Guidelines for Successful Light-Curing
Following basic light-curing concepts should improve restoration longevity
One survey published in 2011 reported that of 46 dental schools, 63% no longer taught the use of amalgam as the preferred posterior restorative material.1 Further, in the United States, current trends in restorative dentistry show that resin-based composites are rapidly replacing amalgam as the material of choice.2 According to the American Dental Association’s 2005-06 Survey of Dental Services Rendered, more than 146 million resin-based composite restorations and sealants were placed annually in the United States alone.3 The clinical success of these light-cured resin composite restorations requires attention to detail in the diagnostic, tooth preparation, and restorative phases of treatment.
While much attention has been paid to the diagnostic and tooth preparation phases of treatment as well as to the development of improved restorative materials, the final procedural step of light-curing is sometimes taken for granted. This may partially explain why some reports indicate that the average lifespan of posterior resin-based composites is only about 6 years,4 and that secondary caries and fracture of the restoration are the two main reasons for replacement.4,5
Contemporary resin composite materials have reached the point where they can successfully fulfill clinical requirements when they are used correctly. Today, the reason composites fail appears to be because of patient or operator variables rather than a problem with the product.2,6,7 This is supported by data showing that when they are placed under controlled conditions, the life expectancy of resin-based composites can reach at least 12 years.6,8
The Consequences of Inadequate Light-Curing
All dental practitioners face significant challenges when light-curing hard-to-reach restorations. We know that the bond strength between the resin and the tooth structure—as well as the resin’s physical properties, polishability, color stability, and the amount of leachates—are all adversely affected when not enough energy is applied to the resin. We also know that when the resin composite is not adequately cured, it cannot resist the forces of mastication and, as a result, there will be greater wear, more postoperative leaching of chemicals that could cause increased sensitivity, and, very likely, more recurrent caries at the margins.9-11
The gingival margin of the proximal box is where the most recurrent caries occurs.12 The resin at the bottom of the proximal box is particularly difficult to cure because it is the least accessible, it is furthest from the light tip, and it receives the lowest level of irradiance (brightness) from the light.11,13-15 For these reasons, using an inadequately powered curing light or an inadequate light-curing technique may be a significant culprit for the premature failure of Class 2 composite resins at the gingival margin of the proximal box.
Choosing the Right Curing Light
In recent years, there have been significant changes and improvements in dental curing lights. Not all lights are the same, so it is important to choose the right curing light for the task at hand. Today, the two most popular types of dental curing lights use either quartz-tungsten-halogen (QTH) bulbs that deliver a broad spectrum of light between 400 nm and 500 nm, or light-emitting diodes (LED).16,17
There are two types of LED curing lights: those that contain blue-only LEDs and provide blue light within a narrow spectral range of 440 nm to 490 nm, and poly-LED curing lights that deliver a broader spectrum of light in the range of 390 nm to 490 nm.16-18 Most manufacturers have stopped making QTH curing lights, partly because LED lights are more efficient than QTH light sources, but also because the incandescent bulbs for QTH devices will soon become scarce as a result of government legislation19 that will start to phase out incandescent bulbs in 2014.
Resin composite’s instructions for use usually state that the curing light’s minimum irradiance output should be at least 300 mW/cm2 to 400 mW/cm2. Currently, most QTH curing lights deliver at least 600 mW/cm2, and some lights that use specialized turbo tips can deliver more than 1,300 mW/cm2.20 The latest generation of LED curing lights can deliver an irradiance output greater than 2,000 mW/cm2, and some poly-LED curing light devices include supplemental LEDs that emit light at additional wavelengths.16-18 When using one of these high-powered curing lights the operator must be careful not to use the light for longer than necessary because too much energy could be delivered, which could cause thermal damage to the pulp or other oral tissues that are exposed to the light.21
Monitoring the Curing Light
The dental radiometers that are currently used in dental practices measure the energy output from curing lights at the tip of the light guide. Using a radiometer and keeping a weekly log of these values will provide a valuable baseline for detecting any changes in the light’s output. However, it is important to understand that even a small distance between the tip and the resin will have an effect on how much light reaches the resin; the greater the distance, the lesser the amount of light energy that will reach the resin.22-25 You can see this effect readily if you move the tip of the curing light around over the dental radiometer sensor.
Delivering Enough Light Energy to the Restoration
Rarely described are the importance of correctly choosing the light tip diameter, orienting the light tip, or matching the irradiance and spectral output from the curing light to the resin composite being used, yet these factors are crucial to successful light-curing. The diameter and design of the light tip can have major implications on the area being light-cured.26,27 While an 8-mm tip can meet the requirements of most restorations, a wider-diameter tip that covers the entire restoration should be considered when placing sealants or resin composites on the occlusal surfaces of permanent molars, when light-curing the complete facial surface of a maxillary anterior tooth when placing porcelain veneers, or when placing direct composite resin veneers. If a smaller-diameter tip is used, multiple light exposures will be required to ensure complete polymerization of the restoration.
Light-cured adhesives and resin-based composites polymerize based on how much light energy they receive, not on the irradiance (ie, the brightness of the light) measured at the light tip. The energy is reported in Joules/cm2. This value is calculated by multiplying the irradiance (mW/cm2) by the light exposure time (seconds). Depending on the shade and brand of composite resin, the minimum energy requirements to photopolymerize resins range from 6 J/cm2 to 24 J/cm2 for a 2-mm increment of composite.28 Thus, if the irradiance output from the light is 1,000 mW/cm2 and the manufacturer’s instructions say to cure for 20 seconds, this means that, under ideal circumstances, 20 J/cm2 should be delivered and this should adequately cure the resin. However, less energy will be delivered if the light tip is damaged, contaminated with resin, or is some distance from the tooth.
The position and orientation of the light guide can have a significant impact on the amount of energy delivered to a restoration.13,29-31 In hard-to-reach areas, the light tip itself often limits how close the light can get to the resin surfaces being light-cured. In these situations the operator must either use a different curing light with a low-profile head that will allow better access to the restoration, or they must use supplementary buccal and lingual light-curing.
Learning How to Use a Curing Light
Most dentists and dental assistants have never been trained in the art and science of light-curing. Many clinical technique articles typically only mention to “light-cure for xx seconds.” Using an innovative device, the MARC® Patient Simulator (Managing Accurate Resin Curing; BlueLight Analytics, http://www.curingresin.com), practitioners can now better understand how to optimally light-cure restorations.13,32
MARC is a laboratory-grade light-curing energy measurement device with sensors that measure the irradiance and wavelength of light energy received by simulated restorations in a typodont head. The irradiance delivered to these simulated restorations is collected and displayed in real time by a chairside computer. Whether users are dentists, dental hygienists, dental assistants, or dental students, MARC provides operators with immediate feedback on how to optimize their technique. MARC quickly shows users how even small changes in technique can have a significant impact on the ability to deliver sufficient light energy to a resin restoration. The obvious, but often overlooked, importance of safely watching what you are doing when light-curing can be easily demonstrated using MARC.
Clinical Tips for Using a Curing Light
These recommendations can help maximize the amount of light energy delivered to resin restorations33:
Before use, inspect the light guide tip and remove any contaminants on the surface (Figure 1).
Be aware that surface barriers on the light will decrease the irradiance (light output) and you will need to increase the light exposure time to compensate.
Protect your eyes using “blue blocking” glasses or shields (orange-colored) (Figure 2).
The light tip should be at right angles to the tooth surface being restored (Figure 7).
Placing the light source at a less than a perpendicular angle to the resin in the cavity preparation can lead to an incomplete cure (Figure 8).
Stabilize the light when curing; use a finger rest.
Begin curing 1 mm away from tooth; after 1 second bring the tip as close to the tooth as possible.
Increase the light exposure time for preparations more than 2 mm to 3 mm deep (especially the proximal box of Class 2 preparations).
Air-cool the tooth and the restoration during each light-curing cycle.
If these guidelines are followed, the clinician and dental assistant can ensure maximum polymerization of the light-cured restorative materials that are being placed.33
Providing adequate energy from the curing light is a key factor to the success of resin-based composite restorations. In many dental practices, the dental assistant has the primary responsibility to maintain the curing light and to cure the restorations placed by the dentist. Do not take light-curing for granted—pay attention to this vital step. Following the guidelines presented in this article will ensure safe and optimum photopolymerization of the restorations being placed in your practice. Also, by taking routine precautions such as the use of orange-colored eye protection and blue-light–blocking shields on the curing light tip, the chairside assistant and the clinician can protect their vision.
About the Author
Richard B. Price, BDS, DDS, MS, PhD
Department of Dental Clinical Sciences
Halifax, Nova Scotia, Canada
Donna Dickie, CDA
Halifax, Nova Scotia, Canada
Howard E. Strassler, DMD
Professor, Director Operative Dentistry
Department of Endodontics, Prosthodontics, and Operative Dentistry
University of Maryland School of Dentistry
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