CURING LIGHTS IN DENTISTRY – THE COMPLETE REVIEW

In recent years, 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).

Quartz Tungsten Halogen VS LED Curing Lights

Most manufacturers have stopped making QTH curing lights, mostly because LED lights are more efficient than QTH light sources: In fact, the bulb in these units consists of a tungsten filament enclosed in a clear, crystalline quartz casing, filled with a halogen-based gas. As electricity flows through the filament, because of the wire resistance, it produces sufficient heat to cause tungsten atoms to literally vaporize from the wire surface.

When this happens, tremendous amounts of electromagnetic energy are released, mostly occurring in the infrared spectral region, where the heat in the target is produced.

Thus, these types of light units typically require tremendous amounts of cooling action to remove that heat, as well as excess visible light not required for photocuring.

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LED Curing Lights: How do they work?

LED light-curing units emit light in the blue part of the visible spectrum, typically between 440 and 490 nm, and do not emit heat. They can be powered by rechargeable batteries because they require low wattage, and they are quieter than QTH units because they do not need a cooling fan. Initial versions of LED units emitted a lower intensity of light, whereas newer versions incorporate multiple LEDs with a variety of ranges of wavelengths to broaden the spectrum of the emitted light and increase the overall intensity in order to adequately polymerize all visible-light activated dental materials.

Some pencil-shaped, curing light models used metal body casings that not only provided structural durability but also provided a large area for thermal dissipation.

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The Chemical Reaction Explained

Visible light-activated dental materials contain an initiator such as camphorquinone (CQ) that absorbs light at the appropriate wavelength (approximately 470 nm for CQ).  When the initiator combines with an organic amine such as dimethylaminoethyl methacrylate (DMAEMA), the resulting reaction generates the free radicals necessary for the initiation of polymerization.

The wavelength, intensity, and duration of exposure to light determine the number of photons absorbed by the initiator and therefore impact optimal polymerization.
Notably, factors such as light-curing unit intensity, angle of illumination, the diameter of the tip of the light source, distance from the light source, and duration of exposure can significantly affect the number of free radicals formed.
Therefore, all those factors make this system highly technique sensitive.

Also, manufacturers of composite filling materials use initiators other than CQ in visible light-activated materials. Because they absorb light at different wavelengths than CQ, it is critical that the light-curing unit used emits light at the requisite wavelength for that particular initiator.

Implications of LED Curing Lights Power

Newer light-curing units have higher intensities, typically greater than 1000 mW/cm². For this reason, the higher intensity permits either shorter durations of the cure for a given depth of cure or increased depth of cure for a given duration of cure. On the other hand, the use of these higher intensity lights can, however, produce higher shrinkage stresses within the restoration.
It is important to remember: the type of light-curing unit and curing mode used to impact the polymerization kinetics, polymerization shrinkage, and associated stresses. It also impacts microhardness, depth of cure, degree of conversion, color change, and microleakage in visible-light activated restorations.
Lastly, precautions such as protective eyewear and light shields are critical for the safety of the patient and clinic personnel when using dental light-curing units.

Variability in the depth of light penetration, differences in curing light intensity, the diameter of the tip of the light source, and time of light exposure can result in variations of polymerization. The benefits of light-activated resin composites include ease of manipulation, control of polymerization, and lack of need for mixing. Since light-activated composites do not require mixing, it is less likely that air will be incorporated and form voids in the mixture.

Understanding Power and Distance

Light intensity at the surface is a critical factor in completeness of cure at the surface and within the material. We highly recommend holding the tip of the light source within 1 mm of the surface to provide optimum exposure.

A standard exposure time using most dental curing lights is 20 seconds. In general, this is sufficient to cure a light shade of resin to a depth of 2 or 2.5 mm, assuming that the light guide is immediately adjacent to the restoration surface.

The anatomy of the tooth often precludes the positioning of the light guide close to the restoration surface.
Important to realize: the light beam is partially collimated and does not spread sufficiently beyond the diameter of the tip at the emitting surface. Therefore, it is necessary to “step” the light across the surface of large restorations.
For this reason, the entire surface receives a complete exposure.

Importance of the Power Supply (Batteries)

Enhancement in the electrical supply to dental LEDs has also made great strides recently.
In fact, the development of lithium polymer battery technology has provided for lighter, and more durable power supplies. However, the introduction of a totally battery-less dental light-curing unit, that operates by charging and discharging an “ultra-capacitor” has greatly expanded the lifetime of dental curing lights.

Curing Lights New Trends

With the advances in LED technology, the second and third generations of LED units were developed.
Indeed, these new devices have a considerably greater light output compared to earlier versions.
As a consequence, the heat generated by the light emitted from such LED units was comparable to or even higher than the heat generated from QTH lights.

In addition, the third generation LED units have blue and violet LED chips, so they emit light with more than one wavelength.