Richard B. Price

Effect of delivering light in specific narrow bandwidths from 394 to 515nm on the micro-hardness of resin composites.

OBJECTIVES This study investigated the wavelength-dependent photosensitivity of eleven resin composites (Admira A2, Heliomolar A2, Herculite XRV A2, Pyramid Dentin A2, Solitaire 2 A2, Z250 A2, AElite LS A2, Vit-l-escence A2, Tetric Ceram Bleach XL, Tetric Ceram A2, Pyramid Enamel Neutral). METHODS Resin composites 1.6mm thick were exposed to narrow bandwidths of light at the following peak wavelengths: 394, 400, 405, 410, 415, 420, 430, 436, 442, 450, 455, 458, 467, 470, 480, 486, 493, 500, 505, and 515+/-5nm. A spectroradiometer was used to ensure that the same irradiance (mW/cm(2)) and total energy density (J/cm(2)) was delivered through each filter. For each resin composite, three specimens were exposed through each filter. The Knoop micro-hardness at the top and bottom of the composites was then measured. The wavelength-dependent photosensitivity of each resin composite was analyzed by plotting the mean hardness achieved at each wavelength. RESULTS The composites responded variably when they received light through the narrow bandpass filters. Six resin composites had a single peak of wavelength-dependent photosensitivity at approximately 470nm. Four resin composites had two peaks of wavelength-dependent photosensitivity at approximately 470 and approximately 405nm. One resin composite had a single peak of wavelength-dependent photosensitivity at approximately 405nm and was only sensitive to light below 436nm. SIGNIFICANCE Using light delivered through narrow bandpass filters is a convenient method to determine the wavelength-dependent photosensitivity of resins and can be used to predict the performance of dental curing lights.

The effect of two configuration factors, time, and thermal cycling on resin to dentin bond strengths

Most in vitro testing of bonding systems is performed using specimens made in a mold with a low configuration (C) factor (ratio of bonded/unbonded surfaces) whereas clinically the C-factor is usually much greater. This study compared the effect of thermal cycling on the measured shear bond strength of 3M Single Bond dental adhesive bonded to dentin using molds with two different C-factors. The hypothesis was that neither C-factor nor thermal cycling would affect measured bond strengths. Resin composite was bonded to human dentin in cylindrical molds with an internal diameter of 3.2mm and either 1mm or 2.5mm deep. The 1mm deep molds had a C-factor of 2.2 and the 2.5mm deep molds had a C-factor of 4.1. Specimens were debonded either 10min after they had been bonded to dentin, or after they had been stored for 7 days in water at 37+/-1 degrees C, or after thermal cycling 5000 times for 7 days. Two-way ANOVA showed that overall both the C-factor and the storage condition had a significant effect on bond strength (p<0.001). There was a significant interaction (p<0.001) between the C-factor and how the specimens had been stored. The GLM/LSMEANS procedure with Sidaks adjustment for multiple comparisons showed that overall the specimens made in the mold with a high C-factor (4.1) had a lower bond strength than those that had been made in the mold with a lower (2.2) C-factor (p<0.001). Thermal cycling had a negative effect on the bond strength only for specimens made in molds with a C-factor of 4.1 (p<0.001).

Irradiance differences in the violet (405 nm) and blue (460 nm) spectral ranges among dental light-curing units.

PROBLEM Previous studies identified nonuniformity in the irradiance at the tip end of a variety of dental light-curing units (LCUs) and correlated those differences with potential clinical implications, but the spectral dependence of the irradiance uniformity has not yet been addressed. PURPOSE This study examined the irradiance uniformity across emitting tips of LCUs at two emission wavelengths, 405 and 460 nm. Two broadband emission light units (quartz-tungsten-halogen [QTH] and plasma arc [PAC]), and four commercial light-emitting diode (LED)-type LCUs were examined. MATERIALS AND METHODS The spectral radiant power from six LCUs was measured using a laboratory grade spectroradiometer (Ocean Optics, Dunedin, FL, USA). The spatial and spectral characteristics of irradiance across the emitting tips of these light units were recorded through 10-nm wide bandpass filters (centered at 405 nm [violet] or 460 nm [blue]) using a laser beam analyzer (Ophir-Spiricon, Logan, UT, USA). Irradiance distributions were reported using two-dimensional contour and three-dimensional isometric color-coded images. Irradiance uniformity at the tip end was determined using the Top Hat Factor (THF) for each filtered wavelength. RESULTS Irradiance distributions from the QTH and PAC units were uniformly distributed across the tip end of the light guide, and THF values, measured through the 405 and 460-nm filters, were not significantly different. However, the three polywave LED units delivered non-uniform irradiance distributions with THF values differing significantly between the 405 and 460-nm emission wavelengths for each unit. Areas of nonuniformity were attributed to the locations of the various types of LED chips within the LCUs. CONCLUSION All three polywave LED units delivered a nonuniform irradiance distribution across their emitting tip ends at the two important emission wavelengths of 405 nm and 460 nm, whereas the broadband light sources (QTH and PAC) showed no evidence of spectral inhomogeneity at these wavelengths.

Localised irradiance distribution found in dental light curing units.

OBJECTIVE To measure the localised irradiance and wavelength distributions from dental light curing units (LCUs) and establish a method to characterise their output. METHODS Using a laboratory grade integrating sphere spectrometer system (Labsphere and Ocean Optics) the power, irradiance, and spectral emission were measured at the light tips of four LCUs: one plasma-arc (PAC) unit, one single peak blue light-emitting diode (blue-LED) unit, and two polywave LED (poly-LED) units. A beam profiler camera (Ophir Spiricon) was used to record the localised irradiance across the face of the light tips. The irradiance-calibrated beam profile images were then divided into 45 squares, each 1mm(2). Each square contained the irradiance information received from approximately 3200 pixels. The mean irradiance value within each square was calculated, and the distribution of irradiance values among these 45 squares across the tip-ends was examined. Additionally, the spectral emission was recorded at various regions across each light tip using the integrating sphere with a 4-mm diameter entrance aperture. RESULTS The localised irradiance distribution was inhomogeneous in all four lights. The irradiance distribution was most uniformly distributed across the PAC tip. Both the irradiance and spectral emission from the poly-LED units were very unevenly distributed. CONCLUSIONS Reporting a single irradiance value or a single spectral range to describe the output from a curing light is both imprecise and inappropriate. Instead, an image of both the irradiance distribution and the distribution of the spectral emission across the light tip should be provided. CLINICAL SIGNIFICANCE The localised beam irradiance profile at the tip of dental LCUs is not uniform. Poly-LED units may deliver spectrally inhomogeneous irradiance profiles. Depending on the photoinitiator used in the RBC and the orientation of the LCU over the tooth, this non-uniformity may cause inadequate and inhomogeneous resin polymerisation, leading to poor physical properties, and premature failure of the restoration.

Irradiance uniformity and distribution from dental light curing units

PROBLEM The irradiance from dental light-curing units (LCUs) is commonly reported as a single number, but this number does not properly describe the light output. PURPOSE This study examined the irradiance uniformity and distribution from a variety of LCUs as well as the effect of different light guides. MATERIALS AND METHODS Five LCUs representing quartz-tungsten-halogen, plasma arc, and light emitting diode units were evaluated. One LCU was evaluated using two different light guides (Standard or Turbo style). The total power emitted from each LCU was measured and the irradiance calculated using conventional methods (I(CM)). In addition, a beam profiler was used to determine the optically active emitting area, the mean irradiance (I(BP)), the irradiance distribution, and the Top Hat Factor (THF). Five replications were performed for each test and compared using analysis of variance with Fishers PLSD tests at a pre-set alpha of 0.05. RESULTS The spatial distribution of the irradiance from LCUs was neither universally symmetrical nor was it uniformly distributed across the tip end. Significant differences in both the emitted power and THF were found among the LCUs. The THF values ranged from a high of 0.74 +/- 0.01 to a low of 0.32 +/- 0.01. Changing from a standard to a turbo light guide increased the irradiance, but significantly reduced beam homogeneity, reduced the total emitted power, and reduced the optical tip area by 60%. CONCLUSIONS Using different light guides on the same LCU significantly affected the power output, irradiance values, and beam homogeneity. For all LCUs, irradiance values calculated using conventional methods (I(CM)) did not represent the irradiance distribution across the tip end of the LCU. CLINICAL SIGNIFICANCE Irradiance values calculated using conventional methods assume power uniformity within the beam and do not validly characterize the distribution of the irradiance delivered from dental light curing units.

Light-Curing Units A Review of What We Need to Know

For improved interstudy reproducibility, reduced risk of premature failures, and ultimately better patient care, researchers and dentists need to know how to accurately characterize the electromagnetic radiation (light) they are delivering to the resins they are using. The output from a light-curing unit (LCU) is commonly characterized by its irradiance. If this value is measured at the light tip, it describes the radiant exitance from the surface of the light tip, and not the irradiance received by the specimen. The value quoted also reflects only an averaged value over the total measurement area and does not represent the irradiance that the resin specimen is receiving locally or at a different moment in time. Recent evidence has reported that the spectral emission and radiant exitance beam profiles from LCUs can be highly inhomogeneous. This can cause nonuniform temperature changes and uneven photopolymerization within the resin restoration. The spectral radiant power can be very different between different brands of LCUs, and the use of irradiance values derived from dental radiometers to describe the output from an LCU for research purposes is discouraged. Manufacturers should provide more information about the light output from the LCU and the absorption spectrum of their resin-based composite (RBC). Ideally, future assessments and research publications should include the following information about the curing light: 1) radiant power output throughout the exposure cycle and the spectral radiant power as a function of wavelength, 2) analysis of the light beam profile and spectral emission across the light beam, and 3) measurement and reporting of the light the RBC specimen received as well as the output measured at the light tip.

The effect of specimen temperature on the polymerization of a resin-composite

OBJECTIVE To use rapid scan FT-IR and Knoop microhardness to determine the effect of specimen temperature on the rate and extent of polymerization of a dental resin. METHODS Two-millimeter thick specimens of shade A2 Tetric EvoCeram were light cured for 20s at 22, 26, 30, and 35°C. The IR spectrum was obtained at the bottom of the specimens at a rate of 3 measurements per second for the first 5 min, and then again 2h later. The Knoop microhardness was measured at the bottom of the samples in the region where the IR spectrum was recorded at 5 min and 2h after light curing. Data were statistically analyzed using mixed model ANOVA (with Fishers PLSD) to examine the effect of temperature, time and their interaction. The rate of conversion was determined using first differences and smoothed using a cubic spline procedure. RESULTS The bottom surfaces of the samples light cured at 22, 26, 30 and 35°C were all significantly different from each other (p<0.05). The higher temperature resulted in higher degree of conversion and Knoop microhardness values, and faster maximum rate of polymerization, which also occurred sooner. One second after the light was turned on, the rate of conversion was 106% faster at 35°C than at 22°C (p=0.003). Regression analysis showed a positive linear correlation between the degree of conversion and Knoop microhardness (r²=0.93). SIGNIFICANCE A relatively small difference in temperature can have a large and significant effect on the rate and extent of polymerization of dental resin. Consequently laboratory studies comparing the performance of resins should be conducted at clinically relevant temperatures.

Knoop Microhardness Mapping Used to Compare the Efficacy of LED, QTH and PAC Curing Lights

This study used a hardness mapping technique to compare the ability of seven curing lights to polymerize five composites. Six curing lights (Sapphire [plasma-arc: PAC], Bluephase16i [light emitting diode: LED], LEDemetron II [LED], SmartLite IQ [LED], Allegro [LED] and UltraLume-5 [Polywave LED]) were compared to an Optilux 501 (halogen: QTH) light. Five resin composites (Vit-1-escence, Tetric Evoceram, Filtek Z250, 4 Seasons and Solitaire 2) were polymerized at 4 mm and 8 mm from the end of the light guide. Four composites were light cured for the following times using these lights: Sapphire (5 seconds), Bluephase16i (5 seconds), LEDemetron II (5 seconds), SmartLite IQ (10 seconds), UltraLume-5 (10 seconds), Allegro (10 seconds) and Optilux 501 (20 seconds). Solitaire 2 required double these irradiation times. On each specimen, the Knoop microhardness (KHN) was measured at 49 locations across a 3 x 3 mm grid to determine the ability of each light to cure each brand of composite. The PAC light delivered the broadest spectrum of wavelengths, the greatest irradiance and hardness values that were 4.7 to 18.1 KHN(50gf) harder than the other lights. The ability of the lights to cure these five composites was ranked from highest to lowest: Sapphire, Optilux 501, Allegro, UltraLume-5, SmartLite IQ, LEDemetron II and Bluephase16i (ANOVA with REGWQ multiple comparison adjustment, p < 0.01).

Temperature excursions at the pulp-dentin junction during the curing of light-activated dental restorations

OBJECTIVES Excessive heat produced during the curing of light-activated dental restorations may injure the dental pulp. The maximum temperature excursion at the pulp-dentin junction provides a means to assess the risk of thermal injury. In this investigation we develop and evaluate a model to simulate temperature increases during light-curing of dental restorations and use it to investigate the influence of several factors on the maximum temperature excursion along the pulp-dentin junction. METHODS Finite element method modeling, using COMSOL 3.3a, was employed to simulate temperature distributions in a 2D, axisymmetric model tooth. The necessary parameters were determined from a combination of literature reports and our measurements of enthalpy of polymerization, heat capacity, density, thermal conductivity and reflectance for several dental composites. Results of the model were validated using in vitro experiments. RESULTS Comparisons with in vitro experiments indicate that the model provides a good approximation of the actual temperature increases. The intensity of the curing light, the curing time and the enthalpy of polymerization of the resin composite were the most important factors. The composite is a good insulator and the greatest risk occurs when using the light to cure the thin layer of bonding resin or in deep restorations that do not have a liner to act as a thermal barrier. SIGNIFICANCE The results show the importance of considering temperature increases when developing curing protocols. Furthermore, we suggest methods to minimize the temperature increase and hence the risk of thermal injury. The physical properties measured for several commercial composites may be useful in other studies.

Contemporary issues in light curing

This review article will help clinicians understand the important role of the light curing unit (LCU) in their offices. The importance of irradiance uniformity, spectral emission, monitoring the LCU, infection control methods, recommended light exposure times, and learning the correct light curing technique are reviewed. Additionally, the consequences of delivering too little or too much light energy, the concern over leachates from undercured resins, and the ocular hazards are discussed. Practical recommendations are provided to help clinicians improve their use of the LCU so that their patients can receive safe and potentially longer lasting resin restorations.