‘Let there be Light,’ and there was Light, but was it Enough? A Review of Modern Dental Light Curing

Light factors

The first law of photochemistry states that light must be absorbed by the photoinitiator system for a photochemical reaction to take place, ultimately resulting in polymer chain crosslinking that provides a sufficiently hard and effective material. It follows that there must be sufficient light (photon energy) reaching the spatial extremities of the resin composite, both in terms of width and depth of the restoration. For optimal material performance, and to avoid premature restoration failure, the operator must carefully consider the parameters of light delivery without complacency, which, quite worryingly, is not often the case.^3,4,5

Let us first consider light ‘transport’ through a dental resin composite material and how light energy is lost during the curing process. A schematic representation of light irradiance variation through a highly filled resin composite material is shown in Figure 1.

Optical properties of the material

Irradiance exiting the light-curing device and incident on the material surface, I₀ (the power per unit area, mW/cm²) will be significantly reduced compared with the irradiance reaching the lower surface (I₁). This is governed by several factors. According to Beer-Lamberts law, molar absorptivity (e), or how efficiently molecules absorb light, and concentration (C) of the absorbing molecule, will change with depth, z. I₀ is also reduced by porosities, filler particle scattering and light refraction at the filler–resin interface. In fact, highly filled resin composites can be made translucent if filler particles are small enough (nanometre range) to prevent scattering. However, such small fillers tend to agglomerate and filler size distributions containing larger fillers are required to fulfil mechanical and cosmetic requirements of dental resin composite materials. Optical property design of both the resin and filler is also critical for optimization of light transmittance. Conventional resin composites are usually manufactured to allow an increase in transmittance as the material cures, which is achieved by consideration of the refractive index mismatch between the glass filler and the resin matrix as it polymerizes and crosslink density increases.^6,7

Resin composite pigmentation is a prerequisite for matching the optical properties of adjacent tooth tissue. Unfortunately, iron oxide pigments (particularly red and yellow), titanium dioxide opacifiers and other colourants used for optical effects, such as fluorescence and opalescence, will compete with the photosensitizer for curing light absorption and significantly lower transmittance, severely limiting depth of cure.⁷ Consequently, for BF materials to achieve a ~4–5 mm curing depth, their translucency must be higher compared with that of conventional resin composites. Several current brand manufacturers of BFs also recommend a more opaque veneering layer to provide appropriate cosmetic tooth mimicry. Although shade choice of BF materials is limited (four or five shades, compared with >15 for conventional resin composites), development of novel photoinitiator system chemistry, such as Ivocerin (Ivoclar Vivadent, Schaan, Liechtenstein) and optimization of glass filler refractive index (Filtek One, 3M, St Paul, MN, USA) purportedly allows one increment placement without significant compromise of tooth colour and opacity, while maintaining sufficient filler load for adequate mechanical and wear properties.

Effect of photon energy

If the ‘dose’ of photon energy (the product of irradiance and exposure time, or radiant exposure (RE), measured in J/s) is considered, then time is the most important factor for ensuring optimally cured materials. Within limitations, reduced output of the light unit and risk of undercuring may be overcome by extending exposure time. Figure 2 shows the ability to effectively cure a conventional resin composite (with a manufacturer's recommended incremental curing thickness of <2 mm) to depths akin to that of modern BF standards (<5 mm) if extended curing times are used.⁷ It should be noted that this experiment served only to demonstrate exposure time as the most influential curing parameter. Such lengthy exposures would not be feasible for clinical practice and, more importantly, would risk overheating.

Operating position of the light source tip

Due to light dispersion, irradiance is affected by the distance of the light source from the material being cured. For this reason, the light source exit portal should be placed as close as possible to the surface of the material being cured (without touching the surface to avoid uncured material adhering to the curing tip or changing its surface morphology), because light intensity significantly decreases with distance and is dependent on the LCU type and its optics (Figure 4).

In clinical terms, the closer the light guide exit window is placed to the surface of the material to be cured, so the irradiance and potential to maximize DC is increased. However, despite the use of energy-efficient LEDs and copper heatsinks in some products, significant thermal energy is emitted from modern LCUs. This heat may be transmitted to the tooth, which could cause thermal trauma to the dental pulp, resulting in irreversible pulpal inflammation.¹³ The operator should also be careful that soft tissues are not inadvertently burned when working with an LCU, especially on a patient who is receiving treatment under local anaesthetic. This can be prevented by the use of rubber dam, which, in any case, is considered essential when manipulating moisture-sensitive resin-based composite and adhesive materials.¹⁴

The light guide exit window should also be positioned parallel to the surface of the material being cured, so that irradiation is even (Figure 5). In certain areas of the mouth, it might be challenging to place the light guide optimally and a direct line of sight to the surface cannot be achieved. This may also be the case in those cavities with undercuts or the presence of existing restorative materials.⁸ The clinician should, therefore, be mindful of these factors and ensure that sufficient light energy is provided to the material being cured.

The importance of irradiance and curing time

An arbitrary value of LCU irradiance required to cure a 1.5–2-mm increment of resin-based composite is often stated as ≥1000 mW/cm².⁸ It has been reported that many curing lights emit less than 300–400 mW/cm²,⁴ which may result in inadequate curing of the material. It should also be noted that irradiance can vary greatly across the light guide exit portal with a higher intensity ‘hot spot’ often present in the central part of the beam.⁸

It is perhaps a misconception among some clinicians and manufacturers that the higher the light output the better. However, too much energy, too quickly may affect the formation of the material polymer network, whereby polymer chains form more quickly, resulting in shorter chain lengths decreasing the mechanical properties of the RBC.^15,16,17 Furthermore, there is less ability for the material to relieve stress by flow during the pre-gelation phase and the increased polymerization stress may compromise the tooth-restoration interface.¹⁸ Modulated irradiance modes such as ‘soft start’, ‘ramp cure’ and ‘pulse-delay’, which involve delivery of an initially lower-power light have been previously suggested in an attempt to delay the onset of gelation and reduce polymerization stress by increasing pre-gel flow. The clinical significance of this approach is now considered questionable.^9,19 Additionally, there is a risk of undercured restorations if increased irradiance or exposure time is not used at the end of the curing regimen, and these modulated curing modes have fallen out of favour.

The concept of high irradiance and short-exposure curing has been debated over several decades, with only limited clinical success and uptake. For example, the development of plasma-arc devices with an irradiance value of ~2000 mW/cm² that advocated 3-s cure times were refuted owing to suboptimal polymerization and the requirement for multiple 3-s exposures.^20,21 Still, the development of modern LED light-curing units often involves the use of very high-power diodes with some manufacturers boasting irradiance values of 7000 mW/cm².²² Similar problems with inadequate polymerization may exist unless the use of high irradiance is coupled with an appropriate adjustment of the chemistry of the material. An alternative approach involves the incorporation of reagents within the resin that modify the highly crosslinked polymer network as it is formed.^23,24,25 ‘Chain transfer’ reagents allow improved network homogeneity. This leads to reduced polymerization stress and enhanced mechanical properties that might not otherwise be achieved with conventional high irradiance and material combinations that do not obey an exposure reciprocity principle. In other words, similar light energy dose achieves equivalent material properties regardless of the ratio of irradiance and exposure time. Examples of modern commercial materials that include these chemistries are Filtek One Bulk Fill (3M) and Tetric PowerFill (Ivoclar Vivadent). The latter material is designed to be used in combination with a high irradiance (~3000 mW/cm²), multispectral (412 and 455 nm) light-curing unit (Bluephase PowerCure, Ivoclar Vivadent) for 3-s cure times. Recent reports suggest that effective polymerization can be achieved at short exposure times under ideal curing conditions, ie minimal light-curing tip to material distance without angulation.^26,27

Compatibility of emission spectrum of the LCU and the spectral absorption profile of the photoinitiator

Another crucial factor is the overlap between the wavelength of the light emitted from the curing light (emission spectrum or spectral band) and the spectral absorption profile of the photoinitiator. If the spectral output does not overlap the peak absorption, the photoinitiator may not absorb sufficient energy, which may limit polymerization. Historically, the most commonly used photoinitiator, camphorquinone (CQ) exhibits an absorbance range between ~400–500 nm with an absorption maxima at ~468 nm.²⁸ Other photoinitiators such as 2,4,6-trimethylbenzoyldiphenyl phosphine oxide (Lucirin-TPO), and bis(4-methoxybenzoyl) diethylgermane (Ivocerin, Ivoclar Vivadent) have been introduced for two main reasons. First, CQ is a canary-yellow dye, which, in too high concentration, may negatively influence bleach or translucent shades of resin-based composites for tooth mimicry; and secondly, the use of multiple photoinitiators at different absorption bands may improve curing efficiency through material depth. This latter group of ‘type 1’ photoinitiators possesses lower peak absorption wavelengths (~405 nm) compared with CQ. Any improvement in polymerization characteristics of resin-based materials containing type 1 photoinitiators would, therefore, not be realized in combination with conventional, single-diode (~450–470 nm) LED light-curing units. Consequently, numerous manufacturers now produce units with multiple diode types that augment polymerization of materials with more than one photoinitiator (Figure 6).

Potential problems with budget light-curing units

It is appropriate at this point to make mention that investing in high-quality products from reputable manufacturers is essential when new hardware is required.²⁹ Dental light-curing units, like many other items of dental equipment, are not regulated and can be purchased very cheaply on well-known online auction sites. A quick online search revealed the availability of many hundreds of low-cost curing lights from overseas manufacturers, some of which were offered at less than £12. Given that modern LCU brands supplied by reputable and well-known manufacturers may be a hundred times more costly, the temptation of significant cost savings by dental practitioners might be expected. With the plethora of LCUs available online, there must certainly remain an active market with dental practitioners buying and using these cheap devices, even if they do not admit to it, or are unaware of their potential shortcomings. Furthermore, such cheap LCUs claim to offer adequate and comparable properties to their more expensive counterparts, eg LED sources, cordless and battery operated, high power output (5W diodes with up to 2000 mW/cm² irradiance). Commonly, such budget LCUs are supplied with a small diameter curing tip (<5 mm) that allows manufacturers to advertise such high irradiance outputs as irradiance may be expressed as the ratio of power and area. This may be problematic since multiple overlapping curing regimens would be required for wide restorations. The optics (diode position, lenses, reflectors) are likely to be less well refined compared with reputable comparators, which may result in non-homogeneous power output across the face of the tip. Previous evaluations of budget LCUs have reported low quality and poorly fitting components³⁰ and significantly lower power outputs at reduced battery levels (without any warning to the operator) compared with LCUs provided by major and reputable manufacturers.³¹ In summary, it is wise to invest in high-quality hardware from manufacturers with a good reputation.

Eye care

It is imperative that those members of the dental team using dental LCUs are aware of the potential damage to eyesight that these devices may present. The retina is very sensitive to blue light, and the ‘blue light hazard’ or phototoxicity can occur with very short exposures and result in acute retinal burns. Chronic exposure to blue light is associated with an increased incidence of macular degeneration and retinal ageing.³⁴ Blue-filtering orange shielding devices should therefore be used clinically (Figure 10). Elastomeric orange filters are available that fit over the end of the light guide (Figure 11); however, their use is not to be recommended because there are that they do not block light effectively, and prevent a clear line of sight. The most effective and safest method of blocking blue light is the use of goggles worn by the dental team, which offer the advantage of freeing up the hands, so allowing proper alignment of the light guide tip and steady light curing. However, owing to the practicalities of working clinically, especially with respect to cross contamination, these are rarely used.