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Following some basic concepts will improve the durability of restorations
The clinical success of composite resin restorations bonded with light-cured adhesive depends on attention to detail in each of the steps required to diagnose, prepare, and restore. While much attention has been given to the details of diagnosis, preparation, and the development of improved adhesives and resins, light-curing is often taken for granted. There are important factors that need to be taken into account when light-curing resin adhesives, resin-based composites, resin cements, and other light-activated restorative materials to ensure the quality and durability of the restorations we are placing.
Practitioners have choices in the light-curing devices they use. Not all light-curing devices are equivalent in variables including—among others—their features, power density and energy delivered to the tooth and light-activated restorative being placed, timing for use, availability of accessories, configuration of curing probes/tips available for a device, and the energy source used to power the curing device. Also, recent research demonstrates that the orientation and diameter of the light probe tip can have a significant impact on the degree of light-curing and a resin’s physical properties and adhesion to tooth structure.1-9 While practitioners are looking for time savings when light-curing, shorter-timed increments for light-curing when placing restorations may not be the best choice.
The adhesives and composites used in restorative dentistry characteristically have photosensitive chemistries in the 460-nm to 480-nm wavelength range using camphorquinone (CQ)—as is typical—for polymerization of composites.10 Light-curing devices need to have a wavelength in this same range. Today, the two main categories of light-curing devices use either broader-light-spectrum, quartz-tungsten-halogen bulbs (QTH) with photo-spectrum emissions in the range of 400 nm to 500 nm, or light-emitting diodes (LED) that provide light in the blue-visible spectrum with a range of 450 nm to 490 nm.11,12 There have been significant improvements in the devices used for light-curing in recent years. QTH devices saw improvements that increased the energy delivered to the restorative material. Most QTH-curing lights delivered at least 600 mW/cm2 and in some cases, using specialized turbo tips, more than 1,300 mW/cm2.13 The latest generation of LED-curing devices provide consistent energy outputs greater than 1,000 mW/cm2, with some of the curing-light devices adding supplemental LEDs, or polyphase curing lights, which expand the range of photo emissions for light-curing.10,12
Clinical Implications of the Physics of Light-Curing
Current in-office curing-light measurement systems, called radiometers, measure the light energy at the tip when the light guide is in immediate contact with the sensor. Using these devices and keeping a monthly log of the values provides a baseline for changes in the curing light. More important though is the understanding of the changes in the light energy as it travels distances from the tip. Adhesives and composites polymerize based on energy delivered to the restoration and not the irradiance (light output) measured at the light tip. For all light-curing units, there is an energy drop-off that decreases the amount of energy delivered to the restoration over distance. Understanding that irradiance multiplied by the duration of light-curing equals total energy in joules/cm2 that a composite would need for curing provides information on additionally needed light-curing energy (increased times for curing). Depending on the shade and brand of composite resin, the minimum energy requirements to photopolymerize resins has been reported to range from 6 J/cm2 to 24 J/cm2 for a 2-mm increment of composite resins.14-17 Considerations for light-curing composites must include knowing the disaggregated irradiance values—the light-spectrum values of the curing light, as well as how the distance, angulation, diameter, and use of barriers on the light guide tip impact polymerization of the restorative. Most adhesives and composites are cured in the spectrum of 450 nm to 480 nm, but some have photoinitiators that are sensitive below 420 nm, so it is advisable to check with the manufacturer about the photoinitiator(s) being used.18 It is important to know the disaggregated irradiance values—the specific wavelengths in the ranges of 380 nm to 540 nm for the curing-light device being used.19 The photosensitivity for light-curing varies between different types and brands of composite resins. Also, for very light shades (bleaching shades), very dark shades of composite resin, flowable composite resin, and microfill composite resins, increased curing times may be necessary.5,20-22 It is only recently that some manufacturers are providing clinicians with guidelines for duration of light-curing based on the total energy needed to polymerize a specific type and shade of composite resin using a curing light with an energy density output of 600 mW/cm2.
Improving Clinical Results
Clinicians may not be aware of how much they depend on light-curing. Proper adhesion to tooth structure and the optimization of the physical properties of composite resins to resist the forces of occlusion and mastication that can cause fracture depend on proper light-curing techniques, as does the ability of teeth to be polished and maintain their luster and color, as well as to resist microleakage, sensitivity, recurrent caries, and wear in function.1-9 By following specific guidelines, a practitioner can ensure maximum polymerization of the light-cured restorative materials that are being placed.20
Light Guide Placement
In recent years, some manufacturers’ claims have included 5-second light-curing and curing composite resins to depths greater than 5 mm. In truth, light-curing times for areas of cavity preparations greater than 4 mm to 6 mm from the light tip require additional light-curing.23-25
Light-curing may in fact be to blame for premature failure of Class II composite resins at the gingival margin of the proximal box. The gingival margin area is the high-risk area for recurrent caries. This is the site where the caries from Class II composite resins first initiate. Clinical evidence has demonstrated that Class II composite resins have significantly higher rates of caries at the gingival margin when compared to amalgam restorations.26-28 Xu and coworkers evaluated adhesion of composite resin as the distance from the light guide increased. Their investigation was prompted by the number of studies demonstrating poor marginal seal and increased microleakage at the gingival margin of these restorations when compared to the occlusal enamel margins. They concluded that, when curing adhesives in deep proximal boxes with a curing light of 600 mW/cm,2 the curing time should be increased to 40 to 60 seconds to ensure optimal polymerization.25 Others have also made similar recommendations for an increase in curing time, even with curing lights with greater than 1,000 mW/cm2 for initial increments of composite resin placement in proximal boxes.25,29
Light-curing Class II composite resins to the gingival margin of the proximal box can be challenging. Reasons cited for significant differences in caries rates at the gingival margins of Class II composite resins include: technique sensitivity of some dentin bonding systems; polymerization shrinkage of composite resin; difficulties encountered using techniques to place highly viscous composite resin into proximal boxes without trapping air bubbles, leading to poor marginal adaptation; contamination of the tooth surfaces due to poor isolation of the field; poor polymerization of the resin adhesive and composite due to inadequate output of a curing light;30,31 and distance of the light guide from the gingival margin.32-34
Light-guide positioning can have a significant impact on the energy delivered to a resin restoration.35-37 While many preparations provide for excellent clinical access for curing lights, some of the areas of the oral cavity are hard to reach. In some cases, the curing-light tip itself places limits on how close the light can get to the surfaces being light-cured and whether or not its orientation is correct. In fact, dentists and dental assistants—many of whom hold and activate the light—may not be well trained in the art and science of light-curing. To illustrate this point, it should be noted that articles on clinical techniques that require light-curing commonly mention only “light-cure for x number of seconds;” the orientation of the light tip, diameter of the light tip, and type of light being used relative to energy output are rarely noted.
The Managing Accurate Resin Curing-Patient Simulator (MARC-PS™ [BlueLight Analytics Inc., www.curingresin.com]) developed by Dr. Richard Price at Dalhousie University in Halifax, Canada, demonstrates how to facilitate optimal light-curing of restorations.35 MARC is a laboratory-grade, clinically relevant, light-curing energy measurement tool with sensors to measure the light energy delivered to simulated restorations in a typodont head and jaws. The irradiance delivered to these simulated restorations is collected and displayed in real-time by a chairside computer. MARC can be used to train dental students, dentists, dental hygienists, and dental assistants to optimize light-curing in the anterior and posterior regions of the mouth. Operators using this device receive immediate feedback on how to improve their light-curing skills.
Based on the evidence, the following recommendations and insights can be used to maximize the light energy delivered:
1. The operator should use “blue blocking” glasses or shields (orange colored).
2. The operator should inspect the light guide tip for any contaminants or damage to the surface.
3. Note that surface barriers can decrease energy delivered.
4. The patient should be positioned for access to light-curing and to see the light tip.
5. The light should be stabilized when curing.
6. The position of the light guide should be positioned to achieve proximity to the surface of the tooth being restored.
10. Curing time should be increased for preparations that are greater than 2 mm to 3 mm in depth (especially the proximal box of Class II preparations).
11. Between each light-curing cycle, air-cool the tooth and restoration or wait several seconds.19,35,37-39
The diameter of the light tip can have implications for the area being light-cured. While an 8-mm-wide diameter tip can meet the requirements of most restorations, there are times a wider-diameter, curing-light tip is necessary. For example, a wider-diameter tip should be considered when placing sealants or composites on the occlusal surfaces of permanent molars or when light-curing the complete facial surface of a maxillary anterior tooth during placement of porcelain veneers or direct-composite resin veneers. In those situations, a smaller-diameter tip would require an overlapping of the tip and multiple curing areas to ensure complete polymerization of the restoration.
Practitioners should be aware that adhesives and composite resins that are not cured completely can lead to problems such as lower bond strengths and increased potential for microleakage, color changes within the composite resin, surface staining, wear, and recurrent caries. The benefits of light-curing can be optimized by following guidelines to ensure maximum photopolymerization of the restorations being placed.
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About the Author
Howard E Strassler, DMD
Department of Endodontics, Prosthodontics and Operative Dentistry
University of Maryland Dental School