Comparison of polymerization contraction stresses between self- and light-curing composites

Application and limitations of configuration factor (C-factor) in stress analysis of dental restorations

OBJECTIVE

The configuration factor (C-factor) is an index used to evaluate the relationship between cavity configuration and the development of polymerization shrinkage stress in dental restorations. Although C-factor has been widely researched, its correlation with stress analysis in dental restorations remains controversial. This review aims to discuss the application and limitations of C-factor and define the restricted conditions under which the C-factor "rule of thumb" is applicable.

METHODS

A thorough literature review was conducted on the application and limitations of C-factor in stress analysis of dental restorations. This was principally based on MEDLINE/PubMed and Web of Science databases and a review of the relevant studies and publications in scientific papers in international peer-reviewed journals for the specific topic of C-factor and polymerization shrinkage.

RESULTS

The C-factor alone cannot provide an accurate prediction of the shrinkage stress of restorations and the mechanical behavior of material-tooth interfaces. C-factor is only applicable under one condition not typically seen in clinical practice: low, near-rigid compliance.

SIGNIFICANCE

Conditions for the application of C-factor have been explicitly defined. A more accurate and precise understanding and utilization of the C-factor is of benefit as it contributes to better understanding of polymerization shrinkage behavior of restorations.

Microleakage and Marginal Integrity of Ormocer/Methacrylate-Based Bulk-Fill Resin Restorations in MOD Cavities: SEM and Stereomicroscopic Evaluation

This in vitro study aimed to compare the microleakage and marginal integrity of methacrylate/ormocer-based bulk-fill composite (BFC) restorations used in cervical marginal relocation with two different layering thicknesses in mesio-occlusal-distal (MOD) cavities exposed to thermo-mechanical loading. Standard MOD cavities were prepared in 60 mandibular molars and assigned into three groups: x-tra fil/AF + x-tra base/XB, Tetric N-Ceram Bulk Fill/TNB + Tetric N-Flow Bulk Fill/TFB, and Admira Fusion x-tra/AFX + Admira Fusion x-base/AFB. Each group was further divided into two subgroups (2 mm and 4 mm) based on the thickness of flowable BFCs (n = 10). The specimens were subjected to thermo-mechanical loading (240,000 cycles) and immersed in 0.2% methylene blue. Following mesiodistal sectioning, the specimens were examined under stereomicroscope (×25) and scored (0-3) for microleakage. Marginal integrity was examined using a scanning electron microscope (SEM). Descriptive statistical methods and the chi-square test were used to evaluate the data (p < 0.05). While there was no statistically significant difference in gingival cement microleakage in the XB and AFB specimens with a 4 mm thickness, microleakage was significantly increased in the TFB specimen (p = 0.604, 0.481, 0.018 respectively). A significantly higher amount of score 0 coronal microleakage was detected in the AFX2 mm + AFB4 mm compared to the TNB2 mm + TFB4 mm (p = 0.039). The SEM examination demonstrated better marginal integrity in groups with 2 mm thick flowable BFCs. Ormocer and methacrylate-based materials can be used in marginal relocation with thin layers.

Can TPO as Photoinitiator Replace "Golden Mean" Camphorquinone and Tertiary Amines in Dental Composites? Testing Experimental Composites Containing Different Concentration of Diphenyl(2,4,6-trimethylbenzoyl)phosphine Oxide

The aim of this research was to compare the biomechanical properties of experimental composites containing a classic photoinitiating system (camphorquinone and 2-(dimethylami-no)ethyl methacrylate) or diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) as a photoinitiator. The produced light-cured composites consisted of an organic matrix-Bis-GMA (60 wt.%), TEGDMA (40 wt.%) and silanized silica filler (45 wt.%). Composites contained 0.27; 0.5; 0.75 or 1 wt.% TPO. Vickers hardness, microhardness (in the nanoindentation test), diametral tensile strength, resistance to three-point bending and the CIE L* a* b* colorimetric analysis was performed with each composite produced. The highest average Vickers hardness values were obtained for the composite containing 1 wt.% TPO (43.18 ± 1.7HV). The diametral tensile strength remains on regardless of the type and amount of photoinitiator statistically the same level, except for the composite containing 0.5 wt.% TPO for which DTS = 22.70 ± 4.7 MPa and is the lowest recorded value. The highest average diametral tensile strength was obtained for the composite containing 0.75 wt.% TPO (29.73 ± 4.8 MPa). The highest modulus of elasticity characterized the composite containing 0.75 wt.% TPO (5383.33 ± 1067.1 MPa). Composite containing 0.75 wt.% TPO has optimal results in terms of Vickers hardness, diametral tensile strength, flexural strength and modulus of elasticity. Moreover, these results are better than the parameters characterizing the composite containing the CQ/DMAEMA system. In terms of an aesthetic composite containing 0.75 wt.%. TPO is less yellow in color than the composite containing CQ/DMAEMA. This conclusion was objectively confirmed by test CIE L* a* b*.

Time-lapse submicrometer particle motion reveals residual strain evolution and damaging stress relaxation in clinical resin composites sealing human root canals

Polymer based composites are widely used for treatment, for example as biofilm resistant seals of root canal fillings. Such clinical use, however, fails more frequently than other dental composite restorations, due to stress-related misfits. The reason for this is that the biomaterials used are inserted as viscous masses that may bond to the substrate, yet shrinkage stresses arising during setting of the cross-linking polymer, work against durable adhesion. Here we combine phase contrast enhanced time-lapse radiography (radioscopy), digital image correlation (DIC) and submicrometer resolution phase-contrast enhanced microtomography (PCE-CT), to reveal the spatial and temporal dynamics of composite polymerization and strain evolution. Radioscopy of cavities located in the upper part of human root canals demonstrates how the composite post-gelation "densification" is dominated by viscous flow with quantifiable motion of both particles and entrapped voids. Thereafter, these composites enter a "stress-relaxation" stage and exhibit several structural adaptations, induced by residual shrinkage stresses. Consequently critical alterations to the final biomaterial geometry emerge: (i) entrapped bubbles expand; (ii) microscopic root filling pull-out occurs; (iii) the cavity walls deform inwards, and (iv) occasionally delamination ensues, propagating out from the root canal filling along buried restoration-substrate interfaces. Our findings shed new light on the interactions between confined spaces and biomedical composites that cross-link in situ, highlighting the crucial role of geometry in channeling residual stresses. They further provide new insights into the emergence of structural flaws, calling attention to the need to find new treatment options. STATEMENT OF SIGNIFICANCE: This work quantifies recurring spatial and temporal material redistribution in composites used clinically to fill internal spaces in teeth. This knowledge is important for both promoting biomaterial resistance against potentially pathologic biofilms and for improving structural capacity to endure years of mechanical function. Our study demonstrates the significant role of geometry and the need for improved control over stress raisers to develop better treatment protocols and new space filling materials. The use of high-brilliance X-rays for time-lapse imaging at submicrometer resolution provides dynamic information about the damaging effects of stress relaxation due to polymerization shrinkage.

Changes of residual stresses on the surface of leucite-reinforced ceramic restoration luted with resin composite cements during aging in water

The objective of this study was to compare the changes in the residual stresses present on the surface of leucite-reinforced dental ceramic restorations luted with a self-adhesive and a conventional resin composite cement during aging in water. Ring specimens made of a leucite-reinforced ceramics were luted to ceramic cylinders using a self-adhesive (Panavia SA Luting Plus) or a conventional resin composite cement (Panavia V5) in dual-cure or self-cure mode. Residual stresses on the ring surface were measured using indentation fracture method at 1 h, 1, 3, 7, 14 and 28 days of the 37 °C water immersion. Water sorption, water solubility and elastic modulus of the cements were also measured. Compressive stress was generated on the surface of the ceramic rings by the polymerization of the resin composite cements, and the stresses appeared to decrease over time by water sorption of the cements. The dual-cured conventional resin composite cement remained compressive stresses on the ceramic surface, while only the self-cured self-adhesive cement, which demonstrated the greatest water sorption, generated tensile stresses during the four weeks of aging in water. The elastic moduli of cements did not significantly change through the immersion, suggesting that the stresses were less affected by the modulus. To prevent the generation of tensile stresses on the leucite-reinforced ceramic restoration, self-adhesive cements exhibiting small water sorption should be clinically selected.

The Photoinitiators Used in Resin Based Dental Composite-A Review and Future Perspectives

The presented paper concerns current knowledge of commercial and alternative photoinitiator systems used in dentistry. It discusses alternative and commercial photoinitiators and focuses on mechanisms of polymerization process, in vitro measurement methods and factors influencing the degree of conversion and hardness of dental resins. PubMed, Academia.edu, Google Scholar, Elsevier, ResearchGate and Mendeley, analysis from 1985 to 2020 were searched electronically with appropriate keywords. Over 60 articles were chosen based on relevance to this review. Dental light-cured composites are the most common filling used in dentistry, but every photoinitiator system requires proper light-curing system with suitable spectrum of light. Alternation of photoinitiator might cause changing the values of biomechanical properties such as: degree of conversion, hardness, biocompatibility. This review contains comparison of biomechanical properties of dental composites including different photosensitizers among other: camphorquinone, phenanthrenequinone, benzophenone and 1-phenyl-1,2 propanedione, trimethylbenzoyl-diphenylphosphine oxide, benzoyl peroxide. The major aim of this article was to point out alternative photoinitiators which would compensate the disadvantages of camphorquinone such as: yellow staining or poor biocompatibility and also would have mechanical properties as satisfactory as camphorquinone. Research showed there is not an adequate photoinitiator which can be as sufficient as camphorquinone (CQ), but alternative photosensitizers like: benzoyl germanium or novel acylphosphine oxide photoinitiators used synergistically with CQ are able to improve aesthetic properties and degree of conversion of dental resin.

Polymerization shrinkage kinetics and degree of conversion of resin composites

This study compared shrinkage strain, polymerization shrinkage kinetics, and degree of conversion (DC) of a set of resin composites and investigated their influencing factors. Ten commercial resin composites were assessed, and 5 specimens (n = 5) were developed for material and subjected to light curing using light emitting diode light at 650 mW/cm² for 40 s. The laser triangulation method was adopted to assess the shrinkage strain, and Fourier transform infrared spectroscopy was used to measure DC. The shrinkage strain was monitored for 5 min in real time and its data were subjected to differential calculations to get the shrinkage strain rate curve with respect to time, obtaining the maximum shrinkage strain rate (R_max) and gel time. The values of shrinkage strain varied from 1.28% to 2.10%. The R_max values were between 5.17 μm/s and 21.83 μm/s. Gel time values varied from 3.08 s to 4.32 s. The DC yielded values ranging from 53.62% to 87.01%. The values of polymerization shrinkage and DC were dependent on the composition of materials, including the monomer matrix and filler system. Compared to the micro-filler materials, the nano-filler resin composites had higher values of DC. Some resin composites are suitable for clinical applications because of their superior polymerization shrinkage properties and DC.

Fluorinated Montmorillonite and 3YSZ as the Inorganic Fillers in Fluoride-Releasing and Rechargeable Dental Composition Resin

Dental caries (tooth decay) is the most frequent oral disease in humans. Filling cavities with a dental restorative material is the most common treatment, and glass ionomer cements are the main fluoride ion release restorative materials. The goal of this study was to develop a restorative compound with superior fluoride ion release and recharge abilities. Previously developed fluorinated bentolite and hydrophobized 3YSZ were used as two different inorganic fillers mixed in a bisphenol A-glycidyl methacrylate (Bis-GMA) matrix. XRD, FTIR, and TGA were used to determine the hydrophobic modification of these two inorganic fillers. In mechanical tests, including diameter tensile strength, flexural strength, and wear resistance, the developed composite resin was significantly superior to the commercial control. A WST-1 assay was used to confirm that the material displayed good biocompatibility. Furthermore, the simulation of the oral environment confirmed that the composite resin had good fluoride ion release and reloading abilities. Thus, the composite resin developed in this study may reduce secondary caries and provide a new choice for future clinical treatments.

Fluorinated Montmorillonite Composite Resin as a Dental Pit and Fissure Sealant

Molar pits and fissures tend to be affected by caries due to cleaning difficulties. As such, the filling of pits and cracks with sealants is common to deter the onset of caries. However, current clinical practices rely on sealants that lack the ability to release and recharge fluoride ions. Thus, we herein report the development of a fluoride-montmorillonite nanocomposite resin that has the potential to provide sustained release of fluoride due to the strong adsorption of fluoride by montmorillonite. X-ray diffractometry, thermogravimetric analysis, and Fourier-transform infrared spectroscopy were employed to confirm the successful insertion of the polymer into the interlayer structure. The mechanical properties (viscosity, hardening depth, hardness, diametral tensile strength, flexural strength, and wear resistance) of the developed composite resin were then examined, and simulation of the oral environment demonstrated a good fluoride ion release and recharge ability for the effective prevention of dental caries. Finally, we demonstrated the non-cytotoxic nature of this material using the water-soluble tetrazolium salt (WST-1) test. We expect that the described fluoride-containing composite resin may become a new clinical option in the near future.

Different depth-related polymerization kinetics of dual-cure, bulk-fill composites

OBJECTIVE

The aim of this study was to evaluate the polymerization kinetics qualitatively and quantitatively for dual-cure bulk-fill composites in comparison with light-cure bulk-fill and traditional incremental composites at two clinically relevant depths.

METHODS

Five commercial dental composites were evaluated, including three dual-cure bulk-fill composites (BulkEZ, HyperFIL and Injectafil), one light-cure bulk-fill composite Filtek Bulk Fill Flowable (FBF) and one traditional incremental composite Filtek Z250 (Z250) as controls. Specimens were prepared in two different depths (0.5 mm and 5 mm) for 20 s light irradiation. Self-cure was also evaluated for the three dual-cure composites. The polymerization kinetics were measured continuously in real-time for at least 10 min using a Fourier-transform infrared spectroscopy (FTIR) with an attenuated total reflectance (ATR) accessory. The experimental kinetic data were fitted using two mathematical models - a sigmoidal function and a superposition of two exponential functions characterizing the gel phase and glass phase. The degree of conversion (DC) and the rate of polymerization were calculated for all test conditions.

RESULTS

Both experimental FTIR measurements and mathematical modeling revealed distinct depth-related polymerization kinetics for BulkEZ compared to the other two dual-cure composites. Specifically, BulkEZ exhibited moderately-paced polymerization kinetics at both depths while HyperFIL and Injectafil exhibited faster polymerization at 0.5 mm and slower polymerization at 5 mm. The bulk-fill FBF and incremental Z250 exhibited relatively fast polymerization at both depths, a characteristic for light-cure. The DC values at the two depths were not significantly different for BulkEZ, but significantly higher at 0.5 mm than at 5 mm for the other four composites (α = 0.05).

SIGNIFICANCE

Polymerization kinetics and their depth variation for dual-cure bulk-fill composites are material dependent. The distinct depth-related polymerization kinetics revealed for BulkEZ compared to other composites may affect their contraction stress and clinical performance.

Delayed Photoactivation of Dual-cure Composites: Effect on Cuspal Flexure, Depth-of-cure, and Mechanical Properties

Objectives:

This study tested whether delayed photoactivation could reduce shrinkage stresses in dual-cure composites and how it affected the depth-of-cure and mechanical properties.

Methods and Materials:

Two dual-cure composites (ACTIVA and Bulk EZ) were subjected to two polymerization protocols: photoactivation at 45 seconds (immediate) or 165 seconds (2 minutes delayed) after extrusion. Typodont premolars with standardized preparations were restored with the composites, and cuspal flexure caused by polymerization shrinkage was determined with three-dimensional scanning of the external tooth surfaces before restoration (baseline) and at 10 minutes and one hour after photoactivation. Bond integrity (intact interface) was verified with dye penetration. Depth-of-cure was determined by measuring Vickers hardness through the depth at 1-mm increments. Elastic modulus and maximum stress were determined by four-point bending tests (n=10). Results were analyzed with two- or three-way analysis of variance and pairwise comparisons (Bonferroni; α=0.05).

Results:

Delayed photoactivation significantly reduced cuspal flexure for both composites at 10 minutes and one hour (p≤0.003). Interface was &gt;99% intact in every group. Depth-of-cure, elastic modulus, and flexural strength were not significantly different between the immediate and delayed photoactivation (p&gt;0.05). The hardness of ACTIVA reduced significantly with depth (p&lt;0.001), whereas the hardness of Bulk EZ was constant throughout the depth (p=0.942).

Conclusions:

Delayed photoactivation of dual-cure restorative composites can reduce shrinkage stresses without negatively affecting the degree-of-cure or mechanical properties (elastic modulus and flexural strength).

µCT-3D visualization analysis of resin composite polymerization and dye penetration test of composite adaptation

This study evaluated the effects of the light curing methods and resin composite composition on composite polymerization contraction behavior and resin composite adaptation to the cavity wall using μCT-3D visualization analysis and dye penetration test. Cylindrical cavities were restored using Clearfil tri-S Bond ND Quick adhesive and filled with Clearfil AP-X or Clearfil Photo Bright composite. The composites were cured using the conventional or the slow-start curing method. The light-cured resin composite, which had increased contrast ratio during polymerization, improved adaptation to the cavity wall using the slow-start curing method. In the μCT-3D visualization method, the slow-start curing method reduced polymerization shrinkage volume of resin composite restoration to half of that produced by the conventional curing method in the cavity with adhesive for both composites. μCT-3D visualization method can be used to detect and analyze resin composite polymerization contraction behavior and shrinkage volume as 3D image in the cavity.

Improvements in self-curing composites

PURPOSE

The purpose of this study was to test the influence of a barbituric acid derivative acting as a catalyst and small amounts of pyrolytic silica in acrylic resins on color stability, solubility and sorption of a composite.

MATERIALS AND METHODS

A series of two-component powder/liquid resin systems were prepared. Monomer-like mixtures (bis-GMA, TEGDMA, tertiary amine 60/40) and a quartz powder with additions of various silica and barbituric acid derivatives were used. Temperature of the material during polymerization was measured with the use of a thermometer. In addition, the material's flexural and compressive strength, sorption and solubility were tested pursuant to ISO4049:2009.

RESULTS

The powder-based acrylic composition in a liquid mixed immediately before use, after an addition of a 0.5% barbituric acid derivative, has a lower temperature during the polymerization process (a reduction from 43°C to 37°C), whereas color stability over time is improved, with ΔE=1.81 for samples of powder mixtures containing between 0.45% of BPO and 0.15% of barbituric acid derivatives. For silanized quartz powder with 0.55% BPO and 0.1% BA+0.5% Aerosil R711, the obtained sorption value was 4.57±0.22μg/mm³, whereas solubility was 1.60±0.32μg/mm³.

CONCLUSIONS

New catalytic system with barbituric acid derivative, improves color stability for samples stored at room condition and under light of high intensity. A two-phase composite (bis GMA TEGDMA/Quartz), with a new catalytic system with barbituric acid derivatives, has a lower self-cured temperature. Adding a small quantity of hydrophobic silica (0.5%) has a significant influence, with reduced sorption and solubility of the material.

Polymerization Shrinkage Influences Microtensile Bond Strength

Shrinkage results from a complex spatial strain network, producing movements within materials. The purpose of this study was to test whether microtensile bond strength (μTBS) of a light-curing resin composite to enamel depends on distance to the center of the curing mass. Labial surfaces of bovine incisors were ground flat, divided into 2 groups (n = 8), acid-etched, and coated with an unfilled resin bond. A resin-based composite was placed in one increment (group A) or separately at gingival, central, and incisal sites (group B), and light-cured. Teeth were sectioned, yielding stick-shaped specimens assigned to one of 9 groups according to distance to incisal edge of restoration (NDistanc). Microtensile bond strength was transformed to percentages of its maximum values within each tooth (PMPa). Comparisons within groups showed (group A) that mean PMPa decreased from central to gingival and from central to incisal (p < 0.01). Comparisons between groups showed that mean PMPa was significantly lower in group A compared with group B, only at gingival and incisal sites. Microtensile bond strength significantly decreased as the distance increased to the center of the curing mass.

Effects of C-factor on bond strength to floor and wall dentin

This study evaluated the effect of C-factor on the bond strength of a resin composite to floor and wall dentin using two adhesive systems. Box-form cavities were prepared on human molars, following the walls of half of the cavities were removed to create flat dentin surfaces for bonding. Each specimen was then restored using one of two adhesives (Clearfil SE Bond or Clearfil tri-S Bond) and filled or built up using Z100 resin composite. After light-curing, the specimen was cut perpendicular to the bonded surface parallel to the floor or wall to obtain beams. The micro-tensile bond strength to the flat floor, flat wall, cavity floor, or cavity wall was determined. Resin composite bonded more strongly to the flat wall dentin than to the flat floor dentin, regardless of the adhesive system used. Bonding to the cavity wall was higher affected by C-factor than to the cavity floor.

Polymerization shrinkage kinetics and shrinkage-stress in dental resin-composites

OBJECTIVES

To investigate a set of resin-composites and the effect of their composition on polymerization shrinkage strain and strain kinetics, shrinkage stress and the apparent elastic modulus.

METHODS

Eighteen commercially available resin-composites were investigated. Three specimens (n=3) were made per material and light-cured with an LED unit (1200mW/cm(2)) for 20s. The bonded-disk method was used to measure the shrinkage strain and Bioman shrinkage stress instrument was used to measure shrinkage stress. The shrinkage strain kinetics at 23°C was monitored for 60min. Maximum strain and stress was evaluated at 60min. The shrinkage strain rate was calculated using numerical differentiation.

RESULTS

The shrinkage strain values ranged from 1.83 (0.09) % for Tetric Evoceram (TEC) to 4.68 (0.04) % for Beautifil flow plus (BFP). The shrinkage strain rate ranged from 0.11 (0.01%s(-1)) for Gaenial posterior (GA-P) to 0.59 (0.07) %s(-1) for BFP. Shrinkage stress values ranged from 3.94 (0.40)MPa for TET to 10.45 (0.41)MPa for BFP. The apparent elastic modulus ranged from 153.56 (18.7)MPa for Ever X posterior (EVX) to 277.34 (25.5) MPa for Grandio SO heavy flow (GSO).

SIGNIFICANCE

The nature of the monomer system determines the amount of the bulk contraction that occurs during polymerization and the resultant stress. Higher values of shrinkage strain and stress were demonstrated by the investigated flowable materials. The bulk-fill materials showed comparable result when compared to the traditional resin-composites.

Environmental SEM and dye penetration observation on resin-tooth interface using different light curing method

The aim of this study was the effects of different light curing methods on marginal sealing and resin composite adaptation to the cavity wall using the dye penetration test and environmental scanning electron microscope (SEM) observations. Cylindrical cavities were prepared on cervical regions. The teeth were restored with Clearfil Liner Bond 2 V adhesive and filled with Clearfil Photo Bright or Palfique Estelite resin composites. These resins were cured with a conventional light-curing method or a slow-start curing method. After thermal cycling, the specimens were subjected to the dye penetration test to evaluate marginal sealing and adaptation of the resin composites to the cavity walls. These resin-tooth interfaces were then observed using environmental SEM. The light-cured resin composite, which exhibited increased contrast ratios during polymerization, suggests high compensation for polymerization stress using the slow-start curing method. There was a high correlation between dye penetration test and environmental SEM observation.

Polymerization stress--is it clinically meaningful?

OBJECTIVES

The objective of this article is to discuss the evidence for polymerization shrinkage and shrinkage stress of dental composite restoratives in terms of its potential relevance to the clinical situation

METHODS

Articles relating to the issue of polymerization contraction stress generation in dental composite materials, and the factors that influence it, were reviewed and included. Particular attention was paid to evidence derived from clinical studies. Articles were identified through PubMed and through the bibliographies of other articles.

RESULTS

There is extensive evidence for the presence of polymerization contraction stress in dental composites, as well as evidence for its deleterious effects, which include marginal leakage, gap formation, cuspal deflection, tooth cracking, reduced bond strength and lowered mechanical properties of the restorative. There is little, if any, direct evidence for the clinical effect of these contraction stresses. No study has directly established a link between these stresses and enhanced postoperative sensitivity or recurrent caries, for example. However, the concern over these stresses and the manner in which they influence the placement of current composite materials demonstrates that they are considered to be very important.

CONCLUSION

Though no direct evidence exists to prove that the generation of contraction stress in dental composite restorations causes reduced clinical longevity, the indirect evidence from numerous in vitro studies and the concern over controlling their effects proves that they are clinically relevant.

Effects of light curing method and resin composite composition on composite adaptation to the cavity wall

This study aimed to evaluate the effects of the light curing method and resin composite composition on marginal sealing and resin composite adaptation to the cavity wall. Cylindrical cavities were prepared on the buccal or lingual cervical regions. The teeth were restored using Clearfil Liner Bond 2V adhesive system and filled with Clearfil Photo Bright or Palfique Estelite resin composite. The resins were cured using the conventional or slow-start light curing method. After thermal cycling, the specimens were subjected to a dye penetration test. The slow-start curing method showed better resin composite adaptation to the cavity wall for both composites. Furthermore, the slow-start curing method resulted in significantly improved dentin marginal sealing compared with the conventional method for Clearfil Photo Bright. The light-cured resin composite, which exhibited increased contrast ratios duringpolymerization, seems to suggest high compensation for polymerization contraction stress when using the slow-start curing method.

Effect of remaining dentin thickness on bond strength of various adhesive systems to dentin

The aim of this study was to evaluate the effect of remaining dentin thickness (RDT) on resin composite bond strength to dentin surfaces when using various adhesive systems. One of three adhesives, Clearfil SE Bond, Single Bond or Clearfil Tri-S Bond, followed by Z100 resin composite were built up on flat dentin surfaces of human molars. The teeth were sectioned obtaining beams with crosssectional areas of approximately 1 mm(2). RDT was measured and microtensile bond strength was determined. Resulting data were categorized into four groups: RDT<2 mm; 2≤RDT<3 mm; 3≤RDT<4 mm; RDT≥4 mm. Clearfil SE Bond showed a correlation between µTBS and RDT. Single Bond showed no significant difference in µTBS for any RDT. The bond strength of resin composite to the different RDT flat dentin surfaces was affected by the adhesive system used.

Acceleration of curing of resin composite at the bottom surface using slow-start curing methods

The aim of this study was to evaluate the effect of two slow-start curing methods on acceleration of the curing of resin composite specimens at the bottom surface. The light-cured resin composite was polymerized using one of three curing techniques: (1) 600 mW/cm(2) for 60 s, (2) 270 mW/cm(2) for 10 s+0-s interval+600 mW/cm(2) for 50 s, and (3) 270 mW/cm(2) for 10 s+5-s interval+600 mW/cm(2) for 50 s. After light curing, Knoop hardness number was measured at the top and bottom surfaces of the resin specimens. The slow-start curing method with the 5-s interval caused greater acceleration of curing of the resin composite at the bottom surface of the specimens than the slow-start curing method with the 0-s interval. The light-cured resin composite, which had increased contrast ratios during polymerization, showed acceleration of curing at the bottom surface.

Thermo-mechanical degradation of composite restoration photoactivated by modulated methods-a SEM study of marginal and internal gap formation

OBJECTIVE

To evaluate the influence of thermal-mechanical degradation on superficial and internal gap formation of composite restorations photoactivated using modulated methods.

MATERIALS AND METHODS

An experimental composite was prepared using a resin matrix containing 65wt% Bis-GMA and 35wt% TEGDMA. Camphorquinone (0.5wt%) and dimethylaminoethyl-methacrylate (0.5wt%) were dissolved in the resin as a photo-initiator system and 65wt.% silanized glass fillers were added to the matrix. Ground buccal surfaces of bovine lower incisors were used to make 160 preparations (3 mm × 3 mm × 2 mm in depth). An adhesive system (Adper Single Bond 2) was applied and the specimens were assigned into 16 groups (n = 10), according to the photoactivation method [high intensity (HI), low intensity (LI), soft-start (SS) and pulse-delay (PD)] and the degradation protocol [(control/no degradation; thermal cycling (TC); mechanical loading (ML); thermo-mechanical loading (TC+ML)]. Marginal and internal interfaces of bonded restorations were replicated in epoxy resin and analyzed by SEM. Gaps were expressed as a percentage of the total length of the margins. Data were submitted to 2-way ANOVA and Tukey's test (α = 0.05).

RESULTS

For the control group no significance was noted among the photoactivation methods. TC had no effect in gap formation. ML and TC+ML increased the incidence of superficial gaps for both HI and SS groups as well as increased the internal gaps for all groups.

CONCLUSION

Although photoactivation methods do not influence gap formation at first, composite restoration photoactivated by low intensity or modulated methods showed improved resistance to thermo-mechanical degradation. Mechanical loading is determinant for interfacial degradation of composite restorations, while thermal cycling has no effect on gap formation.

In Vitro Performance of Class I and II Composite Restorations: A Literature review on Nondestructive Laboratory Trials—Part II

A literature review was conducted on adhesive Class I and II restorations and nondestructive in vitro tests using the PubMed/Medline database for the 1995-2010 period. The first part of this review has presented and critically appraised selected literature dealing with the quality and in vitro behavior of adhesive Class II restorations using photoelasticity, finite element analysis, and microleakage study protocols. This second part reviews additional parameters, which are deformation and fracture resistance to cyclic loading, shrinkage stress and tooth deformation following restoration placement, bond strength (microtensile, tensile, and shear tests), and marginal and internal adaptation. In addition, a “relevance score” has been proposed that aims to classify the different study protocols according, firstly, to the resulting quality, quantity, and consistency of the evidence and then, secondly, to their potential clinical relevance, as estimated by their ability to simulate oral and biomechanical strains. The highest clinical relevance was attributed to marginal and internal adaptation studies, following cyclic loading in a moist environement. However, a combination of in vitro protocols will have an even greater predictive potential and has to be considered as a crucial preclinical research approach with which to investigate the numerous restorative configurations that cannot be efficiently and rapidly tested in vivo.

In Vitro Performance of Class I and II Composite Restorations: A Literature Review on Nondestructive Laboratory Trials—Part I

Posterior adhesive restorations are a basic procedure in general dental practices, but their application remains poorly standardized as a result of the number of available options. An abundant number of study hypotheses corresponding to almost unlimited combinations of preparation techniques, adhesive procedures, restorative options, and materials have been described in the literature and submitted to various evaluation protocols. A literature review was thus conducted on adhesive Class I and II restorations and nondestructive in vitro tests using the PubMed/Medline database for the 1995-2010 period. The first part of this review discusses the selected literature related to photoelasticity, finite element analysis (FEM), and microleakage protocols. Based on the aforementioned evaluation methods, the following parameters proved influential: cavity dimensions and design, activation mode (light or chemical), type of curing light, layering technique, and composite structure or physical characteristics. Photoelasticity has various limitations and has been largely (and advantageously) replaced by the FEM technique. The results of microleakage studies proved to be highly inconsistent, and the further use of this technique should be strictly limited. Other study protocols for adhesive Class II restorations were also reviewed and will be addressed in part II of this article, together with a tentative relevance hierarchy of selected in vitro methods.

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.

Effects of preheating and precooling on the hardness and shrinkage of a composite resin cured with QTH and LED

The aim of this study was to evaluate in vitro the hardness and shrinkage of a pre-cooled or preheated hybrid composite resin cured by a quartz-tungsten-halogen light (QTH) and light-emitting diode (LED) curing units. The temperature on the tip of the devices was also investigated. Specimens of Charisma resin composite were produced with a metal mold kept under 37°C. The syringes were submitted to 4°C, 23°C, and 60°C (n=20) before light-curing, which was carried out with the Optilux 501 VCL and Elipar FreeLight 2 units for 20 seconds. The specimens were kept under 37°C in a high humidity condition and darkness for 48 hours. The Knoop hardness test was carried out with a 50 gram-force (gf) load for 10 seconds, and the measurement of the shrinkage gap was carried out using an optical microscope. The data were subjected to analysis of variance and the Games-Howell test (α=0.05). The mean hardness of the groups were similar, irrespective of the temperatures (p>0.05). For 4°C and 60°C, the top surface light-cured by LED presented significantly reduced shrinkage when compared with the bottom and to both surfaces cured by QTH (p<0.05). It was concluded that the hardness was not affected by pre-cooling or preheating. However, polymerization shrinkage was slightly affected by different pre-polymerization temperatures. The QTH-curing generated greater shrinkage than LED-curing only when the composite was preheated. Different temperatures did not affect the composite hardness and shrinkage when cured by a LED curing unit.

A 7-year randomized prospective study of a one-step self-etching adhesive in non-carious cervical lesions. The effect of curing modes and restorative material

OBJECTIVE

The aim of this study was to evaluate the clinical retention of a one-step self-etching adhesive system (Xeno III) in Class V non-carious cervical lesions and the effect of restorative material and curing techniques on longevity of the restorations.

MATERIALS AND METHODS

A total of 139 Class V restorations were placed with the self-etching primer Xeno III and a resin composite (Tetric Ceram) or a poly-acid modified resin composite (Dyract AP) in non-carious cervical lesions without intentional enamel involvement. The materials were cured with a conventional continuous light, a soft-start or a pulse-delay curing mode. The restorations were evaluated at baseline, 6, 12, 18 and 24 months and then yearly during a 7 year follow-up with modified USPHS criteria. Dentine bonding efficiency was determined by the percentage of lost restorations.

RESULTS

During the 7 years, 135 restorations could be evaluated. No post-operative sensitivity was reported by the participants. Overall relative cumulative loss rate frequencies for the adhesive system at 6 and 18 months and 7 years, independent of curing technique and restorative material, were 0.8%, 6.9% and 23.0%, respectively. The self-etching adhesive fulfilled at 18 months the full acceptance ADA criteria. Tetric Ceram showed at 7 years a 20.9% loss of retention and Dyract AP a 25.0% loss rate (Log rank p = 0.48). The loss rates for the 3 curing techniques: continuous, soft start and pulse delay were 17%, 27.9% and 24.4%, respectively (Log rank p = 0.52). No secondary caries was observed.

SIGNIFICANCE

The single-step self-etching adhesive showed acceptable clinical long-time retention rates to dentine surfaces independent of restorative material and curing technique used.

Evaluation of Light Intensity Output of QTH and LED Curing Devices in Various Governmental Health Institutions

Evaluating the intensity of a light curing unit regularly prior to the application of tooth-colored restorative materials is essential to assure the quality of restorative procedures.

Influence of different restorative techniques on marginal seal of class II composite restorations

Objective

To evaluate the gingival marginal seal in class II composite restorations using different restorative techniques.

Material and Methods

Class II box cavities were prepared in both proximal faces of 32 sound human third molars with gingival margins located in either enamel or dentin/cementum. Restorations were performed as follows: G1 (control): composite, conventional light curing technique; G2: composite, soft-start technique; G3: amalgam/composite association (amalcomp); and G4: resin-modified glass ionomer cement/ composite, open sandwich technique. The restored specimens were thermocycled. Epoxy resin replicas were made and coated for scanning electron microscopy examination. For microleakage evaluation, teeth were coated with nail polish and immersed in dye solution. Teeth were cut in 3 slices and dye penetration was recorded (mm), digitized and analyzed with Image Tool software. Microleakage data were analyzed statistically by non-parametric Kruskal-Wallis and Mann-Whitney tests.

Results

Leakage in enamel was lower than in dentin (p<0.001). G2 exhibited the lowest leakage values (p<0.05) in enamel margins, with no differences between the other groups. In dentin margins, groups G1 and G2 had similar behavior and both showed less leakage (p<0.05) than groups G3 and G4. SEM micrographs revealed different marginal adaptation patterns for the different techniques and for the different substrates.

Conclusion

The soft-start technique showed no leakage in enamel margins and produced similar values to those of the conventional (control) technique for dentin margins.

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 material's 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.

Impact of immediate and delayed light activation on self-polymerization of dual-cured dental resin luting agents

This study investigated the impact of immediate and delayed light activation on self-polymerization of a model dual-cured luting agent. The material presented the following components: base paste - 2,2-bis[4-(2-hydroxy-3-methacryloxyprop-1-oxy)phenyl]propane/triethylene glycol dimethacrylate (TEGDMA), camphorquinone, dimethyl-p-toluidine, butylated hydroxytoluene (BHT), glass fillers; catalyst paste - bisphenol-A ethoxylated dimethacrylate/TEGDMA, benzoyl peroxide, BHT, fillers. The pastes were mixed and seven polymerization scenarios tested: immediate light activation using low (5Jcm(-2)) or high (20Jcm(-2)) energy dose; delayed light activation (after 2min - short delay) using low or high dose; delayed light activation (after 10min - long delay) using low or high dose; and self-polymerization only. The degree of conversion (DC) and rate of polymerization (R(p)) were evaluated for 30min by real-time infrared spectroscopy. The lowest DC was detected for the self-polymerized and immediate-low dose groups, whereas the immediate-high dose and short delay-high dose groups showed the highest values. For the self-polymerized and immediate-high dose samples, R(p)(max) was detected after approximately 7s, whereas this took approximately 14s for the immediate-low dose group. R(p)(max) for the immediate-high dose group was higher than for the self-polymerized sample, which in turn was higher than for the immediate-low dose group. R(p)(max) for the short delay groups was higher than for the long delay groups. In conclusion, the extent of self-polymerization was influenced by the light dose reaching the material, which was dependent on high radiant exposure for optimal polymerization and the moment at which the light was applied; the short delay increased the DC for lower doses, while also generally decreasing the R(p) for all scenarios.

Effect of light-curing units and activation mode on polymerization shrinkage and shrinkage stress of composite resins

The aim of this study was to evaluate the polymerization shrinkage and shrinkage stress of composites polymerized with a LED and a quartz tungsten halogen (QTH) light sources. The LED was used in a conventional mode (CM) and the QTH was used in both conventional and pulse-delay modes (PD). The composite resins used were Z100, A110, SureFil and Bisfil 2B (chemical-cured). Composite deformation upon polymerization was measured by the strain gauge method. The shrinkage stress was measured by photoelastic analysis. The polymerization shrinkage data were analyzed statistically using two-way ANOVA and Tukey test (p≤0.05), and the stress data were analyzed by one-way ANOVA and Tukey's test (p≤0.05). Shrinkage and stress means of Bisfil 2B were statistically significant lower than those of Z100, A110 and SureFil. In general, the PD mode reduced the contraction and the stress values when compared to CM. LED generated the same stress as QTH in conventional mode. Regardless of the activation mode, SureFil produced lower contraction and stress values than the other light-cured resins. Conversely, Z100 and A110 produced the greatest contraction and stress values. As expected, the chemically cured resin generated lower shrinkage and stress than the light-cured resins. In conclusion, The PD mode effectively decreased contraction stress for Z100 and A110. Development of stress in light-cured resins depended on the shrinkage value.

Light- and time-dependent polymerization of dual-cured resin luting agent beneath ceramic

OBJECTIVE

In this study we investigated the degree of conversion (DC) of a dual-cured resin luting agent beneath a ceramic of different thickness.

MATERIAL AND METHODS

The luting material (RelyX) was photo-activated for 40s through ceramic disks (Empress Esthetic, 0.7, 1.4, or 2mm thick). Dual (DUC) and chemically cured (CC) control groups were obtained. A delayed exposure (DE) procedure was tested in which the material was light-activated after 5 min without using ceramic. DC (%) was measured using infrared spectroscopy at 40 s, 1, 2, 4, 6, 8, and 10 min. Light irradiance and spectrum transmitted through ceramic were measured using a power meter and a spectrometer.

RESULTS

A decrease in irradiance was detected with increasing ceramic thickness, but no alteration in the emission profile was observed. At 10 min, all groups showed DC higher than at 40s. No significant differences were detected for DUC compared with the groups indirectly activated through the ceramic disks. Generally, CC showed the lowest DC values. No significant effect in DC was detected for DE after 10 min. A significant increase in DC was generally detected only up to 4 min.

CONCLUSIONS

The luting agent was dependent on light exposure and time to obtain improved DC. Neither the indirect activation nor the DE procedure significantly influenced the final DC.

Buonocore Lecture. Placing dental composites--a stressful experience

The setting of dental composites is accompanied by significant polymerization contraction, resulting in the generation of stresses within the material and at the tooth-restoration interface. These stresses can have a deleterious effect on marginal integrity if they exceed the adhesive strength of the restorative, as well as on the properties of the composite. It has been determined that several factors affect these stresses, including the polymerization rate of the composite, its formulation, including filler and monomer composition and the constraints imposed by the geometry of the cavity preparation. Many strategies have been developed to reduce the effect of these stresses. Changes in the formulation of the composite have included experimentation with a variety of stress relieving additives, modified catalyst compositions and alternative monomer systems. Modifications to the placement techniques have included the use of incremental curing, altered light activation schemes and resilient liners. This manuscript will review many of the important scientific and clinical issues relating to the generation and quantitation of the stresses produced in dental composites during curing.

A mechanism on why slower polymerization of a dental composite produces lower contraction stress

It has been well documented that the rate of polymerization of a dental composite often affects its polymerization contraction stress. In most cases, a slower cure produces a lower stress. To investigate the mechanism behind this, we prepared an unfilled dimethacrylate resin sample and photocured it using two light irradiances, both with the same total irradiation energy. We measured the polymerization-induced shrinkage from the unbonded surface of a class I restoration, contraction stress, extent of polymerization, and flexural modulus. The resin specimens cured under the two irradiances achieved the same extent of polymerization and developed an identical amount of shrinkage from the unbonded surface. But those cured under the lower irradiance possessed a lower contraction stress and a lower flexural modulus than those cured under the higher irradiance. We demonstrated that the stress level did not respond to the extent of viscous flow of the curing resin because the slower polymerization did not produce more shrinkage from the unbonded surface. Instead, the lower stress is likely due to a lower modulus of the cured resin. To explain why the cured resin with an identical extent of polymerization can have different moduli, we proposed that slower polymerization produces a higher level of structural inhomogeneity, which reduces the rigidity of the cured resin.

Influence of Tg, viscosity and chemical structure of monomers on shrinkage stress in light-cured dimethacrylate-based dental resins

OBJECTIVES

The aim of this study was to investigate the influence of the molecular mobility and the chemical structure of dimethacrylates most commonly used in dental composites on shrinkage stress from experimental matrices.

METHODS

Three established neat monomers BisGMA (B), UEDMA (U) and TEGDMA (T), two experimental comonomers BisGMA-based (B-T(70/30) and B-T(50/50)) and two comonomers UEDMA-based (U-T(88/11) and U-T(66/33)) in weight%, were elaborated. Camphorquinone (CQ) and N,N-cyanoethylmethylaniline (CEMA), as photoinitiator and reducing agent, were added. Then the matrices were mixed by centrifugal force at room temperature. The viscosity (eta), the glass transition temperature of the monomers and comonomers systems (T(g(monomer)) and the maximum shrinkage stress (MSS) of each material (five replications) were statistically analysed by one-way ANOVA/Tuckey's test and Pearson's correlation procedure (p = 0.05).

RESULTS

All formulations exhibited a newtonian rheological behavior. The viscosity of the comonomers systems can be divided in two groups: the pair B-T(70/30)/U-T(88/11) with the viscosity 3.5+/-3.10(-3)Pa.s and the pair B-T(50/50)/U-T(66/33) with the viscosity 0.28+/-3.10(-3)Pa.s. This pairs constituted samples allowing to compare the shrinkage stress of the BisGMA and UEDMA-based matrices with each other. The T(g(monomer)) of each group showed equivalent statistically values: -37.1 +/- 0.02 degrees C (U-T(88/11)) with -39.3 +/- 0.02 degrees C (B-T(70/30)) for the 3.5 Pa.s pair, and -53.1 +/- 0.03 degrees C (U-T(66/33)) with -58.5 +/- 0.01 degrees C (B-T(50/50)) for the 0.28 Pa.s one. There was a correlation between eta and T(g(monomer)) (r < 0.45 and p < 0.01). In decreasing order, the shrinkage stress was 14.11 +/- 0.3 MPa (T), 10.64 +/- 0.6 MPa (U-T(66/33)), 8.16 +/- 0.25 MPa (B-T(50/50)) without a significant difference compared to 8.04 +/- 0.5 MPa (U-T(88/11)), 6.83 +/- 0.52 MPa (U), 4.44 +/- 0.25 MPa (B-T(70/30)) and 0.33 +/- 0.3 MPa (B). There was a negative correlation between eta (r < -0.42 and p < 0.01), T(g(monomer)) (r < -0.41 and p < 0.01) and MSS. Whatever the viscosity, the UEDMA-based matrices developed higher shrinkage stresses than the BisGMA homologues.

SIGNIFICANCE

The shrinkage stress development increase with the molecular mobility of the reacting medium. For the same molecular mobility, the large differences in stress values of the matrices studied are correlated to the structure and particularly the functionality of the monomers used.

Impact of similarity in chemical composition of light-polymerized resin composites on post-gel strains and interface integrity

The post-gel strains of two light-polymerized resin composites having similar chemical composition, Filtek P60 and Filtek Z250, was assessed by strain-gauge analysis at the ex vivo level. The restoratives were tested in cavity factor 5 during light-polymerization and water-soaking for 24 h. Strain-gauge signals were digitalized by a data acquisition system and were displayed in a computer by corresponding software at a sample rate of 20 Hz. The strain data were used to compare post-gel strains, polymerization velocity, and total volumetric change of both materials. Evaluation of microleakage and scanning electron microscopy was also undertaken to elucidate effects of post-gel strains at the tooth-restorative interface. Microstrains of Filtek Z250 were lower than those of Filtek P60 and the differences between post-gel strains at several different time intervals were significant (P < 0.009). There was a correlation between polymerization velocity and total volumetric change. Microleakage evaluation and scanning electron microscopy did not reveal any signs of debonding at the interface for both materials. We conclude that the similarity in chemical composition of light-polymerized resin composites is not a determinant for post-gel strains. The tooth-restorative interface can withstand high post-gel strains arising from polymerization of resin composites.