Luis Felipe Jochims Schneider

Influence of photoinitiator type on the rate of polymerization, degree of conversion, hardness and yellowing of dental resin composites

OBJECTIVES To evaluate the degree of conversion (DC), maximum rate of polymerization (Rpmax), Knoop hardness (KHN) and yellowing (b-value) of resin composites formulated with phenylpropanedione (PPD), camphorquinone (CQ), or CQ/PPD at different concentrations. The hypotheses tested were (i) PPD or CQ/PPD would produce less Rpmax and yellowing than CQ alone without affecting DC and KHN, and (ii) Rpmax, DC, and KHN would be directly related to the absorbed power density (PDabs). METHODS CQ/amine, PPD/amine and CQ/PPD/amine were used at low, intermediate and high concentrations in experimental composites. Photoinitiator absorption and halogen-light emission were measured using a spectrophotometer, Rp with differential scanning calorimetry (DSC), DC with DSC and FTIR, KHN with Knoop indentation; and color with a chromameter. The results were analyzed with two-way analysis of variance (ANOVA)/Student-Newman-Keuls test (p<0.05). Correlation tests were carried out between PDabs and each of DC, Rpmax and KHN. RESULTS The PDabs increased with photoinitiator concentration and PPD samples had the lowest values. In general, maximum DC was comparable at intermediate concentration, while Rpmax and KHN required higher concentrations. DC was similar for all photoinitiators, but Rpmax was lower with PPD and CQ/PPD. PPD produced the lowest KHN. Yellowing increased with photoinitiator concentration. PPD did not reduce yellowing at intermediate and/or high concentrations, compared to CQ-formulations. PDabs showed significant correlations with DC, Rpmax and KHN. CONCLUSION PPD or CQ/PPD reduced Rpmax in experimental composites without affecting the DC. The use of PPD did not reduce yellowing, but reduced KHN. DC, Rpmax and KHN were dependent on PDabs.

Shrinkage Stresses Generated during Resin-Composite Applications: A Review

Many developments have been made in the field of resin composites for dental applications. However, the manifestation of shrinkage due to the polymerization process continues to be a major problem. The materials shrinkage, associated with dynamic development of elastic modulus, creates stresses within the material and its interface with the tooth structure. As a consequence, marginal failure and subsequent secondary caries, marginal staining, restoration displacement, tooth fracture, and/or post-operative sensitivity are clinical drawbacks of resin-composite applications. The aim of the current paper is to present an overview about the shrinkage stresses created during resin-composite applications, consequences, and advances. The paper is based on results of many researches that are available in the literature.

Curing efficiency of dental resin composites formulated with camphorquinone or trimethylbenzoyl-diphenyl-phosphine oxide

OBJECTIVES Since photoinitiator systems for dental resins based on camphorquinone (CQ) present color disadvantages, trimethylbenzoyl-diphenyl-phosphine oxide (TPO) has been proposed as an alternative. However, there are remaining considerations about its curing efficiency. The aims of the present investigation were: to characterize the relationship between the photoinitiator absorption spectra and the light spectrum emitted from a QTH light (absorbed power density, PD(abs)); to evaluate the kinetics of polymerization, and the depth of cure for filled dimethacrylate resins formulated with different photoinitiator systems. METHODS CQ+EDMAB (control); TPO and TPO+EDMAB were used in 50:50 Bis-GMA/TEGDMA resins. Photoinitiator absorption and QTH-light emission were evaluated using a spectrophotometer and kinetics of polymerization with differential scanning calorimetry (DSC) (n=3). Depth of cure was analyzed by the scraping method (n=5), as recommended by ISO4049. One-way ANOVA/Tukeys (p<0.05) was used to analyze the results. RESULTS CQ presented higher PD(abs) than TPO (364 and 223 mW/cm(3), respectively). The DSC revealed that TPO and TPO+EDMAB produced a faster reaction than CQ+EDMAB. Composite formulated with CQ+EDMAB produced higher depth of cure (6.3±0.4 mm) than those with TPO (4.3±0.1) or TPO+EDMAB (4.2±0.3). SIGNIFICANCE Although CQ presented higher PD(abs) than TPO, formulations containing TPO exhibited higher reactivity than that with CQ. On the other hand, materials formulated with TPO demonstrated lower depth of cure than that with CQ. Therefore, its use as an alternative photoinitiator requires further investigation, with higher concentrations.

Effect of the increase of energy density on knoop hardness of dental composites light-cured by conventional QTH, LED and xenon plasma arc

The aim of this study was to evaluate the effect of the increase of energy density on Knoop hardness of Z250 and Esthet-X composite resins. Cylindrical cavities (3 mm in diameter X 3 mm in depth) were prepared on the buccal surface of 144 bovine incisors. The composite resins were bulk-inserted and polymerized using different light-curing units and times: conventional QTH (quartz-tungsten-halogen; 700 mW/cm(2); 20 s, 30 s and 40 s); LED (light-emitting diode; 440 mW/cm(2); 20 s, 30 s and 40 s); PAC (xenon plasma arc; 1700 mW/cm(2); 3 s, 4.5 s and 6 s). The specimens were stored at 37 degrees C for 24 h prior to sectioning for Knoop hardness assessment. Three measurements were obtained for each depth: top surface, 1 mm and 2 mm. Data were analyzed statistically by ANOVA and Tukeys test (p<0.05). Regardless of the light source or energy density, Knoop hardness of Z250 was statistically significant higher than that of Esthet-X (p<0.05). Specimens cured with PAC had lower hardness than those cured with QTH and LED (p<0.05). Higher Knoop hardness was obtained when the energy density was increased for LED and PAC (p<0.05). No statistically significant differences (p>0.05) were found for QTH. Knoop hardness values decreased with the increase of depth. The increase of energy density produced composites with higher Knoop hardness means using LED and PAC.

Effect of co-initiator ratio on the polymer properties of experimental resin composites formulated with camphorquinone and phenyl-propanedione

OBJECTIVES To evaluate the effect of amine ratio (ethyl 4-dimethylaminobenzoate, EDMAB) on the maximum rate of polymerization (R(p)(max)), degree of conversion (DC), Knoop hardness (KH), water sorption (Wsp), water solubility (Wsl) and color changes (DeltaE) over time of resin composites formulated with the photoinitiators camphorquinone (CQ), phenylpropanedione (PPD) and CQ-PPD in combination. MATERIALS AND METHODS Experimental resin composites were made with photoinitiator:amine ratios of 2:1, 1:1, 1:1.5 and 1:2 by weight. R(p)(max) was evaluated with differential scanning calorimetry (DSC), DC with DSC and Fourier transformed infrared (FTIR) spectroscopy, KH with Knoop indentation, Wsp and Wsl adapted from ISO 4049; and color with a chromameter. The results were analyzed with two-way ANOVA/Tukeys multiple comparison test (p<0.05). RESULTS The higher the amine ratio in the composite, the higher was DC, R(p)(max), and KH, and the lower was Wsl, regardless of the photoinitiator type. The use of PPD alone resulted in poorer properties than CQ and CQ-PPD. Many factors seem to affect the color changes and the b-axis data revealed that the higher the amine ratio, the higher was the +b value (yellowing) for CQ and CQ-PPD formulations. CONCLUSIONS Higher amine ratios led to improved polymer properties, but also produced more yellowing in resin composites with CQ and CQ-PPD. The use of PPD alone was not advantageous for producing good final properties when compared to CQ and CQ-PPD.

Color stability, conversion, water sorption and solubility of dental composites formulated with different photoinitiator systems

OBJECTIVES The aim of this study was to formulate materials with high color stability and reduced degradation by using photoinitiator systems derived from phosphine oxides alternative to the traditional camphorquinone (CQ)/amine system. METHODS Materials were formulated with the monomers BisGMA and TEGDMA as organic matrix. The photoinitiators tested were CQ+amine (EDMAB), phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO), BAPO+EDMAB, diphenyl(2,4,6-trimethylbenzoyl)-phosphine oxide (TPO) and TPO+EDMAB. A 60% mass of silanated glass filler particles was added. Degree of CC conversion was evaluated by Fourier-transformed infrared spectroscopy 10min after photopolymerization. The color properties were evaluated by a digital spectrophotometer, applying the CIELab parameters, either before photoactivation, immediately after photoactivation, 24h after dry storage, and one month after water immersion. Water sorption and solubility were assessed by mass gain or loss after storage in water for 30 days. Data were analyzed using ANOVA and Tukeys test (5%). RESULTS Degree of conversion did not differ statistically among all the tested formulations. Regarding color, only groups with TPO presented satisfactory color stability after one month of water storage (ΔE<3.3). Water sorption and water stability did not differ among the groups. CONCLUSIONS Degree of conversion, water sorption and water solubility did not differ among all the tested groups. The TPO-based groups were the only materials with satisfactory color stability after one month of water storage. CLINICAL SIGNIFICANCE The use of a photoinitiator system containing TPO might improve the color stability of resin composites compared with the traditional CQ/amine system while attaining similar physico-chemical properties for the composite.

Fluoride-containing adhesive: durability on dentin bonding.

OBJECTIVES To evaluate (1) the influence of fluoride-containing adhesive on microtensile bond strength (microTBS) and (2) in vitro secondary caries inhibition at the resin-dentin interface after 24 h and 3 months water-storage and (3) the degree of conversion of different adhesives after 24h 1 month. METHODS Flat surfaces of human teeth were ground and randomly assigned to six groups: (SBMP-24) Scotchbond Multi-Purpose control [SBMP], 24 h; (SE-24) SBMP etch and primer+Clearfil SE Bond adhesive [SE], 24h; (PB-24) SBMP etch and primer+Clearfil Protect Bond adhesive [PB], 24h; (SBMP-3) SBMP, 3 months; (SE-3) SBMP+SE, 3 months; and (PB-3) SBMP+PB, 3 months. To evaluate the effect of the adhesive resin alone, all teeth were etched with 35% phosphoric acid and primed with SBMP primer prior to applying the adhesive resin. Bonded assemblies were prepared for microTBS and stored in distilled water at 37 degrees C for 24h and 3 months. Sections of restored teeth of each group were exposed to an acid challenge. The specimens were sectioned, polished, and then observed with polarized light microscopy (PLM). Also, the degree of conversion (DC) of the adhesives was measured using Fourier Transform-Infrared spectroscopy (FTIR) at 24 h and 1 month, after polymerization. RESULTS microTBS values obtained in MPa (24h/3m) were: (MP) 61.5+/-10.5/52.9+/-8.9, (SE) 55.5+/-11.8/55.6+/-13, and (PB) 50.3+/-9.9/61.0+/-13.6. For interface analysis by PLM, an inhibition zone (IZ) adjacent to the hybrid layer was created only when the fluoride-containing adhesive (PB) was used. The DC in percentage (24 h, 1 month) were: (MP) 60.5+/-2.8/61.3+/-0.6, (SE) 69.6+/-1.3/70.7+/-0.05, and (PB) 53.1+/-0.4/58.3+/-1.6. SIGNIFICANCE The fluoride-containing adhesive demonstrated significant increase of bond strength values after water-storage. This material was also able to create an acid inhibition zone in dentin. There was a significant increase of degree of conversion after 1 month only for PB.

Alternative photoinitiator system reduces the rate of stress development without compromising the final properties of the dental composite

OBJECTIVES Stress development during the polymerization process continues to be a major factor that limits predictability and longevity of resin composite restorations. This study evaluated the effect of the photoinitiator type on the maximum rate of polymerization (R(p)(max)), stress development (final stress and maximum rate, R(stress)(max)), degree of conversion (DC) and cross-link density (CLD) of materials containing camphorquinone (CQ), phenylpropanedione (PPD) or CQ/PPD. MATERIALS AND METHODS R(p)(max) was evaluated via differential scanning calorimetry (DSC). Contraction force measurement was assessed with a single cantilever device for 5min. The samples were subsequently tested by infrared spectroscopy (FTIR) to evaluate the DC. After, samples were soaked in ethanol to evaluate the swelling coefficient (alpha) as a way to estimate the CLD. The results were analyzed by one-way ANOVA and Tukeys test (p=0.05). RESULTS CQ showed the highest R(p)(max) and R(stress)(max). PPD produced the lowest DC and the highest alpha. The mixture CQ/PPD produced statistically lower R(p)(max) and R(stress)(max) than CQ alone, but similar DC and CLD. CONCLUSION CQ/PPD reduced the R(p)(max) and R(stress)(max) without a reduction in DC and CLD. Therefore, the use of alternative photoinitiator systems could be a promising way to reduce the stress developed during the composites polymerization without affecting the final properties.

Polymerization kinetics and reactivity of alternative initiators systems for use in light-activated dental resins

OBJECTIVES The purpose of this study was to evaluate the reactivity and polymerization kinetics behavior of a model dental adhesive resin with water-soluble initiator systems. METHODS A monomer blend based on Bis-GMA, TEGDMA and HEMA was used as a model dental adhesive resin, which was polymerized using a thioxanthone type (QTX) as a photoinitiator. Binary and ternary photoinitiator systems were formulated using 1mol% of each initiator. The co-initiators used in this study were ethyl 4-dimethylaminobenzoate (EDAB), diphenyliodonium hexafluorophosphate (DPIHFP), 1,3-diethyl-2-thiobarbituric acid (BARB), p-toluenesulfinic acid and sodium salt hydrate (SULF). Absorption spectra of the initiators were measured using a UV-Vis spectrophotometer, and the photon absorption energy (PAE) was calculated. The binary system camphorquinone (CQ)/amine was used as a reference group (control). Twelve groups were tested in triplicate. Fourier-transform infrared spectroscopy (FTIR) was used to investigate the polymerization reaction during the photoactivation period to obtain the degree of conversion (DC) and maximum polymerization rate (R(p)(max)) profile of the model resin. RESULTS In the analyzed absorption profiles, the absorption spectrum of QTX is almost entirely localized in the UV region, whereas that of CQ is in the visible range. With respect to binary systems, CQ+EDAB exhibited higher DC and R(p)(max) values. In formulations that contained ternary initiator systems, the group CQ+QTX+EDAB was the only one of the investigated experimental groups that exhibited an R(p)(max) value greater than that of CQ+EDAB. The groups QTX+EDAB+DPIHFP and QTX+DPIHFP+SULF exhibited values similar to those of CQ+EDAB with respect to the final DC; however, they also exhibited lower reactivity. SIGNIFICANCE Water-soluble initiator systems should be considered as alternatives to the widely used CQ/amine system in dentin adhesive formulations.

Effects of 6 months of aging in water on hardness and surface roughness of two microhybrid dental composites.

PURPOSE This study assessed the effect of 6 months of aging in water on surface roughness and surface/subsurface hardness of two microhybrid resin composites. MATERIALS AND METHODS Filtek Z250 and Charisma were tested. Cylindrical specimens were obtained and stored in distilled water for 24 hours or 6 months, at 37 degrees C. For Knoop hardness evaluation, the specimens were transversely wet-flattened, and indentations were made on surface and subsurface layers. Data were submitted to three-way ANOVA and Tukeys test (alpha < or = 0.05). Surface roughness baseline measurements were made at 24 hours and repeated after 6 months of storage. Data were submitted to repeated measures ANOVA and Tukeys test (alpha < or = 0.05). RESULTS Surface hardness (KHN, kg/mm(2)) means (+/- standard deviation) ranged from 55 +/- 1 to 49 +/- 4 for Z250 and from 50 +/- 2 to 41 +/- 3 for Charisma, at 24 hours and 6 months, respectively. Subsurface means ranged from 58 +/- 2 to 61 +/- 3 for Z250 and from 50 +/- 1 to 54 +/- 2 for Charisma, at 24 hours and 6 months. For both composites, the aged specimens presented significantly softer surfaces (p < 0.01). For the subsurface hardness, alteration after storage was detected only for Charisma, which presented a significant rise in hardness (p < 0.01). Z250 presented significantly harder surface and subsurface layers in comparison with Charisma. Surface roughness (Ra, mum) means ranged from 0.07 +/- 0.00 to 0.07 +/- 0.01 for Z250 and from 0.06 +/- 0.01 to 0.07 +/- 0.01 for Charisma, at 24 hours and 6 months, respectively. For both composites, no significant roughness alteration was detected during the study (p= 0.386). CONCLUSIONS The 6-month period of storage in water presented a significant softening effect on the surfaces of the composites, although no significant deleterious alteration was detected for the subsurface hardness. In addition, the storage period had no significant effect on the surface roughness of the materials.