Shining the Light on Power Bleaching: A Review

Introduction
There is no arguing the fact that we live in a society where appearances are important. When I was growing up, I was told frequently, “you only have one chance to make a first impression, so make it a good one!” A smile is one of the most powerful tools for opening channels of communications, making good first impressions and warming up the atmosphere in the room full of strangers. In North America, a smile showing straight, white teeth came to symbolize health, youth and prosperity. In response to this, cosmetic dentistry has become one of the fastest growing sub-specialties in the practice of dentistry, and has driven the evolution of bonding techniques, composite materials and bleaching formulations; all in the pursuit of that white “Hollywood” smile.

The FDA defines dental whitening as the process which restores teeth to their natural color, whereas dental bleaching is the process which whitens teeth beyond their natural colour. Home bleaching typically uses 10-15% CP (Carbamide Peroxide), although a more powerful 35% CP has recently become available with SDI’s Pola Zing. In-office bleaching uses a more concentrated 30-40% HP (Hydrogen Peroxide), however, recent regulations in Europe have limited the allowed concentration for professional use in EU down to 6% HP. This restriction has not yet reached the North American market, but should this occur, even higher emphasis must be placed upon improving the efficacy of lower concentration bleaching gels using various light sources. Vital tooth bleaching is considered to be the least invasive and the least costly of all the available cosmetic options for changing the color of natural dentition, but it is not without possible side effect. When seeking patient consent, it is important to discuss all of the available treatment options so they may understand, not only the benefits, but risks of each method. Change in tooth surface morphology, micro-leakage of restorations, external root resorption, post-operative sensitivity and pulpal irritation due to temperature rise (when using light activated bleaching) must be addressed. 1

History of Tooth Whitening
Believe it or not, the first attempt at professional dental bleaching of discolored teeth was performed by M’Quillen in 1867. In 1895, Pyrozone became the first commercially available bleaching agent, consisting of a 5:1 mixture of 25% H2O2(HP) and diethyl ether. In 1937, the first attempt at power bleaching was accomplished by heating up 35% HP gel by electromagnetic (EM) radiation. 2 With the development of CH4N2O*H2O2 (CP), in 1989, vital home bleaching kits became available using nightguard-type custom trays and 10% CP, which freed the dentists’ chair time to perform other procedures. The active ingredient in CP is still HP, which is produced when CP is exposed to saliva and is broken down into Urea and HP. HP is a highly unstable molecule which breaks down into oxygen, water and free radicals, like hydroxyl and perhydroxyl radicals. These Reactive Oxygen Species (ROS) cause oxidative decomposition of large stain molecules (chromophores) into smaller white and colorless molecules, resulting in the bleaching effect on the tooth structure. HP must be stored in the fridge, prior to use, otherwise premature decomposition will occur leading to the loss of reactivity, and therefore poor clinical outcome. It is always stored at a low pH because at the neutral pH of 7, it becomes very reactive and has a short shelf life. The most effective bleaching happens at a pH of 9.5-10.8, which is the reasoning behind having commercial gel formulations come with a separate alkalizing agent, which is then mixed with HP immediately prior to in-office application and helps to minimize acidic damage to the surface enamel, while maximizing bleaching efficiency of HP.

Light Sources
In the last 20 years, numerous light sources have been used for power bleaching to speed up the breakdown of HP into ROS during in-office procedures:

Quartz-Tungsten-Halogen (QTH) lamp, e.g. curing light
Light-emitting diode (LED), eg. 3LT green LED by SmartBleach.
LED (organic compound film emits light in response to electric current)
Plasma Arch (PAC) Lamps, e.g. Sapphire by DenMat.
Metal Halide (MH) Lamps, e.g. ZOOM2 by Phillips, Mercury metal halide 350-400nm
Laser light of various wavelengths, Continuous Wave or Pulsed

QHT lamps emit incandescent light in the violet-blue wavelength range of 380-520nm by heated tungsten filament, PAC lamps (380-580nm) emit luminescent light by recombination of electrons with ionized xenon atoms and mercury additives. Similarly, MH lamps (380-580nm) emit light by recombination of electrons with ionized metal atoms, mercury and argon. QHT, PAC and MH lamps are considered broadband emitters. LED light is produced in a similar fashion as the diode laser light, but with one main difference: there is no amplification of the produced radiation. LED’s emit a narrow bandwidth of light, for example 430-490nm blue, and not a single wavelength, like a laser.

Lasers (Light Amplification by Stimulated Emission of Radiation) are unique from all other sources of electro-magnetic (EM) radiation in that they emit light, which is coherent (organized/in phase), monochromatic (one colour/wavelength) and collimated (directional/with little divergence). The above-mentioned characteristics allow lasers to generate the highest possible power density, which in turn may lead to the most efficient activation of the bleaching gels on the tooth surface. The era of laser bleaching officially began in 1996 with FDA approval of Argon laser (488/514nm) and CO2 laser (10600nm), followed by Nd:YAG laser (1064nm) and some visible and near-infrared (IR) diode lasers in 2004, and the remaining infra-red (IR) diode lasers in 2007. Today, all dental diode lasers (790-980nm) are FDA approved for bleaching (Fig. 1).

Fig. 1
EM spectrum and dental lasers. Image from De Moor. 8

Mechanisms of Action
Photo-thermal effects make chemical reaction proceed faster as the temperature increases, which explains why HP decomposition occurs 2.2 times faster when the temperature increases by 10°C. The main concern with power bleaching, when utilizing any type of light source, is the pulp response to the applied heat. We know that a temperature rise of 5.5°C will lead to irreversible pulp damage in 15% of the teeth, and an 11.2°C increase will cause pulp necrosis in 60% of the teeth. 3 There are mechanisms in place which help control and minimize the effects of heat on the pulp, which include the gel itself acting as an insulating layer on the tooth surface, addition of pigment (chromophore) to absorb the specific wavelength of the applied light, and the circulation of blood within the pulp that helps to carry away the applied heat. 4 HP on its own is clear and has a low ability to absorb visible and near-IR light, but does have a good ability to absorb light in the UV, the mid-IR and far-IR areas of EM spectrum. Hard tissues also absorb in these wavelengths, and for this reason, chromophore is added to the gel for the absorption of visible and near-IR spectrum, which then leads to localized heating of the gel to break down HP, effecting the production of ROS and minimizing temperature rise in the pulp.

EM radiation can cause bleaching effects by other mechanisms as well. For example, direct photo-bleaching can occur when covalent bonds of the stain molecules are disrupted by the loss of electron through absorption of the light wavelength compatible with the energy gap in the chromophore molecule. Photolysis can occur by direct absorption of UV light by HP, causing it to split in to hydroxyl radicals, however the use of UV light is not advisable due to other possible collateral tissue damage. Photo-dynamic effect occurs when photo-sensitive dyes are added to HP gel, they donate electrons and help form ROS when excited by light, which can enhance the bleaching process. 5 For example, green light of KTP laser (532nm) is not absorbed by water or hydroxy-apatite (HA) and can, therefore, penetrate deeper to target the chromophore, which absorbs green light (e.g. tetracycline staining), but with the addition of photosensitizer Rhodamine (Smartbleach International, SBI, Herzele, Belgium), the process becomes even more effective. 6

KTP Laser
KTP (Potassium-Titanyl-Phosphate) is derived from Nd:YAG laser (1064nm) by frequency doubling, and hence half the wavelength (532nm), which gives the distinctive green color. It has less penetration into the dental market due to its higher cost, however it has the most supporting literature in the treatment of the toughest tooth discolorations, like tetracycline staining. 6-8 The energy application of 2 Watts for 30 second increases the temperature by only 2.2°C 9; enamel micro-hardness, surface morphology or compositional structure are not affected, in comparison to diode lasers and LED lamps.10 KTP laser is the only device offering all three mechanisms of HP activation: photo-thermal, photo-chemical and photo-dynamic (Fig. 3).

Fig. 2
KTP laser upper vs. 3LT green LED lower. Bennett et al. Lasers Med Sci 2015. 6

Fig. 3
Example of a KTP laser.

Diode lasers
Diode lasers are the most commonly purchased dental lasers due to their affordability and are mainly used for the purposes of minor oral surgery and hemostasis. 810nm (e.g. Picasso by AMD, Odyssey by Ivoclar), 940nm (e.g. Epic, iLase by Biolase) and 980nm (e.g. Sirolase by Sirona, Photon by Zolar) are the three wavelengths commercially available in North America. A study by Lagori G. et al. 11 compared bleaching efficiency of both the KTP laser and diode 810nm laser on teeth stained with coffee, red fruits and tea using a 30% HP gel for 30 minutes, with laser activation of three cycles of 30 seconds each. Both lasers showed equal effectiveness at bleaching away coffee stains, whereas only the KTP laser resulted in significant improvement in bleaching teeth with red fruit and tea stains; tea group exhibited intermediate results between coffee and red fruit groups. 810nm diode laser group was not significantly different from the control group, not receiving any laser activation at all.

Numerous studies have proven the safety of various diode lasers wavelengths, with respect to pulpal temperature rise, when pulpal circulation was accounted for and wavelength-appropriate pigment/chromophore has been added to the HP gel, as a control for the depth of penetration of the laser energy. 7,12 However, it is still unclear if one diode wavelength is better than another for bleaching efficiency and clinical results. A study by Al-Karadaghi T. et al. 13 compared the temperature rise and the change in colour following the exposure of LaserWhite 20 38% HP bleaching gel to a 120 s of laser energy irradiation over two cycles by 940nm and 980nm diode lasers. The temperature rise was reduced by 27-29% from reaching the pulp by the presence of purple pigment in the gel, and 940nm laser showed highest change in colour with less than 2°C pulpal temperature rise (Figs. 4 & 5).

Fig. 4
Bleaching using 940nm diode laser with bleaching wand and LaserWhite20 bleaching gel.

Fig. 5

CO2 and Nd:YAG Lasers
According to the ADA, these lasers are not suitable for power bleaching, even though they were one of the first ones to get FDA approval in 1996 and 2004, respectively. When used for bleaching purposes, these lasers led to a temperature rise of 13-22°C with gel at the enamel surface, and 7-17°C at the pulp. 7

Argon Lasers
Argon lasers come in two wavelengths: 488nm blue and 514nm blue-green. In this area of EM spectrum, the absorption in water and tooth minerals is low, and the absorption in red colour (carotene and hemoglobin) is high, so the risk of thermal damage is relatively low. Even though it was the first laser FDA approved for bleaching in 1996, it has been largely replaced by the less expensive diode lasers. 12

Erbium Lasers
Recent research is pointing towards the mid-IR spectrum lasers as possibly safer and more effective than other light sources. The Erbium lasers, with wavelengths around 3000nm, have very high absorption in water and hydroxy-apatite (HA), and for this reason, only minimally penetrate into dental tissues as they get completely absorbed in the water-based bleaching gels (Fig. 6). This lack of penetration is an important factor for controlling pulpal temperature rise during the bleaching process. Sari et al. showed that Er:YAG 2940nm laser (e.g. LightWalker, Fotona) caused only a very slight increase in pulpal temperature of 1.86°C, compared to 2.61°C with diode 810nm laser and 1.02°C with LED460-480nm curing light. On the other hand, the temperature of the bleaching gel itself rose to 20.11°C with Er:YAG laser, followed by LED at 12.38°C and lowest for diode laser at 6.21°C. This suggests that Er:YAG laser is superior to other light and laser sources at focusing the energy on the gel, where it belongs, and not allowing it to penetrate into the pulp. 15

Fig. 6
Fotona TouchWhite. Photo by Dr. J. Jovanovic dentaltribune.com.

Er,Cr:YSGG 2780nm laser (IPlus, Biolase), a close neighbor of Er:YAG has been shown with SEM observations to cause no change in the surface roughness of enamel or composite restorations.16 Strakas et al. showed no significant rise in pulpal temperature following 60 seconds of laser activation of the LaserWhite20 gel. With this gel on the surface, the temperature increased by less than 2°C, and without by 3.5°C, which is still well within the 5.5°C safety threshold. What we are still lacking is a study showing how the bleaching efficiency of this wavelength compares to other lasers, like the diode or KTP lasers. The FDA has not yet approved the use of this wavelength for power bleaching applications.

Non-Laser Light Sources
LED’s (light emitting diodes), PAC (plasma arch) lamps, QTH (quartz-tungsten-halogen) lamps and MH (metal halide) lamps, with wavelengths raging from 380nm to 780nm use filters to try and suppress IR radiation, but increase in pulpal temperature may still be an issue. Wetter et al. found no significant differences between bleaching efficacy of various bleaching gels (Opalescence and Opus White) when activated by the Xenon lamp or 960nm diode laser, but for the conditions used in the study Xenon arch lamp induced a safer temperature increase. 14 A different study comparing temperature rise from different light sources found that ZOOM 2 (mercury MH lamp 350-400nm) activation of the bleaching gels (Boost, Viva Style 10, 16, 30) resulted in 1.5 to 3-fold increase in both pulpal and tooth surface temperature compared to a 770nm pulsed laser and more than 8-fold when compared to LED405nm and OLED400-760nm. Activation time for all units was 30 minutes, except ZOOM2, which had to be discontinued after 15 minutes for safety reasons. This study confirmed that low-power sources like LED and OLED did not exceed the critical 5.5°C threshold, as opposed to high-power sources like ZOOM2 and laser. However, the laser only increased the temperature beyond the safety margin when focused on the tooth in the absence of the bleaching gel, which acts as an insulator and helps absorb the energy and keeps it from reaching the pulp. 1

Conclusion
Power bleaching has become an important addition to our cosmetic treatment options and has been shown to be safe when the manufacturer’s instructions are precisely followed. Various light sources can be used to speed up the in-office bleaching process, with a savings of valuable chair time. However, in most cases, we cannot guarantee a better outcome from light or laser-activated bleaching, as compared to conventional bleaching, only a faster one. The KTP laser may be better suited for the treatment of tetracycline staining, whereas the Erbium laser family may prove more beneficial in the future in allowing for similar treatment outcomes in the shorter treatment time, or with the use of lower HP concentrations already enforced in Europe. OH

Oral Health welcomes this original article.

References

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About the Author
Dr. Marina Polonsky, DDS, MSc is a gold medal University of Toronto ’99 graduate, she maintains private general practice in Ottawa, Ontario with focus on multi-disciplinary treatment utilizing lasers of different wavelengths. She holds a Mastership from World Clinical Laser Institute (WCLI), Master of Science in Lasers in Dentistry from RWTH University in Aachen, Germany. She is the founder of the Canadian Dental Laser Institute (CDLI), the only study club affiliated with the Academy of Laser Dentistry. She serves on the Executive Committee for Oral Health and is the editor of the Laser Dentistry issue.