Dental Curing Lights

Key Points

  •  The ability to easily and effectively cure polymer-based restorative materials using light energy has revolutionized the field of dentistry over the past 50 years.
  • For a light-cured resin-based restoration to function and last as intended, it must receive the required amount of light energy at the proper wavelength (i.e., the wavelength of the photoinitiator) to facilitate optimal polymerization (or curing).
  • To deliver the required amount of light energy, it is necessary to understand how clinical variables such as curing tip distance and angle of incidence with respect to the restoration surface influence the curing process, as well as exposure time and curing tip diameter.
  • Proper infection control procedures when using curing lights support both patient safety and equipment maintenance.
  • It is important to consider that curing lights can cause an intrapulpal temperature rise. Proper eye protection helps prevent blue-light-induced retinal injury.
Introduction
Photo-activated dental materials, including certain sealants, resin-based cements and composite restorative materials, are an integral part of a general dental practice. Dental light curing units (LCUs) are hand-held light emitting devices used to cure such photo-activated polymer-based restorative materials (PBRMs).1 

Given that practicing dental professionals often spend a large portion of time performing tasks that involve using PBRMs, the convenience of being able to rapidly light-cure these dental materials has revolutionized dentistry over the last 50 years. There are a wide variety of LCUs, and the technology has developed continually since photo-curing was first used in dentistry.2, 3 
 
Curing Light Technology

Photopolymerization works by using light energy to activate a photoinitiator; a molecule that can absorb light and generate reactive species (free radicals) that initiate polymerization.4, 5 As long as the wavelength of the light matches the absorption range of the photoinitiator with sufficient energy, a variety of light sources may be used for photopolymerization in dentistry, as discussed below. The most common photoinitiator used in dental resins today is camphorquinone (CQ).4 The peak absorption range for CQ is from 455 to 481 nm with peak absorption at approximately 469 nm.6

The first light-cured resins used in dentistry date to the early 1970s and were cured using ultraviolet (UV) LCUs.7 The photoinitiators used with these materials were based primarily on benzoin methyl ether or similar types of photoinitiators activated by UV.4 Examples of concerns about early UV-curing LCUs included resin color instability, limited depth of cure, and UV-promoted tissue damage, such as acute and longer-term eye damage.4, 7 However, within a few years of the introduction of UV-curing, dental materials were reformulated to include visible light wavelength photoinitiators, such as CQ.4, 7 As a result, curing units designed to emit UV light were replaced with LCUs that emit light in the visible spectrum, including quartz-tungsten-halogen (QTH) lights.7, 8

In contrast to UV LCUs, QTH curing lights emit blue light as part of their spectral output, require shorter curing times and are associated with lower risk of cataracts. The blue wavelengths emitted by QTH LCUs are not without their own risks, however, such as the risk of direct retinal damage. Thus, dental healthcare workers and patients have been advised to use blue-blocking filters to protect their eyes when using this type of LCU. QTH LCUs continued to be commonly used through the late 1990s.7, 9

Curing Light Use

Training. The type of LCU and the technique employed by the person using it can have a significant effect on the quality of the restoration, and there is potential for a large amount of variability in radiant exposure delivered by different operators.3

A preclinical light-curing simulator called MARC (Managing Accurate Resin Curing)10 was developed to help clinicians learn proper curing techniques. MARC uses simulated restorations and provides values for irradiance received by the restorations during curing along with radiant exposures. MARC also provides the spectral distribution for the curing light. A study using the MARC simulator found that the actual amount of light energy deposited on a restoration was often much less than estimated by the clinician.11

Common terms: Irradiance (Radiant Incidence), Radiant Exitance, Power, and Radiant Exposure (Table 1). The word intensity is often used in discussing curing lights, but the terms irradiance (radiant incidence) and radiant exitance are more precise. Irradiance (radiant incidence) is a measure of the radiant power striking a specific area,12 and radiant exitance is a measure of the power radiated outward from a source of a specific area.12

Regarding LCUs, irradiance is a measure of the power incident on a specific area from the curing unit tip, and radiant exitance is a measure of the power radiated out from the area of the curing unit tip. Irradiance is dependent on the power striking a specific surface area and, thus, can vary with distance from the curing unit tip. By contrast, the radiant exitance of a curing unit is a constant value, since the area of the curing unit tip and the power radiated from this tip are both for the most part constant (“for the most part” is used here because, just like a light bulb in your house, the power can slowly change over time as the bulb ages and then fails; with LCUs power can also change if the tip is damaged or contaminated). Irradiance and radiant exitance are often reported in mW/cm2 by LCU manufacturers (Radiant exitance is required to be included in the manufacturer’s instructions for use according to the American National Standard Institute/American Dental Association (ANSI/ADA) and International Organization for Standardization (ISO) standards for dental LCUs).

Another term commonly used when characterizing LCUs is power. Similar to the constant rate at which water flows out the nozzle of a hose, the power radiated by a LCU can be reported as a rate (energy emitted per unit time), which can be expressed in joules per second (J/s). The power emitted by lights is typically reported in Watts (W), like the light bulbs in your house that have a power rating of 40 W, 60 W, 75 W, etc. Watts can also be used to express the power output of an LCU. However, because dental light curing is done over a time period, such as 10 or 20 seconds, the power output of a curing unit can be thought of as a rate, with 1000 mW being equal to 1 W, which is equal to 1 J/s. When thinking about how much light energy is deposited on a restorative material, it might be easier to think of power as a rate that is multiplied by curing time to yield energy, as described in the next section on radiant exposure.

Yet another important term for understanding the process of curing polymer-based restorative materials is radiant exposure. This term is used to describe the total amount of light energy deposited on the polymer-based restorative material during curing.12 Radiant exposure can be determined by multiplying the irradiance by the curing time. That is, the radiant power striking the area of the resin being cured (the irradiance received at the resin) can be multiplied by curing time to yield radiant exposure. As stated above, thinking about power in terms of rate makes it easier to consider the total amount of light energy deposited on the polymer-based restorative material during curing. For example, when irradiance is expressed in Joules per second per area (J/s/cm2) instead of W/cm2, it can more easily be seen that multiplying irradiance by curing time in seconds, yields radiant exposure, or energy deposited on the restoration during curing, in J/cm2. Therefore, if I know my irradiance value is 1000 mW/cm2 and I cure for 20 seconds, I have delivered 20 Joules of energy to the area of resin that my light is striking. This is because the 1000 mW/cm2 can be expressed as 1 W/cm2 or 1 J/s/cm2, and then multiplied by the 20 second curing time to yields 20 J/cm2, or 20 J of light energy deposited on the area of resin my light is striking.

Table 1. Commonly Used Terms

Term

Curing Unit

Characteristic

Measure

Irradiance

 

(Radiant incidence)

Measures the radiant power striking a specific area

Varies with the distance from the curing unit tip

mW/cm2

Radiant exitance

Measures the power output from a source of a specific area

Essentially* a constant value

mW/cm2

Power

 

 

Light radiating from a curing unit tip

 

Joules per second (J/s)

Radiant exposure

Amount of light energy deposited on the polymer-based restorative material during curing

 

Joules per centimeter squared

(J//cm2)

* “Essentially” is used here because power can slowly change over time as the bulb ages and then fails or if the tip is damaged or contaminated.

Selecting a Dental Curing Light

FDA Clearance of Dental Curing Lights.  To market or sell a dental curing light in the USA, the U.S. Food and Drug Administration (FDA) requires a premarket notification (i.e., 510(k)) submission.13 The FDA has provided a guidance document that identifies the issues they believe should be addressed in a 510(k) submission for a dental curing light.14 Among other items, the document identifies risks to health from the use of these devices and recommends measures for mitigating these risks including labeling, proper infection control procedures, maintenance, and testing according to performance specifications, such as those detailed in ANSI/ADA and ISO standards.15

The FDA specifically prohibits companies from marketing their devices as having been “cleared by FDA;”16 However, a dentist may determine whether a dental device has, indeed, been cleared by FDA by checking the  FDA database for 510(k) Premarket Notification.  This database provides a listing of all devices the FDA has cleared since 1976. (Note:  Many devices are exempt from 510(k) clearance, and devices on the market before May 28, 1976, are grandfathered and do not require FDA clearance).  For each device type, the FDA has assigned a “product code”, which is “EBZ” for dental curing lights.  Entering “EBZ” on the website will provide a list of all dental curing lights the FDA has cleared, including product name, date, 510(k) numbers, manufacturer, and even summaries of the 510(k) submission.  (Note: Product names change so if you do not see your device listed on the list of FDA cleared devices this does not automatically mean the device has not been cleared).  The FDA’s Division of Premarket and Labeling Compliance at the Center for Devices and Radiological Health (CDRH) at 301-796-5770 can provide information on whether a curing light has been cleared by the FDA or is being illegally marketed.

The labeling and instructions for use of a curing unit may provide additional indications about whether a device has FDA clearance.  FDA reviews labeling and instructions for use as part of their clearance process;17  if information is missing, poorly written, or if exaggerated claims are made, it may suggest further investigation is warranted.

Safety Considerations

Blue Light Hazard. Blue-light retinal injury takes place primarily from exposure in the wavelength range between 380 to 550 nm, with the sensitivity of the retina peaking at approximately 440 nm.18, 19 Since the peak absorption for CQ is from about 455 to 481 nm, dental curing units are optimized to perform in the wavelength range that blue-light retinal injury takes place, with many having peak wavelengths near the retina’s sensitivity peak of 440 nm.6 It has been shown that under certain clinically relevant conditions, the light emitted from dental curing units may exceed dose limit values for photochemical retinal exposure over an 8-hour workday with an exposure duration of just under 3 hours, as set in international radiation protection guidelines.18-20

Blue-light filtering eyewear, curing unit tip-mounted shields, and hand-held paddles are all options for eye protection while using curing lights.21 Using quality eye protection in good condition that filters blue light at the same wavelengths as the LCU being used is recommended for all procedures using a light curing unit.22, 23

However, there is evidence that there are commercially available protective filtering devices that allow blue light transmission at significant levels. For example, research performed at the ADA laboratory found that 9 of 22 protective filtering devices allowed transmission of blue light from dental curing units at levels ranging from above 4% to above 15%, when tested under clinically relevant conditions.21 In a similar study performed at the Nordic Institute of Dental Materials, only 9 of 18 protective filtering devices were shown to demonstrate adequate filtering capacity according to international radiation protection guidelines.24

Currently, there is no standard that specifically sets test methods, requirements, and labeling for protective filtering devices intended for protection against retinal blue light exposure from dental curing units. There is a blue light filtering device standard working draft being developed in a subcommittee of ISO Technical Committee 106 Dentistry. In the meantime, there is an existing ISO standard for eyewear used for protection against intense light sources in cosmetic and medical settings.25 This standard specifies transmittance requirements for protective filtering devices based on a “blue light” B-classification scheme of B-1 to B-6 (most to least blue light transmittance) with corresponding labeling instructions. Also, if you are concerned that your blue-light filtering device is not effectively blocking blue light, there is a simple, practical experiment you can perform for yourself. In a dark room, take your light-cured polymer-based restorative material and do the following steps: distribute a clinically relevant increment of the material (about 6 mm in diameter and 2 mm thick) on a pad; place your protective filtering device just above the material; place your curing unit just above the protective filtering device and cure for about 20 seconds. After performing this procedure, if the polymer-based restorative material shows signs of curing, then the protective filtering device is not adequately blocking transmission of blue-light.21

Heat and Temperature Concerns. Temperature rise during the curing process raises concern for the risk of heat-induced pulpal injury. While irradiance and exposure time are important factors in proper curing, they also need to be considered with respect to the risk of thermal injury to pulpal and soft tissues. Special consideration should be taken when curing deep cavities, where there is less dentin to dissipate the total energy deposited on the dentin from the light source, increasing the concern of pulpal tissue injury.3, 26

In an in-vitro experiment testing 7 LED LCUs and 1 QTH LCU, ADA researchers found that the temperature rise of a thermocouple (embedded 1 mm into a 3-mm increment of composite) ranged from 9.8 to 12.9 ºC (49-55 ºF), when curing for 20 seconds with the curing unit tip centered 2 mm above the composite surface.27 It is not clear from the literature above what specific temperature threshold pulpal injury may occur, but tooth temperatures are elevated from curing resin polymers.1, 28, 29 Directing a stream of air over the tooth during the curing process, and/or waiting several seconds between curing cycles, can help prevent overheating the tooth.11   (Note- part of the heat rise is due to the exothermic reaction that accompanies resin polymerization)

Interference with Medical Devices. There has been some concern regarding the potential for interference between various electrical devices used in dentistry and pacemakers and/or defibrillators (for more information, see Oral Health Topic Cardiac Implanted Devices and Electronic Dental Instruments). A 2015 study found, however, that these devices (including LCUs) do not interfere with pacemakers or implantable cardioverter defibrillators, and there is no clinical impact on the safety of patients who have these devices.30

Infection Control. Just as with any other instrumentation that comes into contact with bodily fluids, parts of the LCUs must be disinfected to control for infection and cross-contamination. The ADA recommends that dentists follow the 2003 Centers for Disease Control and Prevention (CDC) Guidelines for Infection Control in Dental Health-Care Settings 31 and the 2016 CDC Summary of Infection Prevention Practices in Dental Settings: Basic Expectations for Safe Care.32

LED lights with autoclavable light guides are the gold standard in terms of infection control but can be easily damaged or contaminated with an accumulation of tip surface scale.22 As mentioned earlier, this can be managed with proper upkeep and polishing. Many modern LED curing units do not use light guides, but instead have the LED chips mounted directly in the light-emitting tip of the curing unit, making autoclaving of the unit unfeasible. The use of barriers that cover either the tip or the entire curing light is another way to help prevent contamination. However, the use of infection control barriers reportedly may reduce irradiance values delivered from the curing unit by as much 40%.23 The light output of the curing unit, both with and without infection control barriers, can be compared using a dental radiometer, and the curing time can be appropriately adjusted based on the percentage decrease in output. Some commercial disinfectant sprays can cause damage to the equipment, which can be mitigated by using only recommended surface disinfectants for the recommended time. ANSI/ADA and ISO standards require curing unit manufacturers to provide appropriate cleaning and disinfection methods in the instructions for use, which should be followed between each patient.3

Reporting Adverse Events. If an adverse event should occur (e.g., thermal  injury) when using a dental curing light, consider reporting it to the FDA through MedWatch, the FDA’s gateway for clinically important safety information, safety alerts and product recalls.33 An ADA Professional Product Review (PPR) article on “The FDA, Medical Recalls and Reporting Adverse Events” provides a summary of the FDA’s MedWatch Program, including “What to Report to FDA MedWatch”, “Voluntary Medical Device Reporting”, and “Who Recalls Medical Devices?”34

Purchasing an LCU

There are a number of considerations to be taken into account when purchasing an LCU (Table 2).

 

Table 2. Considerations When Buying a Light Curing Unit (LCU).

Characteristic

Considerations

Battery life

The current state of the art battery type is lithium-ion. Nickel cadmium (NiCad) batteries do not provide long-lasting charges and are thus avoided by many dentists. Clinicians can think about the longest exposure duration they perform and how many times that exposure is delivered to see if a given battery will last for a given procedure or set of procedures. The amount of curing time that each full charge offers can vary from about 26 minutes to 164 minutes in battery-operated LCUs.27

Beam divergence and footprint of light

If a light is shined on a piece of paper, the uniformity of light can be examined. Some areas may appear brighter than others. Beam spread can also be qualitatively measured by slowly moving the light tip away from the piece of paper and noticing how quickly the size of the circle increases.

Effective light range

The term “blue LED” is not necessarily consistent between lights and does not mean that it will cure all resins. Lights emitting between 455-481 nm are most effective as they span the peak absorption range for CQ

Energy needed for polymerization

Consider the amount of energy needed to polymerize the bottom-most layer of the restoration when selecting from lights with different outputs.

Heat Dispersion

 

LED chips can be driven past their capacity and potentially overheat, and light output of the LCU can be greatly reduced if excess heat is not removed. Metal heat sinks are designed to absorb excess heat generated at the chip. If the LCU feels very light without the battery inside, then there may not be a heat sink in it. Some LED lights have built-in thermostats designed to automatically shut down after reaching a threshold temperature.

Infection control method

The gold standard for infection control is removable light tips that can be autoclaved. Some disinfectants can negatively impact the LCU by harming the light-transmitting ability of glass-fibered light guides or by degrading plastic cases, lenses, light guides, and electronics.

Intraoral ergonomics

Clinicians can check whether the LCU light tip can reach difficult locations in the mouth, especially in children who may not sit still or in the elderly who may have limited range of motion in the jaw.

Intrapulpal temperature

Clinicians can test how much heat is produced by shining a light on the underside of the wrist. If shining the light begins to cause discomfort before the necessary amount of cure time has elapsed, the clinician may want to reconsider using that LCU for that amount of time.

Multiple wavelengths

Poly-wave LED lights emit light at multiple wavelengths, which is useful for curing composites with more than one photoinitiator. It is also worth noting that the different beams in poly-wave LCUs do not mix well, so on a given surface, it is quite possible that one area is receiving light at one wavelength while another area is receiving light at a different wavelength. The clinician may thus need to move the curing light across the surface to help ensure that the composite is receiving light at all of the necessary wavelengths.

Turbo tip and focal effect

Turbo tips focus the power over a smaller area, resulting in an increased irradiance. This smaller area means, however, that repeated, overlapping exposures will be needed to cure across the surface of the restoration. Turbo tips also have a focal effect, where the focal point is the distance away from the site where measured irradiance is greatest. If the tip is held farther away than this, it will deliver less irradiance than a standard tip.

Unit integrity

 

Squeeze the handle of an LED light before buying—if there are cracks or openings between the sections, fluids or disinfectants may be able to enter through those openings. Activation buttons that are blister covered will be less likely to allow these fluids to interfere and cause damage to electronic components.

Use of a hand-held radiometer

Bring a hand-held radiometer to trade shows to compare the light intensities of different LCUs. Radiometers are not always accurate to match the manufacturer’s stated output, but they are consistent enough to compare one light to another.

 

Standard

The ANSI/ADA (American National Standard Institute/American Dental Association) standard for LED curing lights can be purchased through the ADA website.15

References
  1. American Dental Association. Spectral Curing Lights and Evolving Product Technology: An Expert's Buying Guide for Curing Lights. ADA Professional Product Review 2009;4(4):2-16.
  2. Jandt KD, Mills RW. A brief history of LED photopolymerization. Dent Mater 2013;29(6):605-17.
  3. Price RBT. Light Curing in Dentistry. Dent Clin North Am 2017;61(4):751-78.
  4. Stansbury JW. Curing dental resins and composites by photopolymerization. J Esthet Dent 2000;12(6):300-8.
  5. Fouassier J-P. Photoinitiation, photopolymerization, and photocuring: Fundamentals and applications. Munich: Hanser Publishers; 1995.
  6. American Dental Association. An ADA laboratory evaluation of light-emitting diode curing lights. ADA Professional Product Review 2014;9(4).
  7. Rueggeberg FA. State-of-the-art: dental photocuring--a review. Dent Mater 2011;27(1):39-52.
  8. Main C, Cummings A, Moseley H, Stephen KW, Gillespie FC. An assessment of new dental ultraviolet sources and u.v.-polymerized fissure sealants. J Oral Rehabil 1983;10(3):215-27.
  9. Eriksen P, Moscato PM, Franks JK, Sliney DH. Optical hazard evaluation of dental curing lights. Community Dent Oral Epidemiol 1987;15(4):197-201.
  10. Price RB, Strassler HE, Price HL, Seth S, Lee CJ. The effectiveness of using a patient simulator to teach light-curing skills. J Am Dent Assoc 2014;145(1):32-43.
  11. American Dental Association. Effective use of dental curing lights: A guide for the dental practitioner. ADA Professional Product Review 2013;8(2):2-12.
  12. Kirkpatrick SJ. A primer on radiometry. Dent Mater 2005;21(1):21-6.
  13. U.S. Food and Drug Administration How to study and market your device. "https://www.fda.gov/medical-devices/device-advice-comprehensive-regulatory-assistance/how-study-and-market-your-device". Accessed September 2 2020.
  14. U.S. Food & Drug Administration. Guidance document: Dental curing lights-Premarket Notification[510(k)]. "https://www.fda.gov/regulatory-information/search-fda-guidance-documents/dental-curing-lights-premarket-notification-510k". Accessed November2, 2020.
  15. ANSI/ADA. Curing lights (Powered polymerization activators). ANSI/ADA Standard No. 48:2020.
  16. U.S. Food & Drug Administration. 21 CFR 807.97 Misbranding by reference to premarket notification. "https://www.govinfo.gov/app/details/CFR-2012-title21-vol8/CFR-2012-title21-vol8-sec807-97". Accessed November 2, 2020.
  17. Adjodah M, Dental Devices Branch of the CDRH, FDA., Personal Communication.  Aug 17, 2015.
  18. International Commission on Non-Ionizing Radiation Protection (ICNIRP). Guidelines on Limits of Exposure to Incoherent Visible and Infrared Radiation. Health Physics 2013;105(1):74-96.
  19. Lukic H, Megremis SJ. "Blue light" photochemical retinal hazard from dental curing units. The American Association for Dental Research. Ft. Lauderdale, Fla.: American Dental Association; 2018.
  20. Bruzell Roll EM, Jacobsen N, Hensten-Pettersen A. Health hazards associated with curing light in the dental clinic. Clin Oral Investig 2004;8(3):113-7.
  21. Ong VK, Megremis SJ, Shepelak H. Ability of protective filtering devices and shields to block transmission of "blue" light from curing units. Journal of Dental Research 2016;95(Special Issue A).
  22. Shortall AC, Price RB, MacKenzie L, Burke FJ. Guidelines for the selection, use, and maintenance of LED light-curing units - Part II. Br Dent J 2016;221(9):551-54.
  23. Price RB, Ferracane JL, Hickel R, Sullivan B. The light-curing unit: An essential piece of dental equipment. Int Dent J 2020.
  24. Bruzell EM, Johnsen B, Aalerud TN, Christensen T. Evaluation of eye protection filters for use with dental curing and bleaching lamps. J Occup Environ Hyg 2007;4(6):432-9.
  25. International Organization for Standardization. Eyewear for protection against intense light sources used on humans and animals for cosmetic and medical applications--Part I: Specifications for products.ISO 12609-1:2013.
  26. Mouhat M, Mercer J, Stangvaltaite L, Ortengren U. Light-curing units used in dentistry: factors associated with heat development-potential risk for patients. Clin Oral Investig 2017;21(5):1687-96.
  27. Megremis SJ, Ong V, Lukic H, Shepelak H. An ada laboratory evaluation of light-emitting diode curing units. J Am Dent Assoc 2014;145(11):1164-6.
  28. Malhotra N, Mala K. Light-curing considerations for resin-based composite materials: a review. Part II. Compend Contin Educ Dent 2010;31(8):584-8, 90-1; quiz 92, 603.
  29. Bagis B, Bagis Y, Ertas E, Ustaomer S. Comparison of the heat generation of light curing units. J Contemp Dent Pract 2008;9(2):65-72.
  30. Elayi CS, Lusher S, Meeks Nyquist JL, et al. Interference between dental electrical devices and pacemakers or defibrillators: results from a prospective clinical study. J Am Dent Assoc 2015;146(2):121-8.
  31. Centers for Disease and Prevention. Guidelines for infection control in dental healthcare settings--2003 (December 19, 2003/Vol. 52/No. RR-17). U.S. Department of Health and Human Services 2003. "http://www.cdc.gov/mmwr/PDF/rr/rr5217.pdf". Accessed August 31, 2020.
  32. Centers for Disease Control and Prevention. Summary of infection prevention practices in dental settings: Basic expectations for safe care. Atlanta: Centers for Disease Control and Prevention,, 2016. "https://www.cdc.gov/oralhealth/infectioncontrol/pdf/safe-care2.pdf". Accessed August 31, 2020.
  33. U.S. Food and Drug Administration. MedWatch: The FDA Safety Information and Adverse Event Reporting Program. "https://www.fda.gov/safety/medwatch-fda-safety-information-and-adverse-event-reporting-program". Accessed September 2, 2020.
  34. American Dental Association. The FDA, medical recalls and reporting adverse events. ADA Professional Product Review 2014;9(4):6-8.

Last Updated: May 5, 2021

Prepared by:

Department of Scientific Information, Evidence Synthesis & Translation Research, ADA Science & Research Institute, LLC.


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