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Basic Physics of Laser Interaction with Vital Tissue
SCIENTIFIC ARTICLE
Basic Physics of Laser Interaction with Vital Tissue Harvey Wigdor, DDS, MS Harvey Wigdor
Harvey Wigdor, DDS, MS Harvey Wigdor received his DDS from the University of Illinois College of Dentistry in 1976 and his MS in Oral Pathology in 1984. He is presently Chairman and Director of the General Practice Residency, Department of Dentistry, Advocate Illinois Masonic Medical Center in Chicago. He holds academic positions of Clinical Professor at The University of Illinois College of Dentistry, Department of Oral Medicine and Diagnostic Sciences, Chicago, IL, and Adjunct Associate Professor, Department of Biomedical Engineering at Northwestern University in Evanston, IL. He has lectured extensively on laser-oral tissue interactions.
I
t is essential for any practitioner who uses lasers in their clinical practice to understand the basic physics of lasers. It is this knowledge that allows for an educated assessment of the clinical outcomes that lasers produce in our patients. It is also this understanding that provides a scientific basis for the visual feedback the clinician uses to vary parameters as needed to get the desired clinical results. It is the intent of this paper to discuss the very basic reasons why lasers affect tissues the way they do, and to synthesize the plethora of information dental practitioners are seeing regularly in dental journals. HISTORY
Albert Einstein theorized the concept of stimulated emission based on his quantum theory of light. The standard light bulb creates light through spontaneous emission in which an electron from the electrical current interacts with the tungsten in the filament and causes the production of a photon of light. The laser’s energy source, from either an electrical field or from light, produces a photon within the optical cavity which is spontaneously emitted. This photon interacts with a molecule of the medium (CO2, YAG, etc.), causing an electron to jump to a less stable higher energy level. Because this electron position is unstable, it will immediately revert back to its stable state, releasing another photon as it returns to its more stable lower state. Therefore, the single spontaneously emitted photon—through its interaction with the molecule of the medium—will create (or stimulate) 2 photons. These stimulated photons then can interact with other molecules of the medium, producing exponential increases in the number of identical photons, which are also resonating by reflection along the axis of
the medium. Soon, there is an overwhelming amount of collimated, monochromatic (one color or wavelength), and coherent (waves in perfect phase) laser energy that is focused and emitted into the handpiece. The first laser was developed in 1960, and shortly thereafter researchers looked at the possibility of using this new medium in dentistry. Stern and Sognnaes1,2 were pioneers in this area. In the mid1960s, they were very interested in the effects of lasers on dental hard tissues and published many articles on the surface alterations that occurred after laser irradiation. Goldman et al.3,4 experimented with one of the first ruby lasers, and found that this laser required significant energy to create a clinical effect and that the thermal damage was too great to consider this laser as a clinical instrument. Hibst and Keller5 presented results in 1989 from their research with the Er:YAG laser; they theorized that because of the high absorption of this laser in water, it would be a good candidate for cutting dental hard tissue. Much of their research led to the development of
BASIC PHYSICS OF LASER INTERACTION WITH VITAL TISSUE
Figure 1. The active portion of the basic configuration of a laser.
this laser that is now used in dentistry. Featherstone and Nelson6 and Featherstone and Fried7 have extensively researched the surface effects of the CO2 laser (9.3–9.6 mm), and showed that these wavelengths have a very high affinity for hydroxyapatite. They postulated that these wavelengths would have potential for not only enamel and dentin removal but also modifying the surface of enamel to make this surface less susceptible to acid demineralization. ANATOMY OF A LASER
A laser contains an optical cavity or resonating cavity that contains
a medium that can be a crystal, a silicon wafer, or a gas and a pumping energy source, which is commonly a high intensity strobe light or an electrical field or current. There are parallel mirrors on either end of the gas-filled tube or cylindrical crystal (Figures 1 and 2). One of the mirrors is 100% reflective; the other is 95% reflective. The 95% reflective mirror allows the laser light to exit the resonator and is available to irradiate the tissue being treated. These mirrors allow for the photons to reflect off of and re-enter the medium, creating a beam of light that is coherent (all waves are in phase with
each other), collimated (tightly packed), and monochromatic (one color). The wavelength of this monochromatic light is dependent on the medium in the optical cavity. For example, the solid state Erbium: yttrium-aluminum-garnet (Er:YAG) laser—which is an erbium rod doped with yitrium, aluminum, and garnet impurities—produces energy of 2940 nm (sometimes noted as 2.94 mm). One of the principal wavelengths of the CO2 laser is 10,600 nm (10.6 mm). There are 2 additional wavelengths of this laser, which will be discussed later. An neodymium:yttrium-aluminum-garnet (Nd:YAG) laser emits energy at 1064 nm (1.064 mm). It was the first laser wavelength that was introduced to general dentists. In the past few years, diode lasers became available in 3 wavelengths: 810, 940, and 980 nm. These lasers require lower voltages and currents than the gas or solid-state lasers, and can be manufactured in a small compact size. They are very efficient, but each individual diode is not able to generate a significant amount of power to produce
Figure 2. The internal components of a laser.
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BASIC PHYSICS OF LASER INTERACTION WITH VITAL TISSUE their positions in the spectrum both as they relate to each other as a function of wavelength. Figure 3 is a summary chart of the electromagnetic spectrum from the ultraviolet to the far infrared. Figure 4 identifies the location on this spectrum of the wavelengths of lasers used in dentistry. MECHANISM OF ACTION WHEN A LASER IMPACTS WITH TISSUE
Figure 3. Chart of the electromagnetic spectrum, from the ultraviolet to the far infrared.
a clinical effect; therefore, arrays of multiple diode lasers are assembled into one instrument, producing a useful power output. The resonant chamber (Figure 2) is a glass tube with the bright line, the active part of the laser. The instruments around the tube provide the electrical energy source. The totally reflective mirror is at the
left, and the laser beam exits at the right through the partially (95%) reflective mirror. This particular laser produces a blue light, which actually matches the peak absorption of the catalyst that cures most composite resins. The graphs of the electromagnetic spectrum shown here depict the generic names of dental lasers and
A beam of laser light can interact with an object (whether human tissue or other inanimate material) in a number of ways. The light can be transmitted through the material, similar to the way visible light passes through a clear plane of glass. It can be absorbed by the material and cause energy transformation through this absorption, usually into heat. It can be reflected as visible light is in a mirror, or it can be scattered through the material. Human tissue is very heterogeneous, and laser light will interact with this tissue based on how the light is absorbed
Figure 4. The location on the spectrum shown in Figure 3 of the wavelengths of lasers used in dentistry.
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BASIC PHYSICS OF LASER INTERACTION WITH VITAL TISSUE by the principal components of the treated tissue. This absorption causes a very local thermal effect which will determine the clinical effect. The term ‘‘chromophore’’ is applied to the material or tissue component that attracts the laser energy. Dental tissues generally have 4 different possible chromophores. Soft tissue is predominantly composed of water, which would be the principal chromophore. Teeth and bone contain varying amounts of mineral and water, with a carious lesion having the most water and the least mineral. Inflammatory granulation tissue can have markedly increased vascularity and bleeding, and many bacteria are highly pigmented. How do we determine the way light would interact with any material, whether it is human tissue or other material? Using a device called a spectrometer (Figure 6), light of a specific wavelength and predetermined energy is directed at the material, and the amount of light that passes through the material is collected by the spectrometer. The amount of energy that is not accounted for through scatter, reflection, and transmission is the
amount that is absorbed. It is therefore very important to understand the spectral characteristics of any tissue being treated to better understand the changes we would expect to see when the laser interacts with the tissue being treated. Therefore, a chromophore would directly influence the way we select the type of laser for providing care to our patients. As described above, the extent of absorption of laser energy affects the changes observed when a laser is used to cut either soft or hard tissue in the oral cavity. The principle chromophores of teeth are water and hydroxyapatite, and there are a number of lasers that fit this profile: namely, the Er:YAG, Er,Cr:YSGG, and the CO2 (9.3–9.6 mm) lasers. On the absorption curve from the graph below, note that there is a peak of absorption of water at just above the wavelength of both Erbium lasers (Figure 5). Also at this wavelength, there is another peak of the OH radical of the hydroxyapatite. It is this absorption characteristic that makes these lasers of great interest in dentistry. There is also a peak absorption at 9.3 and 9.6 mm of the
Figure 5. The absorption spectra of 4 tissue components. The horizontal axis is the wavelength of light, and the vertical axis is the absorption coefficient, which increases going up the chart.
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PO43– (phosphate) ion which is a component of the hydroxyapatite molecule. Coupled with the fact that the CO2 lasers are also highly attracted to water, current studies are concentrating on producing an effective dental device. Further study of absorption curves show that the diode and Nd:YAG energy is essentially transparent to dental enamel and water, and these lasers have high absorption chromophores in blood products and tissue pigment. Therefore, these wavelengths would be very useful in treating soft tissue inflammatory disease or in sculpting gingiva in very close approximation to healthy tooth structure, because there would be no interaction with the hard tissue. The temporal or the time of the pulses of the laser beam is a very important parameter to understand. Some lasers, such as diodes and some CO2 models, have a continuous light output; that is, the laser beam, like a flashlight, is continuously on. This type of laser is useful for soft tissue surgery, because the thermal effect provides a significant amount of hemostasis. These lasers can also be mechanically or electrically switched on and off for a few milliseconds (103 seconds). The Nd:YAG and the Erbium lasers contain a strobing flashlamp, which produces extremely short pulses (in the range of microseconds [106 seconds]). Laser researchers have developed devices whose pulse duration (sometimes known as pulse width) can be measured in nano-, pico-, or femtoseconds (109, 1012, or 1015seconds, respectively). The amount of energy that is contained in a light dose is expressed in joules. A watt is a measurement of power, which is energy (joule) per unit time. To express this in another way, 1 joule is 1 watt for
Alpha Omegan Volume 101 Number 3
BASIC PHYSICS OF LASER INTERACTION WITH VITAL TISSUE 1 second. When a pulsed laser is being used, each pulse has a certain amount of energy. The power can be determined by multiplying the energy of each pulse by the hertz or repetition rate of the number of pulses per second. For example, if each pulse measured 250 millijoules (mJ), which is a quarter of a joule, and the laser was emitting 20 pulses per second (also known as 20 Hz), the power would be 5 watts (0.250 3 20 ¼ 5). Another interesting calculation is that of peak power per pulse, which is inversely proportional to the pulse duration. That is, the shorter the pulse width, the higher the peak power. An Erbium laser can emit a 400-mJ pulse in 100 microseconds, and that pulse has a peak power of 4000 watts. A femtosecond pulse’s peak power can be in the gigawatt range; however; this is such a short period of time that the amount of ablation is very minimal. As an aside, light travels less than 1 cm during a femtosecond. It is theorized that this high energy actually causes breakage of the chemical bonds of the irradiated material whether animate or inanimate, and as a consequence, very little heat is generated. A report by Girard
et al.8 discussed the effects of a femtosecond laser pulse on bone by looking at the enzymatic activity as a marker of thermal effects. That group showed that the thermal effect only involved a thickness of 14 mm within the bone adjacent to the laser cut. These results seem to suggest that there would be a similar effect on dentin; moreover, it could be postulated that ultra-short laser pulses may be useful in removing existing dental materials, which to date has been a limitation of laser use for tooth preparation. The amount of power that a laser deposits over a specific area is called power density (expressed in watts/ cm2). Using the flashlight analogy, shining a flashlight from a distance of 1 in at a wall, the beam would be very compact and bright. When the flashlight is pulled away from the wall, the beam becomes wider and less bright. In other words, the power of the light in this case becomes less as the beam gets wider per unit of area. This same technique can be used with a laser and is called defocusing the beam. Decreasing the power density allows the surgeon to remove thinner layers of tissue over a broader area. Conversely, as the laser is moved closer
to the tissue, the beam becomes narrower and contains greater power density, and will incise through tissues more readily, providing good hemostasis. The relationship of watts to the clinical effect will at times be different from patient to patient and is part of the learning curve that a dentist will more fully understand over time with his/her experience as a continual teacher. When reviewing research papers that investigate the effect of lasers on tissues, another term similar to power density may be found. Fluence or energy density is the most common unit used when comparing clinical effects as a function of energy used, and is expressed as joules/cm2. Clinicians should understand this principle; but, as stated earlier, the tissue effect as a function of power and energy will be learned over time. Lasers can operate in both contact and non-contact modes; and typically, soft tissue surgery is performed with lasers that operate in the contact mode. However, hard tissue lasers are used in a non-contact technique, which is a different modality from a handpiece in removal of a carious lesion. When using a laser to cut teeth, the feedback is visual, not tactile, and a beginning laser practitioner should proceed slowly to attain confidence and efficiency. CONCLUSION
Figure 6. Integrating sphere of a spectrometer. The dental specimen, either a thin section of dentin or enamel, is placed in front of the entrance port and a specific wavelength of light at a determined energy is directed at the sample. The light from the sample is collected in the sphere, and the absorption is determined by the spectrometer.
Alpha Omegan Volume 101 Number 3
In summary, lasers can be of value in dentistry. The clinician should possess basic knowledge about laser–tissue interactions, and must carefully observe the thermal effects being produced. Contrary to what many laypeople believe—that lasers would make a visit to the dentist easier9—lasers are not the be all and end all, and a single laser will not perform all dental procedures.
131
BASIC PHYSICS OF LASER INTERACTION WITH VITAL TISSUE Education and prudent use of this new methodology will enable the dental profession to provide beneficial treatment to dental patients for now and in the future. References 1. Stern RH, Sognnaes RF. Laser beam effects on dental hard tissues. J Dent Res 1964;43:873–9. 2. Stern RH, Sognnaes RF. Lased enamel: ultrastructural observations of pulsed
132
carbon dioxide laser effects. J Dent Res 1972;51:455–60. 3. Goldman L, Hornby P, Meyer R, Goldman B. Impact of the laser on dental caries. Nature 1964;203:417. 4. Goldman L, Gray GA, Goldman J, Goldman B, Meyer R. Effect of laser beam on teeth. J Am Dent Assoc 1965;70:601–6. 5. Hibst R, Keller U. Experimental studies of the application of the Er:YAG laser on dental hard substances: measurement of the ablation rate. Lasers Surg Med 1989; 9:338–44.
6. Featherstone JDB, Nelson DGA. Laser effects on dental hard tissues. Adv Dent Res 1987;1:21–6. 7. Featherstone JDB, Fried D. Fundamental interactions of lasers with dental hard tissues. Med Laser Appl 2001;16:181–94. 8. Girard B, Yu D, Armstrong MR, Wilson BC, Clokie CMI, Miller DRJ. Effects of femtosecond laser irradiation on osseous tissue. Lasers Surg Med 2007;39:273–85. 9. Wigdor H. Patients’ perception of lasers in dentistry. Lasers Surg Med 1997; 20:47–50.
Alpha Omegan Volume 101 Number 3
Basic Physics of Laser Interaction with Vital Tissue Harvey Wigdor, DDS, MS Harvey Wigdor
Harvey Wigdor, DDS, MS Harvey Wigdor received his DDS from the University of Illinois College of Dentistry in 1976 and his MS in Oral Pathology in 1984. He is presently Chairman and Director of the General Practice Residency, Department of Dentistry, Advocate Illinois Masonic Medical Center in Chicago. He holds academic positions of Clinical Professor at The University of Illinois College of Dentistry, Department of Oral Medicine and Diagnostic Sciences, Chicago, IL, and Adjunct Associate Professor, Department of Biomedical Engineering at Northwestern University in Evanston, IL. He has lectured extensively on laser-oral tissue interactions.
I
t is essential for any practitioner who uses lasers in their clinical practice to understand the basic physics of lasers. It is this knowledge that allows for an educated assessment of the clinical outcomes that lasers produce in our patients. It is also this understanding that provides a scientific basis for the visual feedback the clinician uses to vary parameters as needed to get the desired clinical results. It is the intent of this paper to discuss the very basic reasons why lasers affect tissues the way they do, and to synthesize the plethora of information dental practitioners are seeing regularly in dental journals. HISTORY
Albert Einstein theorized the concept of stimulated emission based on his quantum theory of light. The standard light bulb creates light through spontaneous emission in which an electron from the electrical current interacts with the tungsten in the filament and causes the production of a photon of light. The laser’s energy source, from either an electrical field or from light, produces a photon within the optical cavity which is spontaneously emitted. This photon interacts with a molecule of the medium (CO2, YAG, etc.), causing an electron to jump to a less stable higher energy level. Because this electron position is unstable, it will immediately revert back to its stable state, releasing another photon as it returns to its more stable lower state. Therefore, the single spontaneously emitted photon—through its interaction with the molecule of the medium—will create (or stimulate) 2 photons. These stimulated photons then can interact with other molecules of the medium, producing exponential increases in the number of identical photons, which are also resonating by reflection along the axis of
the medium. Soon, there is an overwhelming amount of collimated, monochromatic (one color or wavelength), and coherent (waves in perfect phase) laser energy that is focused and emitted into the handpiece. The first laser was developed in 1960, and shortly thereafter researchers looked at the possibility of using this new medium in dentistry. Stern and Sognnaes1,2 were pioneers in this area. In the mid1960s, they were very interested in the effects of lasers on dental hard tissues and published many articles on the surface alterations that occurred after laser irradiation. Goldman et al.3,4 experimented with one of the first ruby lasers, and found that this laser required significant energy to create a clinical effect and that the thermal damage was too great to consider this laser as a clinical instrument. Hibst and Keller5 presented results in 1989 from their research with the Er:YAG laser; they theorized that because of the high absorption of this laser in water, it would be a good candidate for cutting dental hard tissue. Much of their research led to the development of
BASIC PHYSICS OF LASER INTERACTION WITH VITAL TISSUE
Figure 1. The active portion of the basic configuration of a laser.
this laser that is now used in dentistry. Featherstone and Nelson6 and Featherstone and Fried7 have extensively researched the surface effects of the CO2 laser (9.3–9.6 mm), and showed that these wavelengths have a very high affinity for hydroxyapatite. They postulated that these wavelengths would have potential for not only enamel and dentin removal but also modifying the surface of enamel to make this surface less susceptible to acid demineralization. ANATOMY OF A LASER
A laser contains an optical cavity or resonating cavity that contains
a medium that can be a crystal, a silicon wafer, or a gas and a pumping energy source, which is commonly a high intensity strobe light or an electrical field or current. There are parallel mirrors on either end of the gas-filled tube or cylindrical crystal (Figures 1 and 2). One of the mirrors is 100% reflective; the other is 95% reflective. The 95% reflective mirror allows the laser light to exit the resonator and is available to irradiate the tissue being treated. These mirrors allow for the photons to reflect off of and re-enter the medium, creating a beam of light that is coherent (all waves are in phase with
each other), collimated (tightly packed), and monochromatic (one color). The wavelength of this monochromatic light is dependent on the medium in the optical cavity. For example, the solid state Erbium: yttrium-aluminum-garnet (Er:YAG) laser—which is an erbium rod doped with yitrium, aluminum, and garnet impurities—produces energy of 2940 nm (sometimes noted as 2.94 mm). One of the principal wavelengths of the CO2 laser is 10,600 nm (10.6 mm). There are 2 additional wavelengths of this laser, which will be discussed later. An neodymium:yttrium-aluminum-garnet (Nd:YAG) laser emits energy at 1064 nm (1.064 mm). It was the first laser wavelength that was introduced to general dentists. In the past few years, diode lasers became available in 3 wavelengths: 810, 940, and 980 nm. These lasers require lower voltages and currents than the gas or solid-state lasers, and can be manufactured in a small compact size. They are very efficient, but each individual diode is not able to generate a significant amount of power to produce
Figure 2. The internal components of a laser.
128
Alpha Omegan Volume 101 Number 3
BASIC PHYSICS OF LASER INTERACTION WITH VITAL TISSUE their positions in the spectrum both as they relate to each other as a function of wavelength. Figure 3 is a summary chart of the electromagnetic spectrum from the ultraviolet to the far infrared. Figure 4 identifies the location on this spectrum of the wavelengths of lasers used in dentistry. MECHANISM OF ACTION WHEN A LASER IMPACTS WITH TISSUE
Figure 3. Chart of the electromagnetic spectrum, from the ultraviolet to the far infrared.
a clinical effect; therefore, arrays of multiple diode lasers are assembled into one instrument, producing a useful power output. The resonant chamber (Figure 2) is a glass tube with the bright line, the active part of the laser. The instruments around the tube provide the electrical energy source. The totally reflective mirror is at the
left, and the laser beam exits at the right through the partially (95%) reflective mirror. This particular laser produces a blue light, which actually matches the peak absorption of the catalyst that cures most composite resins. The graphs of the electromagnetic spectrum shown here depict the generic names of dental lasers and
A beam of laser light can interact with an object (whether human tissue or other inanimate material) in a number of ways. The light can be transmitted through the material, similar to the way visible light passes through a clear plane of glass. It can be absorbed by the material and cause energy transformation through this absorption, usually into heat. It can be reflected as visible light is in a mirror, or it can be scattered through the material. Human tissue is very heterogeneous, and laser light will interact with this tissue based on how the light is absorbed
Figure 4. The location on the spectrum shown in Figure 3 of the wavelengths of lasers used in dentistry.
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BASIC PHYSICS OF LASER INTERACTION WITH VITAL TISSUE by the principal components of the treated tissue. This absorption causes a very local thermal effect which will determine the clinical effect. The term ‘‘chromophore’’ is applied to the material or tissue component that attracts the laser energy. Dental tissues generally have 4 different possible chromophores. Soft tissue is predominantly composed of water, which would be the principal chromophore. Teeth and bone contain varying amounts of mineral and water, with a carious lesion having the most water and the least mineral. Inflammatory granulation tissue can have markedly increased vascularity and bleeding, and many bacteria are highly pigmented. How do we determine the way light would interact with any material, whether it is human tissue or other material? Using a device called a spectrometer (Figure 6), light of a specific wavelength and predetermined energy is directed at the material, and the amount of light that passes through the material is collected by the spectrometer. The amount of energy that is not accounted for through scatter, reflection, and transmission is the
amount that is absorbed. It is therefore very important to understand the spectral characteristics of any tissue being treated to better understand the changes we would expect to see when the laser interacts with the tissue being treated. Therefore, a chromophore would directly influence the way we select the type of laser for providing care to our patients. As described above, the extent of absorption of laser energy affects the changes observed when a laser is used to cut either soft or hard tissue in the oral cavity. The principle chromophores of teeth are water and hydroxyapatite, and there are a number of lasers that fit this profile: namely, the Er:YAG, Er,Cr:YSGG, and the CO2 (9.3–9.6 mm) lasers. On the absorption curve from the graph below, note that there is a peak of absorption of water at just above the wavelength of both Erbium lasers (Figure 5). Also at this wavelength, there is another peak of the OH radical of the hydroxyapatite. It is this absorption characteristic that makes these lasers of great interest in dentistry. There is also a peak absorption at 9.3 and 9.6 mm of the
Figure 5. The absorption spectra of 4 tissue components. The horizontal axis is the wavelength of light, and the vertical axis is the absorption coefficient, which increases going up the chart.
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PO43– (phosphate) ion which is a component of the hydroxyapatite molecule. Coupled with the fact that the CO2 lasers are also highly attracted to water, current studies are concentrating on producing an effective dental device. Further study of absorption curves show that the diode and Nd:YAG energy is essentially transparent to dental enamel and water, and these lasers have high absorption chromophores in blood products and tissue pigment. Therefore, these wavelengths would be very useful in treating soft tissue inflammatory disease or in sculpting gingiva in very close approximation to healthy tooth structure, because there would be no interaction with the hard tissue. The temporal or the time of the pulses of the laser beam is a very important parameter to understand. Some lasers, such as diodes and some CO2 models, have a continuous light output; that is, the laser beam, like a flashlight, is continuously on. This type of laser is useful for soft tissue surgery, because the thermal effect provides a significant amount of hemostasis. These lasers can also be mechanically or electrically switched on and off for a few milliseconds (103 seconds). The Nd:YAG and the Erbium lasers contain a strobing flashlamp, which produces extremely short pulses (in the range of microseconds [106 seconds]). Laser researchers have developed devices whose pulse duration (sometimes known as pulse width) can be measured in nano-, pico-, or femtoseconds (109, 1012, or 1015seconds, respectively). The amount of energy that is contained in a light dose is expressed in joules. A watt is a measurement of power, which is energy (joule) per unit time. To express this in another way, 1 joule is 1 watt for
Alpha Omegan Volume 101 Number 3
BASIC PHYSICS OF LASER INTERACTION WITH VITAL TISSUE 1 second. When a pulsed laser is being used, each pulse has a certain amount of energy. The power can be determined by multiplying the energy of each pulse by the hertz or repetition rate of the number of pulses per second. For example, if each pulse measured 250 millijoules (mJ), which is a quarter of a joule, and the laser was emitting 20 pulses per second (also known as 20 Hz), the power would be 5 watts (0.250 3 20 ¼ 5). Another interesting calculation is that of peak power per pulse, which is inversely proportional to the pulse duration. That is, the shorter the pulse width, the higher the peak power. An Erbium laser can emit a 400-mJ pulse in 100 microseconds, and that pulse has a peak power of 4000 watts. A femtosecond pulse’s peak power can be in the gigawatt range; however; this is such a short period of time that the amount of ablation is very minimal. As an aside, light travels less than 1 cm during a femtosecond. It is theorized that this high energy actually causes breakage of the chemical bonds of the irradiated material whether animate or inanimate, and as a consequence, very little heat is generated. A report by Girard
et al.8 discussed the effects of a femtosecond laser pulse on bone by looking at the enzymatic activity as a marker of thermal effects. That group showed that the thermal effect only involved a thickness of 14 mm within the bone adjacent to the laser cut. These results seem to suggest that there would be a similar effect on dentin; moreover, it could be postulated that ultra-short laser pulses may be useful in removing existing dental materials, which to date has been a limitation of laser use for tooth preparation. The amount of power that a laser deposits over a specific area is called power density (expressed in watts/ cm2). Using the flashlight analogy, shining a flashlight from a distance of 1 in at a wall, the beam would be very compact and bright. When the flashlight is pulled away from the wall, the beam becomes wider and less bright. In other words, the power of the light in this case becomes less as the beam gets wider per unit of area. This same technique can be used with a laser and is called defocusing the beam. Decreasing the power density allows the surgeon to remove thinner layers of tissue over a broader area. Conversely, as the laser is moved closer
to the tissue, the beam becomes narrower and contains greater power density, and will incise through tissues more readily, providing good hemostasis. The relationship of watts to the clinical effect will at times be different from patient to patient and is part of the learning curve that a dentist will more fully understand over time with his/her experience as a continual teacher. When reviewing research papers that investigate the effect of lasers on tissues, another term similar to power density may be found. Fluence or energy density is the most common unit used when comparing clinical effects as a function of energy used, and is expressed as joules/cm2. Clinicians should understand this principle; but, as stated earlier, the tissue effect as a function of power and energy will be learned over time. Lasers can operate in both contact and non-contact modes; and typically, soft tissue surgery is performed with lasers that operate in the contact mode. However, hard tissue lasers are used in a non-contact technique, which is a different modality from a handpiece in removal of a carious lesion. When using a laser to cut teeth, the feedback is visual, not tactile, and a beginning laser practitioner should proceed slowly to attain confidence and efficiency. CONCLUSION
Figure 6. Integrating sphere of a spectrometer. The dental specimen, either a thin section of dentin or enamel, is placed in front of the entrance port and a specific wavelength of light at a determined energy is directed at the sample. The light from the sample is collected in the sphere, and the absorption is determined by the spectrometer.
Alpha Omegan Volume 101 Number 3
In summary, lasers can be of value in dentistry. The clinician should possess basic knowledge about laser–tissue interactions, and must carefully observe the thermal effects being produced. Contrary to what many laypeople believe—that lasers would make a visit to the dentist easier9—lasers are not the be all and end all, and a single laser will not perform all dental procedures.
131
BASIC PHYSICS OF LASER INTERACTION WITH VITAL TISSUE Education and prudent use of this new methodology will enable the dental profession to provide beneficial treatment to dental patients for now and in the future. References 1. Stern RH, Sognnaes RF. Laser beam effects on dental hard tissues. J Dent Res 1964;43:873–9. 2. Stern RH, Sognnaes RF. Lased enamel: ultrastructural observations of pulsed
132
carbon dioxide laser effects. J Dent Res 1972;51:455–60. 3. Goldman L, Hornby P, Meyer R, Goldman B. Impact of the laser on dental caries. Nature 1964;203:417. 4. Goldman L, Gray GA, Goldman J, Goldman B, Meyer R. Effect of laser beam on teeth. J Am Dent Assoc 1965;70:601–6. 5. Hibst R, Keller U. Experimental studies of the application of the Er:YAG laser on dental hard substances: measurement of the ablation rate. Lasers Surg Med 1989; 9:338–44.
6. Featherstone JDB, Nelson DGA. Laser effects on dental hard tissues. Adv Dent Res 1987;1:21–6. 7. Featherstone JDB, Fried D. Fundamental interactions of lasers with dental hard tissues. Med Laser Appl 2001;16:181–94. 8. Girard B, Yu D, Armstrong MR, Wilson BC, Clokie CMI, Miller DRJ. Effects of femtosecond laser irradiation on osseous tissue. Lasers Surg Med 2007;39:273–85. 9. Wigdor H. Patients’ perception of lasers in dentistry. Lasers Surg Med 1997; 20:47–50.
Alpha Omegan Volume 101 Number 3