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Laser Tissue Wavelength

Introduction

During the latter half of the 20^th century, advances in technology led to the development of different types of laser which were able to cut, coagulate, ablate and vaporize biological tissues. Concurrent research over the past 40 years led to the gradual development of fundamental safety parameters and an examination of possible applications of laser instruments in various fields of clinical dentistry, inter alia, periodontology, endodontics, conservative dentistry, tooth whitening, soft tissue surgery, hard tissue applications and dental materials.

The dental profession now benefits from many different laser wavelengths that allow usage in nearly every aspect of dental specialty. Wavelengths used in dentistry are:

• The Er:YAG (Erbium: yttrium-aluminum-garnet )laser, that operates at a wavelength of 2940 nm andin a pulsed waveform. The FDA has cleared it for use on cementumand bone, and it has a variety of hard-tissue applications,including the following:

• The Er,Cr:YSGG (Erbium, Chromium: yttrium-selenium-gallium-garnet ) laser operates at a wavelength of 2790 nm, withan extinction length in water of 1.0 µm (a measure thattranslates into a depth of 90 percent absorption). The waveformfor the Er,Cr:YSGG laser is pulsed.

• The CO₂ (carbon dioxide) lasers operate at a wavelength of 10600 nm. They canbe operated in a gated waveform or continuous waveform.

• The Diode (Aluminium-Gallium-arsenide) laser operates at a wavelength of 810-830 nm and 980 nm, and usesa pulsed or continuous waveform.

• The Nd:YAG (Neodymium: yttrium-aluminum-garnet) laser operates at a wavelength of 1064 nmin a high-intensity pulsed waveform.

• The Argon laser operates at a wavelength of 488 nm and 514 nm,using a pulsed or continuous waveform.

• The Ho:YAG (Holmium:yttrium-aluminum-garnet ) laser operates at a wavelength of 2120 nm, and uses a pulsed waveform.

An individual laser's wavelength is absorbed differently by soft and hard tissues and the laser's efficiency depends on the ability of tissues to absorb or reflect that particular wavelength. This project hopes to give a broad overview of the relevant laser physics; laser-tissue interactions; and highlights the most commonly used wavelengths in dentistry (Er:YAG, Er,Cr:YSGG, CO₂, Diode, Nd:YAG, Argon and Ho:YAG) and their uses for soft and hard tissue procedures and other clinical applications.

Historical development

Based on Albert Einstein's theory of spontaneous and stimulated emission of radiation¹, Maiman developed the first laser prototype in 1960² using a ruby crystal medium that emitted a coherent radiant light from the crystal when stimulated by energy. Thus, the ruby laser was created. Subsequently, in 1961, Snitzer³ published the prototype for the Nd:YAG laser. Thereafter, researchers examined the possibility of utilising lasers for a variety of clinical procedures.Initially, Goldman, in 1961, utilized the ruby laser and he is recognized as the first physician to use laser technology in clinical practice.⁴ Indeed, the seminal application of laser technology in dentistry was reported by Goldman (et al.)⁵ and Stern and Sognnaes⁶, their respective articles describing the effects of the ruby laser on enamel and dentine.

However, the modern relationship between dentistry and the laser originated in an article published in 1985 by Myers and Myers⁷ that detailed the in vivo removal of dental caries using a modified ophthalmic Nd:YAG laser.⁴ The first American Dental Laser for commercial use, using an active medium of Nd:YAG, emitted pulsed light and was developed and marketed in1989 by Dr Terry Myers, who suggested that the Nd:YAG laser could be used for oral soft tissue surgery,⁸ which ultimately led to the current relationship between lasers and clinical dentistry.⁹ Despite its low-power and the inappropriateness of its emission wavelength for use on dental hard tissue¹⁰, the availability of a dedicated laser for oral use was gradually accepted by dentists and the first laser was sold in the UK in 1990 (DLase 300 Nd:YAG Laser). This was followed, in the early nineties, by other laser wavelengths, using machines already deployed for medicine and surgery, with only slight modification for dental use. Unfortunately, being predominately argon, Nd:YAG, CO2 and semiconductor diodes, all these lasers failed to address a growing need amongst dentists and patients for a laser that would ablate dental hard tissue. However, in 1989, work by Keller and Hibst¹¹ using a pulsed erbium YAG (2,940 nm) laser, demonstrated its ability to cut enamel, dentine and bone. This laser became commercially available in the UK in 1995 and, followed shortly thereafter by a similar Er,Cr:YSGG (erbium chromium: yttrium scandium gallium garnet) laser in 1997, comprise a ‘laser armamentarium' capable of fulfilling the surgical needs of clinical dentistry in general practice.

Laser Physics

Photon

Light is a form of electromagnetic energy that travels in waves at a constant speed, with the photon being the basic unit of this radiant energy. A wave of protons can be defined by amplitude (the total height of the wave oscillation from the top of the peak to the bottom) and wavelength (the distance between and…? two corresponding points on the wave). The wavelength of any light beam is measured in metres, with typical values being expressed as nanometres (10^-9 metres).

The relationship of this radiant energy with frequency can be expressed as:

E = hv

where v = frequency (number of wave oscillationswith time) and h = Planck's constant.

The relationship of frequency with wavelength λ can be expressed as:

c = v ÷ λ

where c is the speed of light (a constant).

Substituting wavelength for frequency:

E = (hc) ÷ λ

This relationship thereby establishes an inverse relationship between wavelength and photonic energy. Hence the wavelength of laser energy is important in terms of both how this energy is delivered to the operative site and its effect on the tissue.

Ordinary White Light versus Laser Characteristics

Ordinary white light (e.g. emitted by a light bulb) is the sum of all component wavelengths of the visual electromagnetic spectrum (consisting of multiple colours) visible to the human retina. The waveform of ordinary light is non-coherent, in that there is a confused overlap of successive waves. The spread of such waves results in scattering of light with distance and the multi-direction and interference of successive waves gives rise to divergence and dimming with distance (non- collimated).

Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. The laser is a light source that produces a beam of monochromatic, focused (not diffuse), collimated or non- divergent (has specific spatial boundaries - there is a constant beam size and shape) and coherent (identical waves which are in phase with one another, thus having identical amplitude - peaks and valleys of the same size and identical frequency) light.¹²

Quantum physics of light- absorption and emission

An incident atom can absorb a photon of light ( P1), will result in an electron moving to a higher energy shell. The atom is now in an excited state relative to its ground state. This unstable excited state will result in the emission of photonic energy relative to the stable ground state of the atom, with excess energy being produced as heat. This is known as Spontaneous Emission that was postulated by Bohr in 1922.¹³ If an already energised atom is bombarded with a second photon (P2), this will result in the emission of two, coherent photons of identical wavelength travelling in the same direction at the same time. This was postulated by Einstein as Stimulated Emission ³

In a collection of atoms, where there are more atoms in an excited than resting state, there is said to be a Population Inversion. This is a necessary requirement for production of a laser light.

Spontaneous emission of a photon from one atom will stimulate the release of a second photon from a second atom. These two photons will similarly go on to stimulate the release of a further two more photons; these four then yield eight photons; eight yield 16, and so on (photon chain reaction), producing a brief intense flash of monochromatic coherent light (i.e. a laser beam). The stimulated emission of light is the crucial quantum process necessary for the operation of a laser.

Dental laser wavelengths

Lasers used in dentistry have emission wavelengths ranging from 0.5 to 10.6 microns ( 500nm to 10600nm). They are thus in either the visible or the invisible infra-red, non- ionizing portion of the electromagnetic spectrum. Hence, they emit either a visible wavelength of light or an invisible infrared or UV light.

General Laser System

A conventional laser system consists of a laser medium in a resonant optical cavity with a power supply and a cooling system with some form of control to the unit. Lasers are named after the chemical elements, molecules or compounds that comprise the core, or active medium, that is stimulated. This active medium can be a container of gas, as in the case of a carbon dioxide laser, a solid crystal rod such as an Erbium;YAG laser, or a solid state electronic device in case of a diode.

In order to house the collection of excited atoms and amplify the process of stimulated emission of photons, the laser medium is located within a resonant optical cavity, which typically consists of two mirrors some distance apart aligned so that their reflecting surfaces face each other. Photons bounce back and forth off these mirrors and re-enter the medium to stimulate the release of more photons. If some form of energy is provided to pump and keep atoms in an excited state continuously, then the population inversion can be maintained and generate high-intensity light. The pumping mechanism is usually a light source, either a flashlight or arc-light, but can be a diode laser unit or an electromagnetic coil. The light, travelling in a parallel direction perpendicular to the mirrors, will bounce back and forth many times across the laser medium, increasing in power or being amplified many times before it is powerful enough to be useful.¹⁵ The distal mirror is totally reflective (100%) and the proximal mirror is only partially reflective (90%), so that at a given energy density, laser light will escape to be transmitted to the target tissue as a monochromatic and directional beam of energy. Only part of the power pumped into the laser medium is converted into laser light. Some of the input power is converted into heat, raising the temperature of the laser medium (any light energy that does not pass perpendicularly between the two reflective mirrors will go astray and be lost as heat). This process is very inefficient, with only some 3-10% of incident energy resulting in laser light, ¹⁶ the rest being converted to heat energy. The heat is removed by co-axial coolant systems (air- or water-assisted), which maintain the temperature of the laser medium at an optimum level consistent with maximum lasing efficiency. The laser usually has a controlling system which is microcomputer/microprocessor located inside the unit with a control panel from which the operator dictates the laser power required and other output parameters or wavelength change.

Dependant upon the emitted wavelength, the delivery system may be a quartz fibre-optic, a flexible hollow waveguide, an articulated arm (incorporating mirrors), or a hand-piece containing the laser unit (at present only for low powered lasers). Early attempts to produce delivery systems relied upon the use of fixed mirror and/or lens apparatus. However, it soon became clear that the use of a fine, silica quartz fibre-optic cable maximised the feasibility for medical/dental lasers to reach their target site.

Schematic outline of typical laser device.

In a diode laser, the active medium is sandwiched between silicon wafers. Due to the crystalline nature of the active medium, eg GaAlAs, it is possible to selectively polish the ends of the crystal relative to internal refractive indices to produce totally and partially reflective surfaces, thus replicating the optical resonators of larger lasers. The discharge of current from one silicon wafer to the other, across the active medium, releases photons from the active medium. Individual diode ‘chips' produce relatively low-energy output and so modern, surgically-appropriate diode lasers employ banks of individual diode chips in parallel to achieve the desired power capability.²¹

Schematic outline of a typical diode laser.

However, the suitability of this delivery system is conditional upon the emission wavelength being poorly absorbed by water (hydroxyl groups), present in the quartz fibre. Therefore, shorter wavelengths such as argon, diodes and Nd:YAG can enjoy such fibre delivery, whereas longer wavelengths (Er,Cr:YSGG, Er:YAG and carbon dioxide) give rise to severe power losses through quartz fibre and hence require alternative delivery systems. Examples of such alternatives are articulated arms incorporating internal mirrors and prisms, and hollow waveguides, where the light is reflected along internally-polished tubes. Newer, water-free fibre compounds, e.g. zirconium fluoride, are being developed to overcome this problem.^17-20

Dental lasers can be used either in contact or non- contact mode Contact mode provides easy access to otherwise difficult to reach areas of tissue. The fibre tip can easily be inserted down a root canal to sterilize the canal, or periodontal pocket to remove small amounts of granulation tissue. In non-contact mode, the beam is aimed at the target from a distance, which is useful for following various tissue contours.

Dental lasers are equipped with a separate aiming beam (laser or conventional light) which is delivered along the optic fibre or waveguide in a separate channel and shows the operator the exact spot where the laser energy will be focused, because dental lasers operate at the invisible end of the spectrum.

In either modality, the beam is focused by lenses within the laser itself. With the hollow waveguide, there will be a precise spot at the focal point where the power density (Watts per square centimetre) is greatest. For the optic fibre, the focal point is at or near the tip of the fibre, which again has maximum energy. That spot should be used for incisional or excisional surgery as it results in faster ablation, increased penetration and decreased width of cut. When the hand piece is moved away from the focal point and away from the tissue, the beam is defocused, and becomes more divergent. At a small divergent distance, the beam can cover a wider area, which can be useful in achieving haemostasis. At a greater distance away, the beam loses its energy and dissipates rapidly.

Laser light emission modes

Clinical lasers are often referred to as ‘continuous wave' (CW), ‘gated pulsed' (GP) or ‘free-running pulsed' (FRP), which relates to the rate of emission of laser light with time. The emission mode for any given laser can be either ‘inherent' or ‘acquired'.

Inherent emission modes are related to the nature of the excitation source:

• Free-running pulsed

• Very large energies of laser light are emitted for an extremely short time (few microseconds), followed by a relatively long time in which the laser is off. This process is computer-controlled.

• Laser emission occurs over a pulse width of 100-200 microseconds.

• Continuous wave

• The beam is emitted continuously at one power level for the length of time when the operator depresses the footswitch.

Acquired emission modes are due to a modifying effect (electrical, mechanical, electro-optical or acousto-optical) acting upon the inherent delivery:

• Gated-pulsed

• The laser energy is switched on and off like a strobe light by opening and closing a mechanical shutter in front of the beam path of a continuous wave every few milliseconds.

• Laser emission occurs over tenths (0.1-0.5) of a second.

• Super-pulsed

• It is characterized by very high peak power and short duration.

• Laser emission occurs over 300-400 microseconds.

• Sharp-pulsed or surgi-pulse

• It is characterized by flat shape and longer duration.

• Laser emission occurs over 1 millisecond

The prime benefit of a pulsed delivery mode is the capacity of the target tissue to cool between successive pulses. The thermal relaxation potential (ability to cool of the target) is greatest in FRP emission and least in CW emission. This has a profound bearing on the tissue management during laser-tissue interaction.

Regarding the clinical application of any laser with any target oral tissue, it is important to consider the potential transfer of energy from the laser beam, converted to heat energy in the target, in order that only a sufficient transfer to execute designated tissue change is achieved. For example, 1W means that 1 Joule of energy is generated in 1 second. Hence, the amount of laser energy delivered depends on the length of time or pulse duration that the laser is left on a particular power level. Therefore, the pulse energy generated in a single laser pulse depends on the pulse duration and the output power level during the pulse. For the same output power level, pulsing the laser beam means that greater energy release occurs because of the very short pulse.

In a CO₂ laser, the average power output of a CW machine is readily understood - five Watts of CW output giving five Watts average power. In case of a FRP laser (e.g. Er:YAG), output is often expressed as energy per pulse and the operator can determine the number of pulses - the energy per pulse(e.g. 250 mJ or 250 x 10^-3 J ) multiplied by the number of pulses (e.g. 20 pulses per second) giving an average power delivery of five Watts (0.25 J x 20). However, when considering a FRP laser, the peak power per pulse can be considerable. A value of energy per pulse of 200 mJ with pulse duration of 100 μs (100 x 10^-6 s) can give rise to a peak power of 2,000 Watts for that fraction of time (J ÷ s).

In summary, the mode of emission has a direct effect as folows:

a) The average power (rate of energy with time) being delivered to the target

b) The peak power value of laser light being delivered to the target (observed with FRP modes)

c) The thermal relaxation effect of the target.

References

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• Maiman TH. Stimulated optical radiation in ruby. Nature 1960;187:493-494.

• Snitzer E. Optical maser action of Nd+3 in a barium crown glass. Phys Rev Lett 1961;7:444-446.

• Goldman L. Background to Laser Medicine-History, Principles and Safety. Laser Non-Surgical Medicine.Lancaster: P.A. Technomic Publishing, 1961: p.1.

• Goldman L, Hornby P, Meyer R, Goldman B. Impact of the laser on dental caries. Nature 1964;203:417.

• Stern RH, Sognnaes RF. Laser inhibition of dental caries suggested by first tests in vivo. J Am Dent Assoc 1972;85:1087-1090.

• Myers TD, Myers WD. In vivo caries removal utilizing the YAG laser. J Mich Dent Assoc 1985;67:66-69.

• Myers TD. What lasers can do for dentistry and you. Dent Manage 1989;29:26-28.

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• Myers T D, Myers W D, Stone R M. First soft tissue study utilising a pulsed Nd YAG dental laser. Northwest Dent 1989; 68: 14-17.

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• Bohr N. The theory of spectra and atomic constitution.2nd ed. Cambridge: Cambridge University Press, 1922.

• Einstein A. Zur quantentheorie der strahlung. Physiol Z 1917; 18: 121-128.

• Manni J G. Dental applications of advanced lasers. Burlington MA: JGM Associates,INC. 2000.

• Prakash O, Ram R S. Simple designs to measure efficiency of different types of monochromators. J Opt 1996; 27: 241-245.

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• Yang Y, Chaney C A, Fried N M. Erbium:YAG laser lithotripsy using hybrid germanium/silica optical fibers. J Endourol 2004; 18: 830-835.

• Konorov S O, Mitrokhin V P, Fedotov A B et al. Hollowcore photonic-crystal fibres for laser dentistry. Phys Med Biol 2004; 49: 1359-1368.

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• Parker S.Introduction, history of lasers and laser light production. Br Dent J. 2007 Jan 13;202(1):21-31.

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