Polymerization Shrinkage Forces Research Articles

Change in the Shrinkage Forces of Composite Resins According to Controlled Deflection.

The aim of this study was to investigate how the polymerization shrinkage forces of composite resins change with change in deflection. Five composites, SDR (Dentsply Caulk, Milford, DE, USA), EcuSphere-Shape (DMG, Hamburg, Germany), Tetric N-Ceram Bulk Fill (Ivoclar Vivadent, Schaan, Liechtenstein), CLEARFIL AP-X (Kuraray Noritake Dental Inc., Sakazu, Kurashiki, Okayama, Japan), and Filtek Z350 XT (3M Dental Products, St Paul, MN, USA), were tested in this experiment. The polymerization shrinkage forces of the composites were measured using a custom-made tooth-deflection-mimicking device and software (R&B Inc, Daejon, Korea). In all measurements, six modes were tested: maximum-deflection, zero-deflection, and four deflection-controlled modes. For each deflection mode, the shrinkage forces were recorded continuously every 0.1 second for 180 seconds. Polymerization shrinkage and flexural modulus were also measured. Eight specimens of each material were allocated for each test. For each material, six groups of shrinkage force values were compared using one-way ANOVA and Tukey post hoc tests at a 95% confidence level. The polymerization shrinkage force of each material in each of the six deflection modes was analyzed with 95% confidence using one-way ANOVA and Tukey post hoc tests. The relationship between the force measured in the six deflection modes and the linear polymerization shrinkage and flexural modulus was analysed with 95% confidence using Pearson correlation analysis. For each material, the following held true: the shrinkage force was highest in zero-deflection mode, the force decreased as deflection increased, and the smallest force appeared in maximum-deflection mode (p<0.05). There was a high negative correlation between allowable deflection and shrinkage force in all materials.

Influence of Volume and Activation Mode on Polymerization Shrinkage Forces of Resin Cements

The concern with the polymerization shrinkage of restorative resin composites also applies to resin cements. The aim of this study was to evaluate the influence of volume and polymerization mode on forces generated during polymerization shrinkage (FGPS) of resin cements. Two light-cured resin cements--Variolink II (VL; Ivoclar Vivadent) and Nexus 3 (NX; Kerr)--and two self-cured resin cements - Multilink (ML; Ivoclar Vivadent) and Cement Post (CP; Angelus) - were inserted between two rectangular steel bases (6x2 mm) with distance set at 0.1, 0.3 and 0.5 mm, establishing a variation of volume. These steel bases were attached to a universal test machine with 50 kg load cell and forces (N) were registered for 10 min. Values of maximum forces generated by each material were subjected to two-way ANOVA and Tukey's test for individual comparisons (α=0.05). VL, NX and CP developed increasing FGPS as the volume of material increased, while ML presented the opposite behavior. It may be concluded that higher volume increases FGPS even with a concomitant decrease of C-factor, unless the resin cements present lower force generation rates as a function of time in combination with a low C-factor, resulting in stress relief and consequently lower values of FGPS.

Three-dimensional biomechanical study of functional stresses in composite restorations of masticatory teeth

The aim of the present work was to study and evaluate the intensity of stresses in the adhesive bond in composite restorations of masticatory teeth after light-curing, under temperature changes and masticatory loads. Data for the 3D model generation of a maxillary premolar were obtained during a routine CT head scan. Thirty-three cross-sections of tooth 25 were selected and used to create a 3D geometric model enmeshed using the finite element method (FEM) (made up of 106556 elements and 608724 nodes). The pulp cavity and the periodontal ligament were constructed in the same way and integrated into the premolar model. Eight cavity configurations with converging walls were designed, resistant to masticatory forces (enamel/dentin = 1/1). A comparative computer simulation was carried out of the polymerization shrinkage forces of the composite material (CM), temperature changes in the oral cavity and functional masticatory loads. The distribution of the generated stress on the adhesive bond was evaluated in eight different class I and II cavity configurations. The location of crack formation was assessed in the cases of rupture of the adhesive bond. In all cavity configurations, stress concentration in the adhesive layer is higher at the interface with the dental tissues. Low temperatures (5 degrees C) generate forces that are greater than the strength of the adhesive bond in all studied cavity configurations. The distribution of the generated stresses under the effect of axial and tangent forces of 300 N is similar to that under the effect of temperature factors. The axial masticatory forces have a pronounced adverse effect on the adhesive bond in all cavity configurations. Low temperatures and axial masticatory forces play an important role for the marginal integrity. They exacerbate the adverse effects of polymerization shrinkage in composite restorations of masticatory teeth.

Influence of different light sources and photo-activation methods on degree of conversion and polymerization shrinkage of a nanocomposite resin

The purpose of this study was to evaluate the influence of different light sources and photo-activation methods on degree of conversion (DC%) and polymerization shrinkage (PS) of a nanocomposite resin (Filtek™ Supreme XT, 3M/ESPE). Two light-curing units (LCUs), one halogen-lamp (QTH) and one light-emitting-diode (LED), and two different photo-activation methods (continuous and gradual) were investigated in this study. The specimens were divided in four groups: group 1—power density (PD) of 570 mW/cm2 for 20 s (QTH); group 2—PD 0 at 570 mW/cm2 for 10 s + 10 s at 570 mW/cm2 (QTH); group 3—PD 860 mW/cm2 for 20 s (LED), and group 4-PD 125 mW/cm2 for 10 s + 10 s at 860 mW/cm2 (LED). A testing machine EMIC with rectangular steel bases (6 × 1 × 2 mm) was used to record the polymerization shrinkage forces (MPa) for a period that started with the photo-activation and ended after two minutes of measurement. For each group, ten repetitions (n = 40) were performed. For DC% measurements, five specimens (n = 20) for each group were made in a metallic mold (2 mm thickness and 4 mm diameter, ISO 4049) and them pulverized, pressed with bromide potassium (KBr) and analyzed with FT-IR spectroscopy. The data of PS were analyzed by Analysis of Variance (ANOVA) with Welch’s correction and Tamhane’s test. The PS means (MPa) were: 0.60 (G1); 0.47 (G2); 0.52 (G3) and 0.45 (G4), showing significant differences between two photo-activation methods, regardless of the light source used. The continuous method provided the highest values for PS. The data of DC% were analyzed by Analysis of Variance (ANOVA) and shows significant differences for QTH LCUs, regardless of the photo-activation method used. The QTH provided the lowest values for DC%. The gradual method provides lower polymerization contraction, either with halogen lamp or LED. Degree of conversion (%) for continuous or gradual photo-activation method was influenced by the LCUs. Thus, the presented results suggest that gradual method photo-activation with LED LCU would suffice to ensure adequate degree of conversion and minimum polymerization shrinkage.

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

Objectives The aim of the study was to determine the influence of different light curing units (LCU) and regimes on the polymerization shrinkage stress (PSS) and the mechanical properties of a nano-hybrid composite. Material and methods The polymerization shrinkage force (PSF) was measured continuously with compliance compensation for 300 s after photo-initiating the composite, Tetric EvoCeram (Ivoclar Vivadent, Schaan, FL, Shade A3) in a Stress–Strain Analyser. Astralis 10, Bluephase and MiniL.E.D LCU with exposure times 10, 20 and 40 s were used ( C-factor = 0.33, n = 8 per group). Immediately after the PSF measurements, mechanical properties of the samples were measured at the top and the bottom using a Fischerscope H100C (Helmut Fischer GmbH, Sindelfingen, Germany). Statistical analyses were done using one-way ANOVA ( p < 0.05) and Tukey post hoc test. Results Significant differences in the PSS for 10, 20 and 40 s polymerization using Astralis 10 were found. The MiniL.E.D recorded low stress values. Modulus of elasticity is high after curing the composite with Astralis 10 at 10, 20 and 40 s and for Bluephase 40 s. Low moduli of elasticity were recorded for the MiniL.E.D and for the Bluephase 20 and 10 s. The hardness values (HV) followed the same pattern as the modulus of elasticity. The Ramping mode of the MiniL.E.D had prolonged gel point. Conclusions High intensity LCU produce not just high HV but also high shrinkage, making it important to balance both the effects by choosing the appropriate curing time. Soft-start regimes have no paramount benefit in a LED regarding stresses in the clinical situation.

Effect of stepped light intensity on polymerization force and conversion in a photoactivated composite.

This study evaluated the effect of stepped light intensity on the polymerization shrinkage forces and degrees of conversion of a hybrid composite. Composite specimens were bonded between two steel rods (5.00 mm diameter, 1.25 mm apart, configuration factor = 2) mounted in a universal testing machine using a constant displacement mode. Polymerization contraction force was recorded for 300 seconds under four light exposure conditions: group 1: 40 s x 800 mW/cm2; group 2: 10 s x 100 mW/cm2 + 30 s x 800 mW/cm2; group 3: 60 s x 800 mW/cm2; group 4: 10 s x 100 mW/cm2 + 50 s x 800 mW/cm2. Maximum curing force (N300 s) and maximum force rate of the four groups were compared using one-way analysis of variance (ANOVA) (alpha = 0.05) and the Tukey test. Degree of conversion in all groups was evaluated at two depths (top surface and 2 mm) using Fourier transform infrared spectroscopy (FTIR). Mean maximum shrinkage forces and standard deviations (SD) were: group 1, 177 N (SD = 23); group 2, 172 N (SD = 11); group 3, 213 N (SD = 15); group 4, 197 N (SD = 17). Mean maximum forces for stepped and standard groups with the same duration (1 and 2; 3 and 4) were not statistically different; means for groups 2 and 3 were statistically different. Maximum force rates were not significantly different (p = .1548). Force:time curves were S-shaped. Specimens exposed to stepped curing exhibited longer delays before force was recorded. Mode of curing was shown not to contribute to overall cure, but both duration of cure and the depth (top surface vs. 2.00 mm) were significant with an interaction effect.

Novel approach to measure composite conversion kinetics during exposure with stepped or continuous light-curing.

The objective of this research was to evaluate a novel approach to monitor the polymerization reaction during a light-curing exposure by using infrared (IR) spectroscopy. An IR spectrometer was equipped to use an attenuated total reflectance (ATR) element as an IR-active substrate. The uncured composite (Herculite XRV, Shade A2, Kerr, Orange, California) was placed against the crystal, and the IR spectrum was continuously obtained during various exposure scenarios. The degree of conversion and the maximum rate of reaction were monitored at 0 mm (top surface), 1 mm, 2 mm, and 3 mm beneath the surface. The exposure conditions included continuous 40-second or 60-second exposures at 100% intensity (800 mW/cm2) or a stepped output of 10 seconds at 17% maximal output (133 mW/cm2) followed by full output for the remainder of the 40-second or 60-second exposure (Elipar Highlight, ESPE, Norristown Pennsylvania). The results were analyzed using MANOVA with appropriate post hoc tests (p < or = .05). For 40-second exposures, the peak conversion rates were significantly reduced (p < .05) when using the stepped exposure mode compared to the continuous exposure: 40-second top surface: stepped = 5.5%/s +/- 0.4, continuous = 10.5%/s +/- 1.0; 1 mm step = 3.6%/s +/- 0.4, continuous mode = 4.8%/s +/- 0.2. The same trend was noted when using the 60-second exposure. Equivalent conversion values (p > .05) beneath the surface between stepped and continuous exposure modes at similar depths 60 seconds after light initiation were only attained at 3 mm 4 for the 40-second exposure. However, using the 60-second exposure, equivalent conversion values between step and continuous exposure modes at similar depths were obtained. Even with a reduced conversion rate at the surface using the stepped cure mode, polymerization shrinkage forces were sufficient to debond the specimens from the test crystal after only 20 seconds into the exposure. This result indicated that stress development in the curing composite was non-uniform, and stress values developed at the surface of the restoration were the greatest. Stepped intensity curing for the ESPE Highlight unit was shown to produce significantly lower conversion rates at the surface and at 1-mm depths, but longer exposure times were still required to provide conversion values equivalent to continuous exposure.