Light Intensity of Curing Units in Dental Offices in Zagreb, Croatia

Danijela Matosevic, Vlatko Panduric, Bernard Jankovic, Alena Knezevic, Eva Klaric, Zrinka Tarle


Department of Endodontics and Restorative Dentistry, School of Dental Medicine University of Zagreb

Address for correspondence:

Danijela Matošević DMD
University of Zagreb School of Dental Medicine
Department of Endodontics and Restorative Dentistry
Gundulićeva 5, 10 000 Zagreb
Tel: +385 1 4899 203
Fax: +385 1 4802 159

Received: November 5, 2010

Accepted: February 21, 2011

Available online: March 15, 2011

Acta Stomatol Croat. 2011;45(1):31-40.

Original scientific article

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Objective: Photopolymerization unit is an essential part of every dental office. The intensity of light curing units gradually decreases with time and can lead to poor polymerization, which cannot be detected clinically immediately after illumination. The purpose of this study was to examine whether the intensity of light curing units in dental offices in Zagreb satisfies minimum operational requirements. Materials and methods: The light intensity of 111 curing units was measured using radiometer (Bluephase® meter, Ivoclar Vivadent, Shaan, Liechtenstein). Six measurements were taken for each unit, three at the beginning of illumination and the other three at 35-40 seconds from the beginning. Data were also collected about the type of curing unit, manufacturer, age, frequency of use and the existence of integrated radiometer. Results: Light intensity lower than 300 mW/cm2 had 34% of curing units and 44% lower than 400 mW/cm2. The average light intensity of the remaining curing units was 675.3 mW/cm2. This study included photopolymerization units used in Zagreb which were five years old on average. Conclusion: Though the average light intensity of curing units in Zagreb fulfill the general requirements for efficient polymerization of composite resin materials, the fact that more than one third of curing units are ineffective should alert dentists to regularly monitor their appliances.

Key Words:

Curing Lights, Dental; Polymerization;  Photoinitiators, Dental; Light-Curing Dental Adhesives; Composite Dental Resins


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Tooth colored restorations are among primary dental esthetic demands, which, for most dentists, means the use of composite materials for their patients. A high degree of composite polymerization is essential for the optimal physical properties and the compatibility with biological structures. Not only do the residual unconverted methacrylate groups which may remain in lower parts of poorly polymerized composite fillings present a cytotoxic and genotoxic risk (1-3), but also their solubility might cause the formation of cavities and the occurrence of secondary caries (1, 4-6). The polymerization of composite materials depends on many intrinsic factors. Apart from the composition of organic matrix (7), these involve the type of the photoinitiator (8), shade and the degree of translucency of the material (9).
Together with the material characteristics, the degree of polymerization of light cured composites is significantly influenced by light curing units (LCU). A light curing unit is an unavoidable part of dental equipment in every dental office. A recent study evaluating the efficacy of halogen photopolymerization units in Toronto states that 78% of dental offices participating in the study had more than one light curing unit (10). Since the first introduction of light curing composite resins, various LCUs have been in use, from quartz tungsten halogen (QTH), plasma arc to today’s light emitting diode (LED) and laser curing units. Quartz tungsten halogen curing units have a halogen light bulb which emits the white light which is than filtered and the output is the blue light with wavelengths from 350-520 nm. They are relatively low-cost and still widely used. Curing units based on LED technology have the peak wavelength in range from 455-480 nm. They are lightweight, cordless and portable, have a longer life span and generate less heat (11).
    The light intensity, spectral output of the light source and the curing mode are the most important features associated with the effectiveness of LCUs (12). The spectral range of most LCUs is adjusted to the most commonly used photoinitiator, camphorquinone, which has the peak absorbance at around 468 nm. However, another two photoinitiators phenylpropanedione and lucirin TPO, mostly used in “bleach” shades of some resin materials, are activated by absorption of light in UV part of the spectra (380-430 nm). Most of them are based on variations of the irradiance during curing period in order to minimize negative effects of polymerization contraction, which is strongly associated with a high degree of composite polymerization (13-15). Also, some of the most important variables governing the degree of conversion of composite resin materials are the light energy density (16-18) and the duration of light exposure (19).
    Irradiance is the radiometry term for the power per unit area of electromagnetic radiation at a surface, also known as “light intensity”. Various curing modes are available in different LCUs. There has been an inconsistency in literature regarding the minimal operational irradiance of LCUs. A number of studies use 233 mW/cm2 as minimum, according to the recommendation of Rueggeberg et al. (20). However, the authors point out that routine exposure time periods of 60 seconds are recommended using light-source intensities of at least 400 mW/cm2, providing that incremental layer thickness does not exceed 2 mm. These findings are supported by the results of Yap and Seneviratne, who claim that 500 mW/cm2 is enough for optimal cure after 30 seconds of irradiation (21). The International Organization for Standardization (ISO) suggests the minimum intensity of 300 mW/cm2 in the 400-515 nm wavelength bandwidths at the light curing tip (22).
    The intensity of LCU can be measured directly, using so called radiometers (RM), devices for measurement of radiant flux of electromagnetic radiation. Apart from some precise physical instruments used to measure light intensity, there are also dental radiometers, which can be hand-held or integrated in the curing unit. Their reliability has been repeatedly compromised (23, 24). Indirect test of the efficiency of LCUs is the establishment of the quality of the materials after polymerization, such as the determination of the degree of vinyl conversion of composite materials using spectroscopic methods (25, 26), scrape test (27), microhardness test (28) and others (9).
The purpose of this study was to examine whether the light intensity of curing units used in dental offices in Zagreb satisfies minimum operational requirements and to investigate the distribution of the types of light curing units in private and public dental offices as well as the change of light intensity at the start and the end of curing period. Also, the aim of this study was to compare the results of the survey on the efficiency of light curing units in Zagreb conducted 11 years ago with the present study.


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Materials and methods

In this study, the intensity of 111 different LCUs in 22 public and private dental offices in the area of Zagreb, Croatia was measured. The dental offices were contacted by phone and the dentists were asked for the permission to visit their offices. The procedure and the purpose of the investigation were first explained in each office and the testing of the LCUs followed.
One examiner performed all the measurements. The data about the type of LCU (QTH, plasma-arc or LED), the manufacturer, age, frequency of use and the existence of integrated radiometer were recorded for each LCU. The values of the intensity of emitted light were median values from three consecutive measurements at the beginning of polymerization and three at the period from 35 to 40 sec after the start of irradiation. Considering that many LCUs have more polymerization modes, and that the aim of this study was to get insight in the maximum light intensity of the LCUs in the area of Zagreb, the light intensity was always measured in the strongest mode of operation. If the LCU did not have as option duration of irradiation period of 40 seconds, the regime of work with the longest period of illumination, along with the strongest light intensity was measured.
    Bluephase® meter (Ivoclar Vivadent, Schaan, Liechtenstein) was used for measurement of LCU's light intensity. The sensor in the radiometer determines the surface of the fiber optic tip on the polymerization unit as well as its light power. The irradiance is calculated by dividing the light power and the surface of the light guide tip by means of an integrated microprocessor. It is possible to detect the range of wavelengths from 380 to 520 nm and the light intensity from 300 do 2500 mW/cm2 with it. The measurements were performed by pressing the light guide directly onto the sensor and reading the irradiance values from the screen at the start and at the 35-40 second period of the illumination.
    The credibility of the Bluephase® meter was validated by comparative measurements of 14 LCUs using an integrating sphere (Ulbricht's sphere; Gigahertz Optik GmbH, Puchheim, Germany), an accurate device used in physics which measures the total light power, expressed in watts (W). In order to calculate the irradiance of LCU measured with integrating sphere, it was necessary to divide the value of total light power with the surface of the output tip of each light source. The sample consisted of 14 LCUs (Kavo Polylux II, Astralis 7, Bluephase, ESPE Elipar II, ESPE Elipar Trilight and ESPE Elipar Highlight).

Statistical analysis
Comparison of Bluephase® meter and integrating sphere
Shapiro-Wilk test was used for testing of the regularity of data distribution, and for the testing of homogeneity of variance Levene’s test. The paired-samples t-test was used eor the comparison of the results of measurements made with radiometer (RM) and integrating sphere (IS) and between start and 40 second values, and for the calculation of reliability of measurements, interclass correlation coefficient was used. Measurement error was calculated as square root of the residual mean square from ANOVA table, according to the recommendation of Bland and Altman (29, 30). The variability of all measurements was compared using the coefficient of variability. All the tests were performed with the significance level of p<0.05 using the statistical software SPSS 10.0 (SPSS Inc., Chicago, IL, USA).

Mean research
The sample consisted of 72 LCUs. The initial sample had 111 LCUs, but since 39 of them had irradiance lower than 300 mW/cm2, their absolute irradiance values could not be detected with Bluephase® meter, so the drop-out rate was 35%. For testing of the regularity of data distribution Shapiro-Wilk test was used, and for the testing of homogeneity of variance the Levene test was used. The irradiance values had normal distribution and for their analysis, methods of parametric statistics (t-test and multifactorial anaylsis of variance – ANOVA) were used, while the age of LCUs, which was not normally distributed, was analyzed by non-parametric Mann-Whitney U test. For testing the possible influence of LCUs’ age as a covariance on the differences in irradiance of LED and QTH curing units, taking the type of dental office, the existence of built-in radiometer and the everyday use of LCU into the consideration, the data were tested with analysis of covariance (ANCOVA). The correlation between the type of dental office, LCU type, integration of radiometer, everyday usage and the irradiance at the start and after 35 seconds of irradiation was established by logistic regression. Likelihood ratio test estimated the statistical significance of regression coefficients in the model and predictor variables for multiple logistic regression models were chosen by backward method. Risk ratio with 95% confidentiality intervals was used to express the connection between variables. Pearson’s correlation was used for testing the correlation between normal distributed variables, and for the ones which were not normally distributed - Spearman’s correlation. Procedures of three-factor variance analysis for repeated measurements of general linear model with Sidak’s correction were used for testing the differences between repeated measurements of irradiance at the beginning and after 35 seconds, considering the timing of the measurement, the type of light source and the office. All the tests were performed with the significance level of p<0.05 using SPSS 10.0 statistical software (SPSS Inc., Chicago, IL, USA).



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The comparison of the irradiance values obtained by Bluephase® meter and integrating sphere
Intraclass correlation coefficients (Table 1) show that both measurement methods are reliable ICC=0.92-0.98; p<0.001) and that the measurement error (ME=19.40-38.74) is always lower than the standard deviation (SD=129.39-159.55), namely lower than data dispersion in both measurement instruments and timings (Table 1). The coefficient of variability shows that the values of irradiance for radiometer after 40 seconds are the least variable (CV=2.84%), whereas the measurements with Ulbricht’s sphere at the start of the measurement are the most variable (CV=5.45%).

Irradiance data – main research
The initial sample consisted of 111 LCUs in the area of Zagreb, which comprised 44 different models. Out of that number, 49.55% were LED and 50.45% were QTH. In public institutions (School of Dental Medicine Zagreb, Dental Polyclinic Zagreb, Medical Centre Željezničar, Medical Centre Downtown) 48.65% were measured, and in private dental practices 51.35%. The average age of curing units was 5.55 years. 87.38% are used daily. 42.34% of LCUs had integrated radiometers.
34% of all LCUs had irradiance lower than 300mW/cm2 (Figure 1), so they had to be taken out of the sample which was statistically analyzed (Table 2). Irradiance values were normally distributed, so the parametric statistics was used, but for the age of LCUs, non-parametric statistics was used, since it was not normally distributed.
Comparison of irradiance data considering the LCU type, the time of the measurement and dental office type is shown in Figure 2.
Spearman's correlation showed a weak linear correlation between irradiance and the age of LCU.
Three-factor ANOVA (Table 3) for repeated measurements of General linear model showed the differences in the combination of the time of measurement, type of LCU and dental office type (p=0.029). The analysis of covariance (ANCOVA) points to the significant influence of the average value of irradiance (average of the data from the measurements at the start and at the 35-40 second period), (p<0.001). The age of LCU had influence on the difference between the LCU type (p=0.042) and the dental office type (p=0.032), but did not influence the combination of the LCU type and the type of dental office.
The first model of logistic regression shows that public dental practices are different from private practices only by the age and the average irradiance of LCUs. Public dental practices have 1.3 times higher chance to have an older LCU (95% CI 1.1-1.5; p<0.001), which also have higher start irradiance than LCUs in private practices, but odds ratio is very small and hardly exceeds 1. The logistic regression model describes only 21% of the variability (Cox & Snell pseudo r2=0.214).
The second model of logistic regression shows that QTH LCU have seven times higher chance to be older than LEDs. The chance to find a QTH LCU in public dental practices is 41 times higher than in private practices (95% CI=1.8-931.6; p=0.020). That model describes almost 65% of variability (Cox & Snell pseudo r2=0.645).

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Table 1 The parameters of the reliability of measurements. RM – radiometer, IS – integrating sphere, ICC - intra-class correlation coefficient, CI – confidence interval, ME – measurement error, CV - coefficient of variability.

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Figure 1 The distribution of curing units according to the measured irradiance. Vertical lines determine minimum operational requirements according to different authors: full line - 300 mW/cm2, an ISO standard (22); dashed line - 400 mW/cm2, according to Yap and Seneviratne (21).

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Table 2 Average age and irradiance of LCUs, with respect to the dental office and LCU type.

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Figure 2. Comparison of irradiance data considering the LCU type, the time of the measurement and dental office type. Columns represent average values and whiskers 95% confidence interval. The same letters indicate groups with statistically significant difference among them. Horizontal lines determine minimum operational requirements according to different authors: full line - 300 mW/cm2, an ISO standard (22); dashed line - 400 mW/cm2, according to Yap and Seneviratne (21).

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Table 3 The results of t-tests for differences in irradiance considering the type of curing unit and the existence of integrated radiometer. * independent samples t-test. # paired samples t-test.

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Proper functioning and an adequate light intensity of photopolymerization units are necessary for the longevity and biocompatibility of composite fillings. The intensity of light curing units gradually decreases over time and can lead to poor polymerization (31). Immediate clinical detection of inadequate curing is not possible because the material surface seems hardened, so it is necessary to regularly monitor the irradiance of LCUs. We measured the irradiances of the LCUs in dental offices in Zagreb and recorded general data on the type of curing unit, dental office, age and the presence of the integrated radiometer.
    The results of this study showed that 34% of LCU had the light intensity below 300 mW/cm2, which is the minimum irradiance recommended by the ISO for proper polymerization of composite resin materials. It is known that for the hardening of the upper side of the composite resin, only 20 seconds of polymerization with a device of 200 mW/cm2 intensity is sufficient. On the other hand, for the same degree of cure 2 mm under the surface 120 seconds of curing with 300 mW/cm2 irradiance is needed. The same study indicates that effective cure at the bottom of the 2 mm layer is achieved after 40 seconds with 400 mW/cm2, 30 seconds with 500 mW/cm2 or 20 seconds with 600 mW/cm2 (21). Therefore, it is necessary to apply these facts in clinical use. Even if the LCU has low irradiance (but not lower than 300 mW/cm2), it is theoretically possible to ensure the optimal cure of the composite resin material throughout its depth if sufficient irradiation time is invested. In terms of reasonable time used for polymerization of each composite increment, we might conclude that 400 mW/cm2 should be taken as a minimum. With this new value set as threshold, 43% of the photopolymerization units in the sample are not able to cure effectively.
    Eleven years ago, similar survey was also conducted in Zagreb in order to determine the effectiveness of LCUs in dental practices. It was concluded that 44% of curing devices had the irradiance lower than 233 mW/cm2. All tested LCUs were QTH and it was observed that the irradiance of some LCUs decreased after 40 seconds to the values below 233 mW/cm2 (32). With current knowledge, we might presume that the number of inefficient LCUs at that time would have been even higher. When compared to the present study, it is evident that the quality of curing units in the area of Zagreb has improved over 11 years period. At the time of conducting the current study, 38% of LCUs had the irradiance lower than 300 mW/cm2, which is lower than 44% of inadequate curing units 11 years ago, although with the lower threshold of 233 mW/cm2. This study does not support the findings of the previous study that the light intensity of LCUs decreases towards the end of polymerization period of 40 seconds. Also, the distribution of the type of LCUs has significantly changed. Almost equal numbers of QTH and LED units were recorded in this study, while in the previous study all the LCUs were halogen.
The measurement of light intensity of curing units in dental offices was conducted in Canada (10), Israel (4), Japan (33), Australia (34) and Germany (35). The 2005 Canadian study reported 30% of units with intensities lower than 400 mW/cm2 (10), while in an 1998 Australian study, 52% of LCUs were ineffective under the same conditions (34). In 1999, Pilo et al. chose a limit of 200 mW/cm2 and reported that 33% of LCUs in Tel Aviv had a reading below that value (4).  Miyazaki et al. (1998) took the highest threshold; 42% of units had irradiance lower than 500 mW/cm2 (33). In contrast to the present study, the sample of all of them consisted only of QTH units. These studies conducted in other countries also chose different standards as a minimum operational irradiance. Hence, it is very difficult to compare the results, but an overall improvement over the time can be seen. Apart from the present investigation, a German study from 2006 was the only one which also tested the LED LCUs, although they took a smaller part of the sample. The amount of LCU below 400 mW/cm2 was 26%, which is the best result among these studies (35).  
    Since the credibility of the radiometers has been brought into question in the past (23), the accuracy of the Bluephase® meter has been compared with the integrating sphere and it was shown that both devices were reliable. The advantage of used radiometer was that it allowed the measurements of curing units with various tube diameters. However, the lack of precise readings for irradiation values below 300 mW/cm2 causes difficulties in scientific work although it may be a very useful tool in dental offices.
    In this study, no statistical difference was observed between the irradiance values of LCUs in private and public dental offices, nor between LED and QTH LCUs. Regarding the age of LCUs, public practices generally had older devices than private practices. Older QTH devices are predominant in public practices, but the newest LED units as well. Private practices have relatively newer QTH units and the LEDs are older than in public practices. We might presume that the reason is the effort of the dentists in private practices to keep up with current advances in technology and that is why they were the first to purchase LED LCUs when they appeared on the market.
     The statistically significant drop in the irradiance values at the end of the usual irradiation period, 40 seconds, is observed in LED curing units in the sample. More specifically, the drop was observed in LED devices in private practices and in QTH units in public practices. One possible explanation of this phenomenon might be the fact that LED units in public dental offices and the QTH units in private practices are newer. However, this presumption should be further investigated in future studies.


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Under the conditions of the current study:
34% of examined curing units had the light intensity lower than 300 mW/cm2 and 44% of them lower than 400 mW/cm2, which is in range with the studies conducted in other countries; When compared to the previous study conducted 11 years ago in Zagreb, Croatia, the percentage of curing units inadequate for effective polymerization has decreased; Almost equal numbers of LED and QTH units suggest that the technological advances in the field of light polymerization have arrived in dental practices; It is recommended that dentists should regularly monitor the light intensity of photopolymerizaction devices.

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We gratefully acknowledge the financial support from the Ministry of Science, Education and Sports of Croatia (grant “Nanostructure of restorative materials and interactions with hard dental tissues”, number 065-0352851-0410).

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