Braden Sullivan

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.

Effect of the irradiance distribution from light curing units on the local micro-hardness of the surface of dental resins.

OBJECTIVE An inhomogeneous irradiance distribution from a light-curing unit (LCU) can locally cause inhomogeneous curing with locally inadequately cured and/or over-cured areas causing e.g. monomer elution or internal shrinkage stresses, and thus reduce the lifetime of dental resin based composite (RBC) restorations. The aim of the study is to determine both the irradiance distribution of two light curing units (LCUs) and its influence on the local mechanical properties of a RBC. METHODS Specimens of Arabesk TOP OA2 were irradiated for 5, 20, and 80s using a Bluephase® 20i LCU in the Low mode (666mW/cm(2)), in the Turbo mode (2222mW/cm(2)) and a Celalux® 2 (1264mW/cm(2)). The degree of conversion (DC) was determined with an ATR-FTIR. The Knoop micro-hardness (average of five specimens) was measured on the specimen surface after 24h of dark and dry storage at room temperature. RESULTS The irradiance distribution affected the hardness distribution across the surface of the specimens. The hardness distribution corresponded well to the inhomogeneous irradiance distributions of the LCU. The highest reaction rates occurred after approximately 2s light exposure. A DC of 40% was reached after 3.6 or 5.7s, depending on the LCU. The inhomogeneous hardness distribution was still evident after 80s of light exposure. SIGNIFICANCE The irradiance distribution from a LCU is reflected in the hardness distribution across the surface. Irradiance level of the LCU and light exposure time do not affect the pattern of the hardness distribution--only the hardness level. In areas of low irradiation this may result in inadequate resin polymerization, poor physical properties, and hence premature failure of the restorations as they are usually much smaller than the investigated specimens. It has to be stressed that inhomogeneous does not necessarily mean poor if in all areas of the restoration enough light intensity is introduced to achieve a high degree of cure.

Examining exposure reciprocity in a resin based composite using high irradiance levels and real-time degree of conversion values

OBJECTIVE Exposure reciprocity suggests that, as long as the same radiant exposure is delivered, different combinations of irradiance and exposure time will achieve the same degree of resin polymerization. This study examined the validity of exposure reciprocity using real time degree of conversion results from one commercial flowable dental resin. Additionally a new fitting function to describe the polymerization kinetics is proposed. METHODS A Plasma Arc Light Curing Unit (LCU) was used to deliver 0.75, 1.2, 1.5, 3.7 or 7.5 W/cm(2) to 2mm thick samples of Tetric EvoFlow (Ivoclar Vivadent). The irradiances and radiant exposures received by the resin were determined using an integrating sphere connected to a fiber-optic spectrometer. The degree of conversion (DC) was recorded at a rate of 8.5 measurements a second at the bottom of the resin using attenuated total reflectance Fourier Transform mid-infrared spectroscopy (FT-MIR). Five specimens were exposed at each irradiance level. The DC reached after 170s and after 5, 10 and 15 J/cm(2) had been delivered was compared using analysis of variance and Fishers PLSD post hoc multiple comparison tests (alpha=0.05). RESULTS The same DC values were not reached after the same radiant exposures of 5, 10 and 15 J/cm(2) had been delivered at an irradiance of 3.7 and 7.5 W/cm(2). Thus exposure reciprocity was not supported for Tetric EvoFlow (p<0.05). SIGNIFICANCE For Tetric EvoFlow, there was no significant difference in the DC when 5, 10 and 15J/cm(2) were delivered at irradiance levels of 0.75, 1.2 and 1.5 W/cm(2). The optimum combination of irradiance and exposure time for this commercial dental resin may be close to 1.5 W/cm(2) for 12s.

Characterizing the output settings of dental curing lights.

OBJECTIVES For improved inter-study reproducibility and ultimately improved patient care, researchers and dentists need to know what electromagnetic radiation (light) is emitted from the light-curing unit (LCU) they are using and what is received by the resin. This information cannot be obtained from a dental radiometer, even though many studies have used a dental radiometer. METHODS The light outputs from six LCUs (two QTH and four broad-spectrum LED units) were collected in real-time using an integrating sphere connected to a fiberoptic spectrometer during different light exposures. RESULTS It was found that the spectral emissions were unique to each LCU, and there was no standardization in what was emitted on the various ramp (soft-start) settings. Relative to the normal use setting, using the ramp setting reduced the radiant energy (J) delivered from each LCU. For one of the four broad-spectrum LED LCUs, the spectral emissions in the violet range did not increase when the overall radiant power output was increased. In addition, this broad-spectrum LED LCU emitted no light from the violet LED chip for the first 5s and only emitted violet light when the ramp phase finished. CONCLUSIONS A single irradiance value derived from a dental radiometer or from a laboratory grade power meter cannot adequately describe the output from the LCU. Manufacturers should provide more information about the light output from their LCUs. Ideally, future assessments and research publications that include resin photopolymerization should report the spectral radiant power delivered from the LCU throughout the entire exposure cycle.

Effect of a broad-spectrum LED curing light on the Knoop microhardness of four posterior resin based composites at 2, 4 and 6-mm depths

OBJECTIVE To measure the Knoop microhardness at the bottom of four posterior resin-based composites (RBCs): Tetric EvoCeram Bulk Fill (Ivoclar Vivadent), SureFil SDR flow (DENTSPLY), SonicFill (Kerr), and x-tra fil (Voco). METHODS The RBCs were expressed into metal rings that were 2, 4, or 6-mm thick with a 4-mm internal diameter at 30°C. The uncured specimens were covered by a Mylar strip and a Bluephase 20i (Ivoclar Vivadent) polywave(®) LED light-curing unit was used in high power setting for 20s. The specimens were then removed and placed immediately on a Knoop microhardness-testing device and the microhardness was measured at 9 points across top and bottom surfaces of each specimen. Five specimens were made for each condition. RESULTS As expected, for each RBC there was no significant difference in the microhardness values at the top of the 2, 4 and 6-mm thick specimens. SureFil SDR Flow was the softest resin, but was the only resin that had no significant difference between the KHN values at the bottom of the 2 and 4-mm (Mixed Model ANOVA p<0.05). Although the KHN of SureFil SDR Flow was only marginally significantly different between the 2 and 6-mm thickness, the bottom at 6-mm was only 59% of the hardness measured at the top. CLINICAL SIGNIFICANCE This study highlights that clinicians need to consider how the depth of cure was evaluated when determining the depth of cure. SureFil SDR Flow was the softest material and, in accordance with manufacturers instructions, this RBC should be overlaid with a conventional resin.

Effect of High Irradiance on Depth of Cure of a Conventional and a Bulk Fill Resin-based Composite

OBJECTIVES This study evaluated the effect of using three commercial light curing units (LCUs) delivering a range of irradiance values, but delivering similar radiant exposures on the depth of cure of two different resin-based composites (RBCs). METHODS A conventional hybrid RBC (Z100 shade A2, 3M ESPE) or a bulk fill RBC (Tetric EvoCeram Bulk Fill shade IVA, Ivoclar Vivadent) was packed into a 10-mm deep semicircular metal mold with a 2-mm internal radius. The RBC was exposed to light from a plasma-arc-curing (PAC) light (Sapphire Plus, DenMat) for five seconds, a quartz-tungsten-halogen (QTH) light (Optilux 501, Kerr) for 40 seconds, or a light-emitting-diode (LED) light (S10, 3M ESPE) for 20 seconds and 40 seconds (control). The Knoop microhardness was then measured as soon as possible at the top surface and at three points every 0.5 mm down from the surface. For each RBC, a repeated measures analysis of variance (ANOVA) model was used to predict the Knoop hardness in a manner analogous to a standard regression model. This predicted value was used to determine at what depth the RBC reached 80% of the mean hardness achieved at the top surface with any light. RESULTS The PAC light delivered an irradiance and radiant exposure of 7328 mW/cm(2) and 36.6 J/cm(2), respectively, to the RBCs; the QTH light delivered 936 mW/cm(2) and 37.4 J/cm(2) and in 20 seconds the LED light delivered 1825 mW/cm(2) and 36.5 J/cm(2). In 40 seconds, the control LED light delivered a radiant exposure of 73.0 J/cm(2). For Z100, using 80% of the maximum hardness at the top surface as the criteria for adequate curing, all light exposure conditions achieved the 2.0-mm depth of cure claimed by the manufacturer. The LED light used for 40 seconds achieved the greatest depth of cure (5.0 mm), and the PAC light used for five seconds, the least (2.5 mm). Tetric EvoCeram Bulk Fill achieved a 3.5-mm depth of cure when the broad-spectrum QTH light was used for 40 seconds delivering 37.4 J/cm(2). It required a 40-second exposure time with the narrow-spectrum LED, delivering approximately 73 J/cm(2) to reach a depth of cure of 4 mm. CONCLUSIONS When delivering a similar radiant exposure of 37 J/cm(2), the QTH (40 seconds) and LED (20 seconds) units achieved a greater depth of cure than the PAC (five seconds) light. For both resins, the greatest depth of cure was achieved when the LED light was used for 40 seconds delivering 73 J/cm(2) (p<0.05).

Post-curing in dental resin-based composites

OBJECTIVE To determine the post-curing in six commercial contemporary resin-based composites (RBCs) using axial shrinkage, the degree of conversion, and Vickers hardness. METHODS Five Bulk Fill and one conventional RBCs from three companies were selected with a wide range of filler volume content. The axial shrinkage of samples that were 1.00mm thick by 9-10mm diameter was measured using a modified bonded disk method over a time between 15h and 19h at temperatures of 26°C and 34°C (mouth temperature). The degree of conversion (DC) was collected continuously for 10min using mid-infrared spectroscopy in the attenuated total reflectance geometry. Vickers hardness was measured at 1h post-irradiation using a load of 300gf. For all three tests, the samples were irradiated at five exposure times, 20, 5, 3, 1.5 and 1s with a light curing unit radiant exitance of 1.1W/cm2. Three samples (n=3) were used for each experimental condition. RESULTS After light exposure, the axial shrinkage and degree of conversion exhibited a functional time dependence that was proportional to the logarithm of time. This suggests an out-of-equilibrium polymer composite glass that is transitioning to thermal equilibrium. At a sufficiently long time and among the RBCs investigated, the shrinkage related physical aging rate was found to vary between 1.34 and 2.00μm/log(t). The rate was a function of the filler content. Furthermore, 15h after light exposure, the post-curing shrinkage was estimated to be an additional 22.5% relative to the shrinkage at 100s for one RBC at T=34°C. The hardness in the photo-cured RBC was varied by using different light exposure times. The first two experimental techniques show that the higher the initial DC 10min after light exposure, the smaller is the post-curing shrinkage related and DC related physical aging rates. A direct correlation was observed between the shrinkage related and the DC related physical aging rates. SIGNIFICANCE Post-curing shrinkage should be evaluated for longer than 1h. The post-curing shrinkage 15h after light exposure in dental RBCs can be appreciable. The long-term development of built-in stress within the tooth wall structure may shorten the restorations lifespan.