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 Table of Contents  
ORIGINAL RESEARCH
Year : 2018  |  Volume : 10  |  Issue : 4  |  Page : 206-209

Evaluation of light-curing units used in dental clinics at a University in Malaysia


1 Department of Oral Diagnosis, School of Dental Sciences, Universiti Sains Malaysia, Malaysia
2 Department of Conservative Dentistry, School of Dental Sciences, Universiti Sains Malaysia, Department of Dentistry, Hospital Universiti Sains Malaysia, 16150 Kubang Kerian, Kelantan, Malaysia
3 Department of Conservative Dentistry, School of Dental Sciences, Universiti Sains Malaysia, Malaysia

Date of Web Publication28-Aug-2018

Correspondence Address:
Dr. Nik Rozainah Nik Abdul Ghani
Conservative Dentistry Unit, School of Dental Sciences, Universiti Sains Malaysia, 16150 Kubang Kerian, Kelantan
Malaysia
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jioh.jioh_114_18

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  Abstract 

Aim: The purpose of this study was to evaluate the efficiency of all light-curing units (LCUs) used in dental clinics of a university in Malaysia. Materials and Methods: Light intensity output (LIO) of each LCU was first determined using a light radiometer. A total of 88 LCUs were used in this study which were divided into three groups based on their LIO as follows: Group 1 (G1) ≥1000 mW/cm2, Group 2 (G2) 701–999 mW/cm2, and Group 3 (G3) ≤700 mW/cm2. Prepared composite resin specimens were then cured using these LCUs and were subjected to compressive strength test. Kruskal–Wallis test was used to compare the compressive strength of composite resin specimens cured by the three LCU groups. Independent t-test was used to compare LIO between corded and cordless light-emitting diode (LED) LCUs. The level of significance was set at P ≤ 0.05. Results: About 77.27% of LCUs had LIO ≥1000 mW/cm2 (G1), 14.77% had LIO ranging from 701 to 999 mW/cm2 (G2), and 7.95% ≤700 mW/cm2 (G3). There was no significant difference in the compressive strengths of composites cured by G1 and G2 (P > 0.05). However, compressive strength of composites cured by G1 was significantly higher than that of G3 (P < 0.05). Furthermore, compressive strengths of composite cured by G2 were also significantly higher than that of G3 (P < 0.05). The LIO of cordless LED LCU was significantly higher than that of the corded ones (P < 0.05). Conclusion: LCU with LIO above 700 mW/cm2 outperformed those with LIO < 700 mW/cm2. Cordless LED LCU demonstrated better performance than corded ones.

Keywords: Composite resin, compressive strength test, light-curing units, light intensity output


How to cite this article:
Lee YR, Nik Abdul Ghani NR, Karobari MI, Noorani TY, Halim MS. Evaluation of light-curing units used in dental clinics at a University in Malaysia. J Int Oral Health 2018;10:206-9

How to cite this URL:
Lee YR, Nik Abdul Ghani NR, Karobari MI, Noorani TY, Halim MS. Evaluation of light-curing units used in dental clinics at a University in Malaysia. J Int Oral Health [serial online] 2018 [cited 2020 Apr 2];10:206-9. Available from: http://www.jioh.org/text.asp?2018/10/4/206/240011


  Introduction Top


Adequate light intensity output (LIO) is a vital factor which contributes to the physical properties of cured composite resin. In recent times, the demand for light-cured tooth-colored restorative materials has increased significantly due to their ease of handling, command set property, and esthetically appealing final appearance.[1] This, in turn, has caused an increase in the usage of visible light-curing system over the last few decades.

The success of light-cured restorative materials highly depends on the efficacy of light-curing units (LCUs). Inadequate polymerization of light-cured restorative materials can lead to complications such as discoloration, marginal breakdown, hardness deterioration, and poor flexural and compressive strengths.[2] Furthermore, it also contributes to a wide range of clinical complications such as pulpal irritation, postoperative sensitivity, and restoration failure.[3] Clinicians should be aware that at high irradiance, there is a risk of superficial tissue damage and damage to pulp when only a thin layer of dentine is left.[4]

Sufficient exposure of visible light is necessary in order to activate the photosensitizer present in light-cured material to its excited state.[5] Should there be an inadequate output of light emitted by the LCU, inadequate polymerization of resins will occur, despite the recommended curing times being obeyed.[6],[7] There are several factors that affect the output of LCU, mainly due to the damage of internal components of curing units; for instance, line voltage, bulb and filter failure, contamination of light tip end, and breakage of conductive fibers.[8]

Quartz tungsten halogen (QTH) was the standard for clinical use due to its dependability and satisfactory performance until the late 1990s.[9] Followed by the era of light-emitting diode (LED), LED LCUs rapidly replaced QTH due to their various advantages which include a longer lifespan of up to 1000 h, narrower light spectrum as compared to QTH, and minimal heat generation.[10],[11],[12] LED LCUs can be further divided into corded and cordless LED LCUs. Cordless LED LCUs have the benefit of lower energy consumption as compared to their corded counterparts.[12] Nevertheless, regular maintenance of LED LCU, be it corded or cordless, is extremely important to ensure their adequate and appropriate functioning. A study revealed that as many 30% of LCUs checked had LIOs below 200 mW/cm2.[13] This study shows that most dentists were unaware that the output of their LCU was inadequate. Hence, this study may indicate the trend that the maintenance of LCU is neglected by many dental practitioners.

In recent years, the exact LIO of LCU in our dental clinics was undetermined since there are limited experiments carried out in the past to evaluate the success rate of cured materials. Therefore, the efficiency of LCU used in Universiti Sains Malaysia (USM) dental clinics remains undetermined. A study was carried out to determine the LIO of various LCUs used in USM.[14] However, the effects of various LIOs on the strength of cured composite material were not determined. Hence, the purpose of this study was to evaluate the efficiency of all functional LCUs being used in dental clinics of a university in Malaysia and to assess the effects of LIO on the compressive strength of cured composite resin.


  Materials And Methods Top


This prospective study was conducted in the various dental clinics of USM in Malaysia over a period of 6 months. All the confirmed functional LCUs available in dental clinics at USM were included in this study. The selected 88 curing units were further evaluated under controlled conditions and then included in this study. LCUs were numbered and their brands, manufacturers, and types were identified and recorded [Table 1]. The LIOs of these curing units were measured by employing a light radiometer unit (CureRite, Dentsply Caulk, USA) as per the manufacturer's instructions. Ten seconds after the start of irradiation, the LIO was recorded. The readings were taken three times using the same protocol and the average intensity output was calculated. Based on their intensity output, these LCUs were divided into three groups as follows: Group 1 (G1) ≥1000 mW/cm2, Group 2 (G2) 701–999 mW/cm2, and Group 3 (G3) ≤700 mW/cm2. These groups of LCU were then used to cure composite resin specimens, and the compressive strength of cured specimens was determined.
Table 1: Types, brands, and manufacturers of light‑curing units (n=88)

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DiaFil composite (Shade A1, DiaDent Group International Inc., Korea) was used in this study. Resin composite was condensed into a Teflon mold with 4 mm internal diameter and 6 mm height and then compressed between two sheets of celluloid strips with pressure applied using a specific glass slab. The glass slab was removed and specimens were cured with various LCUs by light curing for 40 s. The specimens were removed from the mold after curing and stored away from moisture and light exposure. These specimens were then subjected to compressive strength test using a Universal Testing Machine (AG-X plus 20 kN, Shimadzu Cooperation, Japan) at a crosshead speed of 1.0 mm/min. Data analysis was performed using SPSS statistics version 22.0 (IBM Corporation, New York, USA). Kruskal–Wallis test complemented by Mann–Whitney test was used to compare the compressive strength of composite resin specimens cured by the three groups of LCU. Independent t-test was used to compare LIO between corded and cordless LED LCUs. The level of statistical significance was set at P ≤ 0.05.


  Results Top


Among the 88 LCUs, 58 were cordless and 30 were corded LED LCUs [Table 1]. Sixty-eight (77.27%) of them had LIO ≥1000 mW/cm2 (G1), 13 (14.77%) had LIO ranging between 701 and 999 mW/cm2 (G2), and 7 (7.95%) had LIO ≤700 mW/cm2 (G3). In general, these three groups (G1, G2, and G3) show significant difference in terms of compressive strengths of their composite resin specimens [Table 2]. However, intergroup comparisons show no significant difference in compressive strengths of composite resin specimens cured by LCU in G1 and G2 (P > 0.05), but compressive strength of composite resin specimens cured by LCU in G1 and G2 was significantly higher than those cured by LCU in G3 (P < 0.05) [Table 3]. The mean LIO of cabled LED LCU was 1198.03 mW/cm2, whereas that of cordless LED LCU was 1457.03 mW/cm2. Independent sample t-test revealed that LIO of cordless LED LCU was significantly higher than that of the corded LED LCUs (P < 0.05) [Table 4].
Table 2: Results of Kruskal-wallis test comparing the compressive strength (MPa) of composite resin specimens cured by different light-curing unit groups

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Table 3: Mann-whitney test comparing the compressive strength (MPa) of composite resin specimens cured by different light-curing unit groups

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Table 4: Independent sample t-test comparing light intensity output based on types of light-emitting diode light-curing units

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  Discussion Top


This study shows that LED LCUs are the main curing systems used in the dental clinics of USM. QTH LCUs seem to have been eliminated from these clinics. Even though QTH technologies have been a standard curing unit for several years due to their lower cost compared with other units, the curing technology industries still underwent a paradigm shift when LED curing units were introduced to the industry due to their longer lifespan of 10,000 h compared to that of QTH LCU (40-100 h). LED produces less heat and degrades less over time, consumes less power, and produces higher intensity output.[15]

This study was conducted in moisture-free and contamination-free environment, under maintained temperature to overcome the failures. Anyhow, the study setup has some limitations that it can never completely reflect the clinical situations. Furthermore, A review conducted by Price et al. stated that most LCUs have non uniform irradiance, that is, LCUs have hot and cold spots of irradiance, this results in some regions of resin composite receiving more irradiance then other regions. This results in inhomogeneous photopolymerization and in turn affects the development of polymerization stress resulting in reduced physical properties.[16] In another review, authors stated that physical properties of resin composite are affected mainly by wavelength, local power, and the beam power profile of the LCU used. Furthermore, it was concluded that LED units with polywave system are preferable, as this is the method of choice for curing broader range of resin composites.[17]

In this study, LIO of cabled and cordless LED LCU was compared. The results indicated that cordless LED LCU generally outperformed cabled LED LCU in terms of LIO, even though cabled LED LCUs were newer than the cordless ones. Similar results were shown in a previous study in 2014, which was carried out in the same setting as our study. It was stated that the performance of cordless LED and QTH LCU was better than that of cabled LED LCU, despite not statistically significant relative to QTH LCU. Nearly 92% of cordless LED LCUs were recorded to have acceptable LIO ≥400 mW/cm2 as compared to 42% of cabled LED units.[14] Our study not only involved the assessment of LIO, but also investigated the effects of various LIOs on the compressive strength of cured composite resin specimens. However, our study still lacks precision because in our study, cordless LED LCUs were only tested once (in situ). It is preferable to test and compare fully charged cordless LED LCUs, as well as in situ (not fully charged) to increase the accuracy of the results.

A similar study has been carried out in Japan as well to evaluate the LCU used in private dental offices based on the specimen's compressive strength. The results are similar to our study in which the compressive strength of resin specimens tends to decrease as LIO decreases. This is because inadequate LIO will not be able to polymerize composite resin completely. However, the results of our study should be interpreted with caution. This is so because as compared to the previous study, only LED LIOs were used in this study, whereas in the previous study, QTH LCUs were included as well. Furthermore, in this study, only 4 of 88 LCUs had LIO lower than 400 mW/cm2 (marginal intensity). This in turn affects the accuracy of the results since the specimens produced by poorly performing LCUs were limited. Therefore, it is recommended that more LCUs with LIO <400 mW/cm2 be included in the future studies, and cordless LED LCUs should be tested twice in situ and in fully charged status to increase the accuracy of the results.

Types of curing units is not the only factor which contributes to the performance of curing units. It is also affected by their age and frequency of use. A survey conducted mentioned that resin buildup on the light-curing tip may affect curing by the partial light exposure. However, our study lacks to record the same.[18] In a review, authors state about the importance of the distance between the curing tip and resin composite and temperature of material. They concluded that it is recommended to minimize the distance between the LCU tip and resin composite surface as much as possible; if this cannot be done, then increase in curing time or higher irradiance level LCU is recommended. To enhance the physical properties of resin composite, preheating was also recommended.[19] This study also revealed that the LIOs are lower in LCUs that are more frequently used even though they are of the same manufacturer, type, and brand. Furthermore, LCUs with earlier year of purchase also show lower LIO as compared to those that are recently purchased.

In this study, the performance of LCUs was evaluated by assessing their ability to polymerize composite resin completely. It is known that light intensity plays a significant role in determining the degree of polymerization of resin composites.[20] Usually, assessment of effectiveness of curing unit depends on surface hardness of light-cured resin. However, surface hardness is not a reliable index since even an inferior curing unit will polymerize the surface as well as an effective unit.[21] Therefore, compressive strength test was employed to measure the ability of cured composite resin to withstand the compressive force. Capability to withstand the compressive strength may indicate the mechanical integrity of the cured material.[5] Curing units with lower LIO tend to produce cured resin specimens with lower mean compressive strengths. A previous study also revealed that the compressive strength of the resin composite decreased with the decrease of light intensity.[5]

The performance of LCU involved in this study can be considered as acceptable as almost all of them have LIOs above the marginal intensity, which is more than 400 mW/cm2. There were only 4 units with LIO <400 mW/cm2 out of the 88 curing units tested in this study. These results are satisfactory as compared to a similar study carried out in 1994. A previous survey revealed that nearly 30% of the dental office curing units in Texas had an intensity output <200 mW/cm2.[13] A survey among dentists in Australia also found that half of the LCUs were not functioning satisfactorily, and nearly 50% of practitioners never had periodic checkup and maintenance of their LCU.[22] Therefore, despite the fact that curing units in Hospital USM (HUSM) dental clinics perform well, the intensity may deteriorate over time if periodic maintenance is continuously neglected.


  Conclusion Top


Cordless LED demonstrates better performance than cabled LED in terms of light intensity outputs. LCU with higher LIO produced cured composite resins with higher compressive strength. Age of units and frequency of usage affect the performance of units. Periodic maintenance of units should be practiced in order to achieve optimal performance.

Acknowledgments

Support from Mr. Mohd Yusof Soon Abdullah during the compressive strength test procedures is highly acknowledged. Advice of Associate Professor Dr. Wan Muhamad Amir W Ahmad, for statistical analysis, is gratefully appreciated. Furthermore, we thank the management of the Hospital Universiti Sains Malaysia, Kubang Kerian, Kelantan, for granting the permission to the investigators to use space and assets belonging to the hospital during the process of conducting this research.

Financial support and sponsorship

Financial support for this study was provided by USM under the short-term research grant scheme no. 304.PPSG.6131306.

Conflicts of interest

There are no conflicts of interest.

 
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Ab Rahman A, Husein A, Ahmed HM, Mohamad D, Bakar WZ, Farea M, et al. A survey on light intensity outputs of QTH, cabled and cordless LED light curing units. Arch Orofac Sci 2014;9:85-90.  Back to cited text no. 14
    
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  [Table 1], [Table 2], [Table 3], [Table 4]



 

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