage temperature, maximum storage time of teeth, and dentin depth in group A; type and stiffness of composite and bonding area in group B; storage of bonded samples (medium, temperature, and time) in group C, and testing mode and crosshead speed in group D. A significant positive correlation was observed between the mean bond strength and the rate of cohesive failure. It can be concluded from this study that some of these parameters should be controlled by the use of a standardized protocol. Unfortunately, the substrate-related variables are more difficult to control, even though their influence is consistent.

Dynamic and static moduli of elasticity of resin-based materials

OBJECTIVES The purpose of this study was to assess and compare the elastic moduli of 34 resin-based materials using a dynamic and a static method. The effect of water storage was also studied up to 6 months. METHODS Five samples of each material were prepared according to ISO-4049. The dynamic moduli were first determined non-destructively from the fundamental period of the vibrating specimen, then the static moduli were determined by a three-point bending test. The percentages of fillers by weight were determined by ashing in air at 900 degrees C. RESULTS Low values were obtained with flowable composites as well as with two packable resin composites. Correlations were found between the static and the dynamic modulus of elasticity (r = 0.94; p = 0.0001) as well as between the weight percentage of fillers and the moduli of elasticity (r = 0.82; p < 0.05 for static modulus and r = 0.90; p < 0.05 for the dynamic modulus) both at 24h. Water storage significantly affected both static and dynamic moduli of elasticity (ANOVA two factors; p < 0.05). SIGNIFICANCE The low moduli of the flowable composites do not allow their use in posterior cavities under high stress. However, this does not exclude their use for minimally-invasive Class I cavities when the opposing tooth is stabilized to a large amount on the natural enamel. The Grindosonic method is very useful and simple for determining the dynamic moduli although it gives higher values than the static one. The elastic modulus evolution of resin-based materials after water storage is unpredictable since different patterns were observed as a function of time.

The micro-Raman spectroscopy, a useful tool to determine the degree of conversion of light-activated composite resins.

Light-activated composites are now among the most popular dental restorative materials. Nevertheless, concerns exist about the so-called depth of cure. Infrared spectroscopy (FTIR) has traditionally been used to quantify this problem by evaluating the degree of conversion of dental resins. However, Raman scattering provides an alternate method. This article describes the advantages and the limitations of micro-Raman spectroscopy, as compared to FTIR and other techniques, for calculating the local degree of conversion and the depth of cure of light-cured composites.

Investigating filler morphology and mechanical properties of new low-shrinkage resin composite types

Three types of low-shrinkage composites are today commercially available: Ormocers, cationic ring-opening curing systems and highly filled methacrylate-based materials, which cure via free-radical polymerization mechanisms. The aim of this study was to characterize the inorganic fraction of materials belonging to each type and to compare their mechanical properties. Two Ormocers (Admira and an experimental Ormocer V35694), one ring-opening composite (Filtek Silorane) and five methacrylate-based composites [Filtek Supreme XT, Tetric EvoCeram, Grandio, Synergy D6 (Coltène-Whaledent, Langenau, Germany) and an experimental material, V34930] were tested. Inorganic fillers were quantified by thermogravimetric analysis and morphologically characterized by scanning electron microscopy. Dynamic modulus was determined by an impulse excitation technique, static elastic moduli and flexural strength by a three-point bending method. The results were analyzed using ANOVA tests (P < 0.05) and linear correlations. Grandio, V34930 and V35694 exhibited significantly higher filler mass fractions. Both dynamic and static moduli of Grandio and V34930 were significantly higher than the other materials (P < 0.05), although no significant difference in flexural strength was observed between material type (P > 0.05). From the present findings, it was suggested that V35694 and Filtek Silorane exhibit comparable properties to conventional methacrylate-based composites, although clinically the cavity type and location must guide material choice. Under high occlusal load, the use of Grandio and V34930 might be favoured. For small cavities, alternative technologies could be preferred as the need for mechanical resistance is lower and the potential for stress generation is greater.

Photoinitiator type and applicability of exposure reciprocity law in filled and unfilled photoactive resins.

OBJECTIVES To test the influence of photoinitiator type and filler particle inclusion on the validity of exposure reciprocity law. MATERIALS AND METHODS 50/50 wt% Bis-GMA/TEGDMA resins were prepared with equimolar concentrations of camphorquinone/DMAEMA (0.20/0.80 mass%) (CQ) or Lucirin-TPO (0.42 mass%), and were used either unfilled or filled to 75 mass%. Specimens were cured with a halogen Swiss Master Light (EMS, Switzerland) using four different curing protocols: 400 mW/cm² for 45 s as reference protocol (18 J/cm²), 1500 mW/cm² for 12 s (18 J/cm²), 3000 mW/cm² for 6 s (18 J/cm²) and 3 s (9 J/cm²). Degree of conversion (DC) was measured in real time for 70 s by FT-NIRS and temperature rise using a thermocouple. Depth of cure was determined with a penetrometer technique. RESULTS With respect to DC and depth of cure, exposure reciprocity law did not hold for any tested material, except for the depth of cure of filled CQ-based materials. At similar radiant exposure, DC was significantly higher (p<0.05) for all unfilled and filled TPO-based materials compared with CQ-based materials. As exposure time was reduced and irradiance increased, TPO-based materials exhibited higher DC whilst an opposite trend was observed for CQ-based materials (p<0.05). For similar curing regimes, depth of cure of CQ-based materials remained significantly greater than that of TPO-based materials. Adding fillers generally reduced DC, except at higher irradiance for CQ-based materials where a positive effect was observed (p<0.05). SIGNIFICANCE The validity of exposure reciprocity law was dependent on several factors, among which photoinitiator type and filler content were important. Lucirin-TPO is a highly reactive and efficient photoinitiator, which may allow the potential for a reduction in curing time of TPO-based photoactive materials in thin sections.

Influence of curing protocol on selected properties of light-curing polymers: Degree of conversion, volume contraction, elastic modulus, and glass transition temperature

OBJECTIVES The purpose of this study was to investigate the effect of light-curing protocol on degree of conversion (DC), volume contraction (C), elastic modulus (E), and glass transition temperature (T(g)) as measured on a model polymer. It was a further aim to correlate the measured values with each other. METHODS Different light-curing protocols were used in order to investigate the influence of energy density (ED), power density (PD), and mode of cure on the properties. The modes of cure were continuous, pulse-delay, and stepped irradiation. DC was measured by Raman micro-spectroscopy. C was determined by pycnometry and a density column. E was measured by a dynamic mechanical analyzer (DMA), and T(g) was measured by differential scanning calorimetry (DSC). Data were submitted to two- and three-way ANOVA, and linear regression analyses. RESULTS ED, PD, and mode of cure influenced DC, C, E, and T(g) of the polymer. A significant positive correlation was found between ED and DC (r=0.58), ED and E (r=0.51), and ED and T(g) (r=0.44). Taken together, ED and PD were significantly related to DC and E. The regression coefficient was positive for ED and negative for PD. Significant positive correlations were detected between DC and C (r=0.54), DC and E (r=0.61), and DC and T(g) (r=0.53). Comparisons between continuous and pulse-delay modes of cure showed significant influence of mode of cure: pulse-delay curing resulted in decreased DC, decreased C, and decreased T(g). Influence of mode of cure, when comparing continuous and step modes of cure, was more ambiguous. SIGNIFICANCE A complex relationship exists between curing protocol, microstructure of the resin and the investigated properties. The overall performance of a composite is thus indirectly affected by the curing protocol adopted, and the desired reduction of C may be in fact a consequence of the decrease in DC.

Pulpal-temperature Rise and Polymerization Efficiency of LED Curing Lights

This paper assesses the effects of light characteristics and irradiation time on the Vickers microhardness (VH) of a dual-photoinitiator commercial composite and on temperature increase in the pulp chamber (deltaT). Four recent light-emitting diodes (LEDs)--bluephaseG2 (BG2), bluephase16i (B16i), G-Light (G) and Freelight2 (F2)--and one control halogen light (XL3000-X) were tested on two shades of Tetric EvoCeram (A2 and Bleach XL), whose respective commercial formulations differed based on their concentration of camphorquinone and lucirin TPO. Three different irradiation times were applied--10, 20 and 40 seconds-and VH was measured on the upper and lower surfaces of 2-mm thick samples. The deltaT was measured by using a K-type thermocouple inserted into the pulp chamber of a molar that had been prepared to obtain a 2-mm thickness of dentin. The measurements were made either during polymerization of a 2 mm composite (Shade A2 or Bleach) or with an empty mold. The data were analyzed with the two-way ANOVA (p < 0.05) test. For shade A2, all but one irradiation condition (F2-10 seconds, lower surface) generated VH values that were statistically equal to or better than the standard chosen for this study (X-40 seconds). For Bleach shade, the VH values obtained with G and BG2-20 and 40 seconds were statistically comparable to X-40 seconds for both the upper and lower surfaces. This was not the case with either G and BG2-10 seconds or for all the procedures with other LCUs for which a VH of at least one of the surfaces was significantly lower than the reference. The results also highlight differences between the two material shades, whether the upper or lower surface is considered. Regarding temperature measurements for shade A2, B16i-20-40 seconds, BG2-40 seconds and G-40 seconds induced significantly higher deltaTs (3.98, 5.98, 5.21 and 4.95, respectively) than X-40 seconds (3.09). For Bleach shade, B16i-20 and 40 seconds, F2-20 and 40 seconds, BG2-40 seconds and G-40 seconds generated deltaTs significantly higher than the control values (2.70, 4.05, 3.03, 4.58, 2.74 and 2.44, respectively). The deltaT values obtained with uncovered tooth were generally higher than those obtained with a 2-mm layer of composite. In conclusion, this research emphasizes that a perfect correspondence between light and material spectra is of prime concern, both to insure optimal polymerization and to limit heating in the pulp chamber. Some reduction in curing time is possible, but only within certain limits.

Physical, mechanical and rheological characterization of resin-based pit and fissure sealants compared to flowable resin composites

OBJECTIVES The purpose of this study was to compare the mechanical and rheological properties of resin-based pit and fissure sealants to flowable resin composites in order to define clinical indications based on these properties. METHODS Eight flowable resin composites (Admira Flow, Filtek Supreme XT Flow, FlowLine, Grandio Flow, Point-4 Flowable, Premise Flowable, Revolution Formula 2, X-Flow) and four resin-based pit and fissure sealants (Clinpro, Delton FS(+), Estiseal F, Guardian Seal) were used in this study. Their filler weight content was measured by thermogravimetric analysis. Mechanical properties were measured: dynamic and static moduli of elasticity, flexural strength and Vickers microhardness. Rheological measurements were performed using a dynamic oscillation rheometer. RESULTS Flowable resin composites have by far better mechanical properties than pit and fissure sealants, except for Delton FS(+). All the materials tested are non-Newtonian, shear thinning fluids. They all showed elasticity even at the lowest frequencies but elasticity differs pretty much from one material to another. SIGNIFICANCE Resin-based pit and fissure sealants seem appropriate for preventive pit and fissure sealing. For enlarged fissures, it can be assumed that flowable resin composites with low elasticity at low frequency are more appropriate.

Irradiation Modes' Impact on Radical Entrapment in Photoactive Resins.

Different irradiation protocols are proposed to polymerize dental resins, and discordances remain concerning their impact on the material. To improve this knowledge, we studied entrapment of free radicals in unfilled Bis-GMA/TEGDMA (50:50 wt%) resin after light cure. The tested hypothesis was that various irradiation parameters (curing time, irradiance, and radiant exposure) and different irradiation modes (continuous and pulse-delay) led to different amounts of trapped free radicals. The analysis of cured samples (n = 3) by electron paramagnetic resonance (EPR) revealed that the concentrations of trapped free radicals significantly differed according to the curing protocol. When continuous modes with similar radiant exposure were compared, higher concentrations of trapped free radicals were measured for longer times with lower irradiance. Concerning pulse modes, the delay had no influence on trapped radical concentration. These results give new insights into the understanding of the photopolymerization process and highlight the relevance of using EPR when studying polymerization of dimethacrylate-based materials.

Ultra-fast light-curing resin composite with increased conversion and reduced monomer elution

OBJECTIVES To test the null hypotheses that photoactive resin composites containing a Type I photoinitiator would exhibit reduced DC or increased monomer elution at substantially short curing times compared with materials based on a Type 2 ketone/amine system. METHODS Two experimental resin composites were prepared, using either Lucirin-TPO or camphorquinone/DMAEMA. Specimens were light-cured using appropriate spectral emission that coincided with the absorption properties of each initiator using different irradiation protocols (0.5, 1, 3, 9s at 500, 1000 and 2000mW/cm(2) for Lucirin-TPO based composites and 20 or 40s at 1000mW/cm(2) for Lucirin-TPO and camphorquinone-based composites). Degree of conversion (DC) was measured by Raman spectroscopy, propagating radical concentrations were collected by means of electron paramagnetic resonance (EPR) and monomer leaching was characterized using high-performance liquid chromatography (HPLC). RESULTS The null hypotheses were rejected, except for a single irradiation protocol (0.5s @ 500mW/cm(2)). Lucirin-TPO-based composites could cure 20 times faster and release at least 4 times less monomers in comparison to camphorquinone-based composites. At 1000mW/cm(2), and 1s irradiation time for curing times of 1s, Lucirin-TPO based composites displayed 10% higher DC. The difference in polymerization efficiency of Lucirin-TPO compared with camphorquinone-based resin composites were explained using EPR; the former showing a significantly greater yield of radicals which varied logarithmically with radiant exposure. SIGNIFICANCE Lucirin-TPO is substantially more efficient at absorbing and converting photon energy when using a curing-light with an appropriate spectral emission and otherwise a limitation noted in several previous publications. At concentrations of 0.0134mol/L, Lucirin-TPO-based composites require a minimum light intensity of 1000mW/cm(2) and an exposure time of 1s to provide significantly improved DC and minimal elution compared with a conventional photoinitiator system. The use of a wide range of curing protocols in the current experiment has realized the significant potential of Lucirin-TPO and its impact for clinical applications, in replacement to materials using camphorquinone.