Use of carbon dioxide lasers in dentistry
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This paper aims to perform a descriptive analysis by reviewing publications concerned with the production of carbon dioxide (CO2) lasers and their technological advancement, optical properties, and parameters in relation to clinical applications in dentistry.
This review was based on the literature search in Scopus, Google Scholar, and PubMed in English. The last search was made in December 2017 and there was no publication-year limit.
: The 10,600-nm CO2 laser is a readily available dental laser on the market. It enables performance of a bloodless surgical procedure and reduces post-operative discomfort in soft tissue dental surgery. Due to the advancement of technology, CO2 lasers with short pulse duration and high peak power are available. This new-parameter CO2 laser causes less collateral thermal damage to soft tissue than conventional lasers with continuous wave mode. Recently, a 9,300-nm wavelength CO2 laser has been introduced for clinical use in dental hard tissue removal. These developments make CO2 lasers fitting for dental hard tissue preparation.
The 10,600-nm CO2 laser is widely accepted for soft tissue surgery applications. Although CO2 lasers have been studied extensively in caries prevention, they have not been applied in clinical practice. The optical properties of 9,300-nm and 9,600-nm CO2 wavelengths are suitable for dental hard tissue treatment. Technological advancements in software and laser parameters will aid in new clinical application and technique development. CO2 lasers as hard tissue lasers will become more popular and more widely accessible to researchers and clinicians.
KeywordsCO2 laser Carbon dioxide laser Dental caries
Laser is an acronym that stands for light amplification by stimulated emission of radiation . The photons that make up a laser beam are coherent, amplified in phase (standing wave) of a specific wavelength (monochromatic). Laser has been used in dentistry for over two decades . Dental lasers are categorised according to their active medium and wavelengths. The currently available dental lasers are diode lasers (445 nm, 635 nm, and 810–980 nm), potassium titanyl phosphate (KTP) lasers (532 nm, Green), neodymium-doped yttrium aluminium garnet (Nd:YAG) lasers (1064 nm), erbium lasers (2780 nm and 2940 nm), and carbon dioxide (CO2) lasers (9300 nm and 10,600 nm). Each wavelength of the lasers has a specific thermal output and a particular tissue interaction.
Dental lasers of different wavelengths are used to perform different procedures. Blue lasers, diode lasers, Nd:YAG lasers, and CO2 lasers are primarily used in soft tissue surgery to provide good coagulation [3, 4, 5, 6]. Because CO2 lasers are well absorbed by water, they are absorbed on the surface of the soft tissue. The visible lasers (445–660 nm) are absorbed within the first centimetre of the soft tissue because they are best absorbed by pigmented chromophores such as melanin and haemoglobin. Lasers with 810- to 1064-nm wavelengths in the near infrared spectrum can penetrate into the soft tissue by a few centimetres because they are comparatively less well absorbed by melanin and haemoglobin. Erbium lasers, operating in free running pulse mode, are highest in water absorption, enabling their use for soft tissue ablation as well as for hard dental tissue and osseous preparation. The two erbium wavelengths commonly used in dentistry are erbium, chromium-doped yttrium, scandium, gallium, and garnet (Er,Cr:YSGG) lasers (2780 nm) and erbium-doped yttrium aluminium garnet (Er:YAG) (2940 nm) lasers. Although erbium lasers can also be used for soft tissue procedures, bleeding control is less effective than with diode and CO2 lasers, which offer better visualisation of the surgical site .
Some 10,600nm CO2 lasers on the market
Surgical CO2 laser
DOCTOR MED Co., Ltd.
2015 Korea fractional CO2 laser
Daeshin Enterprise DSE
Bothell, WA, USA
The transversely excited atmospheric pressure (TEA) CO2 laser  was developed by energising a gas laser with a high-voltage electrical discharge in a gas mixture, generally above atmospheric pressure. A pulsed low-energy CO2 laser is available with very short pulse durations of a few microseconds with a high repetition rate (frequency) over 1000 Hz per second. These developments make CO2 lasers fitting for dental hard tissue preparation . In this paper, the production of CO2 lasers and their technological advancement, optical properties, and parameters in relation to clinical applications in dentistry will be discussed.
Production of CO2 lasers
The CO2 laser was one of the earliest gas lasers to be developed in 1964 . It is one of the most useful and continuous wave lasers currently available. The lasing medium is a gas discharge, and the three main filling gases within the discharge tube are CO2, nitrogen (N2), and helium (He). With electrical discharge, microwave, or radiofrequency, electron impact excites the vibrational motion of N2 molecules. This marks the beginning of the population inversion, where molecules in the system are in their excited states. N2 cannot lose this energy by photon emission because it is a homonuclear diatomic molecule.
Excited vibrational levels are relatively long-lived and in a metastable state. The energy transfer that occurs due to the collision between N2 molecules and CO2 molecules causes vibrational (resonant) excitation of CO2 molecules, with sufficient efficiency to lead to the required population inversion of CO2 for laser operation (collision of the second kind). The N2 molecules are then returned to ground state. The CO2 molecules are still at a higher energy level after emission of photons. They return to ground state by colliding with cold He atoms. The resulting hot He atoms can be cooled by striking the bore (wall of the tube). The pressure in the tube must be low for adequate flow of photons. This limits the amount of CO2 molecules in the tube, producing a low-power laser. The photons emitted due to transition between energy levels have low energy and a longer wavelength than visible and near-infrared light because the energy levels of molecular vibration and rotation are similar.
Technological advancements of CO2 lasers
More than one laser wavelength can be produced by a CO2 gas laser. The wavelength depends on the isotope and resonator amplifying the wavelength desired. In dentistry, the 10,600 nm (12C16O2 molecule) wavelength is the earliest and most commonly produced wavelength. A CO2 laser is more efficient than other lasers because of its comparatively higher ratio of output power to pump power. Higher peak powers of CO2 lasers can be achieved by slow flowing of the gas instead of using a sealed tube. Another method to achieve higher peak power is to increase the density of excited CO2 molecules (i.e. the gas pressure). However, the voltage needed to achieve gas breakdown and couple energy into the upper laser levels also increases. The method to prevent producing a high voltage is to pulse the voltage transversely to the laser axis. Because electrical discharge can move transversely perpendicular to the laser axis, the electrons can travel at a substantially shorter distance and collide with more molecules . Such a design is called the TEA CO2 laser. The TEA CO2 laser can achieve high peak power in short pulses (~ 2 μs) and at a high repetition rate.
The 9300 nm CO2 laser was approved by the US Food and Drug Administration (FDA) and recently introduced in 2010 for both hard and soft tissue surgery (SOLEA, Convergence Dental, Inc., USA). The 9300 nm wavelength is produced by using an isotope 12C18O2 gas molecule instead of the normal 12C16O2 molecule. Both 18O and 16O are naturally stable CO2 molecules. Because 18O is heavier, with extra two neutrons, the frequency and energy level of molecular vibration are different from those of 16O .
Optical properties and laser parameters
Absorption coefficient and depth of enamel/dentine with carbon dioxide lasers
Chemical and morphological changes of enamel at different temperatures (adapted from Fowler and Kuroda 1986 )
Chemical and morphological changes in enamel during heating in furnace
Above 1100 °C
1225 °C β-Ca3 (PO4)2 converted to α′-Ca3 (PO4)2, 1250 °C Ca4 (PO4)2O melting
1450 °C disproportionate to α′-Ca3 (PO4)2
1600 °C α′-Ca3 (PO4)2 and Ca4 (PO4)2O melts. Conversion of OH− to O2−
Recrystallization, crystal growth of β-Ca3 (PO4)2 formed in tooth enamel
Decrease in OH− and conversion of OH− to O2−
Loss of H2O and CO32− and loss of trapped CO2 + NCO−
Decomposition and denaturation of proteins
Formation of pyrophosphate P2O7 from acid phosphate HPO42−
CO32− loss (−66%)
Laser interactions with dental hard tissue and their clinical applications
Although many laboratory and clinical studies have been conducted with CO2 lasers on dental hard tissue, only recently could these findings be clinically implemented because there is currently only one 9300 nm CO2 dental laser approved for hard tissue application by the FDA. Laser interactions with dental hard tissue fall into three major categories, namely, (1) interaction with the mineral, (2) interaction with the protein and lipid, and (3) interaction with the water . CO2 lasers can be used in tooth ablation and caries prevention. For ablation, the fluence must be above the ablation threshold, the point above which sufficient energy has been added to the surface in a short enough period of time to cause expansion and/or vaporisation of the tissue. In the case of CO2 lasers, absorption in both the mineral and water will occur with some melting and vaporisation of the mineral at around 1000 °C and above, as well as heating and expansion of subsurface water. It has been reported that the use of a 9300 nm CO2 laser with a fluence of 9 to 42 J cm−2 at a higher repetition rate (300 Hz) can ablate enamel and dentine effectively .
The role of CO2 lasers in dental caries prevention has been explored since the 1960s. For caries prevention purposes, it is likely that the most effective wavelengths are those that are most strongly absorbed by the mineral of dental hard tissues. The CO2 laser wavelengths of 9300 nm, 9600 nm, 10,300 nm, and 10,600 nm overlap with the strong phosphate absorption bands of the mineral. To prevent dental caries, the laser light must alter the composition or solubility of the dental substrate and the energy must be strongly absorbed and efficiently converted to heat without damage to underlying or surrounding tissues . Studies on the effects of CO2 lasers have focused on increasing the resistance to caries by reducing the rate of subsurface enamel and dentine demineralization [26, 27]. A greater depth of carbonate loss in enamel by a 10,600 nm CO2 laser was observed compared to that by a 9600 nm CO2 laser . Featherstone et al. reported that using a pulsed 9600 nm CO2 laser produced an 84% inhibition of demineralization in an intra-oral crossover study . Furthermore, some studies have combined the effects of lasers with fluoride [28, 29]. In an in vivo study, Rechmann et al. showed that occlusal fissures irradiated by a 9600 nm CO2 laser followed by fluoride varnish application twice a year are more resistant to caries than fissures without irradiation . Another study using a 9300 nm CO2 laser showed that mineral loss was reduced by 55% compared to fluoride application . However, it was reported that there was no increase in acid resistance in dentine when using 9300 nm CO2 lasers . Further studies are needed to determine the clinical application of CO2 lasers in caries prevention because there are vast variations in the parameters used.
Currently, the 9300 nm CO2 laser (SOLEA) is the only CO2 laser on the market that is FDA approved for dental hard tissue ablation. Dental hard tissue ablation is possible with minimal collateral tooth and pulpal damage [33, 34, 35, 36, 37]. Power, pulse duration, and frequency as adjustable parameters were discarded from the panel. They were replaced by spot size, power percentage, and water percentage. This makes the unit user friendly for operators without much understanding of laser parameters. The novel idea of using a digital rheostat foot pedal changes the percentage of power, thereby controlling the speed of ablation. Dentists are familiar with using a foot pedal to control turbine speed. The presence of a continuous water spray is essential to prevent a rise in temperature and the possibility of irreversible damage to the pulp . Clinical application of CO2 lasers in preventive and restorative dentistry may be closer to being a reality .
Laser interactions with oral soft tissues and their clinical applications
Oral soft tissues are largely composed of water, which absorbs lasers, such as CO2 lasers, well in the mid-infrared (erbium lasers) and far-infrared spectra. Penetration depth in water by CO2 laser energy is in the region of 10 μm. This results in tissue interaction predominantly on the surface of soft tissue at 50 μm. Volumetric expansion from liquid to steam is in the ratio of 1:1600. This rapid expansion results in vaporisation (ablation) of the soft tissue. Rapid thermal conduction of tissue around the vaporised zone results in protein denaturation, desiccation and shrinkage, and carbonisation of tissue.
There are many advantages to performing soft tissue surgery with a 10,600 nm CO2 laser. Capillaries are effectively sealed and coagulated during ablation in surgical sites, resulting in minimal bleeding with a clearly visible operating field, which may reduce operation time. The laser surgical wound heals by secondary intention. The surgical site is decontaminated by laser energy with a low chance of bacteraemia and less suturing need. In all laser wounds beyond the ablation and coagulation zones, there is a zone of photobiomodulation, which improves wound healing compared to scalpel and electrosurgery. Hyaluronic acid is a chemical which plays a key role in wound repair. A higher level of hyaluronic acid is found in a CO2 laser wound compared to a scalpel wound. Reduction in post-operative swelling, pain, and scarring is achieved with the appropriate laser parameters and clinical technique. Patient acceptance is high, with less post-operative discomfort. Hence, the CO2 laser is first used in oral surgery and in implant surgery, such as for excision, incision of soft tissues, premalignant lesion removal, and preprosthetic surgical procedures [39, 40]. In orthodontics, a CO2 laser can be used to perform frenectomies in children and teenagers  and removal of hyperplastic tissues around orthodontic brackets . Gingivectomies, gingivoplasties , de-epithelialisation for periodontal tissue regeneration , soft tissue crown lengthening, and cosmetic gingival recontouring  are periodontal procedures for which a CO2 laser can be used. Furthermore, CO2 lasers can be used for mucocele removal in soft tissues . Premalignant lesions such as leukoplakia and oral lichen planus may be treated by excision for biopsy or ablation . CO2 lasers have also been used for removal of hyperplastic soft tissues and soft tissue management around the implant in cases of peri-implantitis and implant uncovering of submerged healed implants . In addition, CO2 lasers can be used for tissue removal layer by layer (i.e. peeling) in melanin depigmentation of gingiva and vaporisation of vascular lesions. The advanced laser parameters in the 9300 nm SOLEA CO2 laser will give the operator even greater control in soft tissue surgery .
The 10,600 nm CO2 laser is widely accepted for soft tissue surgery applications. Although CO2 lasers have been studied extensively in caries prevention, they have not been applied in clinical practice. The optical properties of 9300 nm and 9600nm CO2 wavelengths are suitable for dental hard tissue treatment. Technological advancements in software and laser parameters will aid in new clinical application and technique development. CO2 lasers as hard tissue lasers will become more popular and more widely accessible to researchers and clinicians.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
This article does not contain any studies with human participants or animals performed by any of the authors.
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