Dental Curing Lights

Photoactivated dental materials, including certain sealants, resin-based cements and composite restorative materials, are an integral part of general dental practice. Dental light curing units (LCUs) are handheld light-emitting devices used to cure such photoactivated, polymer-based restorative materials (PBRMs).¹

Dental professionals spend considerable time performing tasks that involve using PBRMs, and the convenience of being able to rapidly light cure these dental materials has transformed dentistry over time. In the contemporary marketplace, there are a wide variety of dental LCUs, and the technology has developed continually since photocuring was first used in dentistry.^{2, 3}

Photopolymerization is a light-activated reaction that uses visible light energy to activate a photoinitiator system, which absorbs light photons and produces reactive species (free radicals) that initiate the polymerization process.^4-7 In dentistry, resin-based composite materials are commonly comprised of a polymer resin matrix (typically containing dimethacrylate monomers, photoinitiators, accelerators and other compounds) and inorganic filler particles (e.g., silica, alkaline glass).⁸ As long as the wavelength of the light matches the absorption range of the photoinitiator (in the presence of activators) with sufficient energy, a variety of light sources may be used for photopolymerization in dentistry, as discussed below. One of the most commonly used photoinitiators in dental resins is camphorquinone (CQ).⁴ The peak absorption range for CQ is from 455 to 481 nm, with peak absorption at approximately 469 nm.^{9, 10}

The first light-cured resins used in dentistry date to the early 1970s and were cured using ultraviolet (UV) LCUs.¹¹ 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, 11} However, shortly after the introduction of UV-curing, dental materials were reformulated to include visible light wavelength photoinitiators, such as CQ.^{4, 11} 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.^{11, 12} 

In contrast to UV LCUs, QTH curing units emit blue light as part of their spectral output, require shorter curing times and are associated with lower risk of cataracts. However, the blue wavelengths emitted by QTH LCUs are not without their own risks, such as the risk of direct retinal damage.¹¹ In the mid-1980s (when QTH LCUs were commonly used), researchers advised clinicians to wear blue blockers for ocular protection,^{11, 13, 14} and in 1986, the ADA issued a recommendation to wear appropriate protective filtering eyeglasses when using this type of LCU.¹⁵ Recommendations for ocular protection extend to modern-day use of light-emitting diode curing lights, which also emit blue light, and several groups have called for use of orange (i.e., blue-light blocking) glasses or shields to be worn during all light-curing procedures (see “Blue Light Hazard” section for more information).^16-18

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 considerable variability in radiant exposure delivered by different operators.³

A preclinical light-curing simulator called MARC (Managing Accurate Resin Curing)¹⁹ 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 that estimated by the clinician.²⁰

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 and emitting from the curing unit tip; radiant exitance is a measure of the power radiated outward from a source of a specific area (e.g., from the curing unit tip).²¹

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 home-use light bulb, 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 also recommended 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 home-use light bulbs 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 period of time, 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, power can be considered as a rate that is multiplied by curing time to yield energy, as described in the next section on radiant exposure.

Another important term for understanding the process of curing polymer-based restorative materials is radiant exposure, which is used to describe the total amount of light energy deposited on the material during curing.²¹ 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 the irradiance value is 1000 mW/cm² and the curing time takes 20 seconds, then 20 joules of energy have been delivered to the area of resin that the curing light is striking. This is because the 1000 mW/cm² can be expressed as 1 W/cm² or 1 J/s/cm², and then multiplying by the 20-second curing time yields 20 J/cm², or 20 J of light energy deposited on the area of resin the curing light is striking.

* “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.

FDA Clearance of Dental Curing Lights.  To market or sell a dental curing light in the United States, the U.S. Food and Drug Administration (FDA) requires a premarket notification (i.e., 510(k)) submission.²² 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.²³ 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.²⁴

The FDA specifically prohibits companies from marketing their devices as having been “cleared by FDA.”²⁵ 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 Office of Product Evaluation and Quality at the Center for Devices and Radiological Health (CDRH) 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;²⁶ if information is missing, poorly written, or if exaggerated claims are made, it may suggest further investigation is warranted.

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.^{27, 28} 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.¹⁰ 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.^27-29

Blue-light filtering eyewear, curing unit tip-mounted shields, and handheld paddles are all options for eye protection while using curing lights.^{18, 30} 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.^{16, 31}

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.³⁰ 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.³²

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.³³ 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 clinicians are concerned that their blue-light filtering device is not effectively blocking blue light, there is a simple, practical experiment that could be done to test the filtering device’s efficacy. In a dark room, take the 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 the protective filtering device just above the material; place the 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.³⁰

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, 34}  

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.⁹ 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.^35-37 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.²⁰ (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 Implantable 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 that there is no clinical impact on the safety of patients who have these devices.³⁸

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 Guidelines for Infection Control in Dental Health-Care Settings³⁹ and the 2016 CDC Summary of Infection Prevention Practices in Dental Settings: Basic Expectations for Safe Care.⁴⁰

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.¹⁶ 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. Using 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 as 40%.³¹ 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 recommend that curing unit manufacturers 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.⁴¹ 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?”⁴²

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

The ANSI/ADA (American National Standard Institute/American Dental Association) standard for LED curing lights can be purchased through the ADA website.²⁴

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16. 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.
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ADA Clinical Evaluators Panel Reports

Dental light-curing units: An American Dental Association Clinical Evaluators Panel survey (July 2020)

ADA Professional Product Reviews (PPRs)

ADA PPR Video: Dental Curing Units: Factors Influencing the Effective Use (2015)

An ADA Laboratory Evaluation of Light Emitting Diode Curing Lights (2014)

Effective Use of Dental Curing Lights: A Guide for the Dental Practitioner (2013)

Spectral Curing Lights and Evolving Product Technology/An Expert’s Buying Guide for Curing Lights (Fall 2009)

LED Curing Lights (Fall 2006)

Journal of the American Dental Association (JADA)

Search JADA for articles related to curing lights