Maximum Stress Rate Research Articles

Shrinkage stress kinetics of Bulk Fill resin-based composites at tooth temperature and long time

ObjectiveTo determine the shrinkage stress kinetics at up to 12h after light exposure and at tooth temperature during placement of selected Bulk Fill resin-based composites (RBCs). MethodsFive representative Bulk Fill RBCs from four companies were chosen with a wide range of viscosity and filler volume content. The shrinkage stress kinetics at T=33°C was measured continuously over a period of 12h using a modified tensometer with the ability to measure the cantilever beam deflection to better than 40nm accuracy at a sampling rate of up to 200 samples/s, and thermally stable resulting in a measurement accuracy better than 0.05MPa at 12h. The tensometer compliance was 0.105μm/N. A custom made heater was used to control the RBC sample temperature at T=33°C with a temperature gradient across the sample of less than 1°C. The samples were irradiated for 20s with irradiance of 1.1W/cm2 and total energy density of 22J/cm2. Three samples (n=3) were used for each RBCs. ResultsThe shrinkage stress at 12h for the five Bulk Fill RBCs ranged from 2.21 to 3.05MPa, maximum stress rate ((dS/dt)M) varied from 0.18 to 0.41MPa/s, time at which the maximum stress rate occurred (tMax) were between 1.42 to 3.24s and effective gel time (tgel) varied from 50 to 770ms. Correlations were observed between (dS/dt)M and tMax (r=−0.946), tMax and filler volume fraction (r=−0.999), and between the shrinkage stress at 12h and tgel (r=0.994). However, no correlation was observed between the stress at 12h and filler volume fraction. SignificanceThe shrinkage stress for four of the five Bulk Fill RBCs were not significantly different (p<0.05) at 6h and beyond after photo-curing and that fully developed stress induced by photo-cured RBCs may only be reached at times longer than 12h.

Polymerization Shrinkage Stress Kinetics and Related Properties of Bulk-fill Resin Composites

The present study assessed the polymerization shrinkage stress kinetics of five low-shrinkage light-cured bulk-fill resin composites: Surefil SDR flow (SF, Dentsply), Tetric EvoCeram Bulkfil (TE, Ivoclar Vivadent), Venus Bulk Fill (VB, Heraeus Kulzer), x-tra fil (XF, Voco), and experimental bulk fill (FB, 3M ESPE). Filtek Z250 (FZ, 3M ESPE) was used as a control. Real-time shrinkage stress of investigated composites was measured using a tensometer; maximum shrinkage stress, stress rate (Rmax), and time to reach maximum stress rate (tmax) were recorded. Flexural strength and modulus were measured using a standard procedure, and curing efficiency of 4-mm long specimens was determined using bottom/top percentage Knoop microhardness. Data were analyzed using one-way analysis of variance and Bonferroni multiple range tests at a significance level of α=0.05. Results of shrinkage stress, Rmax, and tmax of all bulk-fill materials were significantly lower (p<0.05) than those of the control except for XF. All tested bulk-fill materials were able to achieve acceptable curing efficiency (≥80% bottom/top percentage) at 4-mm depth. In conclusion, this study reports a significant reduction in polymerization shrinkage stress while maintaining comparable curing efficiency at 4 mm for some bulk-fill composites and supports their potential use in posterior clinical situations.

Frequency Content of Cartilage Impact Force Signal Reflects Acute Histologic Structural Damage

Objective:The objective of this study was to determine if acute cartilage impact damage could be predicted by a quantification of the frequency content of the impact force signal.Design:Osteochondral specimens excised from bovine lateral tibial plateaus were impacted with one of six impact energies. Each impact force signal underwent frequency analysis, with the amount of higher-frequency content (percentage of frequency spectrum above 1 kHz) being registered. Specimens were histologically evaluated to assess acute structural damage (articular surface cracking and cartilage crushing) resulting from the impact.Results:Acute histologic structural damage to the cartilage had higher concordance with the high-frequency content measure than with other mechanical impact measures (delivered impact energy, impact maximum stress, and impact maximum stress rate of change).Conclusions:This result suggests that the frequency content of an impact force signal, specifically the proportion of higher-frequency components, can be used as a quick surrogate measure for acute structural cartilage injury. Taking advantage of this relationship could reduce the time and expense of histologic processing needed to morphologically assess cartilage damage, especially for purposes of initial screening when evaluating new impaction protocols.

Comparison between a silorane-based composite and methacrylate-based composites: Shrinkage characteristics, thermal properties, gel point and vitrification point

A silorane-based composite was compared against methacrylate-based composites in terms of shrinkage characteristics, thermal properties, gel point, and vitrification point. Shrinkage strain was measured using a laser triangulation method. Shrinkage stress was measured using a stress analyzer. Heat flow during photopolymerization was measured using photo-DSC. Statistical analysis was performed using one-way ANOVA and Tukey's test (p=0.05). Silorane exhibited significantly lower shrinkage strain than the methacrylate-based composites. It also presented the lowest stress values during light exposure, but the highest maximum stress rate after light exposure. It showed the highest heat flow rate, and it took the longest time to reach gel and vitrification points. Silorane demonstrated improved performance over the methacrylate-based composites with delayed gel and vitrification points as well as reduced shrinkage strain and stress. However, a high quantity of heat was liberated during the curing process, causing silorane to show significantly higher stress rate (p<0.05) than the methacrylate-based composites after light exposure.

Contraction stress, elastic modulus, and degree of conversion of three flowable composites

The aim of this study was to measure the contraction stress of three flowable resin composites and to correlate the stress with the elastic modulus and the degree of conversion. One low-shrinkage (Venus Diamond Flow) and two conventional (Tetric EvoFlow and X-Flow) flowable composites were polymerized for 40s with a light-emitting diode (LED) curing unit. Contraction force was continuously recorded for 300s using a stress-analyser, and stress values were calculated at 40s and at 300s. The maximum stress rate was also calculated for each specimen. The elastic modulus of each composite was assayed using a biaxial flexural test, and degree of conversion was analysed with Raman spectroscopy. X-Flow exhibited higher stress values than the other tested materials. Venus Diamond Flow showed the lowest stress values at 40s and at 300s, and the lowest maximum stress rate. Stress values were correlated with elastic modulus but not with degree of conversion, which was comparable among all tested materials.

Degree of Conversion and Contraction Stress Development of a Resin Composite Irradiated Using Halogen and LED at Two C-factor Levels

This study verified the influence of curing methods and light sources on contraction stress, stress rate and degree of conversion (DC) of a restorative composite at two C-factor (CF) levels. For the stress test, composite (0.84 mm thick) was applied between two glass rods 5-mm in diameter mounted in a servohydraulic testing machine. Stress rates were calculated as the change in stress vs time at each second. DC was measured by micro-FTIR. Five curing methods were tested at two C-factor levels (1.5 and 3.0): High Intensity LED (LED HI), Continuous Light (QTH CL), Medium Intensity LED (LED MI), Low Intensity LED (LED LI) and Pulse Delay (QTH PD). The results were analyzed by ANOVA and Tukey's test (alpha = 0.05). For the stress test at CF 1.5, QTH PD presented lower values than LED HI, QTH CL and LED LI. At CF 3.0, no difference was observed among the curing methods. For all curing methods, stress values at CF 3.0 were statistically higher than those at CF 1.5. LED HI presented the highest maximum stress rate, followed by QTH CL, LED MI, LED LI and QTH PD for both C-factors. In the DC test, no difference was observed among the methods and between the C-factor levels.

Contraction stress and physical properties development of a resin-based composite irradiated using modulated curing methods at two C-factor levels

Objectives The objective of this study was to verify the influence of curing methods on contraction stress, stress rate and degree of conversion (DC) of a restorative composite at two C-factor levels. Methods For the stress test, composite was applied between two for 15 min from diameter glass rods mounted in a servohydraulic machine, and stress was monitored for 10 min from the beginning of light curing. Stress rates were calculated as the change in stress versus time at each second. DC was measured by micro-FTIR. Four curing methods were tested at two C-factor levels (1.5 and 3.0): continuous light (CL), soft-start (SS) and two pulse delay methods using different initial irradiances—150 mW/cm 2 (PD150) and 80 mW/cm 2 (PD80). Results were analyzed by ANOVA and Tukey's test ( α = 0.05). Results For the stress test, at CF 1.5, PD80 presented the lowest mean value, statistically different from the others. PD150 also showed a mean value statistically inferior to CL. At CF 3.0, no statistical difference was observed among CL, SS and PD150. PD80 presented statistically lower stress values compared to CL and SS. Stress values at CF 3.0 were statistically higher than those at CF 1.5 for all curing methods. CL presented the highest maximum stress rate, followed by SS, PD150 and PD80, for both C-factors. In the DC test, no difference was observed among the methods and between the C-factor levels. Significance Modulated curing methods were shown to be effective in reducing contraction stress rate, without compromising the DC of the restorative composite. C-factor was shown to influence negatively the stress rate and the amount of stress generated.

Modulated photoactivation methods: Influence on contraction stress, degree of conversion and push-out bond strength of composite restoratives

Verify the influence of curing methods on contraction stress, stress rate, and degree of conversion (DC) of a restorative composite and on bond strength of composite restoratives. For the stress test, composite (0.84 mm thick) was applied between two 5-mm diameter glass rods, mounted in a servohydraulic machine. Stress rate was taken by the value of stress/time at each second. DC was measured by micro-FTIR. Bond strength testing was performed using a push-out test. The C-factor in all tests was 3.0. Four curing methods were tested: continuous light (CL), soft-start (SS), and two pulse delay methods using different initial irradiances--150 mW/cm(2) (PD150) and 80 mW/cm(2) (PD80). Results were analyzed by ANOVA and Tukey's test (alpha=0.05). Stress values ranged from 7.9 MPa (PD80) to 10.3 MPa (CL). No statistical difference was verified among CL, SS, and PD150. PD80 presented statistically lower stress values compared to CL and SS. CL presented the highest maximum stress rate, followed by SS, PD150 and PD80. Mean DC values ranged from 54.2% (PD150) to 55.9% (PD80), with no difference observed among the methods. For the bond strength test, values ranged from 26.4 MPa (CL) to 35.5 MPa (PD150). PD150 and PD80 were both statistically superior to SS and CL. SS presented statistically higher bond strength compared to CL. Modulated curing methods were shown to be effective in reducing contraction stress rate and improving the strength of the bonded interface, and without compromising the DC of the restorative composite.

A New Method for Determination of Proportional Limit and Machine Stiffness

Abstract A new method for determining the proportional limit of engineering materials, characterized by maximum stress rate, is presented. Simultaneous measurement of stress rate and strain rate also permits a new means of measuring machine stiffness which has certain advantages over present techniques.