Relationship Between the Cost of 12 Light-curing Units and Their Radiant Power, Emission Spectrum, Radiant Exitance, and Beam Profile
To correlate the radiant power (mW), radiant exitance (or tip irradiance in mW/cm2), emission spectrum (mW/cm2/nm), and beam irradiance profile of 12 light-curing units (LCUs) available in the Brazilian market with their market cost. Six LCUs that cost more than US$900 (Bluephase G4,VALO Grand, VALO Cordless, Radii Xpert, Elipar DeepCure-S, and Radii plus) and six low-cost LCUs costing less than US$500 (Radii Cal, Optilight Max, High Power LED 3M, Emitter D, Emitter C, and LED B) were examined. Radiant power (mW) and emission spectrum (mW/nm) were measured using an integrating sphere connected to a fiber-optic spectroradiometer. The internal tip diameter (mm) of each LCU was measured using a digital caliper and was used to calculate the average radiant exitance (mW/cm2). Irradiance profiles at the light tip were measured using a commercial laser beam profiler. The cost of each LCU in Brazil was correlated with internal tip diameter, radiant power, and tip irradiance. None of the low-cost LCUs were broad spectrum multiple peak LCUs. There was no correlation between the cost of the LCUs and their averaged tip irradiance; however, there was a high positive correlation between the cost of the LCUs and the radiant power and tip diameter. The VALO Grand, Elipar DeepCure-S, VALO Cordless, and Bluephase G4 all emitted a higher radiant power. They also had a significantly greater tip diameter than other LCUs. For the LCUs with a nonuniform output, some areas of the light tip delivered less than 400 mW/cm2, while other areas delivered more than 2500 mW/cm2. In general, LCUs that had a higher cost (US$971–US$1800) delivered more power (mW) and had a greater tip diameter (mm), which covered more of a tooth. In general, the low-cost LCUs (US$224–US$470) emitted a lower radiant power and had a smaller tip diameter.SUMMARY
Objectives:
Methods and Materials:
Results:
Conclusions:
INTRODUCTION
The light-curing process is an important step when delivering both indirect restorations1,2 and direct resin-based composite (RBC) restorations.3,4 Only when an adequate amount of light at the correct wavelength is delivered from the light-curing unit (LCU) to the RBC will the resin manufacturer's claimed esthetic and mechanical properties be achieved.5,6 The light emitted from the LCU should be uniformly distributed across the entire light tip,3,7 and it should also fully cover the RBC.7–9 When deciding which LCU to purchase, clinicians are influenced by the claimed irradiance and the price. However, there are other factors to consider, such as the power output, emission spectrum, beam collimation, beam profile, ergonomics of the LCU, and the ability to disinfect the unit.8,10
The commonly used tip irradiance value is a calculated value that is based on the radiant power (mW) and the internal tip diameter (mm) from which the tip area (mm2) is calculated.7 Since handheld dental radiometers are inaccurate11 and unable to measure the emission spectrum (mW/nm), it is recommended that the total output from the LCU be measured using laboratory-grade equipment and then the average irradiance (mW/cm2) across the light tip can be calculated. Dental radiometers will often measure the light output in a small area just at the center of the light tip.11 If the light from the LCU is greater at the center area of the light tip, the tip irradiance (radiant exitance) can be artificially increased when measured with most dental radiometers. Another reason is related to the inability of dental radiometers to detect and then adjust for the different emission spectra from dental LCUs.
In vitro studies often follow the ISO 4049 standard that uses a 4.0-mm diameter metal mold.12,13 When such a small diameter mold is used, the effects of any lack in the homogeneity of the light distribution from the LCU are unlikely to be evident. In contrast, if the samples are prepared using larger molds that represent the size of a molar tooth, for example, between 10 and 12 mm in diameter, the beam width, beam homogeneity, and characteristics of the LCUs will have a significant effect on the photoactivation of the RBCs.3
The photo-polymerization process can influence the longevity of both direct and indirect restorations. Unfortunately, most studies do not include a sufficient description of the characteristics and quantity of the light received by the resin composite or resin cement specimens to allow studies to be replicated.14 Consequently, the results of such studies using inappropriate LCUs can propagate scientific misinformation. The Brazilian dental community produced the second-largest number of papers in 2018 (USA, 3053; Brazil, 2218). Most of these publications are in the fields of are operative dentistry/cariology, dental materials, and endodontics;15 thus, the information about LCUs used in Brazil not only affects the restorations produced in Brazil, but also those produced in the rest of the world.
There are substantial differences in the cost of LCUs. To the best of the authors' knowledge, no study has related the market cost to the different parameters that are used to characterize different LCUs. Most of the LCUs tested in the present study are sold in other countries or are available for purchase on the Internet. Thus, the correlation between market cost and the characteristics of LCUs is important information for all dentists. Therefore, the aim of this study was to measure the radiant power, emitted spectrum, internal and external diameter, and calculated irradiance, and correlate them with the market cost of several low-cost LCUs that are sold in Brazil, and to also show the beam profile of LCUs and how much of a maxillary central incisor and a molar tooth would be covered by the light emitted from the LCU. The research hypotheses tested were that: (1) the cost of the LCU will correlate with its power, tip diameter, and tip irradiance (radiant exitance); and (2) the emitted light from different LCUs will provide similar coverage of a maxillary central incisor and the occlusal surface of a molar tooth.
METHODS AND MATERIALS
Characterization of the LCUs
Twelve commonly purchased LCUs in Brazil were tested (Table 1). Six of the LCUs cost more than US$900 (Bluephase G4,VALO Grand, VALO Cordless, Radii Xpert, Elipar DeepCure-S, and Radii plus) and six low-cost LCUs cost less than US$500 (Radii Cal, Optilight Max, High Power LED 3M, Emitter D, Emitter C, and LED B). The LCU's internal and external tip diameters were measured using a digital caliper (Mitutoyo, Tokyo, Japan). The tip area was calculated from the inner diameter of the light tip.
Power and Radiant Exitance (Tip Irradiance)
The total radiant power emitted from each LCU was measured 5 times using a 6-inch integrating sphere (Labsphere, North Sutton, NH, USA) attached to a fiber optic spectrometer (USB 4000; Ocean Insight, Orlando, FL, USA).16 The light tip was placed at the 12.5-mm diameter entrance to the integrating sphere so that all of the light from the LCU tip was captured. The measurement system, comprising the spectrometer, optical fiber, and integrating sphere, was calibrated before use. The mean radiant exitance across the light-emitting surface of the tip was then calculated as the quotient of the average of the 5 radiant power values (from the integrating sphere) and the internal optical area of the LCU tip. This result provided the averaged single radiant exitance (tip irradiance value) that is commonly reported by manufacturers and used in the ISO 10650.17
Beam Profile
The irradiance distribution across the light tip was captured using a laser beam profiler charge-coupled device (CCD) digital camera with a 50-mm focal length lens (SP620U; Ophir-Spiricon, Logan, UT, USA) that was fixed at the focal distance from the diffusing surface of a 60° holographic diffuser target (54–505; Edmund Optics, Barrington, NJ, USA). Two blue glass bandpass filters (34–434; Edmund Optics) and 1 reflective neutral density filter (Edmund Optics) were required to flatten the spectral response of the CCD camera. So that the beam profile images of the LCUs could be compared qualitatively, all the images were made at the same distance using the same exposure time.
After the LCU was turned on, the beam profiler camera captured an image that was recorded using the beam analyzer software (BeamGage Professional version 6.14, Ophir-Spiricon). Using the internal tip diameter (mm) of each LCU, the “Optical Scaling” tool in the BeamGage Professional (Ophir-Spiricon) calibrated the beam profile data in millimeters. The mean radiant power values (mW) previously obtained were then entered into the beam analyzer software to produce color-coded calibrated tip irradiance in mW/cm2 images. The calibrated data from BeamGage Professional (Ophir-Spiricon) were then exported into OriginPro 2019 version 9.6. (OriginLab, Northampton, MA, USA), where the images were all scaled to the same irradiance levels and x and y dimensions. These images were then superimposed over the image of a central incisor laminate veneer and over the mesialocclusal-distal (MOD) cavity in molar for comparison using the software CorelDRAW 2018 (Corel, Ottawa, Ontario, Canada).7
Statistical Analysis
Data were first analyzed for normal distribution (Shapiro-Wilk test) and homoscedasticity (Levene's test). Analysis of variance, followed by the Tukey post hoc multiple comparison test, was then used to compare the light output from the LCUs. The Pearson correlation test was used to verify if there was a correlation between power, tip irradiance, and tip diameter and market cost of the LCUs. All tests used a significance level of α = 0.05, and all analyses were carried out with the statistical package Sigma Plot version 13.1 (Systat Software Inc, San Jose, CA, USA).
RESULTS
The external tip diameter, internal tip diameter, radiant power, irradiance, and market cost of the 12 LCUs are reported in Table 2. The external tip diameter ranged from 7.2 to 15.1 mm. The internal tip diameter from where light was emitted ranged between 6.8 and 12.0 mm. The radiant power ranged between 224 and 1092 mW, and the calculated average tip irradiance ranged between 585 and 1467 mW/cm2. The market cost ranged from US$224 to US$1800.
The radiant power and the emission spectra from the LCUs into the integrating sphere, and the radiant power that the 12-mm diameter specimens received during an exposure time of 20 seconds, are shown in Figures 1 and 2, respectively. None of the low-cost LCUs were broad spectrum multiple peak LCUs. The calibrated three-dimensional beam profiles from the LCUs showing the tip diameter and average radiant exitance (tip irradiance in mW/cm2) distribution across the light tip are shown in Figure 3. In general, the more expensive LCUs (those costing more than US$900) such as VALO Grand, Bluephase G4, VALO Cordless, and Elipar DeepCure-S delivered more power, had a tip diameter greater than 9 mm, and had a more homogeneous irradiance, with radiant exitance values above 800 mW/cm2 across most of the tip area. In general, the lower-cost LCUs (those costing less than US$500) such as Emitter D, Optilight Max, High Power, Radii cal, LED B, and Emitter C delivered had less power, a tip diameter that was at most 7.4 mm, a light output that was not homogeneously distributed across the tip, and there was often a peak of high irradiance in a small area at the center of the light tip. Some areas across a light tip that had a nonuniform output delivered less than 400 mW/cm2, while other areas delivered more than 2500 mW/cm2. The Radii Xpert was considered to be in the group of more expensive LCUs, but this LCU had a low power output of 426 mW.
Citation: Operative Dentistry 46, 3; 10.2341/19-274-L
Citation: Operative Dentistry 46, 3; 10.2341/19-274-L
Citation: Operative Dentistry 46, 3; 10.2341/19-274-L
Table 2 shows that there can be almost a 5 mm difference between the outer tip diameter of the LCU and the internal tip diameter from where light is emitted. The spatial irradiance uniformity of the light from LCUs is illustrated by two everyday clinical situations: (1) the MOD cavity in a molar tooth, and (2) across the facial surface of a veneer on a maxillary central incisor (Figure 4). Figure 4 shows that the irradiance beam profiles of many of the tested LCUs only partially cover these tooth surfaces. In general, LCUs that cost more than US$900, such as VALO Grand, Bluephase G4, VALO Cordless, and Elipar DeepCure-S, covered more area of the central incisor surface and more of the molar tooth. Radii Xpert, which was also classified as a more expensive LCU, failed to cover a similar area as other LCUs of the same cost.
Citation: Operative Dentistry 46, 3; 10.2341/19-274-L
The Pearson correlations between LCU cost and tip diameter, radiant power, and tip irradiance are shown in Figure 5. There was a significant positive correlation between the cost of the LCU and the active tip diameter (p<0.001, r=0.82) and between the cost of the LCU and the radiant power (p<0.001, r=0.76). However, no correlation was found between cost of the LCU and tip irradiance (p=0.234, r=−0.019).
Citation: Operative Dentistry 46, 3; 10.2341/19-274-L
DISCUSSION
This study was designed to determine if there was any correlation between the light tip diameter, power, calculated tip irradiance (radiant exitance), and beam profile of LCUs with their cost (Table 2). The 12 LCUs delivered a wide range of power and tip irradiance values. The fact that there is such a wide range in light outputs among the LCUs tested may help explain why so many resin restorations fail after only 6–7 years, as the dentist will often use these different lights for the same exposure time.18–20 Doing so will deliver a wide range of energy to photocure the resin. Based on the results of our study, the first research hypothesis was partially confirmed. The cost of the LCU had a significant positive correlation with tip diameter (p<0.001, r=0.82) and power (p<0.001, r=0.76); these LCUs had a market cost above US$900 (US$971–US$1800) and were classified as more expensive LCUs (Table 2). The more expensive the LCU, the more power it delivered and had a greater active tip area; however, no correlation was observed between the cost of LCU and radiant exitance (tip irradiance). The lack of any correlation between the cost of LCU and the radiant exitance (tip irradiance) can be explained by how the irradiance value from the LCU can be increased without increasing the power output simply by reducing the internal tip diameter. The lower cost LCUs that had a market cost between US$224 and US$470 had light tips with a smaller diameter, and the light was generally focused on the center area of the LCU tip. Some areas across the light tip delivered less than 400 mW/cm2, while other areas delivered more than 2500 mW/cm2. Basic mathematics shows that if the tip diameter is decreased from 10 to 7 mm, this 3-mm reduction will halve the area and, as a result, double the radiant exitance (tip irradiance) if the same power is delivered. This also means that if the light tip is reduced by 3 mm, only half the power is required to deliver the same tip irradiance.21 Although the averaged radiant exitance from some LCUs, such as the Elipar DeepCure-S, Bluephase G4, VALO Grand, and VALO Cordless, tend to appear lower than that from other LCUs, Table 2 shows that the radiant power and tip diameters are greater on these light units compared with the low-cost lights units. Figure 3 shows that the Optilight Max, Emitter D, and High Power LCUs also have orange or red areas that indicate a high irradiance at the center of the light tip. In contrast, the lower radiant exitance from the more expensive LCUs is evenly distributed across the light tip and will cover a larger surface area of the restoration.7
The results reported in Figures 1 and 2 and Table 2 reinforce the need to know the power, emission spectra, active light tip diameter, and irradiance of the LCU. Many manufacturers recommend a 10-second exposure time for their composite resins and resin cements, but this time does not consider the wide range of power, tip area, and beam homogeneity found in the 12 LCUs included in this study. Although the low-cost LCUs (US$224–US$470) tested in this study are very popular, they should be used with caution. Some LCUs, such as the High Power (3M), Radii cal (SDI), and Emitter D (Schuster), delivered an unstable light output during a 20-second exposure. This behavior is probably caused by a lower quality of electric components that cannot compensate for any power fluctuations from the battery.22–24 Some low-cost LCUs will deliver less power, have an active tip diameter close to 7 mm, and have an inhomogeneous light distribution across the tip, which may compromise the polymerization process across different areas of the resin.21 The effects of light output inhomogeneity on the surface microhardness of different resins can be further accentuated if a shortened exposure time is used;3,25 this negative aspect will be further accentuated if all of the restoration is not covered by an adequate amount of light from the tip of the LCU. Figure 4 shows that there was a large range in the area covered by the LCUs tested over the MOD cavity in the molar tooth and over the facial surface of the central incisor. Based on these findings, the second research hypothesis was not accepted. The consequences of a lack of spatial irradiance uniformity from the LCUs are illustrated by two everyday clinical situations. Figure 4 shows the inhomogeneous spatial distribution of the irradiance beam profile of the LCUs when superimposed over a molar Class II MOD preparation and a veneer on a maxillary central incisor. Notably, for the low-cost LCUs, the active light tip from where light is emitted does not cover the tooth. Thus, some lights will deliver a wide range of radiant power and energy to different areas on the tooth, which will produce inhomogeneous photocuring of resin materials, especially if a short exposure time is used for all these LCUs. The effects of these differences will be further accentuated by the cavity depth in MOD restorations3,26 or the thickness, shade, and composition of the resin.27,28
The decisions made by clinicians when purchasing a new LCU should not be based on market cost and a questionable tip irradiance value. The clinician should also not be misled into believing that this averaged tip irradiance value is distributed equally across the tip end of LED curing lights.24 Future research should study the potential problems caused by purchasing a low-cost LCU so that this information can be disseminated in dental school curricula and continuing education programs. Studies that are designed to test bond strengths or the mechanical properties of resins should also consider the diameter of the samples tested.12,13 For example, the microshear bond strength test is frequently used to test the bonding interaction between resin cement and ceramic materials. However, the area from which the specimens for studies are made is generally small and comes from under the center of the light tip. Specimens made using low-cost lights will likely produce results that are comparable or even better than the more expensive lights because more energy will be delivered from the center of the light tip. The bond strength from specimens made from under the outer areas of the light tip will likely be lower when using low-cost lights because the resin will have received less energy.29 This may be why a previous study that tested two LCUs with a 2.5-mm difference in diameter tip (VALO Cordless, 9.7 mm; Radii Cal, 7.2 mm) found that the choice of LCU had no effect on the microhardness and color change of composite resins when the diameter of the composite resin samples was only 5.0 mm, as these areas of the specimens would have received a relatively homogeneous amount of light from both LCUs.30 A different conclusion may have been reached had the outer areas of the specimens tested been 9-mm in diameter.
Figures 3 and 4 show that for the Emitter D, Optilight Max, High Power 3M, Emitter C and LED B units tested in this study, different radiant powers were emitted at various areas across the tip of this LCU. When resinous materials are photopolymerized using LCUs that deliver a highly inhomogeneous beam of light, resin polymerization, wear resistance, and microhardness can all be adversely affected.23,31 Consequently, if larger samples that are 9–12 mm in diameter are prepared using mold sizes that represent teeth, the effect of the smaller diameter light tips, which are found in several LCUs tested in this study such as Radii plus, Radii cal, Optilight Max, High Power, Emitter D, Emitter C, and LED B, may be more clinically relevant.3 Consequently, clinicians who purchase low-cost LCUs should probably use longer exposure times and multiple exposures from different locations to ensure that they completely cover the resin restoration with the active light tip. This should minimize concerns regarding undercuring of composite restorations when using an LCU with a smaller diameter tip.32
High cost does not always mean high quality; however, the use of a single averaged tip irradiance (radiant exitance) value for describing the quality of the LCU should be avoided. Manufacturers should provide more information about the range of irradiance values emitted at the light tip and the radiant power to help clinicians decide which LCU to purchase. Additionally, when researchers describe the methodology used in their experiments that involve light curing, a better description of the light received by the specimens is recommended to help clarify the results. A cost-benefit analysis should be conducted to better select an LCU that is used in both a private and public oral health care context and to determine a cost-benefit analysis of undercuring the RBC.
CONCLUSIONS
Within the limitations of a study that only examined 12 LCUs, those that were more costly (US$971–US$1800) delivered more power (mW) and had a greater tip diameter (mm), which covered a greater area of a tooth. The lower-cost LCUs (US$224–US$470) generally emitted a lower radiant power and had a smaller tip diameter and area. Care should be taken when using these low-cost LCUs since they are less powerful (mW). They may only deliver both a high irradiance in a small area at the center of a small diameter light tip and a much lower irradiance at the outer regions of the light tip.
Radiant power (mW) emitted during a 20-second exposure from the 12 LCUs: (A): Bluephase G4; (B): VALO Grand; (C): VALO Cordless; (D): Elipar DeepCure-S; (E): Emiter D; (F): Optlight Max; (G): Radii plus; (H): LED B; (I): Radii Cal; (J): Radii Xpert; (K): High Power; (L): Emitter C. Note the difference in power outputs from these 12 lights. Abbreviations: LCU, light-curing unit; s, seconds.
Emission spectrum (mW/nm) from the 12 LCUs: (A): Bluephase G4; (B): VALO Grand; (C): VALO Cordless; (D): Elipar DeepCure-S; (E) Emitter D; (F): Optlight Max; (G): Radii plus; (H): LED B; (I): Radii Cal; (J): Radii Xpert; (K): High Power; (L): Emitter C. Note the different spectra from these 12 lights. Abbreviation: LCU, light-curing unit.
Calibrated beam profiles from the LCUs showing the tip diameter and averaged irradiance (mW/cm2) distribution across the light tip. Some regions across the light tips with a nonuniform output deliver less than 400 mW/cm2, while other regions deliver more than 2500 mW/cm2. Note the wide range in beam uniformity. Abbreviations: 3D, three-dimensional; LCU, light-curing unit.
Calibrated irradiance (mW/cm2) beam profiles from all LCUs superimposed over a mesial-occlusal-distal preparation in a molar and on the facial surface of a maxillary central incisor tooth. Note the difference in light coverage over the teeth. Abbreviation: LCU, light-curing unit.
Pearson correlations for cost and tip diameter, power, and irradiance of the LCUs tested. (A): Correlation between tip diameter (mm) and cost (US$) of the LCU (p<0.001, r=0.82). (B): Correlation between radiant power (mW) and cost (US$) of the LCU (p<0.001, r=0.76). (C): Correlation between tip irradiance (mW/cm2) and cost (US$) of the LCU (p=0.234, r=−0.019). Abbreviation: LCU, light-curing unit.
Contributor Notes
*Carlos J Soares, DDS, MS, PhD, School of Dentistry, Federal University of Uberlândia, Operative Dentistry and Dental Materials, Uberlândia, Minas Gerais, Brazil
Stella SL Braga, DDS, MSC, PhD, School of Dentistry, Federal University of Uberlândia, Operative Dentistry and Dental Materials, Uberlândia, Minas Gerais, Brazil
Richard B Price, BDS, DDS, MS, PhD,FDS RCS (Edin), FRCD(C), Dental Clinical Sciences, Dalhousie University, Halifax, NS, Canada
Clinical Relevance
Higher cost light-curing units deliver more power (Watts) and have a wider tip that covers more of the tooth. When choosing their curing light, clinicians should consider evaluating the emitted power, active tip diameter, and tip irradiance value.