Composite depth of cure obtained with QTH and LED units assessed by microhardness and micro-Raman spectroscopy

This study analyzed the depth of cure of a composite assessed by microhardness and the degree of conversion as a function of the light cure unit (LCU) used. Two light cure units, one LED (Ultraled-Dabi Atlante) and one quartz-tungsten-halogen (QTH, Optilux 401-Demetron) unit were used to cure 4.0 x 4.0 mm and 5.0 mm deep composite specimens (Filtek Z250, 3M ESPE). After 24 hours storage at 37 degrees C, Knoop microhardness and degree of conversion were measured on the irradiated surface and at each millimeter of the sample's depth. The degree of conversion was determined by using micro-Raman spectroscopy. The specimens cured with the QTH unit presented uniform decay in microhardness up to 4 mm in depth. Beyond 4 mm, the drop was abrupt. With LED photoactivation, uniform decay was observed only up to 2 mm. At higher depths, the decay in microhardness increased rapidly, especially beyond 3 mm. Depth of cure assessed by micro-Raman revealed that the degree of conversion behaved similarly to microhardness for both LCUs. A strong linear regression between microhardness and the degree of conversion, including both LCUs, was established with R2 = 0.980.     

Physical properties of two new crown and bridge veneering resins

The degree of conversion and physical properties of two contemporary resin veneers based on light-cured microfilled composite formulations and employing proprietary curing systems were evaluated. Visio-gem (V) and Dentacolor (D) were polymerized using the appropriate curing systems. Polymer structure and degree of conversion were determined by Fourier transform infra-red (FTIR) techniques. Physical properties, including depth of cure, compressive and diametral tensile strengths, hardness, thermal expansion and colour stability were determined by standard test modalities. Both materials appear to have the properties of typical microfilled resins including low compressive yield strengths and high thermal expansion coefficients. The curing system for V produced an increased depth of cure compared to conventional light curing techniques and the D system, but no clinically significant increase in the physical properties.     

Curing potential of experimental resin composites filled with bioactive glass: A comparison between Bis-EMA and UDMA based resin systems

ObjectivesTo evaluate the degree of conversion, light transmittance, and depth of cure of two experimental light-curable bioactive glass (BG)-containing composite series based on different resin systems.MethodsExperimental composite series based on either Bis-EMA or UDMA resin were prepared. Each series contained 0, 5, 10, 20, and 40 wt% of BG 45S5. Reinforcing fillers were added up to a total filler load of 70 wt%. The degree of conversion was evaluated using Raman spectroscopy, while light transmittance was measured using visible light spectroscopy. The depth of cure was estimated from the degree of conversion data and using the ISO 4049 scraping test.ResultsReplacement of reinforcing fillers with BG can diminish the degree of conversion, light transmittance, and depth of cure. The effect of BG on the aforementioned properties was highly variable between the experimental series. While in the Bis-EMA series, the degree of conversion was significantly impaired by BG, all of the composites in the UDMA series attained clinically acceptable degree of conversion values. The reduction of the degree of conversion in the Bis-EMA series occurred independently of the changes in light transmittance. The UDMA series showed better light transmittance and consequently higher depth of cure than the Bis-EMA series. The depth of cure for all composites in the UDMA series was above 2 mm.SignificanceWhile the Bis-EMA series demonstrated clinically acceptable curing potential only for 0–10 wt% of BG loading, an excellent curing potential in the UDMA series was observed for a wide range (0–40 wt%) of BG loadings.     

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

PURPOSE: This study evaluated the effect of ramped light intensity on the polymerization shrinkage forces and degrees of conversion (DC) of a hybrid composite. MATERIALS AND METHODS: Composite samples were bonded between two steel rods (2.50 mm diameter, 1.25 mm apart, configuration factor = 1.0) mounted in a universal testing machine using a constant displacement mode. Polymerization contraction force was recorded for 250 seconds under four light exposure conditions: group 1, STD: (40 s x 800 mW/cm2); group 2, EXP: (150 mW/cm2 logarithmic increase to 800 mW/cm2 over 15 s) + (25 s x 800 mW/cm2); group 3, 2-STEP: (10 s x 150 mW/cm2) + (30 s x 800 mW/cm2); group 4, MED: (80 s x 400 mW/cm2). Maximum curing force (N250s) and maximum force rate of the four groups were compared using one-way analysis of variance (ANOVA) (alpha = 0.05) and the Tukey test. Degrees of conversion obtained with STD, EXP, and MED cure modes were evaluated at three depths (top surface, 1 mm, and 2 mm) using Fourier transform infrared spectroscopy (FTIR). RESULTS: Maximum rates of polymerization shrinkage force development and standard deviations (SD), in ascending order, were group 4, MED: 0.33 +/- 0.03 N/s; group 2, EXP: 0.35 +/- 0.06 N/s; group 1, STD: 0.44 +/- 0.03 N/s; and group 3, 2-STEP: 0.46 +/- 0.07 N/s. Maximum rates of polymerization shrinkage force development of group 2, EXP and group 4, MED were statistically equivalent and lower than those of group 1, STD and group 3, 2-STEP. Maximum shrinkage forces (+/- SD), in ascending order, were group 2, EXP: 20.4 +/- 2.5 N; group 4, MED: 25.8 +/- 1.0 N; group 3, 2-STEP: 27.4 +/- 5.8 N, and group 1, STD: 30.5 +/- 2.7 N. Maximum force of the EXP mode was statistically lower than MED, 2-STEP, and STD curing modes. The EXP ramp was successful in reducing the conversion rate at the top surface and at 1.0-mm depth, but it did not affect the total conversion compared to the STD 40-second cure mode. There was no difference in DC at the top surface and 1-mm depth with mode of cure. The MED cure mode resulted in a higher DC than the EXP mode at a depth of 2 mm. CLINICAL SIGNIFICANCE: Maximum shrinkage force and force rate exhibited during the first 250 seconds of polymerization were significantly lower using a ramped light intensity exposure. Ramped light intensity decreased conversion rate at the top surface and at 1.0-mm depth and did not affect the total extent of conversion compared to a standard 40-second, single-intensity cure mode. The slower conversion rate resulting from ramped light intensity helped to reduce the rate and maximum polymerization stress, but would not be expected to compromise the physical properties for the restorative material, since similar degrees of conversion were obtained.     

Effect of Ramped Light Intensity on Polymerization Force and Conversion in a Photoactivated Composite

Purpose: This study evaluated the effect of ramped light intensity on the polymerization shrinkage forces and degrees of conversion (DC) of a hybrid composite. Materials and Methods: Composite samples were bonded between two steel rods (2.50 mm diameter, 1.25 mm apart, configuration factor = 1.0) mounted in a universal testing machine using a constant displacement mode. Polymerization contraction force was recorded for 250 seconds under four light exposure conditions: group 1, STD: (40 s ± 800 mW/cm²); group 2, EXP: (150 mW/cm² logarithmic increase to 800 mW/cm² over 15 s) + (25 s ± 800 mW/cm²); group 3, 2‐STEP: (10 s × 150 mW/cm²) + (30 s × 800 mW/cm²); group 4, MED: (80 s × 400 mW/cm²). Maximum curing force (N_250s) and maximum force rate of the four groups were compared using one‐way analysis of variance (ANOVA) (α= 0.05) and the Tukey test. Degrees of conversion obtained with STD, EXP, and MED cure modes were evaluated at three depths (top surface, 1 mm, and 2 mm) using Fourier transform infrared spectroscopy (FTIR). Results: Maximum rates of polymerization shrinkage force development and standard deviations (SD), in ascending order, were group 4, MED: 0.33 ± 0.03 N/s; group 2, EXP: 0.35 ± 0.06 N/s; group 1, STD: 0.44 ± 0.03 N/s; and group 3, 2‐STEP: 0.46 ± 0.07 N/s. Maximum rates of polymerization shrinkage force development of group 2, EXP and group 4, MED were statistically equivalent and lower than those of group 1, STD and group 3, 2‐STEP. Maximum shrinkage forces (± SD), in ascending order, were group 2, EXP: 20.4 ± 2.5 N; group 4, MED: 25.8 ± 1.0 N; group 3, 2‐STEP: 27.4 ± 5.8 N, and group 1, STD: 30.5 ± 2.7 N. Maximum force of the EXP mode was statistically lower than MED, 2‐STEP, and STD curing modes. The EXP ramp was successful in reducing the conversion rate at the top surface and at 1.0‐mm depth, but it did not affect the total conversion compared to the STD 40‐second cure mode. There was no difference in DC at the top surface and 1‐mm depth with mode of cure. The MED cure mode resulted in a higher DC than the EXP mode at a depth of 2 mm. CLINICAL SIGNIFICANCE Maximum shrinkage force and force rate exhibited during the first 250 seconds of polymerization were significantly lower using a ramped light intensity exposure. Ramped light intensity decreased conversion rate at the top surface and at 1.0‐mm depth and did not affect the total extent of conversion compared to a standard 40‐secondY single‐intensity cure mode. The slower conversion rate resulting from ramped light intensity helped to reduce the rate and maximum polymerization stress, but would not be expected to compromise the physical properties for the restorative material, since similar degrees of conversion were obtained.     

Curing efficiency of different polymerization methods through ceramic restorations

The aim of this in vitro study was to examine the curing efficiency of three different polymerization methods through ceramic restorations by determination of the depth of cure and the universal hardness of a composite resin luting material. Therefore, 36 ceramic specimens [Empress 2 (Ivoclar), color 300, diameter 4 mm, height 2 mm] were prepared and inserted in steel molds (diameter 4 mm, height 6 mm) using a composite resin luting material [Variolink II (Vivadent)] with and without catalyst. The polymerization through six specimens of each group was done conventionally (40 s), by softstart polymerization (40 s), or by plasma arc curing (10 s). Depth of cure under the ceramic specimens was assessed according to ISO 4049. Additionally, universal hardness was determined at 0.5 and 1.0 mm from the ceramic using a universal testing machine (Zwick 14040). Curing without a catalyst, using conventional and softstart polymerization, resulted in greater hardness in both layers, compared to plasma arc curing. The use of a catalyst always produced a greater hardness and depth of cure with all polymerization methods. Depth of cure was always greater using conventional polymerization and softstart polymerization, compared to plasma arc curing. The curing efficiency of plasma arc curing through ceramic was lower compared to conventional and softstart-polymerization.     

Raman scattering determination of the depth of cure of light-activated composites: influence of different clinically relevant parameters

The purpose of this research was to determine the depth of cure of light-activated composites in relation with different clinically relevant parameters. A Raman spectroscopic method has been used. The measurement of cure is made on a relative basis by comparing the vibration band of the residual unpolymerized methacrylate C=C bond at 1640 cm-1 against the aromatic C=C stretching band at 1610 cm-1 used as an internal standard. The information gained draw attention to the importance of light transmission during the exposure. The influence of sample's thickness on the depth of cure is illustrated by a second order polynomial regression. The shade and translucency of the resin composite also modify the light transmission and thus have a significant influence on the degree of conversion. Moreover the light-source intensity and the distance from the curing tip are important parameters of influence. A significant reduction of the depth of cure is observed for all sample thickness of resin composite tested when using a light device with an intensity of 300 mW cm-2 as well as using a distance from the curing tip higher than 20 mm.     

Depths of cure and effect of shade using pulse-delay and continuous exposure photo-curing techniques

This study investigated the extent of cure (monomer conversion into polymer) of a variety of photo-initiated resin composites and different shades. Cure values were measured at the top surface and at simulated lighting conditions 0.5, 1.0 and 2.0 mm below the top. The exposure methods used were continuous output at 600 mW/cm2 (10, 20 or 40 seconds), initial component of the pulse-delay technique (pulse) (3 seconds at 200 mW/cm2) and the entire pulse-delay technique (pulse, 3-minute delay, 10 seconds at 600 mW/cm2). The results showed very little difference in conversion values between A2 and D2 shades of the same composite with respect to depth. Conversion values using only the pulse method were remarkably low at the top surface and diminished rapidly at depths. Conversion using the pulse-delay technique produced similar values as that of the continuous 10-second exposure at similar depths but still decreased remarkably at depth. Conversion values using the pulse-delay technique and a 20-second continuous exposure were significantly lower than those obtained using continuous 40-second exposure.     

New insight into the "depth of cure" of dimethacrylate-based dental composites

ObjectivesTo demonstrate that determination of the depth of cure of resin-based composites needs to take into account the depth at which the transition between glassy and rubbery states of the resin matrix occurs.MethodsA commercially available nano-hybrid composite (Grandio) in a thick layer was light cured from one side for 10 or 40 s. Samples were analyzed by Vickers indentation, Raman spectroscopy, atomic force microscopy, electron paramagnetic imaging and differential scanning calorimetry to measure the evolution of the following properties with depth: microhardness, degree of conversion, elastic modulus of the resin matrix, trapped free radical concentration and glass transition temperature. These measurements were compared to the composite thickness remaining after scraping off the uncured, soft composite.ResultsThere was a progressive decrease in the degree of conversion and microhardness with depth as both properties still exhibited 80% of their upper surface values at 4 and 3.8 mm, respectively, for 10 s samples, and 5.6 and 4.8 mm, respectively, for 40 s samples. In contrast, there was a rapid decrease in elastic modulus at around 2.4 mm for the 10 s samples and 3.0 mm for the 40 s samples. A similar decrease was observed for concentrations of propagating radicals at 2 mm, but not for concentrations of allylic radicals, which decreased progressively. Whereas the upper composite layers presented a glass transition temperature – for 10 s, 55 °C (±4) at 1 mm, 56.3 °C (±2.3) at 2 mm; for 40 s, 62.3 °C (±0.6) at 1 mm, 62 °C (±1) at 2 mm, 62 °C (±1.7) at 3 mm – the deeper layers did not display any glass transition. The thickness remaining after scraping off the soft composite was 7.01 (±0.07 mm) for 10 s samples and 9.48 (±0.22 mm) for 40 s samples.SignificanceAppropriate methods show that the organic matrix of resin-based composite shifts from a glassy to a gel state at a certain depth. Hence, we propose a new definition for the “depth of cure” as the depth at which the resin matrix switches from a glassy to a rubbery state. Properties currently used to evaluate depth of cure (microhardness, degree of conversion or scraping methods) fail to detect this transition, which results in overestimation of the depth of cure.     

Comparative depths of cure among various curing light types and methods

This study evaluated the depth of cure associated with commercial LEDs (light-emitting diodes) (Elipar FreeLight [FL], 3M-ESPE; GC e-Light [EL], GC), high intensity (Elipar TriLight [TL], 3M-ESPE) and very high intensity (Astralis 10 [AS], Ivoclar Vivadent) Quartz Tungsten Halogen (QTH) curing lights. Depth of cure of the various lights/curing modes were compared to a conventional QTH light (Max [Mx], Dentsply-Caulk). Ten exposure regimens were investigated: FL1 - 400 mW/cm2 [40 seconds]; FL2 - 0-400 mW/cm2 [12 seconds] --> 400 mW/cm2 [28 seconds]; EL1 - 750 mW/cm2 [10 pulses x 2 seconds], EL2 - 350 mW/cm2 [40 seconds]; EL3 - 600 mW/cm2 [20 seconds]; EL4 - 0 - 600 mW/cm2 [20 seconds] --> 600 mW/cm2 [20 seconds]; TL1 - 800 mW/cm2 [40 seconds]; TL2 - 100- 800 mW/cm2 [15 seconds] --> 800 mW/cm2 [25 seconds]; AS1 - 1200 mW/cm2 [10 seconds]; MX - 400 mW/cm2 [40 seconds]. Depth of cure was determined by penetration, scraping and microhardness techniques. The results were analyzed using one-way ANOVA/Scheffe's post-hoc test and Pearson's correlation at significance level 0.05 and 0.01, respectively. All light curing regimens met the ISO depth of cure requirement of 1.5 mm with the exception of EL1-EL3 with the microhardness technique. Curing with most modes of EL resulted in significantly lower depths of cure than the control [MX]. No significant difference in depth of cure was observed among the control and the two modes of FL. Curing with TL1 resulted in significantly greater depth of cure compared to MX with all testing techniques. No significant difference in depth of cure was observed between the control and AS1 for all testing techniques except for the penetration technique. The depth of composite cure is light unit and exposure mode dependent. Scraping and penetration techniques were found to correlate well but tend to overestimate depth of cure compared to microhardness.     

Effectiveness of polymerization of a prosthetic composite using three polymerization systems.

STATEMENT OF PROBLEM: Although properties of laboratory-polymerized composite materials are influenced by the type of polymerizing unit, little information is available regarding the comparison between use of a high-intensity light source and application of secondary heat treatment. PURPOSE: This study examined properties of a prosthetic veneering composite polymerized with 3 polymerizing systems to evaluate the effects of varying polymerization modes on hardness, solubility, and depth of cure. MATERIAL AND METHODS: A composite material designed for a prosthetic veneer (Conquest Crown and Bridge) was polymerized using 3 methods: (1) exposure in the proprietary photopolymerizing unit with 2 halogen lamps (Cure-Lite Plus), followed by heating in an oven (Conquest Automatic Curing Unit); (2) exposure in a photopolymerizing unit with a xenon stroboscopic light source (Dentacolor XS); and (3) exposure in a photopolymerizing unit with 2 metal halide lamps (Hyper LII). Knoop hardness, water solubility, and depth of cure were determined for groups of 5 specimens, according to standardized testing methods. Data were compared using analysis of variance and the Duncan new multiple range test (P <.05). RESULT: The hardness number generated with the metal halide unit was statistically greater than those produced by the other 2 methods, and material component released into water was minimal when the material was exposed with the metal halide unit (P <.05). Among the 3 photopolymerizing units, the metal halide unit consistently exhibited the greatest depth of cure. CONCLUSION: Certain properties generated with the use of the high-intensity polymerizing unit exceeded those obtained from a proprietary system that requires a postheat treatment.     

Polymerization depths of contemporary light-curing units using microhardness

PURPOSE: This research investigated composite depths of cure using a variety of light-curing units and exposure protocols. MATERIALS AND METHODS: Composite (Herculite XRV, shade A2, Kerr, Orange, California) was exposed in opaque compules to conventional quartz tungsten halogen (QTH) units, soft-start units, high-intensity QTH and plasma arc (PAC) curing lights, and one argon laser. Cured compules were sonicated to remove uncured composite and were sectioned and polished along the long axis to expose cured composite. Knoop hardness was measured 0.5 mm from the irradiated, top surface and then at 1.0 mm and in 1.0-mm increments until reliable readings could no longer be obtained. Hardness values were compared by analysis of variance at similar depths within a specific curing-light classification, using the hardness of the standard 40-second conventional QTH exposure as comparison (Dunnett's t-test). Depth of cure was defined as the deepest hardness value found equivalent to that at 0.5-mm depth for a specific curing light and scenario. RESULTS: Conventional QTH lights provided similar hardness profiles. At 2-mm depth, use of a different unit or curing tip made no difference in hardness compared with the standard. At this depth, soft-start (pulse-delay and step-cure) methods yielded hardness similar to that of the standard. High-intensity QTH lights provided similar hardness at 2-mm depth in 10 seconds to that of the standard 40-second exposure. Plasma arc exposure for less than 10 seconds produced inferior hardness compared with the standard. A 10-second PAC and a 5-second laser exposure gave hardness at 2-mm depth equivalent to that of the 40-second standard. Depth of cure for almost all curing scenarios was not greater than 2 mm. CLINICAL SIGNIFICANCE: Similar-type conventional QTH lights with different tip diameter (8 and 12 mm) provide similar composite cure characteristics. Soft-start techniques provide similar cure profiles to those achieved with conventional QTH technique when used according to manufacturer's recommendations. High-intensity QTH units and the argon laser can reduce exposure time while providing composite with similar hardness to that of conventional QTH curing. Plasma arc exposure should be at least of 10 seconds duration to provide hardness equivalent to that achieved with conventional 40-second QTH exposure. Even with consideration of high-intensity curing units, composite increments should still be no greater than 2 mm to provide homogeneous hardness.     

Thiol-ene-methacrylate composites as dental restorative materials

Objectives

The objective of this study was to evaluate composite methacrylate-thiol-ene formulations with varying thiol:ene stoichiometry relative to composite dimethacrylate control formulations. It was hypothesized that the methacrylate-thiol-ene systems would exhibit superior properties relative to the dimethacrylate control resins and that excess thiol relative to ene would further enhance shrinkage and conversion associated properties.

Methods

Polymerization kinetics and functional group conversions were determined by Fourier transform infrared spectroscopy (FTIR). Volume shrinkage was measured with a linometer and shrinkage stress was measured with a tensometer. Flexural modulus and strength, depth of cure, water sorption and solubility tests were all performed according to ISO 4049.

Results

All of the methacrylate-thiol-ene systems exhibited improvements in methacrylate conversion, flexural strength, shrinkage stress, depth of cure, and water solubility, while maintaining equivalent flexural modulus and water sorption relative to the dimethacrylate control systems. Increasing the thiol to ene stoichiometry resulted in further increased methacrylate functional group conversion and decreased volume shrinkage. Flexural modulus and strength, shrinkage stress, depth of cure, water sorption and solubility did not exhibit statistically significant changes with excess thiol.

Significance

Due to their improved overall functional group conversion and reduced water sorption, the methacrylate-thiol-ene formulations are expected to exhibit improved biocompatibility relative to the dimethacrylate control systems. Improvements in flexural strength and reduced shrinkage stress may be expected to result in composite restorations with superior longevity and performance.     

Use of near-IR to monitor the influence of external heating on dental composite photopolymerization.

OBJECTIVES: This study was conducted to determine the effect of modest external heating on the photopolymerization kinetics and conversion of commercial dental composite restorative materials. METHODS: A transmission-mode, real-time near-infrared spectroscopic technique was used to monitor the photopolymerization process in the composite materials at various temperatures between 23 and 70 degrees C. Several light curing units, differing in spectral output and power densities were compared at the different cure temperatures. Several significantly different commercial composites were compared for their response. RESULTS: Regardless of the curing light or composite material used, photopolymerization at a moderate curing temperature of 54.5 degrees C resulted in significantly higher immediate and final conversion values compared with room temperature photocuring. Contrary to the room temperature cured materials, at the elevated cure temperature the extent of post-cure was minor and different curing lights produced very uniform conversion values within a given material. The time required to reach a given level of conversion, established as full conversion with the room temperature cure, was reduced typically by 80-90% using the elevated curing conditions. Complementary kinetic studies confirmed the effect of cure temperature on increasing the polymerization rate in dental composites as significant. SIGNIFICANCE: Increasing the temperature of composite resin within potentially biologically compatible limits can significantly influences resin polymerization. These increased rates and conversion could lead to improved properties of composite restorative materials.     

Effect of mold diameter on the depth of cure of a resin‐based composite material

The purpose of this study was to examine the effect of mold diameter on depth of cure of a resin‐based composite material for varying amounts of irradiation. A resin‐based composite was light‐cured for 10–80 s in stainless‐steel molds of either 6 mm or 4 mm in diameter and then dark‐stored for 24 h. Specimens were then scraped back and the length of the cured specimens was measured to provide depth of cure (D_SB). Radiant exposure to each of the mold diameters was determined by measuring the power. The D_SB values using the 4‐mm molds were lower than those of the 6‐mm molds. The average difference between the two groups for each irradiation time was 0.45 ± 0.02 mm. A fixed depth of cure required about 39% more irradiation time for the 4‐mm mold than for the 6‐mm mold but 75% more radiant exposure. The difference in cure depth for a fixed radiant exposure was 0.79 mm. A better comparison of depth of cure is obtained by using identical radiant exposures for different mold diameters. It is believed that greater loss of light by absorption at the stainless‐steel cylinder walls for the 4‐mm‐diameter cylinders accounts for the lower depth of cure when compared with the 6‐mm molds.     

The effect of infection-control barriers on the light intensity of light-cure units and depth of cure of composite.

AIM: Dental curing lights are vulnerable to contamination with oral fluids during routine intra-oral use. This controlled study aimed to evaluate whether or not disposable transparent barriers placed over the light-guide tip would affect light output intensity or the subsequent depth of cure of a composite restoration. METHODS: The impact on light intensity emitted from high-, medium- and low-output light-cure units in the presence of two commercially available disposable infection-control barriers was evaluated against a no-barrier control. Power density measurements from the three intensity light-cure units were recorded with a radiometer, then converted to a digital image using an intra-oral camera and values determined using a commercial computer program. For each curing unit, the measurements were repeated on ten separate occasions with each barrier and the control. Depth of cure was evaluated using a scrape test in a natural tooth model. RESULTS: At each level of light output, the two disposable barriers produced a significant reduction in the mean power density readings compared to the no-barrier control (P<0.005). The cure sleeve inhibited light output to a greater extent than either the cling film or the control (P<0.005). Only composite restorations light-activated by the high level unit demonstrated a small but significant decrease in the depth of cure compared to the control (P<0.05). CONCLUSION: Placing disposable barriers over the light-guide tip reduced the light intensity from all three curing lights. There was no impact on depth of cure except for the high-output light, where a small decrease in cure depth was noted but this was not considered clinically significant. Disposable barriers can be recommended for use with light-cure lights.     

Polymerization Depths of Contemporary Light-Curing Units Using Microhardness

Purpose: This research investigated composite depths of cure using a variety of light‐curing units and exposure protocols. Materials and Methods: Composite (Herculite XRV, shade A2, Kerr, Orange, California) was exposed in opaque compules to conventional quartz tungsten halogen (QTH) units, soft‐start units, high‐intensity QTH and plasma arc (PAC) curing lights, and one argon laser. Cured compules were sonicated to remove uncured composite and were sectioned and polished along the long axis to expose cured composite. Knoop hardness was measured 0.5 mm from the irradiated, top surface and then at 1.0 mm and in 1.0‐mm increments until reliable readings could no longer be obtained. Hardness values were compared by analysis of variance at similar depths within a specific curing‐light classification, using the hardness of the standard 40‐second conventional QTH exposure as comparison (Dunnett's t‐test). Depth of cure was defined as the deepest hardness value found equivalent to that at 0.5‐mm depth for a specific curing light and scenario. Results: Conventional QTH lights provided similar hardness profiles. At 2‐mm depth, use of a different unit or curing tip made no difference in hardness compared with the standard. At this depth, soft‐start (pulse‐delay and step‐cure) methods yielded hardness similar to that of the standard. High‐intensity QTH lights provided similar hardness at 2‐mm depth in 10 seconds to that of the standard 40‐second exposure. Plasma arc exposure for less than 10 seconds produced inferior hardness compared with the standard. A 10‐second PAC and a 5‐second laser exposure gave hardness at 2‐mm depth equivalent to that of the 40‐second standard. Depth of cure for almost all curing scenarios was not greater than 2 mm. CLINICAL SIGNIFICANCE Similar‐type conventional QTH lights with different tip diameter (8 and 12 mm) provide similar composite cure characteristics. Soft‐start techniques provide similar cure profiles to those achieved with conventional QTH technique when used according to manufacturer's recommendations. High‐intensity QTH units and the argon laser can reduce exposure time while providing composite with similar hardness to that of conventional QTH curing. Plasma arc exposure should be at least of 10 seconds duration to provide hardness equivalent to that achieved with conventional 40‐second QTH exposure. Even with consideration of high‐intensity curing units, composite increments should still be no greater than 2 mm to provide homogeneous hardness.     

Light exposure required for optimum conversion of light activated resin systems

OBJECTIVE: The aim of this study was to evaluate the relationship between the degree of conversion (DC) of composites and the light intensity using LED-curing units and also to determine the amount of exposure required to achieve optimal curing. METHOD: The light outputs of light-curing units and the depths of cure of composites exposed to these units were determined using the methods outlined in modified ISO standards, ISO/TS10650 and ISO 4049, respectively. The distributions of DC in composites were investigated by IR spectra of microareas obtained at various depths from the irradiated surface of thin specimens cut out from the cured composites. IR spectra were measured using a Fourier transform infrared spectrometer equipped with a microscopic unit. DC was calculated from the changes in the amount of C=C double bonds in the IR spectra. RESULTS: The light intensity at various depths through the cured composite was calculated from the attenuation coefficient of each material, obtained from the linear relationship between the depth of cure and the logarithm of the amount of exposure, which is defined as the product of the irradiance and irradiation time. There was a third or fourth order regression relationship between DC and the logarithm of total light energy at a particular depth. SIGNIFICANCE: The minimum light energy required to produce a saturated DC was about 1000 s mW/cm2.     

Depth of cure of light-cured glass-ionomer cements

The depth of cure of three light-cured glass-ionomer cements was examined immediately after polymerization under the curing light and 12 hours after polymerization. The duration of illumination was also varied among groups to determine its effect on the depth of cure. Light-Cured Zionomer showed a greater immediate depth of cure than did either Vitrabond or XR Ionomer. All three materials demonstrated greater depth of cure 12 hours after application of the light source, indicating that they possess dual-curing characteristics. Increasing the duration of illumination from 30 to 60 seconds significantly (P less than .05) increased the depth of cure for all three materials.     

Effect of power density of curing unit, exposure duration, and light guide distance on composite depth of cure

This in vitro study compared the depth of cure obtained with six quartz tungsten halogen and light-emitting diode curing units at different exposure times and light tip-resin composite distances. Resin composite specimens (Tetric Ceram, A3; diameter 4 mm, height 6 mm) were exposed from 0-, 3-, and 6-mm distance. The curing units (200–700 mW/cm²) were used for standard (20 and 40 s), pulse-delay mode (initial exposure of 3 s at 200 mW/cm², followed by a resting period of 3 min and a final exposure of 10 or 30 s at 600 mW/cm²), or soft-start curing (40 s; exponential ramping). Curing depth was determined by measurement of Wallace hardness for each half millimeter starting at 0.5 mm from the top surface. For each specimen, a mean H_W value was calculated from the H_W values determined at the depths of 2.0 mm and less (0.5, 1.0, 1.5, and 2.0 mm, respectively). The depth of cure for each specimen was found by determining the greatest depth before an H_W value exceeding the minimal H_W value by 25% occurred. For all curing units, an increase in exposure time led to significantly higher depth of cure. Increasing the light tip-resin composite distance significantly reduced the depth of cure. With a light tip-resin composite distance of 6 mm, median values of depth of cure varied between 2.0 and 3.5 mm following a 20-s (or 3+10 s) exposure and between 3.0 and 4.5 mm following a 40-s (or 3+30 s) exposure. The composite situated above the depth of cure value cured equally well with all curing units. At both exposure times, Luxomax resulted in the significantly lowest depth of cure, and Astralis 7 yielded significantly higher depth. At both exposure times, a significant linear correlation was found between the determined power densities of the curing units and the pooled depth of cure values obtained. It seems that for the resin composite tested, the recommended exposure time of 40 s per 2-mm increment may be reduced to 20 s, or that increments may be increased from 2 to 3.5 mm. It may be that the absolute values of depth of cure found are material specific, but we believe that the relationships found between curing units, between exposure times, and between light guide distances are universal.