Avoiding Pitfalls When Using a Light-Curing Unit

The dental profession must consider a number of issues when it comes to selecting and using LCUs:

• Broad-spectrum quartz-tungsten halogen (QTH) lights are being replaced by narrow-spectrum light-emitting diode (LED) units.
• Some resin manufacturers are using photoinitiators that require light to be in the 410 nm range for optimum polymerization, yet many LED units do not deliver any light below 420 nm.
• LCUs are becoming increasingly more powerful, with curing times being reduced to as little as 1 second, although there are no good clinical trials that clearly show the effectiveness of fast-curing protocols, or one LCU over another LCU.
• Due to concerns over blue light damaging their eyes, operators often avoid watching what they are doing when light-curing.
• An influx of inexpensive, poorly designed LCUs has occurred.

Safe and successful light-curing depends on a number of factors, as outlined in the following paragraphs.

Performance—Surveys published worldwide on LCUs used in dental offices have shown that many deliver less than 300 mW/cm^2, which is insufficient for satisfactory light-curing. This is likely due to inadequate maintenance. Additionally, the light tips of many LCUs were either damaged or covered in resin, which will also reduce the light output. Clinicians should regularly check their LCU for adequate output and inspect the light tip for damage or debris before each use.

Spectral Emission—If the LCU is a QTH unit, the range of wavelengths is sufficiently broad to adequately polymerize any dental RBC material. However, most LED units produce a very narrow spectral emission⁵ and are usually optimized to cure the commonly used camphorquinone photoinitiator that is most reactive to light at ~468 nm. Since some RBCs use alternative photoinitiators that require very different wavelengths (~410 nm), it is possible to use an LED unit that is not ideally matched to the RBC. Recently, broadband LED units have been introduced that use two or more different colors of LED, meaning that their spectral output includes both blue (~460 nm) and violet wavelengths (~410 nm) of light. These lights can polymerize RBCs containing both conventional and alternative photoinitiators. However, in some polywave LED units the spectral emission is not uniformly distributed across the light tip.⁵ Thus, some areas of the resin may not receive the required wavelengths.

Irradiance Value—The irradiance from an LCU, also referred to as power density, is usually expressed as one number in units of mW/cm². However, the light output from dental LCUs is not uniformly distributed over the end of the light tip.⁵ Conventional methods of measuring the light output, such as a dental radiometer, a thermopile, or an integrating sphere, do not detect how uniform the light beam is, and, in fact, may be misleading since a single irradiance value does not show if there are “hot spots” within the light beam. This problem is greater with LED units compared to QTH units. Manufacturers should provide the beam profile from their LCU over clinically relevant distances (0 mm to 8 mm), as some are now doing.⁶

Irradiance Over Distance—In some LCUs, the irradiance may be high close to the tip, but then it declines rapidly as the distance from the tip increases. Clinicians need to look for data that reports the output or performance of the LCU not only at 0 mm distance from the tip, but also at clinically relevant distances. Manufacturers and researchers are now starting to provide this information.^6,7

Clinicians must use the correct amount of curing time and should follow the manufacturer’s instructions for each brand and shade of RBC, because not all shades and types of RBCs require the same amount of exposure time. There can be an eight-fold difference in the curing time recommended by the manufacturer to effectively light-cure different shades and types of their own RBCs with their own light.⁸

Clinicians must avoid overheating the tooth and should not arbitrarily increase the curing time beyond the manufacturer’s recommendations; the extra radiant energy delivered to the tooth can cause an excessive increase in intrapulpal temperature or burn the oral tissues. Patients, who are often anaesthetized, cannot be relied upon to indicate if their tissues are getting too hot.

Contrary to popular belief, doubling the irradiance does not necessarily halve curing time. Although the same amount of energy is delivered to the RBC, the same degree of resin curing may not be achieved because the relationship between irradiance and the degree of resin curing is only linear within a certain range.

Despite the preceding concerns over temperature increase, it is important to consider the restoration characteristics and location when determining the exposure time.⁹ Ideally, the light guide should have direct line of sight access to the resin, with the tip as close as possible and at a 90-degree angle to the RBC surface. In a Class II RBC restoration, the apical portion of the proximal box is often in the shadow of the matrix band or remaining tooth structure and furthest from the LCU. Consequently, the resin in this area will receive less light than at the occlusal surface and may be undercured. To overcome this problem, it is recommended to increase the exposure time when curing the initial layers of RBC at the apical portion of the proximal box.¹⁰ Of note, it is the margin in this area where secondary caries is most often found.

When performing light-curing of RBC restorations, clinicians often place the tip of the curing light in proximity to the restoration, turn on the unit, and then look away to protect their eyes from the damaging effects of the blue light.¹¹ As a consequence of looking away and other distractions, recent research has shown that many light-cured restorations placed in teeth could be receiving an inadequate amount of light energy and the resins may never reach their manufacturers’ intended properties.⁹

A light-curing simulator, MARC^® (Managing Accurate Resin Curing) (BlueLight Analytics, www.curingresin.com), was developed to teach proper light-curing technique. Prior to receiving training on MARC, using the same LCU for the same exposure time, there was a large variation in energy delivered among operators: 27% delivered less energy than was necessary to adequately cure a Class I preparation, and 82% delivered less energy than was necessary to adequately cure a posterior Class V preparation.⁹ This was far less energy than the clinicians assumed. Further research has shown that after training using MARC, including using appropriate eye protection, the amount of energy delivered to the simulated restoration was 63% greater (p < 0.01).¹²

Protecting eyes from the blue light hazard is critical as the light from LCUs can be very dangerous to eyes.¹¹ The most damaging wavelength for the retina is light near 440 nm, which is within the spectral emission from dental LCUs. High levels of blue light cause immediate and irreversible retinal burning, and chronic exposure to low levels of blue light causes premature retinal aging and degeneration. To minimize ocular health risks, the operator should wear appropriate “blue-blocker” protective glasses.

In order to deliver the correct wavelengths, irradiance, and exposure duration to the RBC, the clinician needs to know the uniformity of the spectral emission and the irradiance both across the tip and at clinically relevant distances from the LCU, together with the energy requirement of the RBC. Clinicians should know how to most effectively use the LCU, minimize the amount of heat generated, and best protect their eyes, the patient’s eyes, as well as the patient’s oral tissues.

Richard B. Price, BDS, DDS, MS, PhD
Department of Dental Clinical Sciences
Faculty of Dentistry, Dalhousie University
Halifax, Nova Scotia, Canada

Dr. Price is the inventor of MARC^®, a device that has been assigned to Dalhousie University and subsequently licensed to BlueLight Analytics Inc., a company in which he owns shares.