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Purpose: To assess the detrimental effects that polymerization contraction causes on the interfacial microtensile bond strength (μTBS) to dentin of three light-curing resin composites: two conventional bis-GMA-based composite resins (Filtek Z250, TPH Spectrum) and a low shrinkage material (Filtek Silorane [SIL]). Materials and Methods: Flat surfaces of labial dentin were made in 46 bovine teeth and restored in single (group A) or in three separate (group B) one-increment visible-light cured resin composite blocks, with similar locations among teeth. After 24 h, restored teeth were sectioned perpendicular to interfaces, producing rectangular compound specimens (385 specimens were produced in group A, and 132 in group B), and submitted to tension (crosshead speed 1mm/min) until failure. μTBS results were adjusted to bonded area and transformed to percentages of maximum values within each tooth (PTens). In A groups, distances of specimens to the gingival ends of each restoration were transformed to PDistances: the percentage of that distance within its specimen. Results: Spearman's nonparametric correlation test showed that for Z250, TPH and SIL, in A groups, a material's correlation coefficient was positive for the first half of restorations (0% to 50% PDistances) and negative for the second (50% to 100%). For all materials, PTens values in extremes of restorations (pooled 0 and 100% PDistances) in A groups were smaller than corresponding values in B groups. These differences were statistically significantly (Student's t-test) only for Z250 and TPH. Conclusion: Even in cases of favorable configuration factor, polymerization shrinkage in large restorations can reduce interface mechanical characteristics. The relevance of this decrease has still to be established. [ABSTRACT FROM AUTHOR]

Objectives: This study aimed to assess the effects of fiber incorporation on the contraction stress (CS), time to gelation (TG), and temperature rise (TR) features of two experimental resin-based composites (RBCs) in comparison to those of bulk-fill RBCs and conventional RBCs in simulated, clinically relevant cavities. Materials and methods: CS over 300 s, TG and TR were assessed by a stress-strain analyzer (SSA T80) in two cavity configurations (2 × 4 × 4 mm3/4 × 4 × 4 mm3/configuration value 1 2 and 1 3 ) for two experimental RBCs, eight bulk-fill RBCs (EverX Posterior, Filtek Bulk Fill, SDR, SonicFill, Tetric EvoCeram Bulk Fill, Venus Bulk Fill, X-tra base, and X-tra Fill) and one RBC (Filtek Supreme XTE) by light curing (1200 mW/cm2) (n = 10). The experimental materials used were based on EverX Posterior with modifications made to the fiber and filler content. Statistical analyses were performed via ANOVA; multiple pair-wise comparisons were performed via Tukey's test, and homogeneous subsets were identified ("multcomp" package, R). Results: CS values ranged from 1.00 to 4.07 MPa (2 × 4 × 4 mm3) and from 0.97 to 2.49 MPa (4 × 4 × 4 mm3); TG values ranged from 1.31 to 4.05 s (2 × 4 × 4 mm3) and from 1.39 to 3.84 s (4 × 4 × 4 mm3). SDR values showed lowest stress values in both cavity configurations. The experimental composite with fibers presented significantly higher stress values than did the experimental composite without fibers. Conclusion: Bulk-fill RBCs showed lower levels of stress than did conventional RBCs. The incorporation of fibers had no positive impact on the CS and TG. Clinical relevance: Appropriate material selection may be essential to clinical success because certain RBCs exhibit higher CS, TG, and TR values. [ABSTRACT FROM AUTHOR]

The aim of the study was to assess the effect of delayed photo-initiation on the polymerization contraction stress (PCS) and degree of conversion (DC) of a dual-cure resin-luting agent. Thirty-five disk (6 mm × 1 mm) samples (n = 10 each group) of dual cure resin luting agent for PCS assessment were fabricated and polymerized using two illuminated quartz rods. Based on the delay in photo-initiation, 30 disks were divided among six groups [group A-0 min (min) delay, group B-2 min, group C-4 min, group D-6 min, group E-8 min and group F-10 min]. A non-photoinitiated group (group G – chemical cure –n = 5) was included as control. The PCS for all specimens was assessed using a Tensometer. For DC evaluation thirty-five specimens were divided into seven groups with delays in photo-initiation (group H-0 min, group I-2 min, group J-4 min, group K-6 min, group L-8 min and group M-10 min, group N-chemical cure). DC was assessed using attenuated total reflectance spectroscopic technique. Statistical comparison among groups was performed using analysis of variance (α = 0.05). The maximum and minimum PCS and DC values with delayed photo-initiation was observed in group-C (3.34 MPa) & group-F (2.44 MPa); and group-M (0.78 MPa) and group-H (0.55 MPa) respectively. Chemically cured samples showed the least PCS (group-G, 1.94) and DC (group-N, 0.53) values in their respective categories. PCS significantly decreased with delayed photo-initiation. A significant increase in DC was noticed when photo-initiation was delayed in the dual cure resin luting agent. [ABSTRACT FROM PUBLISHER]

Abstract: Objectives: The incidence of non-carious cervical lesions (NCCLs) has been increasing. The clinical performance of resin composites in NCCLS was previously unsatisfactory due to their non-retentive forms and margins lying on dentin. In order to address this problem, a lot of effort has been put into developing new dentin adhesives and restorative techniques. This article discusses these challenges and the criteria used for evaluating clinical performance as they relate to clinical studies, especially long-term clinical trials. Polymerization contraction, thermal changes and occlusal forces generate debonding stresses at adhesive interfaces. Methods: In laboratory studies, we have investigated how these stresses can be relieved by various restorative techniques and how bond strength and durability can be enhanced. Lesion forms, restorative techniques, adhesives (adhesive strategies, bond strengths, bond durability, and the relationship between enamel and dentin bond strengths) were found to have a complex relationship with microleakage. With regard to some restorative techniques, only several short-term clinical studies were available. Results: Although in laboratory tests marginal sealing improved with a low-viscosity resin liner, an enamel bevel or prior enamel etching with phosphoric acid, clinical studies failed to detect significant effects associated with these techniques. Long-term clinical trials demonstrated that adhesive bonds continuously degraded in various ways, regardless of the adhesion strategy used. Significance: Early loss of restoration may no longer be the main clinical problem when reliable adhesives are properly used. Marginal discoloration increased over time and may become a more prominent reason for repair or replacement. Reliable and standardized criteria for the clinical evaluation of marginal discoloration should be established as soon as possible and they should be based on evidence and a policy of minimal intervention. [Copyright &y& Elsevier]

Abstract: Objectives: Polymerization contraction of dental composite produces a stress field in the bonded surrounding substrate that may be capable of propagating cracks from pre-existing flaws. The objectives of this study were to assess the extent of crack propagation from flaws in the surrounding ceramic substrate caused by composite contraction stresses, and to propose a method to calculate the contraction stress in the ceramic using indentation fracture. Methods: Initial cracks were introduced with a Vickers indenter near a cylindrical hole drilled into a glass-ceramic simulating enamel. Lengths of the radial indentation cracks were measured. Three composites having different contraction stresses were cured within the hole using one- or two-step light-activation methods and the crack lengths were measured. The contraction stress in the ceramic was calculated from the crack length and the fracture toughness of the glass-ceramic. Interfacial gaps between the composite and the ceramic were expressed as the ratio of the gap length to the hole perimeter, as well as the maximum gap width. Results: All groups revealed crack propagation and the formation of contraction gaps. The calculated contraction stresses ranged from 4.2MPa to 7.0MPa. There was no correlation between the stress values and the contraction gaps. Significance: This method for calculating the stresses produced by composites is a relatively simple technique requiring a conventional hardness tester. The method can investigate two clinical phenomena that may occur during the placement of composite restorations, i.e. simulated enamel cracking near the margins and the formation of contraction gaps. [Copyright &y& Elsevier]

Abstract: Objective: In a previous study on of polymerization contraction stress determinations of adhesives bonded to dentin a continuous decline of stress was observed after the adhesives had been light-cured. The decline was ascribed to stress relief caused by diffusion into the adhesive layer of water and/or solvents, left in the impregnated dentin surface after drying and/or evaporation in the application procedure. The purpose of the present study was to test the hypothesis that the contraction stress of adhesives bonded to enamel will not decline after light-curing, based on the assumption that water and/or solvents are more efficiently removed from impregnated enamel surfaces in the drying and/or evaporation step. Materials and methods: Contraction stress was determined in a tensilometer for three total-etching adhesives Scotchbond multi-purpose, Single bond and One-step plus and four self-etching adhesives Clearfil SE Bond, Clearfil Protect Bond, AdheSE, and Xeno III. The adhesives were placed in a thin layer between a glass plate and a flat dentin or enamel surface pre-treated with phosphoric acid or self-etching primer and light-cured under constrained conditions. Results: All adhesives bonded to enamel showed a stress decline, but significantly less than for dentin with the exception of two self-etching adhesives. The greatest decline was found for the total-etching adhesive systems bonded to dentin. The presence of hydrophobic monomers in the adhesives had a significant influence on the decline. Significance: The experiments indicate that fluids are withdrawn from the resin impregnated tooth structures, which may result in small defects in the tooth–resin interfaces. [Copyright &y& Elsevier]

Summary: Objective: The present study determined the influence of polymerization shrinkage of eight commercially available hybrid, micro-filled and nano composites (Z100 and Filtek Supreme, 3M-ESPE; Charisma and Durafill, Heraeus Kulzer; Tetric and InTen-S, Ivoclar Vivadent; Enamel plus HFO, GDF; Palfique Estelite Low Flow, Tokuyama) placed in large class 1 cavities on the bond strength to dentin. Methods: Polymerization shrinkage was recorded for 300s at room temperature with a Stress–Strain-Analyzer (C FACTOR=0.3). The maximum contraction stresses after 300s, the time until gelation (t 0.5N) and the coefficient of near linear fit of contraction force/time (gradient) were analysed. For the evaluation of the micro-tensile-bond-strength (μ-TBS), hourglass shape samples obtained from a total of 32 no carious extracted human third and second molars, randomly divided into 8 groups, were used. Micro tensile bond strengths were determined by computing the ratio of maximum load by the adhesion area of the hourglass shape. In order to analyze the quality of the polymerization within fillings, hardness profiles of a surface cut thought the middle of the restored tooth and along the tooth axis were made. Further, the modulus of elasticity, determined in a three-point-bending test, as well as the variation of the modulus of elasticity and of the hardness within a filling were considered. The statistical analyses were conducted by ANOVA (α=0.05) and post-hoc Tukey''s test. Results: A significant correlation between polymerization shrinkage and μ-TBS was found (Pearson; −0.44). The correlation between modulus of elasticity in bending test and shrinkage stress (Pearson; 0.77), coefficient of near linear fit m (Pearson; 0.72), time until gelation (Pearson; −0.52) and micro-tensile-bond-strength (Pearson; −0.45) was also significant. A high polymerization tension and modulus of elasticity negatively affected the adhesion of the composite to the tooth hard substance. Further, the number of samples lost during the slice cutting and the hourglass shape preparation was found to correlate highly significantly with the shrinkage stress (Pearson; 0.97), coefficient of near linear fit m (Pearson; 0.94) and modulus of elasticity (Pearson; 0.82). All materials showed sufficient polymerization within the filling in comparison to the filling surface, the hardness at the bottom of the filling was greater than 80% of the maximum hardness value. The modulus of elasticity was not considerably reduced within the filling. Significance: High contraction stress and modulus of elasticity, fast development of contraction force and an early start of stress build-up of materials placed in restrictive cavities cause tension in the material with a possible subsequent distortion of the bond to the tooth structure. A low modulus of elasticity is not necessarily associated with high bond strength. However, it causes a more uniform stress distribution at the restorative composite-tooth interface. This is also evident in a reduced sample loss during the different stages of sample preparation. [Copyright &y& Elsevier]

Summary: Objective: The present study determined the influence of different light curing regimes (four light-emitting diode (LED) units (Freelight 1 and 2, 3M-ESPE; e-light, GC; Bluephase (prototype), Ivoclar Vivadent), two quartz–tungsten–halogen (QTH) lights (Astralis 10, Ivoclar Vivadent; Swiss Master Light, EMS) and one plasma-light curing unit (Easy Cure, DMDS)) on the curing behavior of a resin-based composite material (InTen-S, Ivoclar Vivadent). Methods: Polymerization shrinkage was induced by light curing the tested material with 14 different regimes of the curing units mentioned above. The contraction stress was recorded for 300s at room temperature with a Stress–Strain-Analyzer (c FACTOR=0.3). The maximum contraction stresses after 300s, the time until gelation (t 0.5N), and the coefficient of near linear fit of contraction force/time (gradient) were analyzed. The statistical analysis was conducted using ANOVA (α=0.05) and Tukey''s post hoc test. Results: The five tested regimes of the LED unit e-light revealed the lowest statistically significantly maximum contraction stress followed by the low intensity LED unit Freelight 1 and the plasma curing unit Easy Cure. The high intensity LED unit Freelight 2 exhibited a significantly higher contraction stress compared to Freelight 1. No significant differences between the standard and exponential modes within these curing units were found. No significant differences were found between the LED unit Freelight 2 and the pulse program of the halogen light curing unit Astralis 10. The highest polymerization stresses were observed for the high energy curing units, either QTH (Swiss Master Light and Astralis 10) or LED (Bluephase). Significance: Fast contraction force development, high contraction stress and an early start of stress build-up cause tension in the material with possible subsequent distortion of the bond to the tooth structure. The lowest polymerization stress was observed for the low energy LED lamps, while the plasma unit and the high energy QTH and LED curing units produced two to three times higher stress. [Copyright &y& Elsevier]

Objective. The magnitude and kinetics of polymerization contraction stress build-up may be potential predictors of bond failure of adhesive restorations. The present study determined these properties of seven commercial compomers (Dyract, Dyract AP, F2000 Rasant, Hytac, Compoglass F, Luxat, Glasiosite).Methods. Polymerization shrinkage was generated by 40 s light curing the test materials (800 mW/cm2). The contraction force induced was recorded for 300 s at room temperature (23–24 °C) by means of a Stress–Strain-Analyzer (C factor=0.33). Maximum contraction stress (MPa), coefficient of near linear fit of contraction force/time (gradient) and relative force rate (%/s) of each material were compared with that of two hybrid composites (Tetric Ceram, Prodigy). The statistical analysis was conducted by ANOVA (α=0.05) and post hoc Tukey''s test.Results. No statistically significant differences in the maximum stress between Glasiosite (2.27±0.06 MPa), Hytac (2.31±0.07 MPa) and Tetric Ceram (2.21±0.11 MPa), and between Compoglass F (2.60±0.18 MPa) and Prodigy (2.70±0.06 MPa) were found. The contraction stress of F2000 Rasant (3.41±0.09 MPa) and Luxat (3.33±0.08 MPa) were significantly highest, whilst Dyract exhibited the significantly lowest shrinkage stress (1.27±0.08 MPa) among the tested materials.Significance. High contraction stress, early start of stress build-up and rapid contraction force development may lead to failure of bond to tooth structure. This study suggested that the contraction stress and kinetic behavior of compomers are generally similar to those of hybrid composites in a dry condition. Dyract might be superior in maintaining the bond with cavity walls compared to conventional hybrid composites in view of its low shrinkage stress. [Copyright &y& Elsevier]

Objectives: The goal of this study was to assess the reduction of polymerization contraction stress of composites during a two-step light-activation process and to relate this reduction to the process of polymerization shrinkage and specimen thickness.Methods: Three test procedures were performed to compare two-step light-activation with delay with one-step continuous irradiation of composites: polymerization contraction stress using a closed-loop servohydraulic testing instrument, polymerization shrinkage by a mercury dilatometer, and degree of conversion by FTIR. For the one-step continuous curing method, the samples were light-activated for 60 s at 330 mW/cm2. For the two-step curing method, a 5 s light exposure at 60 mW/cm2 was followed by 2 min without light exposure, and then a second light exposure for 60 s at 330 mW/cm2. The same light parameters were used for measurements of stress, shrinkage, and degree of conversion. Three composites, Heliomolar, Herculite and Z100 were evaluated. The contraction stress experiments were repeated with varying thickness for Herculite using the one-step and two different two-step techniques.Results: Polymerization contraction stress 10 min after light-activation was significantly reduced (P<0.05) by the two-step method: 29.7% for Heliomolar, 26.5% for Herculite, and 19.0% for Z100. Total volumetric shrinkage and degree of conversion were not significantly different for composites cured by the two different techniques. Increasing the thickness of the composite sample reduced the measured contraction stress, especially for one of the two-step curing methods.Significance: A combination of low initial energy density followed by a lag period before a final high-intensity light irradiation provides a reduction of polymerization contraction stresses in dental composites. The stress reductions cannot be attributed to reductions in degree of conversion or unrestrained volumetric shrinkage. [Copyright &y& Elsevier]