Q U I N T E S S E N C E I N T E R N AT I O N A L
RESTORATIVE DENTISTRY
M. Murat Mutluay
Effect of using proper light-curing techniques on energy delivered to a Class 1 restoration M. Murat Mutluay, DDS, PhD1/Fred A. Rueggeberg, DDS, MS2/ Richard B. Price, BDS, DDS, MS, FRCD(c), FDS RCS(Edin), PhD 3 Objectives: To determine the effect of proper light-curing instruction on the radiant exposure (energy density) delivered by dentists using six dental curing lights to a posterior Class 1 restoration. Method and Materials: Twenty-five dentists attending a professional meeting were instructed to position a patient simulator (MARC-PS, BlueLight), as they would for a patient, and then to expose the simulated Class 1 maxillary second molar preparation for a specified amount of time. At this point, the dentists were unaware of the purpose of the experiment. Each participant used three different curing lights, and the irradiance and radiant exposure (J/cm2) delivered to the preparation was recorded. Participants were then informed of the purpose of the exercise, and given specific light-curing instructions and training using the patient simulator, after which they re-exposed the same preparation using the same curing lights. Pre- and post-instruction radiant exposure values
were compared using one-way ANOVA (α = .05), and for each light among all operators using a two-tailed, paired Student’s t test. Results: There was a wide variation in the radiant exposure delivered by the dentists and by the six curing lights, from 2.9 to 15.4 J/cm2. Before receiving additional light-curing instruction, 68% of dentists delivered less than 10 J/cm2. The radiant exposure delivered increased significantly (P < .001) by up to 30%, as a result of training using MARC-PS. Conclusion: The results indicate that some of the dentists participating in the present study delivered an inadequate amount of radiant exposure before instruction. More energy was delivered after a short training session using the MARC-PS. Reinforcing the proper photo-curing techniques may improve the outcome when placing resin-based restorations. (Quintessence Int 2014;45:549–556; doi: 10.3290/j.qi.a31959)
Key words: clinical performance, composite resin, light curing, operative dentistry, polymerization
Considerable evidence exists demonstrating that delivering inadequate radiant exposure (energy density) to a resin restoration will result in a restoration with less than optimal properties and decreased clinical perfor1
Lecturer, Department of Cariology, Institute of Dentistry, University of Turku, Turku, Finland.
2
Professor and Section Director, Dental Materials, Department of Oral Rehabilitation, College of Dentistry, Georgia Regents University, Augusta, Georgia, USA.
3
Professor and Division Head, Division of Fixed Prosthodontics, Department of Dental Clinical Sciences, Dalhousie University, Halifax, Nova Scotia, Canada.
Correspondence: Dr M. Murat Mutluay, Adhesive Dentistry Research Group, Institute of Dentistry, University of Turku, Lemminkaisenkatu 2, 20520 – Turku, Finland. Email: [email protected]
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mance.1,2 The strength, stiffness, and hardness of a resin that can only be light-cured will be adversely affected if the resin does not receive sufficient radiant energy.1,3 There will also be greater wear,4-6 reduced bonding to the preparation wall7 or to orthodontic brackets,8 enhanced resin “washout” at the gingival margin,9 increased bacterial colonization,10 reduced color stability,11,12 and increased leachates with cytotoxic potential from the resin restoration.13-15 A combination of operator-dependent factors and restoration material properties are critical for the longterm success of a restoration.16 These factors include
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a
b
c
d
e
Figs 1a to 1e The black oval shape inside the left maxillary second molar represents the detector. Light-curing units without (a) and with (b) light guides, shown in an ideal situation where the detector is directly under the light-curing tip at the minimum distance, good angulation, and good mesiodistal position. Note the close proximity of the light guide to mandibular anterior teeth (yellow arrows in (a) and (b)). This is a limiting factor for units with a light guide when polymerizing restorations in posterior teeth. Common mistakes such as incorrect angulation (c), location (d), and distance (e) of the curing tip result in reduced energy delivery to the restoration.
using an appropriate curing light, the operator technique, the location of the restoration, and the type of resin used.17 In a photo-curable resin, the chemical reaction leading to polymerization is initiated by lightcuring devices, and the resins must receive sufficient energy to drive this process. How light-curing devices are used combined with their design, spectral emission, and power output, defines the amount of useful energy delivered to the resin. It has been reported that 6 to 12 J/cm2 of radiant exposure should be sufficient to adequately polymerize some composite resins to a depth of 1.5 mm,18 although depending on material shade and opacity of the resin, up to 35 J/cm2 may be required.19,20 To maximize the cure of a 2-mm layer of composite, Phillips’ Science of Dental Materials textbook recommends a value of 16 J/cm2.21 In laboratory settings, various quartz tungsten halogen and light emitting diode (LED) lights were compared and no difference was found between the depth of cure and Vickers hardness of composite resins cured using different lights.22 This suggests that all modern curing lights could potentially give satisfactory results irrespective of their design.
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It is difficult to prove a direct relationship between inadequate light curing and clinical failure of resinbased restorations using a randomized clinical trial, due to the unethical nature of purposely “under-curing” a restoration placed in a human patient. However, a clinical study performed in patients who had light-cured resin-based restorations placed in the teeth of their dentures5 verified that purposely under-cured resinbased restorations showed significantly greater and clinically unacceptable occlusal wear after 2 years of function. Also, a wide range of hardness values of two photo-curable resins was reported when they were exposed using 214 different light-curing devices found in a selection of dental offices in Toronto. When the resins were exposed for 40 seconds under ideal conditions using the curing lights found in the dental offices, only 10% of the resin specimens reached 80% of the required hardness value.22 Such an inconsistency indicates that the use of curing lights in clinical settings may be influenced by various factors that may not be predicted using data created in laboratory settings. Recently, using the MARC patient simulator (MARCPS, BlueLight Analytics) two studies investigated the
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influence of instruction on the light-curing abilities of dental students.23,24 These articles suggest that instruction has a significant effect on the amount of energy delivered by students to a restoration. However, the ability of dentists to deliver energy to a restoration and how this can be improved has not been reported. The objective of the current study was to use the MARC-PS to identify the contribution of operator factors and factors related to the design of the light-curing unit on the amount of light energy delivered by dentists. The null hypothesis tested was that all dentists will deliver at least 10 J/cm2 of radiant exposure and that neither the instructions given nor the type of lightcuring unit used would have any influence on the amount of energy delivered.
METHOD AND MATERIALS The MARC-PS measures the irradiance, spectral emission, and radiant exposure delivered from light-curing devices to simulated dental restoration sites in a mannequin head. The MARC-PS uses a 3.9-mm diameter cosine-corrected irradiance probe (CC3-UV, Ocean Optics) to capture the radiant power delivered to the bottom of a cavity preparation within a dental mannequin (Kilgore) contained in a mannequin head (Fig 1). The detector is located inside a Class 1 preparation of the left maxillary second molar (tooth 27 according to FDI notation), with the top surface of the detector 2 mm from the occlusal surface, and 4 mm from the cusp tips. The 3.9-mm active diameter of the probe is similar in size to a Class 1 preparation in a molar tooth. The CC3-UV irradiance probe is attached to a laboratory-grade, fiber-optic spectroradiometer (USB 4000, Ocean Optics) and the MARC software (BlueLight Analytics) reports the irradiance (mW/cm2), spectral emission (nm), and radiant exposure (J/cm2) delivered to the floor of the simulated preparation during simulated photo-curing. The interincisal mouth opening can be adjusted, and in this experiment it was fixed at 43 mm. To evaluate the ability of the clinician to deliver energy to the simulated preparation using a curing light, the following precautions were taken in accor-
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dance with the guidelines from the institutional review board at the University of Turku, Finland. Verbal consent was obtained from the participants, no personal information was collected, and subjects remained completely anonymous during and after the data had been collected. The subjects were informed about their own results only. The project plan concerning the present study was reviewed and accepted by the Ethic Committee of University of Turku (Statement #9/2013). Before using the light-curing units, each of 25 dentists was instructed to position the head of the patient simulator as they would a patient’s head and then to perform simulated light curing for 10 seconds, using a variety of commercially available, blue LED-type curing lights (Table 1). Because of the low radiant exposure delivered from the Magna curing light, this unit was used for 20 seconds. The dentists were divided into two groups, with each group using three of the six different curing lights. The irradiance (mW/cm2) delivered to the preparation site was recorded in real-time by the MARC software (BlueLight Analytics), but was not visible to the dentist operating the light, only to the research assistant. The MARC software calculated the resulting real-time radiant exposure delivered as the mathematical product of irradiance and exposure duration recorded by the MARC software. The duration of exposure was controlled by the internal timing mechanism of each curing light. After all of the “before” instruction data were collected, the dentists were given more information about the purpose of the exercise. They were given simple instructions on the basic concepts of proper chairside light curing: they were told to use “blue blocker” glasses, to directly observe what they were doing, and to stabilize the light close to and perpendicular to the preparation site. Following these instructions that took less than a minute to deliver, the same dentists then re-exposed the same preparation area in the second molar using the same lights, and the radiant exposure value was again obtained: the “after” instruction value. To gauge the relative power output of the different units, the radiant power (Watts) from the curing lights, irrespective of the tip diameter, was measured using a
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Table 1
Description of the light units used
Light type
Output mode
Unit-controlled exposure time (s)
Yes
LED (blue)
Standard
10
Ultradent
No
LED (polywave)
Standard
10
Optilux 501
Kerr
Yes
QTH
Boost
10
Elipar S10
3M
Yes
LED (blue)
Standard
10
Bluephase 20i
Ivoclar Vivadent
Yes
LED (polywave)
High
10
FLASHlight Magna
Discus Dental
No
LED (blue)
Standard
20
Manufacturer
Light guide
Demi
Kerr
VALO
Brand name
Product profile
QTH, quartz tungsten halogen.
laboratory grade thermopile (PM-10, Coherent). This process was performed by holding the light at a minimum distance from the sensor, but without contacting the thermopile surface, until repeatable results were obtained. Pre- and post-instruction radiant exposure values were compared using one-way ANOVA (α = .05). In addition, values were compared within each light brand, among all operators using a two-tailed, paired Student’s t test at a family-wise α of .05. Within a brand of curing light, a significance level of .008 (.05/6) was used.
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RESULTS The radiant power from each light-curing device is presented in Fig 2. The data do not contain standard deviations because the radiant power values recorded by the thermopile were very consistent, with no variation among subsequent measurements. There was a wide variation in the amount of radiant exposure delivered: from 2.9 to 15.4 J/cm2 (Fig 3). Although power emitted from the curing lights differed among units, the power value was not indicative of the radiant exposure that was delivered by the dentists. A significant difference between the amounts of radiant exposure delivered using each light-curing device was found (P < .05).
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1500
Power output [mW]
1250 1000 750 500 250 0 Demi Fig 2
VALO
Optilux 501
ELIPAR S10
Bluephase 20i
FLASHlight Magna
Power output of the curing lights measured using a laboratory grade thermopile.
16 Radiant Exposure [J/cm2]
14 12 10 8 6 4 2 Average increase after instruction: 0
30.5% *
6.2%
8.5%
20.7%
29.7% *
14.3%
before after
before after
before after
before after
before after
before after
Demi* 10s
VALO 10s
Optilux 501 10s
ELIPAR S10 10s
Bluephase 20i* 10s
FLASHlight Magna 20s
Fig 3 Maximum, minimum, and median radiant exposure delivered before and after instruction using MARC-PS. *Indicates significant change in percentage improvement in the radiant exposure delivered. Note the general reduction in standard deviation and outliers of the energy values after instruction.
Instruction and receiving immediate feedback on the amount of energy being delivered resulted in a more than a 1 J/cm2 increase in the amount of energy delivered on 43 occasions, as opposed to decreased reading on five occasions. Irrespective of the curing device, the number of dentists who delivered less than 10 J/cm2 decreased from 68% to 40% as a result of instruction. Overall, there was an 18% increase (P < .05) in radiant exposure delivered by dentists after instruction compared to pre-instruction values. The radiant
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exposure delivered increased significantly (P < .001) by up to 30%, as a result of training using MARC-PS. The greatest improvement (P < .05) was found using the Bluephase 20i and Demi curing units, with a 30% and 31% increase respectively (Fig 3). The VALO light-curing device resulted in more consistent results both before and after instruction and was not influenced by instruction as much as the other curing lights (P > .05).
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a
b
c
d
Figs 4a to 4d (a) The intense reflection from the target area when light curing a restoration is usually distracting for the clinician. (b) The limited protection provided by the orange shield on the light-curing unit is not effective in the majority of light-curing positions and the clinician often looks away. Furthermore, the small size of the orange shield on the curing light does not protect other clinical personnel in the vicinity from occupational exposure to blue light. (c) A light-shield or (d) “blue blocking” glasses will shield the operator from all the reflected blue light, thus allowing the clinician to safely watch what they are doing.
DISCUSSION The MARC-PS proved effective in quantifying the ability of dentists to consistently deliver high levels of radiant exposure. In addition, the device provided immediate feedback to the participants on their light-curing technique so that the participant could see their improvement. Before receiving additional instruction, 68% of dentists delivered less than 10 J/cm2. Thus the hypothesis that all dentists would deliver a minimum of 10 J/cm2 was not upheld and was rejected. There was a wide variation in the average amount of radiant exposure delivered: 2.9 to 15.4 J/cm2. This suggests that many resin-based restorations that are light cured by dentists will have a wide range of clinically relevant properties such as wear resistance, depth of cure, hardness, and fracture resistance.25-27 This may explain why many clinical studies investigating the longevity of composite restorations have reported that the operator factors influence the results significantly.28-30 None of the dentists delivered the 16 to 35 J/cm2 that has been reported as necessary to adequately cure some resins.19-21,27 However, it should be noted that after training, all but one of the 25 dentists delivered more than the minimum 6 J/cm2. The second null hypothesis,
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which assumed that radiant exposure delivered would not significantly increase following additional training using the MARC-PS, was not upheld for the overall results; however, on an individual basis five of the 25 participants showed more than a 1 J/cm2 decrease in the amount of energy they delivered compared to their pre-training values. The radiant exposure delivered to a Class 1 preparation in the maxillary second molar in MARC may not be applicable to all clinical restorative scenarios. Compared to Class 2 or Class 5 configurations with distal extensions on molar teeth, the access to a Class 1 preparation is relatively easy.17 Conversely, compared to anterior Class 5 or 3 configurations, the access to a second molar Class 1 preparation is relatively more difficult. Thus, the radiant exposure delivered in the current study may be even less under more complex clinical situations, or greater for more accessible restorations.31-35 The specific light-curing units used in the study were well maintained and were working optimally. This is in sharp contrast to curing lights found in many dental offices that often have resin contamination on the light tip, or decreased light output due to long-term clinical use.31-35 There was a wide variation in the amount of radiant exposure delivered by the dentists: 2.9 to 15.4 J/cm2.
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Since the long-term success of photo-cured dental restorations depends on how well they are polymerized and this is a direct reflection of the initial amount of radiant exposure delivered to the resin, the low radiant exposure delivered by these dentists is a cause for concern.36-41 The present study shows that radiant exposure delivered to a Class 1 preparation site may be significantly influenced by the type of curing light used as well as by the training received by the operator, confirming previous results.42 However, individual operator factors also caused significant differences. Even though the amount of instruction given to these dentists was minimal (60 seconds), the influence of the education at the time of the experiment was significant. Further testing for long-term retention of the principles imparted during instruction should be performed. Figures 1c to 1e show commonly observed errors when curing a Class 1 molar restoration in sagittal plane. A combination of these errors using recommended exposure times will result in a resin-based restoration receiving an inadequate amount of energy. The result of a typical angulation error (Fig 1c) of 45 degrees has been reported to result in a 56% decrease in radiant exposure.42 This confirms the importance of watching what you are doing when light curing. To protect the eyes, “blue blocker” glasses or a shield must be used when observing the curing light tip. Even when the dentist instantly looks away after placing the curing light, the accumulated exposure in a day can exceed the maximum daily exposure limit according to International Commission on Non-Ionizing Radiation Protection guidelines.43 Figure 4a shows the reflected blue light from the curing tip, which makes it impossible for the dentist to observe the procedure directly. Typically, if a “blue blocker” is not used, the dentist will have to look away during the curing procedure (Fig 4b), possibly resulting in a misaligned curing tip. In an ideal situation, the dentist should be protected from the blue light with a “blue blocker” glass or shield (Fig 4c), to observe the curing procedure properly (Fig 4d). It is interesting to note that the improvement obtained as a result of instruction ranged between 6% (VALO) and 31% (Demi) (Fig 3). The VALO light has a
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straight handpiece-shaped design, and does not use a fiber-optic light guide. This design showed less improvement after education. This result may be an indication of a successful design, which is relatively less prone to energy-delivery variation, compared to “gun” style lights with separate light guides that are more difficult to position over the restoration (Fig 1). Gun-style (pistol-gripped) units or curing units with angled light guides, like Bluephase 20i and Demi respectively, showed greater improvement in the results after instruction, possibly because they may be more prone to deviation from correct angulation. The length and the angle of the light guide makes positioning the tip properly more difficult compared to units that have no guide and that locate the LED chips directly at the distal end of the device.
CONCLUSIONS Within the limitations of the present study, the following conclusions may be made: • The factors that were identified to contribute to the clinical radiant exposure of composite resins on a Class 1 cavity are: light-curing technique and curing unit design. • Irrespective of the measured power outputs from the curing lights, reinforcing the proper clinical curing technique seems to significantly improve the radiant exposure of Class 1 cavities. • Proper light-curing technique should decrease the number of under-exposed resin-based restorations and improve their clinical longevity.
REFERENCES 1. Ferracane JL, Berge HX, Condon JR. In vitro aging of dental composites in water: effect of degree of conversion, filler volume, and filler/matrix coupling. J Biomed Mater Res 1998;42:465–472. 2. Halvorson RH, Erickson RL, Davidson CL. Energy dependent polymerization of resin-based composite. Dent Mater 2002;18:463–469. 3. Calheiros FC, Daronch M, Rueggeberg FA, Braga RR. Degree of conversion and mechanical properties of a BisGMA:TEGDMA composite as a function of the applied radiant exposure. J Biomed Mater Res B Appl Biomater 2008;84:503–509. 4. Condon JR, Ferracane JL. In vitro wear of composite with varied cure, filler level, and filler treatment. J Dent Res 1997;76:1405–1411. 5. Ferracane JL, Mitchem JC, Condon JR, Todd R. Wear and marginal breakdown of composites with various degrees of cure. J Dent Res 1997;76:1508–1516.
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6. Bhamra GS, Fleming GJ. Influence of halogen irradiance on short- and longterm wear resistance of resin-based composite materials. Dent Mater 2009;25:214–220. 7. Kim SY, Lee I B, Cho BH, Son HH, Um CM. Curing effectiveness of a light emitting diode on dentin bonding agents. J Biomed Mater Res B Appl Biomater 2005;77:164–170. 8. Staudt CB, Krejci I, Mavropoulos A. Bracket bond strength dependence on light power density. J Dent 2006;34:498–502. 9. Vandewalle KS, Ferracane JL, Hilton TJ, Erickson RL, Sakaguchi RL. Effect of energy density on properties and marginal integrity of posterior resin composite restorations. Dent Mater 2004;20:96–106. 10. Brambilla E, Gagliani M, Ionescu A, Fadini L, García-Godoy F. The influence of light-curing time on the bacterial colonization of resin composite surfaces. Dent Mater 2009;25:1067–1072. 11. Brackett MG, Brackett WW, Browning WD, Rueggeberg FA. The effect of light curing source on the residual yellowing of resin composites. Oper Dent 2007;32:443–450. 12. Janda R, Roulet JF, Latta M, Kaminsky M, Rüttermann S. Effect of exponential polymerization on color stability of resin-based filling materials. Dent Mater 2007;23:696–704. 13. Franz A, König F, Anglmayer M, et al. Cytotoxic effects of packable and nonpackable dental composites. Dent Mater 2003;19:382–392. 14. Sigusch BW, Völpel A, Braun I, Uhl A, Jandt KD. Influence of different light curing units on the cytotoxicity of various dental composites. Dent Mater 2007;23:1342–1348. 15. Knezevic A, Zeljezic D, Kopjar N, Tarle Z. Cytotoxicity of composite materials polymerized with LED curing units. Oper Dent 2008;33:23–30. 16. Jokstad A, Bayne S, Blunck U, Tyas M, Wilson N. Quality of dental restorations. FDI Commission Project 2–95. Int Dent J 2001;51:117–158. 17. Price RB, Felix CM, Walen JM. Factors affecting the energy delivered to simulated class I and class V preparations. J Can Dent Assoc 2010;76:a94. 18. Fan PL, Schumacher RM, Azzolin K, Geary R, Eichmiller FC. Curing-light intensity and depth of cure of resin-based composites tested according to international standards. J Am Dent Assoc 2002;133:429–434. 19. Moore BK, Platt JA, Borges G, Chu TM, Katsilieri I. Depth of cure of dental resin composites: ISO 4049 depth and microhardness of types of materials and shades. Oper Dent 2008;33:408–412. 20. Rueggeberg FA, Cole MA, Looney SW, Vickers A, Swift EJ. Comparison of manufacturer-recommended exposure durations with those determined using biaxial flexure strength and scraped composite thickness among a variety of light-curing units. J Esthet Restor Dent 2009;21:43–61. 21. Anusavice K. Phillips’ Science of Dental Materials, 12th edition. St Louis: WB Saunders, 2013:290. 22. Mousavinasab SM, Meyers I. Curing efficacy of light emitting diodes of dental curing units. J Dent Res Dent Clin Dent Prospects 2009;3:11–16. 23. Federlin M, Price R. Improving light-curing instruction in dental school. J Dent Educ 2013;77:764–772. 24. Seth S, Lee CJ, Ayer CD. Effect of instruction on dental students; ability to light-cure a simulated restoration. J Can Dent Assoc 2012;78:c123.
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25. Komori PC, de Paula AB, Martin AA, Tango RN, Sinhoreti MA, Correr-Sobrinho L. Effect of light energy density on conversion degree and hardness of dualcured resin cement. Oper Dent 2010;35:120–124. 26. Santos GC Jr, El-Mowafy O, Rubo JH, Santos MJ. Hardening of dual-cure resin cements and a resin composite restorative cured with QTH and LED curing units. J Can Dent Assoc 2004;70:323–328. 27. Shortall A, El-Mahy W, Stewardson D, Addison O, Palin W. Initial fracture resistance and curing temperature rise of ten contemporary resin-based composites with increasing radiant exposure. J Dent 2013;41:455–463. 28. Sunnegardh-Gronberg K, van Dijken JW, Funegard U, Lindberg A, Nilsson M. Selection of dental materials and longevity of replaced restorations in Public Dental Health clinics in northern Sweden. J Dent 2009;37:673–678. 29. Opdam NJ, Bronkhorst EM, Roeters JM, Loomans BA. A retrospective clinical study on longevity of posterior composite and amalgam restorations. Dent Mater 2007;23:2–8. 30. Demarco FF, Correa MB, Cenci MS, Moraes RR, Opdam NJ. Longevity of posterior composite restorations: not only a matter of materials. Dent Mater 2012;28:87–101. 31. El-Mowafy O, El-Badrawy W, Lewis DW, et al. Efficacy of halogen photopolymerization units in private dental offices in Toronto. J Can Dent Assoc 2005;71:587. 32. Strydom C. Dental curing lights: maintenance of visible light curing units. SADJ 2002;57:227–233. 33. Strydom C. Curing lights: the effects of clinical factors on intensity and polymerisation. SADJ 2002;57:181–186. 34. Burke FJ, Earp DP, Farrell S. Effectiveness of light-curing units. Br Dent J 1996;181:283. 35. Maghaireh GA, Aizraikat H, Taha NA. Assessing the irradiance delivered from light-curing units in private dental offices in Jordan. J Am Dent Assoc 2013;144:922–927. 36. Caldas DB, de Almeida JB, Correr-Sobrinho L, Sinhoreti MA, Consani S. Influence of curing tip distance on resin composite Knoop hardness number, using three different light curing units. Oper Dent 2003;28:315–320. 37. Correr AB, Sinhoreti MA, Sobrinho LC, Tango RN, Schneider LF, Consani S. Effect of the increase of energy density on Knoop hardness of dental composites light-cured by conventional QTH, LED and xenon plasma arc. Braz Dent J 2005;16:218–224. 38. Harris JS, Jacobsen PH, O’Doherty DM. The effect of curing light intensity and test temperature on the dynamic mechanical properties of two polymer composites. J Oral Rehabil 1999;26:635–639. 39. Lohbauer U, Rahiotis C, Krämer N, Petschelt A, Eliades G. The effect of different light-curing units on fatigue behavior and degree of conversion of a resin composite. Dent Mater 2005;21:608–615. 40. Xu X, Sandras DA, Burgess JO. Shear bond strength with increasing lightguide distance from dentin. J Esthet Restor Dent 2006;18:19–27. 41. St-Georges AJ, Swift EJ Jr, Thompson JY, Heymann HO. Curing light intensity effects on wear resistance of two resin composites. Oper Dent 2002;27:410–417. 42. Price RB, McLeod ME, Felix CM. Quantifying light energy delivered to a Class I restoration. J Can Dent Assoc 2010;76:a23. 43. Labrie D, Moe J, Price RB, Young ME, Felix CM. Evaluation of ocular hazards from 4 types of curing lights. J Can Dent Assoc 2011;77:b16.
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RESTORATIVE DENTISTRY
M. Murat Mutluay
Effect of using proper light-curing techniques on energy delivered to a Class 1 restoration M. Murat Mutluay, DDS, PhD1/Fred A. Rueggeberg, DDS, MS2/ Richard B. Price, BDS, DDS, MS, FRCD(c), FDS RCS(Edin), PhD 3 Objectives: To determine the effect of proper light-curing instruction on the radiant exposure (energy density) delivered by dentists using six dental curing lights to a posterior Class 1 restoration. Method and Materials: Twenty-five dentists attending a professional meeting were instructed to position a patient simulator (MARC-PS, BlueLight), as they would for a patient, and then to expose the simulated Class 1 maxillary second molar preparation for a specified amount of time. At this point, the dentists were unaware of the purpose of the experiment. Each participant used three different curing lights, and the irradiance and radiant exposure (J/cm2) delivered to the preparation was recorded. Participants were then informed of the purpose of the exercise, and given specific light-curing instructions and training using the patient simulator, after which they re-exposed the same preparation using the same curing lights. Pre- and post-instruction radiant exposure values
were compared using one-way ANOVA (α = .05), and for each light among all operators using a two-tailed, paired Student’s t test. Results: There was a wide variation in the radiant exposure delivered by the dentists and by the six curing lights, from 2.9 to 15.4 J/cm2. Before receiving additional light-curing instruction, 68% of dentists delivered less than 10 J/cm2. The radiant exposure delivered increased significantly (P < .001) by up to 30%, as a result of training using MARC-PS. Conclusion: The results indicate that some of the dentists participating in the present study delivered an inadequate amount of radiant exposure before instruction. More energy was delivered after a short training session using the MARC-PS. Reinforcing the proper photo-curing techniques may improve the outcome when placing resin-based restorations. (Quintessence Int 2014;45:549–556; doi: 10.3290/j.qi.a31959)
Key words: clinical performance, composite resin, light curing, operative dentistry, polymerization
Considerable evidence exists demonstrating that delivering inadequate radiant exposure (energy density) to a resin restoration will result in a restoration with less than optimal properties and decreased clinical perfor1
Lecturer, Department of Cariology, Institute of Dentistry, University of Turku, Turku, Finland.
2
Professor and Section Director, Dental Materials, Department of Oral Rehabilitation, College of Dentistry, Georgia Regents University, Augusta, Georgia, USA.
3
Professor and Division Head, Division of Fixed Prosthodontics, Department of Dental Clinical Sciences, Dalhousie University, Halifax, Nova Scotia, Canada.
Correspondence: Dr M. Murat Mutluay, Adhesive Dentistry Research Group, Institute of Dentistry, University of Turku, Lemminkaisenkatu 2, 20520 – Turku, Finland. Email: [email protected]
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mance.1,2 The strength, stiffness, and hardness of a resin that can only be light-cured will be adversely affected if the resin does not receive sufficient radiant energy.1,3 There will also be greater wear,4-6 reduced bonding to the preparation wall7 or to orthodontic brackets,8 enhanced resin “washout” at the gingival margin,9 increased bacterial colonization,10 reduced color stability,11,12 and increased leachates with cytotoxic potential from the resin restoration.13-15 A combination of operator-dependent factors and restoration material properties are critical for the longterm success of a restoration.16 These factors include
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a
b
c
d
e
Figs 1a to 1e The black oval shape inside the left maxillary second molar represents the detector. Light-curing units without (a) and with (b) light guides, shown in an ideal situation where the detector is directly under the light-curing tip at the minimum distance, good angulation, and good mesiodistal position. Note the close proximity of the light guide to mandibular anterior teeth (yellow arrows in (a) and (b)). This is a limiting factor for units with a light guide when polymerizing restorations in posterior teeth. Common mistakes such as incorrect angulation (c), location (d), and distance (e) of the curing tip result in reduced energy delivery to the restoration.
using an appropriate curing light, the operator technique, the location of the restoration, and the type of resin used.17 In a photo-curable resin, the chemical reaction leading to polymerization is initiated by lightcuring devices, and the resins must receive sufficient energy to drive this process. How light-curing devices are used combined with their design, spectral emission, and power output, defines the amount of useful energy delivered to the resin. It has been reported that 6 to 12 J/cm2 of radiant exposure should be sufficient to adequately polymerize some composite resins to a depth of 1.5 mm,18 although depending on material shade and opacity of the resin, up to 35 J/cm2 may be required.19,20 To maximize the cure of a 2-mm layer of composite, Phillips’ Science of Dental Materials textbook recommends a value of 16 J/cm2.21 In laboratory settings, various quartz tungsten halogen and light emitting diode (LED) lights were compared and no difference was found between the depth of cure and Vickers hardness of composite resins cured using different lights.22 This suggests that all modern curing lights could potentially give satisfactory results irrespective of their design.
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It is difficult to prove a direct relationship between inadequate light curing and clinical failure of resinbased restorations using a randomized clinical trial, due to the unethical nature of purposely “under-curing” a restoration placed in a human patient. However, a clinical study performed in patients who had light-cured resin-based restorations placed in the teeth of their dentures5 verified that purposely under-cured resinbased restorations showed significantly greater and clinically unacceptable occlusal wear after 2 years of function. Also, a wide range of hardness values of two photo-curable resins was reported when they were exposed using 214 different light-curing devices found in a selection of dental offices in Toronto. When the resins were exposed for 40 seconds under ideal conditions using the curing lights found in the dental offices, only 10% of the resin specimens reached 80% of the required hardness value.22 Such an inconsistency indicates that the use of curing lights in clinical settings may be influenced by various factors that may not be predicted using data created in laboratory settings. Recently, using the MARC patient simulator (MARCPS, BlueLight Analytics) two studies investigated the
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influence of instruction on the light-curing abilities of dental students.23,24 These articles suggest that instruction has a significant effect on the amount of energy delivered by students to a restoration. However, the ability of dentists to deliver energy to a restoration and how this can be improved has not been reported. The objective of the current study was to use the MARC-PS to identify the contribution of operator factors and factors related to the design of the light-curing unit on the amount of light energy delivered by dentists. The null hypothesis tested was that all dentists will deliver at least 10 J/cm2 of radiant exposure and that neither the instructions given nor the type of lightcuring unit used would have any influence on the amount of energy delivered.
METHOD AND MATERIALS The MARC-PS measures the irradiance, spectral emission, and radiant exposure delivered from light-curing devices to simulated dental restoration sites in a mannequin head. The MARC-PS uses a 3.9-mm diameter cosine-corrected irradiance probe (CC3-UV, Ocean Optics) to capture the radiant power delivered to the bottom of a cavity preparation within a dental mannequin (Kilgore) contained in a mannequin head (Fig 1). The detector is located inside a Class 1 preparation of the left maxillary second molar (tooth 27 according to FDI notation), with the top surface of the detector 2 mm from the occlusal surface, and 4 mm from the cusp tips. The 3.9-mm active diameter of the probe is similar in size to a Class 1 preparation in a molar tooth. The CC3-UV irradiance probe is attached to a laboratory-grade, fiber-optic spectroradiometer (USB 4000, Ocean Optics) and the MARC software (BlueLight Analytics) reports the irradiance (mW/cm2), spectral emission (nm), and radiant exposure (J/cm2) delivered to the floor of the simulated preparation during simulated photo-curing. The interincisal mouth opening can be adjusted, and in this experiment it was fixed at 43 mm. To evaluate the ability of the clinician to deliver energy to the simulated preparation using a curing light, the following precautions were taken in accor-
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dance with the guidelines from the institutional review board at the University of Turku, Finland. Verbal consent was obtained from the participants, no personal information was collected, and subjects remained completely anonymous during and after the data had been collected. The subjects were informed about their own results only. The project plan concerning the present study was reviewed and accepted by the Ethic Committee of University of Turku (Statement #9/2013). Before using the light-curing units, each of 25 dentists was instructed to position the head of the patient simulator as they would a patient’s head and then to perform simulated light curing for 10 seconds, using a variety of commercially available, blue LED-type curing lights (Table 1). Because of the low radiant exposure delivered from the Magna curing light, this unit was used for 20 seconds. The dentists were divided into two groups, with each group using three of the six different curing lights. The irradiance (mW/cm2) delivered to the preparation site was recorded in real-time by the MARC software (BlueLight Analytics), but was not visible to the dentist operating the light, only to the research assistant. The MARC software calculated the resulting real-time radiant exposure delivered as the mathematical product of irradiance and exposure duration recorded by the MARC software. The duration of exposure was controlled by the internal timing mechanism of each curing light. After all of the “before” instruction data were collected, the dentists were given more information about the purpose of the exercise. They were given simple instructions on the basic concepts of proper chairside light curing: they were told to use “blue blocker” glasses, to directly observe what they were doing, and to stabilize the light close to and perpendicular to the preparation site. Following these instructions that took less than a minute to deliver, the same dentists then re-exposed the same preparation area in the second molar using the same lights, and the radiant exposure value was again obtained: the “after” instruction value. To gauge the relative power output of the different units, the radiant power (Watts) from the curing lights, irrespective of the tip diameter, was measured using a
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Table 1
Description of the light units used
Light type
Output mode
Unit-controlled exposure time (s)
Yes
LED (blue)
Standard
10
Ultradent
No
LED (polywave)
Standard
10
Optilux 501
Kerr
Yes
QTH
Boost
10
Elipar S10
3M
Yes
LED (blue)
Standard
10
Bluephase 20i
Ivoclar Vivadent
Yes
LED (polywave)
High
10
FLASHlight Magna
Discus Dental
No
LED (blue)
Standard
20
Manufacturer
Light guide
Demi
Kerr
VALO
Brand name
Product profile
QTH, quartz tungsten halogen.
laboratory grade thermopile (PM-10, Coherent). This process was performed by holding the light at a minimum distance from the sensor, but without contacting the thermopile surface, until repeatable results were obtained. Pre- and post-instruction radiant exposure values were compared using one-way ANOVA (α = .05). In addition, values were compared within each light brand, among all operators using a two-tailed, paired Student’s t test at a family-wise α of .05. Within a brand of curing light, a significance level of .008 (.05/6) was used.
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RESULTS The radiant power from each light-curing device is presented in Fig 2. The data do not contain standard deviations because the radiant power values recorded by the thermopile were very consistent, with no variation among subsequent measurements. There was a wide variation in the amount of radiant exposure delivered: from 2.9 to 15.4 J/cm2 (Fig 3). Although power emitted from the curing lights differed among units, the power value was not indicative of the radiant exposure that was delivered by the dentists. A significant difference between the amounts of radiant exposure delivered using each light-curing device was found (P < .05).
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1500
Power output [mW]
1250 1000 750 500 250 0 Demi Fig 2
VALO
Optilux 501
ELIPAR S10
Bluephase 20i
FLASHlight Magna
Power output of the curing lights measured using a laboratory grade thermopile.
16 Radiant Exposure [J/cm2]
14 12 10 8 6 4 2 Average increase after instruction: 0
30.5% *
6.2%
8.5%
20.7%
29.7% *
14.3%
before after
before after
before after
before after
before after
before after
Demi* 10s
VALO 10s
Optilux 501 10s
ELIPAR S10 10s
Bluephase 20i* 10s
FLASHlight Magna 20s
Fig 3 Maximum, minimum, and median radiant exposure delivered before and after instruction using MARC-PS. *Indicates significant change in percentage improvement in the radiant exposure delivered. Note the general reduction in standard deviation and outliers of the energy values after instruction.
Instruction and receiving immediate feedback on the amount of energy being delivered resulted in a more than a 1 J/cm2 increase in the amount of energy delivered on 43 occasions, as opposed to decreased reading on five occasions. Irrespective of the curing device, the number of dentists who delivered less than 10 J/cm2 decreased from 68% to 40% as a result of instruction. Overall, there was an 18% increase (P < .05) in radiant exposure delivered by dentists after instruction compared to pre-instruction values. The radiant
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exposure delivered increased significantly (P < .001) by up to 30%, as a result of training using MARC-PS. The greatest improvement (P < .05) was found using the Bluephase 20i and Demi curing units, with a 30% and 31% increase respectively (Fig 3). The VALO light-curing device resulted in more consistent results both before and after instruction and was not influenced by instruction as much as the other curing lights (P > .05).
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a
b
c
d
Figs 4a to 4d (a) The intense reflection from the target area when light curing a restoration is usually distracting for the clinician. (b) The limited protection provided by the orange shield on the light-curing unit is not effective in the majority of light-curing positions and the clinician often looks away. Furthermore, the small size of the orange shield on the curing light does not protect other clinical personnel in the vicinity from occupational exposure to blue light. (c) A light-shield or (d) “blue blocking” glasses will shield the operator from all the reflected blue light, thus allowing the clinician to safely watch what they are doing.
DISCUSSION The MARC-PS proved effective in quantifying the ability of dentists to consistently deliver high levels of radiant exposure. In addition, the device provided immediate feedback to the participants on their light-curing technique so that the participant could see their improvement. Before receiving additional instruction, 68% of dentists delivered less than 10 J/cm2. Thus the hypothesis that all dentists would deliver a minimum of 10 J/cm2 was not upheld and was rejected. There was a wide variation in the average amount of radiant exposure delivered: 2.9 to 15.4 J/cm2. This suggests that many resin-based restorations that are light cured by dentists will have a wide range of clinically relevant properties such as wear resistance, depth of cure, hardness, and fracture resistance.25-27 This may explain why many clinical studies investigating the longevity of composite restorations have reported that the operator factors influence the results significantly.28-30 None of the dentists delivered the 16 to 35 J/cm2 that has been reported as necessary to adequately cure some resins.19-21,27 However, it should be noted that after training, all but one of the 25 dentists delivered more than the minimum 6 J/cm2. The second null hypothesis,
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which assumed that radiant exposure delivered would not significantly increase following additional training using the MARC-PS, was not upheld for the overall results; however, on an individual basis five of the 25 participants showed more than a 1 J/cm2 decrease in the amount of energy they delivered compared to their pre-training values. The radiant exposure delivered to a Class 1 preparation in the maxillary second molar in MARC may not be applicable to all clinical restorative scenarios. Compared to Class 2 or Class 5 configurations with distal extensions on molar teeth, the access to a Class 1 preparation is relatively easy.17 Conversely, compared to anterior Class 5 or 3 configurations, the access to a second molar Class 1 preparation is relatively more difficult. Thus, the radiant exposure delivered in the current study may be even less under more complex clinical situations, or greater for more accessible restorations.31-35 The specific light-curing units used in the study were well maintained and were working optimally. This is in sharp contrast to curing lights found in many dental offices that often have resin contamination on the light tip, or decreased light output due to long-term clinical use.31-35 There was a wide variation in the amount of radiant exposure delivered by the dentists: 2.9 to 15.4 J/cm2.
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Since the long-term success of photo-cured dental restorations depends on how well they are polymerized and this is a direct reflection of the initial amount of radiant exposure delivered to the resin, the low radiant exposure delivered by these dentists is a cause for concern.36-41 The present study shows that radiant exposure delivered to a Class 1 preparation site may be significantly influenced by the type of curing light used as well as by the training received by the operator, confirming previous results.42 However, individual operator factors also caused significant differences. Even though the amount of instruction given to these dentists was minimal (60 seconds), the influence of the education at the time of the experiment was significant. Further testing for long-term retention of the principles imparted during instruction should be performed. Figures 1c to 1e show commonly observed errors when curing a Class 1 molar restoration in sagittal plane. A combination of these errors using recommended exposure times will result in a resin-based restoration receiving an inadequate amount of energy. The result of a typical angulation error (Fig 1c) of 45 degrees has been reported to result in a 56% decrease in radiant exposure.42 This confirms the importance of watching what you are doing when light curing. To protect the eyes, “blue blocker” glasses or a shield must be used when observing the curing light tip. Even when the dentist instantly looks away after placing the curing light, the accumulated exposure in a day can exceed the maximum daily exposure limit according to International Commission on Non-Ionizing Radiation Protection guidelines.43 Figure 4a shows the reflected blue light from the curing tip, which makes it impossible for the dentist to observe the procedure directly. Typically, if a “blue blocker” is not used, the dentist will have to look away during the curing procedure (Fig 4b), possibly resulting in a misaligned curing tip. In an ideal situation, the dentist should be protected from the blue light with a “blue blocker” glass or shield (Fig 4c), to observe the curing procedure properly (Fig 4d). It is interesting to note that the improvement obtained as a result of instruction ranged between 6% (VALO) and 31% (Demi) (Fig 3). The VALO light has a
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straight handpiece-shaped design, and does not use a fiber-optic light guide. This design showed less improvement after education. This result may be an indication of a successful design, which is relatively less prone to energy-delivery variation, compared to “gun” style lights with separate light guides that are more difficult to position over the restoration (Fig 1). Gun-style (pistol-gripped) units or curing units with angled light guides, like Bluephase 20i and Demi respectively, showed greater improvement in the results after instruction, possibly because they may be more prone to deviation from correct angulation. The length and the angle of the light guide makes positioning the tip properly more difficult compared to units that have no guide and that locate the LED chips directly at the distal end of the device.
CONCLUSIONS Within the limitations of the present study, the following conclusions may be made: • The factors that were identified to contribute to the clinical radiant exposure of composite resins on a Class 1 cavity are: light-curing technique and curing unit design. • Irrespective of the measured power outputs from the curing lights, reinforcing the proper clinical curing technique seems to significantly improve the radiant exposure of Class 1 cavities. • Proper light-curing technique should decrease the number of under-exposed resin-based restorations and improve their clinical longevity.
REFERENCES 1. Ferracane JL, Berge HX, Condon JR. In vitro aging of dental composites in water: effect of degree of conversion, filler volume, and filler/matrix coupling. J Biomed Mater Res 1998;42:465–472. 2. Halvorson RH, Erickson RL, Davidson CL. Energy dependent polymerization of resin-based composite. Dent Mater 2002;18:463–469. 3. Calheiros FC, Daronch M, Rueggeberg FA, Braga RR. Degree of conversion and mechanical properties of a BisGMA:TEGDMA composite as a function of the applied radiant exposure. J Biomed Mater Res B Appl Biomater 2008;84:503–509. 4. Condon JR, Ferracane JL. In vitro wear of composite with varied cure, filler level, and filler treatment. J Dent Res 1997;76:1405–1411. 5. Ferracane JL, Mitchem JC, Condon JR, Todd R. Wear and marginal breakdown of composites with various degrees of cure. J Dent Res 1997;76:1508–1516.
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6. Bhamra GS, Fleming GJ. Influence of halogen irradiance on short- and longterm wear resistance of resin-based composite materials. Dent Mater 2009;25:214–220. 7. Kim SY, Lee I B, Cho BH, Son HH, Um CM. Curing effectiveness of a light emitting diode on dentin bonding agents. J Biomed Mater Res B Appl Biomater 2005;77:164–170. 8. Staudt CB, Krejci I, Mavropoulos A. Bracket bond strength dependence on light power density. J Dent 2006;34:498–502. 9. Vandewalle KS, Ferracane JL, Hilton TJ, Erickson RL, Sakaguchi RL. Effect of energy density on properties and marginal integrity of posterior resin composite restorations. Dent Mater 2004;20:96–106. 10. Brambilla E, Gagliani M, Ionescu A, Fadini L, García-Godoy F. The influence of light-curing time on the bacterial colonization of resin composite surfaces. Dent Mater 2009;25:1067–1072. 11. Brackett MG, Brackett WW, Browning WD, Rueggeberg FA. The effect of light curing source on the residual yellowing of resin composites. Oper Dent 2007;32:443–450. 12. Janda R, Roulet JF, Latta M, Kaminsky M, Rüttermann S. Effect of exponential polymerization on color stability of resin-based filling materials. Dent Mater 2007;23:696–704. 13. Franz A, König F, Anglmayer M, et al. Cytotoxic effects of packable and nonpackable dental composites. Dent Mater 2003;19:382–392. 14. Sigusch BW, Völpel A, Braun I, Uhl A, Jandt KD. Influence of different light curing units on the cytotoxicity of various dental composites. Dent Mater 2007;23:1342–1348. 15. Knezevic A, Zeljezic D, Kopjar N, Tarle Z. Cytotoxicity of composite materials polymerized with LED curing units. Oper Dent 2008;33:23–30. 16. Jokstad A, Bayne S, Blunck U, Tyas M, Wilson N. Quality of dental restorations. FDI Commission Project 2–95. Int Dent J 2001;51:117–158. 17. Price RB, Felix CM, Walen JM. Factors affecting the energy delivered to simulated class I and class V preparations. J Can Dent Assoc 2010;76:a94. 18. Fan PL, Schumacher RM, Azzolin K, Geary R, Eichmiller FC. Curing-light intensity and depth of cure of resin-based composites tested according to international standards. J Am Dent Assoc 2002;133:429–434. 19. Moore BK, Platt JA, Borges G, Chu TM, Katsilieri I. Depth of cure of dental resin composites: ISO 4049 depth and microhardness of types of materials and shades. Oper Dent 2008;33:408–412. 20. Rueggeberg FA, Cole MA, Looney SW, Vickers A, Swift EJ. Comparison of manufacturer-recommended exposure durations with those determined using biaxial flexure strength and scraped composite thickness among a variety of light-curing units. J Esthet Restor Dent 2009;21:43–61. 21. Anusavice K. Phillips’ Science of Dental Materials, 12th edition. St Louis: WB Saunders, 2013:290. 22. Mousavinasab SM, Meyers I. Curing efficacy of light emitting diodes of dental curing units. J Dent Res Dent Clin Dent Prospects 2009;3:11–16. 23. Federlin M, Price R. Improving light-curing instruction in dental school. J Dent Educ 2013;77:764–772. 24. Seth S, Lee CJ, Ayer CD. Effect of instruction on dental students; ability to light-cure a simulated restoration. J Can Dent Assoc 2012;78:c123.
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25. Komori PC, de Paula AB, Martin AA, Tango RN, Sinhoreti MA, Correr-Sobrinho L. Effect of light energy density on conversion degree and hardness of dualcured resin cement. Oper Dent 2010;35:120–124. 26. Santos GC Jr, El-Mowafy O, Rubo JH, Santos MJ. Hardening of dual-cure resin cements and a resin composite restorative cured with QTH and LED curing units. J Can Dent Assoc 2004;70:323–328. 27. Shortall A, El-Mahy W, Stewardson D, Addison O, Palin W. Initial fracture resistance and curing temperature rise of ten contemporary resin-based composites with increasing radiant exposure. J Dent 2013;41:455–463. 28. Sunnegardh-Gronberg K, van Dijken JW, Funegard U, Lindberg A, Nilsson M. Selection of dental materials and longevity of replaced restorations in Public Dental Health clinics in northern Sweden. J Dent 2009;37:673–678. 29. Opdam NJ, Bronkhorst EM, Roeters JM, Loomans BA. A retrospective clinical study on longevity of posterior composite and amalgam restorations. Dent Mater 2007;23:2–8. 30. Demarco FF, Correa MB, Cenci MS, Moraes RR, Opdam NJ. Longevity of posterior composite restorations: not only a matter of materials. Dent Mater 2012;28:87–101. 31. El-Mowafy O, El-Badrawy W, Lewis DW, et al. Efficacy of halogen photopolymerization units in private dental offices in Toronto. J Can Dent Assoc 2005;71:587. 32. Strydom C. Dental curing lights: maintenance of visible light curing units. SADJ 2002;57:227–233. 33. Strydom C. Curing lights: the effects of clinical factors on intensity and polymerisation. SADJ 2002;57:181–186. 34. Burke FJ, Earp DP, Farrell S. Effectiveness of light-curing units. Br Dent J 1996;181:283. 35. Maghaireh GA, Aizraikat H, Taha NA. Assessing the irradiance delivered from light-curing units in private dental offices in Jordan. J Am Dent Assoc 2013;144:922–927. 36. Caldas DB, de Almeida JB, Correr-Sobrinho L, Sinhoreti MA, Consani S. Influence of curing tip distance on resin composite Knoop hardness number, using three different light curing units. Oper Dent 2003;28:315–320. 37. Correr AB, Sinhoreti MA, Sobrinho LC, Tango RN, Schneider LF, Consani S. Effect of the increase of energy density on Knoop hardness of dental composites light-cured by conventional QTH, LED and xenon plasma arc. Braz Dent J 2005;16:218–224. 38. Harris JS, Jacobsen PH, O’Doherty DM. The effect of curing light intensity and test temperature on the dynamic mechanical properties of two polymer composites. J Oral Rehabil 1999;26:635–639. 39. Lohbauer U, Rahiotis C, Krämer N, Petschelt A, Eliades G. The effect of different light-curing units on fatigue behavior and degree of conversion of a resin composite. Dent Mater 2005;21:608–615. 40. Xu X, Sandras DA, Burgess JO. Shear bond strength with increasing lightguide distance from dentin. J Esthet Restor Dent 2006;18:19–27. 41. St-Georges AJ, Swift EJ Jr, Thompson JY, Heymann HO. Curing light intensity effects on wear resistance of two resin composites. Oper Dent 2002;27:410–417. 42. Price RB, McLeod ME, Felix CM. Quantifying light energy delivered to a Class I restoration. J Can Dent Assoc 2010;76:a23. 43. Labrie D, Moe J, Price RB, Young ME, Felix CM. Evaluation of ocular hazards from 4 types of curing lights. J Can Dent Assoc 2011;77:b16.
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