Application of Laser in Dentistry – A Literature Review

Introduction

Laser is an acronym for light amplification by stimulated emission of radiation.¹ Light travels in waves at a constant velocity. The basic unit of this energy is called a photon. Amplitude and wavelength are the two properties that define waves of a photon. Amplitude is the vertical height of the wave from the zero axis to the peak² . Wavelength is the horizontal distance between two adjacent parts of a wave.²

Laser technology was introduced in dentistry to make procedures simple and pain free. Properties of lasers like bactericidal effect, detoxification, hemostasis, and ablation enable it to serve as an adjunct or alternative to conventional treatment by virtue of limited adverse effects and pain, better visualization and improve healing.

This review summarizes current relevant literature on the application of laser in dentistry as a minimally invasive form of treatment and focuses on postoperative wound healing.

Background

In 1917 Albert Einstein gave a theory on stimulated emission on radiation.^3,4In 1951, MASER (microwave amplification by stimulated emission of radiation) was introduced by Charles H Townes.³ .The term laser was first used in 1957 by Gordon Gould.² Maiman developed the first ruby laser in 1960.3 Javan, Bennett, and Herriott in 1961 described the first gas laser, helium-neon (HeNe).⁵ . In 1964, Geusic, Marcos, and Van Uitert at Bell Labs demonstrated neodymium-doped yttriumaluminum-garnet (Nd: YAG) laser.² In 1964, at bell labs, Patel developed the carbon dioxide (CO₂) laser.⁶ Argonion laser was developed in 1964 (Bridges of Hughes Research Laboratories). In 1974 Er: YAG lasers were first introduced by Zharikov. In 1997, the US Food and Drug Administration (FDA)approved its use lasering the oral cavity for removal of caries and cavity preparation and soft tissue surgery in 1999 like sulcular debridement and osseous surgery in 2004.. In 1995, FDA approved oral soft tissue surgery and in 1998 sulcular debridement with a diode laser. In 1990 the FDA approved Nd: YAG laser for removal of soft tissue, in 1997 for sulcular debridement, and in 1999 for enamel caries removal. In 1976, CO₂ laser was approved by the US Food and Drug Administration for soft tissue surgery.⁷

Laser Device Components

The laser cavity existing at the center of the laser light consists of three components: an active medium, pumping mechanism and an optical resonator. The active medium is composed of a chemical element, compound or molecules. Laser are named based on their active medium a) gas (CO₂ laser) (b) solid crystal (Nd: YAG, Er: YAG) (c) solid-state semiconductor (diode laser) d) liquid (not used in dentistry). The pumping mechanism serves to pump energy into the active medium. The (- laser cavity has two mirrors on either side of the optical cavity which acts as an optical resonator (reflects waves back and forth).²

Delivery System Of Laser

In a target tissue, the laser energy delivery system for shorter wavelength instruments (e.g., diode, Nd: YAG) has small, flexible fiber optic systems with bare glass fiber. The laser wavelength is absorbed by water (e.g., CO₂ , Er: YAG) that cannot pass through the conventional glass fibers therefore special fibers are constructed which are capable of transmitting the wavelengths, with semiflexible hollow waveguide, or with articulated arms. Water spray is employed in the erbium laser for cooling the hard tissues.²

Laser Tissue Interaction

Laser light energy has four different interaction with the target tissue;reflection, transmission , scattering and absorption.^2,3 Reflection is the first tissue interaction and is simply the beam that is redirected off the surface and has an effect on the target tissue. If redirected to an unintentional target, such as the eye it could be dangerous. This highlights the importance for every person present in the laser treatment room to wear wavelength-specific safety glasses. Transmission is the second tissue interaction of the laser energy directly through the tissue without any effect on the target tissue. The transmission effect depends upon the laser light wavelength. Scattering is the third tissue interaction in which the laser light weakens the intended energy. It could cause unwanted damage to the tissue by transfer of heat adjacent to the surgical site. Absorption of the laser energy on the tissue depends upon the tissue characteristics such as pigmentation and water content, and the wavelength of the laser.⁴

Tissue Temperature

At the temperature of 100˚c, vaporization of water occurs within the tissue and this process is called ablation. Soft tissue excision/incision is commenced at this temperature. At temperatures between 60-100˚c without any vaporization of tissue, proteins begin to denature. This temperature enables removal of diseased granulomatous without affecting healthy tissue. Edges of the Soft tissue can be welded back at the temperature of 70˚ to 80˚c without applying sutures. At 200˚c the tissue gets dehydrated and burned and the final product is carbon, so heat sink occurs as the lasing continues.^2,4

Characteristics of Laser

Wavelength

In dentistry lasers used consist of a diverse wavelength. A beam of a single color (i.e., monochromatic) is generated whenever laser energy is emitted. Each wave in laser light is coherent or identical which means that the amplitude and frequency of all waves of photons are identical. Laser beams emitted by some of the instruments are collimated. But beams that are produced from optical fibers are divergent. Dental laser devices based on the emission mode can emit light energy either in constant-on or pulsed on and off. The pulsed mode is further divided into gated and free running modes. The three different emission modes (i) continuous-wave mode (ii) gated-pulse mode (iii) free-running pulsed mode or true-pulsed mode.³ . The lasers commonly used in dentistry and their wavelengths are summarized in Table 1.

Penetration Depth of Laser

There are four different interactions in biological tissue, among which degree of absorption determines the performance of a laser and is dependent on its wavelength. Depending on the laser wavelength, it is classified into two types: (i) Deep penetrating: deeper penetration into biological tissue occurs when there is a lower absorption coefficient into the water, such as neodymium-doped yttrium-aluminium-garnet (Nd: YAG) and diode lasers; these laser lights can penetrate and scatters deeply into the tissue. (ii) Superficially absorbed type (shallowly penetrating type): it exhibits a higher absorption coefficient into water such as carbon dioxide (CO2), Er: YAG and Er, Cr: YAG lasers; these lasers do not penetrate or scatter deeply and laser light is absorbed only in the superficial layer.^4,8

Tissue Ablation

1. Soft Tissue

Soft tissue evaporates by the thermal effect, as the photothermal effect is one of the properties of laser on the tissue. A direct photothermal effect has been seen in CO₂ and erbium lasers, which can easily evaporate soft tissue. Nd: YAG and diode lasers create a condition called hot tip by converting its emitting light into heat by refraction or diffuse reflection at the tip end. Thus, secondary thermal effects of the hot tip can incise the soft tissues. The tissue coagulates and gets vaporized with the overheated tip rather than by laser energy itself.⁹

2. Hard Tissue

Based on photothermal interaction, hard tissue ablation with Er: YAG laser has been hypothesized to occur due to the thermo-mechanical effect.¹⁰Tissue ablation by Er: YAG laser begins with the mechanism of thermal evaporation as this laser is easily absorbed in water within biological tissues. In hard tissue, vaporization occurs when water molecules absorb laser energy, which increases intra-tissue pressure and provokes microexplosions, thus causing the mechanical breakdown of tissue and contributing to the ablation process¹¹.

Thermal Side Effects and Hemostasis

When a laser deeply penetrates soft tissue, it produces a layer of thick coagulation on the lased surface and exhibits strong hemostasis. Perry et al and White et al. reported that while incising bovine oral soft tissue by Nd: YAG laser at 3-10 W, the width of the coagulation layer was 0.3-0.8 in vitro.^12,13 Walsh et al reported that in a noncontact mode, when Er:YAG laser incised porcine skin, the width of the thermally changed layer was only 10-50µm.¹⁴ When Er: YAG laser is used with water cooling, it is effectively capable of ablating hard tissues, producing a thermally affected layer approximately 5-30 μm thickness. A major thermal change, such as carbonization, melting, and resolidification has also been seen on the hard tissue, which is generally produced by lasers such as Nd: YAG and CO₂ .¹⁵

Disinfection And Detoxification Effects

The photo thermal effect of lasers enables killing of bacteria. On irradiation by the laser they evaporate, denatured resulting in devitalization or inactivation.¹⁶ During surgery, lasers are capable of creating a disinfected field, reducing the risk of infection, and advantageous for postoperative wound healing because of the bactericidal effect of laser therapy¹⁷. The laser could be effective for debilitating some of the pigmented bacteria (e.g., porphyromonas gingivalis), which is linked with the periodontal disease because Nd: YAG laser show selective pigmented bacteria absorption.

Biostimulation

Biostimulation (photobiomodulation) is one of the properties of laser therapy of tissues and cells post irradiation. This therapy is related to a photochemical reaction within cells, , however, the mechanism is still unclear. Faster wound healing, reduction in inflammation, and relief in pain are the various biological effects of laser irradiation.^18,19

Advantages of Laser

• Have bactericidal and detoxification effect
• Less discomfort than conventional approaches ²⁰
• Minimally invasive
• Usually, no need for sutures
• Tissue easily gets ablated through laser than a conventional scalpel
• Visualization is better at the surgical site due to hemostasis
• Laser provides less post-operative tissue edema and swelling.

Disadvantages of Laser

• High financial cost of laser apparatus²⁰
• Improper irradiation can damage teeth and the surfaces of the root
• May also damage the underlying bone and dental pulp^20,21

Risk and Precaution of Laser

• Risk and precautions in the clinical use of the laser are summarized in (Table 2).²¹

Laser Application In Dentistry

The laser has become routine in dental practice. Ablation or vaporization, hemostasis, and the sterilization effect are some of its various characteristics that may serve as either an adjunctive treatment methodology or as stand-alone additions to the dental armamentarium.⁷ The current and potential laser application in dentistry are summarized in Table 3.

1. Scaling and Root Planing

Several studies have been published elaborating the use of laser as an adjunct to scaling and root planing. Numerous studies using lasers (diode, CO₂ , Nd: YAG, Er: YAG, and Er,Cr: YSGG) have shown a decrease in inflammation and/or pocket depths (Table 4). Tucker et al. reported that a CO₂ laser in a pulsed mode at 6 W was capable of eliminating dental plaque on the surface of the root.²² Coffelt et al. observed that in a defocused mode when the CO₂ laser was used at an energy density between 11 and 41 MJ/cm2 destroyed microbial colonies without inflicting undue damage on the surface of the root.²³ Crespi et al reported that pulsed defocus mode CO₂ laser at 2 W, 1 Hz increased fibroblast attachment after root conditioning.²⁴

2. Laser Assisted New Attachment Procedure (LANAP) Using Nd: YAG

LANAP was specially developed as an alternative to conventional periodontal surgery and for teeth with severely damaged periodontium. Several evolutions and changes in the name from laser ENAP to LPT to LANAP were done over many years and finally, the definitive LANAP protocol was established.²⁷ Based on human histologic evidence that LANAP is the only laser procedure which in the absence of a long Junctional epithelium gives the cementum-mediated periodontal ligament attachment and was approved by the United States FDA 510 (K) marketing clearance (K030290).²⁷ Some studies support new attachment following the use of LANAP (Table 4).

Dentinal Hypersensitivity

Low output power lasers (helium-neon and gallium/ aluminium/arsenide [diode]) and middle output power lasers (Nd: YAG, Er: YAG, and carbon dioxide) are t used in treatment of dentinal hypersensitivity.¹ Several studies support using lasers in dentinal hypersensitivity (Table 4). In 1972, Kantola observed craters in dentin which were created while using a CO₂ laser.³² On analysis with microradiography and electron probe it was revealed that increased levels of calcium and phosphorus existed in the fused or recrystallized dentine walls of the crater compared with normal dentin. One year later, using radiographic diffraction analysis in a follow-up study, Kantola observed that recrystallization had occurred in the laser-irradiated fused dentin and the dentin had changed structurally so that the crystalline structure of normal enamel hydroxyapatite can closely resemble. Glaucheet et al. reported the melting and resolidification of dentin in the presence of craters and cracks using CO₂ laser and Ciaramicoli et al using Nd: YAG laser. Dederich et al were the first to describe melting and recrystallization by the exposure of Nd:YAG laser on root canal wall dentin .^33,34,37

Caries Diagnostics

In the 1980s, the natural green fluorescence of tooth tissue by visual detection method was developed.³⁸ The method used an argon-ion laser (488nm) excitation wavelength to differentiate bright - green - fluorescing healthy tooth tissue from poorly fluorescing carious lesions. In early 1990, this technique was further refined by replacing argon-ion laser from a xenonbased arc lamp, with the emission light shone through a blue-transmitting filter and known as quantitative light-induced fluorescence (QLF), which quantify the observed green-fluorescence loss as an indirect measure of loss of mineral by using the digitization of images ³⁹. 405 nm excitation wavelength which is performed by the QLF system allows quantification and visualization of both intrinsic green fluorescence of dental tissues whereas in calculus, plaque, and advanced caries the red fluorescence is observed² . Substantial red fluorescence phenomenon using laser wavelengths between 650 and 800 nm in carious lesions has resulted in a hand-held device to detect dental caries² . To limit the possibility of false-positive results, the QLF system is best incorporated as an addition to other diagnostic methods (tactile, visual, and radiographic)

Pulp Diagnosis

Laser Doppler flowmetry (LDF) was developed for the diagnostic measurement of blood flow in the dental pulp40 and to examine blood flow in microvascular systems. This technique uses a low power of 1 or 2 mW from helium-neon and diode laser⁴¹. There are some limitations to laser Doppler flowmetry. In an anterior teeth region, the enamel and dentine are thin which generally do not create a problem. While in the molar region the enamel and dentin are thick and variability in the pulp position within the tooth may cause variation in pulpal blood flow⁴². There are some problems with laser Doppler flowmetry which was reported by Frentzen et al. that in a posterior region of teeth, it causes inconvenience in acquiring laser reflection.⁴³ Also in restored teeth, it was difficult to gain laser reflection because of limited transmission. In traumatized teeth excitability of the pulp is reduced, but LDF could present an acceptable alternative to the conventional method of vitality test.

When clinical application improves, and equipment costs decrease, then this technology could be used in a patient who cannot communicate or for young children who may not respond reliably.

Pulp Capping and Pulpotomy

The treatment done with CO₂ laser of the exposed pulp tissues in dogs to achieve hemostasis was first described by Melcer et al Shoji et al. who applied focused and defocused mode CO₂ laser and a wide range of energy levels (3, 10, 30, and 60 W) to expose the pulps of dogs.^44,45 There was no damage noticed in the radicular pulp area and coagulation, necrosis, and deterioration of the odontoblastic layer occurred. Jukic et al. observed carbonization, necrosis, an inflammatory response, edema, and hemorrhage in pulp tissue by using CO₂ and Nd: YAG lasers with energies densities of 4J/cm2 and 6.3 J/cm2.⁴⁶ Moritz et al treated a patient in whom direct pulp capping was indicated by using a CO₂ laser of 1W at an exposure time of 0.1-second with a pulse interval of 1 second and was applied until the exposed pulps were not completely sealed and dressed with calcium hydroxide.⁴⁷ In the control group, pulps were crapped with calcium hydroxide only. And 1 week later and monthly for 1-year, it was observed that 89% of them had not reported any symptoms and normally responded to the vitality test versus the control group (68%).

Incision/Excision Procedures

A variety of intraoral soft tissue lesions can be treated with laser therapy using continuous and pulsed CO₂ and Nd: YAG systems such as haemangioma, tongue papilloma, tongue lipoma, pyogenic granuloma, lymphangioma, lichen planus, and focal melanosis (Table 4). White et al proposed that the patient who was treated with the laser excision was well tolerated with no adverse effect.

Conclusion

In dentistry, the laser is becoming a common phenomenon and maybe even a form of routine treatment. Before using lasers, the practitioner must have knowledge on its scientific basis and tissue effects, proper training in use of the device, and sound clinical experience. In dental practice, the laser is extremely adaptable and can frequently be used instead of the conventional methods.

Conflict of Interest

None.