Nigerian Journal of Clinical Practice

ORIGINAL ARTICLE
Year
: 2019  |  Volume : 22  |  Issue : 4  |  Page : 469--477

Investigation of the reliability of light-curing units in Sivas City, Turkey


D Eren1, F Tutkan2,  
1 Department of Restorative Dentistry, Faculty of Dentistry, Cumhuriyet University, Sivas, Turkey
2 Sivas Oral and Dental Healt Center, Sivas, Turkey

Correspondence Address:
Dr. D Eren
Department of Restorative Dentistry, Faculty of Dentistry Cumhuriyet University, Sivas
Turkey

Abstract

Background: The number of studies investigating the physical properties of light-curing units used in city centers in terms of the light intensity, presence of residues fractures at the tips, how long they have been used, and hardness measurements of the composite resins polymerized by these is limited. There is no such study in Turkey and Sivas province. The objective of this study is to examine the light-curing units used in Sivas city center and determine the reliability of light-curing units by measuring the surface hardness of composite samples polymerized with these devices. Materials and Methods: The power of the light-curing units that used in all private clinics in Sivas city center was measured. Then, the Vickers surface hardness measurements of the composite resin samples polymerized with these devices were made, and they were statistically evaluated. Results: The light intensity was found to be below from the acceptable value of 400 mW/cm2 in 10.7% of the devices. It was observed that with increasing years of usage, the light intensity of light-curing units decreased (P < 0.05). Conclusion: It was observed that as the power of light-curing units increased, the hardness values of the bottom and top surfaces increased significantly.



How to cite this article:
Eren D, Tutkan F. Investigation of the reliability of light-curing units in Sivas City, Turkey.Niger J Clin Pract 2019;22:469-477


How to cite this URL:
Eren D, Tutkan F. Investigation of the reliability of light-curing units in Sivas City, Turkey. Niger J Clin Pract [serial online] 2019 [cited 2019 Apr 23 ];22:469-477
Available from: http://www.njcponline.com/text.asp?2019/22/4/469/255925


Full Text



 Introduction



With the development of resin-based materials that are polymerized by light, the control of the operating time, better color stability, and reduced porosity have been achieved. The polymerization of composite resins by light can provide the desired appropriate morphology, in addition to facilitating the application of composites of different colors in layers and obtaining a more aesthetic restoration.[1] Although ultraviolet light sources have been previously used as a light source to ensure polymerization, devices that emit visible blue light have been used for more than 20 years due to being more reliable. Conventional quartz tungsten halogen (QTH) light sources, high-intensity QTH light sources, light-emitting diode (LED), soft start halogen and plasma-arc light sources, and laser units are tools made available for dentists.[2] One of the most important factors affecting the clinical success of resin-based composites is the degree of monomer exchange of the resin. Low monomer exchange leads to the high rates of unreacted residual double bonds in the material, and in addition to causing decreased physical properties of the composite resin, these residual double bonds lead to increased water absorption and water dissolution properties, which cause the resin to be colored, cytotoxicity, and edge leakage.[3] Furthermore, residual monomers passing to the pulp through dentin tubules due to inadequate polymerization can lead to irreversible damage to the pulp.[4]

For optimum polymerization, the light must be applied in an appropriate wavelength range, at an effective intensity, and for a sufficient time. Moreover, factors such as the positioning of the light-curing units as close to the restoration as possible and at a right angle, the type of the device, the size, cleanness, and durability of the tip of the light-curing unit, and the disinfection method applied also affect the degree of polymerization.[5],[6],[7],[8]

The light power per unit area where the light is applied is called light intensity. Light intensity affects many physical properties of restoration. For adequate polymerization, light intensity must be at least 400 mW/cm.[2],[9]

In understanding whether light-curing units have sufficient intensity, the visual inspection of devices is not sufficient. Thus, radiometers produced to be used in clinics should be used to check whether light-curing units operate properly.[10] The long-term use of these devices may result in a gradual decrease in their efficiency and, thus, inadequate polymerization, especially in the deepest layer. Therefore, measurements should be made with radiometers at certain intervals.[11],[12]

In the literature, the number of studies investigating the physical properties of light-curing units used in city centers (in terms of the light intensity, presence of residues fractures at the tips, how long they have been used, and hardness measurements of the composite resins polymerized by these) is limited. There is no such study in Turkey and Sivas province. The objective of this study is to examine the light-curing units used in Sivas city center and determine the reliability of light-curing units by measuring the surface hardness of composite samples polymerized with these devices.

 Materials and Methods



Ethical considerations in accordance with the Helsinki Declaration have been observed throughout this series of studies. All participants gave written informed consent. This study was approved by the Ethics Committee of the University of Cumhuriyet Process No. 2015-04/16.

A total of 121 light sources found in the private clinics in Sivas city center (Sivas, Turkey), at Sivas Oral and Dental Health Center and the Faculty of Dentistry of Cumhuriyet University, were evaluated.

Evaluation of light sources in terms of the presence of deformation and residue, year of usage, diameter of the tip, and light intensity

Light-curing units used by dentists in Sivas city center were examined in terms of the presence of deformation and composite residues at the light output of the device. The dentists were asked how long the devices had been used, and the years of usage of the devices were noted. Furthermore, the diameter of the tip of each device was measured using a caliper and recorded.

The light intensities of light-curing units were measured using an LED radiometer (Peng Lim Enterprise Co., Ltd., Taiwan). This radiometer was calibrated for intensity measurements of 0–2000 mW/cm2. In order to capture full light intensity, measurements were made after the operation of the light-curing units for 10 s. The measurements were made by positioning the tip of the device as close to the sensor surface as possible without touching it. Three measurements were made with each light-curing unit, and the average of these three values was taken so that the final light intensity values could be obtained. All tests were performed by a single researcher to eliminate the error caused by personal differences. A total of 121 light-curing units were examined.

Surface hardness measurement

Preparation of test samples

Finally, the Vickers test was conducted to evaluate the light-curing units. Teflon molds with a diameter of 5 mm and a depth of 2 mm were used to prepare the samples. While preparing the samples, a polyester strip, then a Teflon mold was placed on glass, the composite resin (Filtek Z550, 3M ESPE, Germany) was placed in the mold, and surplus was taken by an insertion spatula. Afterward, first, a polyester strip, then the glass was placed and pressed with constant pressure. The devices were kept in such a way that the light beam was perpendicular to the composite resin and polymerized for 40 s. About 121 samples were prepared with a total of 121 light-curing units [Table 1] in private clinics, at the Oral and Dental Health Center, and the Faculty of Dentistry of Cumhuriyet University.{Table 1}

Polishing procedures were performed using a Sof-Lex XT (3M ESPE, St. Paul, MN, USA) ultrathin disk system. The disks were applied for 15–20 s under water cooling from thick-grained to fine-grained, and they were replaced with a new one in every four samples. After each disk use, the surface was washed.

Before the hardness measurement, the samples were kept in distilled water at 37°C for 7 days in containers that did not receive light.

Surface hardness measurement

The microhardness test was performed using a DuraScan Vickers Tester (Emcotest, Kuchl-Salzburg/Austria). Three measurements were made on top and on the bottom of the specimen, using a load of 500 g for 10 s. The results of the measurements made with the Vickers tip positioned perpendicular to the surface of the samples were calculated using a computer program. The average of three measurements made from each surface was determined as the surface hardness value of that surface.

Statistical analysis

The data obtained from the present study were uploaded to the SPSS IBM Statistic for Windows, (Ver. 22, NY, USA) program, and when the parametric test assumptions were fulfilled, the (Kolmogorov–Smirnov) analysis of variance, Tukey's test, and parametric test were used in the data evaluation, and the Mann–Whitney U test was used when the assumptions of the significance test of the difference between the two media were not fulfilled, and the margin of error was taken as 0.05.

 Results



Findings on the light-curing units

A total of 121 light sources found in the private clinics in Sivas city center (Sivas, Turkey), at Sivas Oral and Dental Health Center and the Faculty of Dentistry of Cumhuriyet University, were evaluated. Of the devices, 116 (95.9%) were LED-curing lights and 5 (4.1%) were QTH light devices [Table 1]. There were devices of 26 different models of 22 different brands in total. Of these, 46 (38%) were Woodpecker, 23 (19%) were Turbo Top Light, 14 (11.6%) were Valo, 7 (5.8%) were Cicada, 5 (4.1%) were SVD Dental, 3 (2.5%) were Elipar, and 3 (2.5%) were LY-B 200 Built-in brand devices. There were two of each Penguin, Great Start, Gnatus, Visible Light CU-80, and Monitex brand devices. There were one of each Optilux, Seasky, Bluephase, Hilux, Elca T LED, De-Ga, Lunar, Ajax, Rainbow, and Phonex brand light-curing units.

The tips of the light-curing units were evaluated in terms of the adhesion of composite-adhesive residues and the presence of fracture wear, and it was found out that 52.9% of the devices had residues fractures [Table 2].{Table 2}

The dentists were asked for how many years the light-curing units had been used. The years of use ranged from 1 week to 21 years. The years of the devices were divided into four groups and examined. It was observed that 43% of the devices were new 0–1-year-old devices. About 35 (28.9%) devices were used for 2–3 years, 22 (18.2%) for 4–5 years, and 12 (9.9%) for 6 years and longer.

Upon evaluating the tips of light-curing units in terms of diameters, there were 80 (66.7%) devices with thin tips (7 mm) and 40 (33.3%) devices with thick tips (10 mm). The tip of one light-curing unit was completely broken, and there was no tip at all.

Upon measuring the light intensity of the devices, it was observed that the light intensity of 13 devices was bottom than the acceptable value of 400 mW/cm2 and 80 (66.1%) devices had a light intensity of 1000 mW/cm2 and above [Table 3]. Furthermore, the intensities of the light-curing units were compared in terms of years, the presence of residues/fractures at the tip, and the diameters [Table 4], [Table 5], [Table 6]. Upon comparing the values in pairs by years, although the difference between the intensities of devices between 0–1 (1377.88) and 4–5 (960) years and between 0–1 and 6+ years (841.66) was found to be significant (P < 0.05), the difference between the other years was found to be insignificant. The highest average intensity value was obtained from 0 to 1-year devices.{Table 3}{Table 4}{Table 5}{Table 6}

Upon comparing the light intensity values of devices with/without residues/fractures at the tip and devices with different diameters, the difference between them was found to be insignificant.

Findings of the Vickers microhardness test

The top and bottom surface hardness values were compared by years of use of light-curing units.

Upon comparing the top surface hardness measurements of the samples by years, although the difference between the devices used for 2–3 years and 4–5 years and 2–3 years and 6 years and more was found to be statistically significant (P < 0.05), the difference between the other years was found to be insignificant [Table 7]. Upon comparing the bottom surface hardness measurements of the samples by years of use of the devices, no statistically significant difference was found [Table 8].{Table 7}{Table 8}

No statistically significant difference was found upon comparing the hardness values of light-curing units with/without residues fractures by years of use [Table 9].{Table 9}

Upon comparing the hardness measurements by the light intensity of the devices, the difference was found to be significant for both the top surface and bottom surface hardness values (P < 0.05). Upon comparing the light intensity values in terms of the top surface hardness in pairs, although the difference between those with 0–399 mW/cm2 and 1000 mW/cm2 and above and between 400–699 mW/cm2 and 1000 mW/cm2 and above was found to be significant (P < 0.05), others were found to be insignificant. Upon comparing the bottom surface hardness by the light intensity in pairs, although the difference between those with 0–399 mW/cm2 and 1000 mW/cm2 and above, between 400–699 mW/cm2 and 1000 mW/cm2 and above, and between 700–999 mW/cm2 and 1000 mW/cm2 and above was found to be significant (P < 0.05), others were found to be insignificant [Table 10].{Table 10}

 Discussion



Polymerization is a process during which small molecules called monomers are chemically bonded to form a large chain or molecular network and form a larger molecule. Polymerization is one of the most important factors affecting the clinical success of composite resin restorations.[13]

Inadequate polymerization may lead to negativities such as composite discoloration, reduced fracture resistance, microleakage, increased wear, increased water absorption, decreased restoration connection, and loss of restoration.[14] The adequacy of polymerization depends on the composition of the material, the application method of the composite, and the properties of the light-curing unit.[15]

In this study, the light-curing units used in Sivas city center (Sivas, Turkey) were evaluated in terms of the presence of deformation, residues, year of use, the diameter of the tip, and light intensity. Afterward, the surface hardness degrees of the composite samples polymerized using these devices were measured.

As in every profession group, there are new developments and advancements in dentistry. Depending on the developments in technology, new materials are constantly put on the market, and new techniques are being developed.

QTH light-curing units lose their old popularity over time, and most clinicians now prefer LED light-curing units. In fact, many manufacturers are slowly pulling QTH devices they have manufactured from the market.[6],[16] The disadvantages of halogen light-curing units such as the short lifetime (40–100 h), the use of a cooling fan to reduce heat generated during polymerization, and the decrease in the power of the light source over time have led dentists to prefer these devices less. The clinical lifespan of LED light-curing units is long (10,000 h), and there is no decrease in the light power over time. Furthermore, they have become widespread in short time due to reasons such as not generating heat during polymerization and not conducting heat to teeth, having short polymerization periods, and being wireless, light, and ergonomic.[17],[18]

In the clinics visited, light-curing units of 26 various models of 22 different brands were evaluated. The vast majority of the devices comprised Woodpecker brand light sources (Guilin Woodpecker Medical Instrument Co). A large majority of the devices used are not devices produced by renowned major companies and proven to be effective and are therefore bottom in price than these.

The study evaluating 11 light-curing units that can be obtained inexpensively on the internet, in 2013, reported that many components of devices, such as bodies and electrical parts, were of bottom quality or poorly installed and that the diameters of light tips were smaller compared to those of larger manufacturers.[19] However, it was also reported that all 11 LED devices had a surprisingly high light intensity and exhibited good performance in polymerization tests. The researcher pointed out that the long-term reliability of these devices is questionable and that there are no security certifications and recommended to use light curing units with proven safety and clinical effectiveness.

Al Shaafi et al.[20] reported that there are significant differences in terms of the power output between large manufacturers' devices and low-priced devices, although they have similar light brightness. They showed that inexpensive devices had tips with a smaller diameter compared to others and that the light does not radiate uniformly from the tip of the device. Moreover, they stated that some inexpensive devices could not show the first light intensity after repeated light applications and that there was a significant decrease in the light efficiency over time.

In a study conducted by Hao et al.[18] in 2013, the researchers stated that there were 64 LED and 132 halogen devices, that the number of LED devices purchased in recent years increased, and that the purchase of halogen devices showed a downward trend. Maghaireh et al.[21] determined that there were 154 LED devices and 141 halogen devices in the clinics they visited in 2013.

The results of the present study showed that the number of LED devices in Sivas (Turkey) province is more dominant (95%) compared to halogen devices. While the number of halogen devices in previous studies was higher, it is observed that there are more LED devices in the present study. The reason why LED devices have been preferred more to this extent in the short term may be due to both clinical and financial advantages.[22]

The intensity of the dental light-curing unit must be high enough to initiate the polymerization of resin-based composite restorations and to ensure adequate curing. The literature shows that the light intensity values ranging from 200 to 600 mW/cm2 are required to obtain adequate polymerization in a resin-based composite.[22],[23] Some researchers reported that 300 mW/cm2 irradiation is the minimum intensity required for the effective polymerization of resin-based composites when appropriate curing times are used.[10],[18],[24] Some researchers argue that the minimum light intensity must be 400 mW/cm2.[20],[23] Rueggeberg et al.[25] recommended the minimum light intensity of 400 mW/cm2. The intensity of at least 400 mW/cm2 was selected as the appropriate intensity for this study.

In their study conducted in 1999 to determine whether the light intensities of clinically used devices were appropriate, Demirköprülü et al.[10] determined that out of 80 devices, 31 devices (<200 mW/cm2) required maintenance and were not appropriate for clinical use, 21 devices (200–300 mW/cm2) required an additional polymerization time, and 28 devices (>300 mW/cm2) had the light intensity appropriate for clinical use. As a result, 65% of the controlled light-curing units were found to have bottom light intensity values than the appropriate ones.

In the study carried out by Al-Samadani et al.[24], it was observed upon examining the light-curing units used in terms of their intensities that 20.5% of the devices had unacceptable values (<300 mW/cm2), 46% had acceptable values (300–400 mW/cm2), and only 33% had a good quality (>400 mW/cm2) for clinical use. Again, in three similar studies conducted in different locations, it was reported that approximately 45% of the devices had insufficient light intensity.[22],[26]

In this study, while the lowest light intensity was found to be 100 mW/cm2, the highest intensity was found to be 2000 mW/cm2. Only one device had the value of 100 mW/cm2. Devices with a bottom value than the minimum acceptable intensity of 400 mW/cm2 constitute 10.7% of all devices. Devices with a light intensity above 1000 mW/cm2 constitute the greatest ratio, which is far above the level required for the polymerization of the devices used. Contrary to other studies, the reason for the higher ratio of devices with acceptable intensity in the present study may be due to the increase in the use of LED devices. A periodic control of devices with a radiometer can lead to a further decrease in the inadequate ratio of 10.7%.

In many studies conducted, it has been shown that the decrease in the light intensity of devices is related to the duration of use, and the light intensity decreases with the increasing year of use.[10],[27],[28],[29]

Similar results were obtained in the present study. Light intensities of 0–1-year-old devices were found to be higher than those of devices used for more than 4 years. It was reported in a study conducted in the Damascus (Syria) region that the intensity of 163 light-curing units examined decreased with the year of use.[30]

In addition to the year of use, one of the factors reducing the light intensity is composite residues that accumulate at the tip of the device and damages.[31],[32] This accumulation has a negative effect on brightness because the resin-based composite material partially blocks the light output. Removing stains at the light tip is likely to improve brightness.[21]

In their study, Hao et al.[18] showed that contaminated or damaged light tips were observed in 80% of the light-curing units used for more than 3 years and that the power intensity of half of these units was less than 300 mW/cm2. Therefore, they recommended cleaning the tips of the light-curing unit regularly with a cotton swab soaked in alcohol and removing those with the high resin accumulation using a rubber disk with a drill rotating at a slow speed. They also stated that heavily damaged light tips must be replaced. In two similar studies, composite accumulation and damage were observed at the tips of most devices, but they could not determine the effect of them on light intensity.[21],[30],[31]

We determined composite accumulation or damage at the tip of 52.9% of the devices. Unlike the above studies, we compared the devices with and without damage and accumulation in terms of light intensity and microhardness by dividing them by years in order to determine the effect of composite accumulation and damage on light intensity. However, we did not find a statistically significant difference. The high light intensity of LED devices may have compensated for the damage caused by residues.

One of the factors affecting the light intensity of the light-curing unit is the diameter of the tip of the device. Although light-curing units with a larger light tip have more power, the light intensity decreases because the unit area where the light is diffused is larger.[18]

Among the devices we have examined, those with a diameter of 7 mm are dominant. Although 66.7% of the devices consist of devices with a tip 7 mm in diameter, 33.3% consist of devices with a diameter of 10 mm. There was no statistically significant difference in terms of the light intensity between devices 7 mm in diameter and devices 10 mm in diameter. We consider that this is due to the fact that the number of third-generation Valo Ortho-curing lights among devices with big tips is high.

Various methods are used to determine the polymerization degrees of composite resins. Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and microhardness measurements are only a few of these techniques. FTIR and Raman spectroscopy are expensive and time-consuming methods.[33] The surface hardness measurement, which is easier to apply compared to these methods, has been used by many researchers to determine the polymerization degree.[34],[35] In this study, the Vickers hardness measurement method, which is a reliable method, was used to evaluate the effect of light-curing units on the polymerization degrees of composite test samples.

In parallel to previous studies, in this study, bottom surface hardness values were found to be bottom than top surface hardness values in all samples.[23],[36],[37] Pilo et al.[26] conducted a similar study in 1999 and examined the top and bottom surface hardness in the test samples they prepared. In their study, they observed that while the bottom surface hardness values continued to increase with the increasing light intensity value, the bottom surface hardness was at relatively bottom intensity levels in the plateau slope. According to this result, the authors reached the conclusion that when the top surface hardness of the composite samples was compared with the bottom surface hardness, the light tip distance was less dependent on the light intensity and irradiation time.

Morimoto et al.[38] examined 67 devices in 55 clinics in Santos city, Brazil, and after measuring light intensities, the researchers studied the Knoop hardness values of the composite samples they prepared. They examined the hardness values in two groups, being at the intensities below and above 300 mW/cm2. The bottom and top surface hardness values of above 300 mW/cm2 were found for both LED and halogen devices. Although the researchers stated that even at low intensities, the top surface hardness values were acceptable, they expressed that the bottom surface hardness values at low intensity were inadequate for LED devices. They found bottom hardness values in LED devices compared to halogen devices at intensities less than 300 mW/cm2 and attributed this to measuring the light intensity of two LED devices as 0 (zero). Park et al.[39] used LED light-curing units with 980, 560, and 310 mW/cm2 light intensities for the polymerization of composite resin samples. While they could not find any difference between the hardness values obtained from the top surfaces of the samples, they reported that there was a difference between the bottom surface values. Although the highest hardness was found for 980 mW/cm2, the lowest hardness value was found for 310 mW/cm2.

In this study, the hardness values of the top and bottom surfaces were examined according to the light intensities of the devices. It was observed that as the light intensity increased both on the top surface and the bottom surface, the hardness values also increased. Upon comparing the light intensity values in terms of the top surface hardness in pairs, the difference between the devices with the intensity of 0–399 and 400–699 mW/cm2 and with 1000 mW/cm2 and above was found to be significant (P < 0.05). Upon comparing the bottom surface hardness by light intensity in pairs, the difference between devices with the light intensity of 0–399, 400–699, and 700–999 mW/cm2 and those with the light intensity of 1000 mW/cm2 and above was found to be significant (P < 0.05). In this study, the bottom surface hardness continued to increase with increasing light intensity, supporting the studies conducted by Pilo et al.[26] and El-Mowafy et al.[23]

 Conclusıons



In the study, 10.7% of the devices examined were found to have a light intensity less than the acceptable value of 400 mW/cm2. This means that the restorations made were negatively affected at the same rate. The power of the device decreased with the year of use and this affected the hardness grades of the bottom and top surfaces of the composite resins. It is suggested that physicians should have radiometers in dental clinics that they can easily use and periodically measure the light intensity of light-curing units, that devices with decreased light intensity should be maintained, and those with a light intensity that is too low to be used should be replaced. The introduction of this habit during the period of education will increase the continuation rate when the professional life starts. Low-cost light-curing units, which are widely used in clinics, can be investigated for efficiency in future studies.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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