In the past few years, the use of diode lasers in clinical dentistry has increased considerably, but more research is needed concerning the events that occur when the tip of the laser comes into contact with oral tissues. When a "naked" fiber tip contacts tissue and fluid, debris will immediately accumulate on the tip, absorbing the laser energy propagating through the fiber. This will cause the tip to heat and immediately carbonize energy from the infrared photons will continue to be absorbed by the carbonized tip, which will become red hot. This secondary emission of the "hot tip" energy to the tissue is accompanied by fundamentally different heat transfer and photobiologic events to the oral tissues and fluids (including blood).
The purpose of this paper is to describe the quantum and thermodynamic events associated with energy emission from diode lasers with "hot contact tips," and the tissue response to this contact with the laser. Only with an understanding of these concepts can clinicians make the most appropriate choices for diode laser treatment of their patients.
When a clinician decides to use a laser as part of treatment, it is essential that the user has a clear understanding of the discipline and applications of photobiology. Laser photobiology is the science describing the unique interactions of nonionizing electromagnetic radiation (laser generated photons) with tissues and the molecular components that comprise tissues (ie, hemoglobin, water, collagen). As an example, the Er:YAG laser is one of the more commonly used lasers in dentistry today, and produces a beam at 2.94 um in the mid infrared portion of the electromagnetic spectrum. At this wavelength, the Er:YAG has the highest coefficient of absorption for water in the mid-infrared range, and correspondingly, the lowest depth of tissue penetration.1 To cut or ablate hard or soft tissue effectively, the Er:YAG laser targets the chromophore of water selectively instead of the extracellular matrix of collagen or hydroxyapatite, and produces an instantaneous vaporization of the water to a depth of about 4 um/pulse (4/1000ths of a millimeter) in front of the beam.2 The result of this interaction is a thermally driven, instantaneous, controlled tissue degradation or ablation, with an explosive ejection of the degraded cellular components and heated vaporous material, with poor hemostasis in soft tissue.2
A second example is the superpulsed CO2 laser, which is a gas laser that produces a beam at 10.6 um in the far infrared portion of the electromagnetic spectrum. At this wavelength, the super-pulsed CO2 has the highest coefficient of absorption for water in the far-infrared range, and correspondingly has a penetration depth of about 50 um with a greater thermal interaction and diffusion than the Er:YAG.1 The super-pulsed CO2 is an excellent laser for hemostasis and coagulation as opposed to the Er:YAG, which is used for cutting and thermal mechanical ablation without significant tissue thermal diffusion to surrounding tissues.3
Diode lasers, the focus of this paper, have a wide variety of medical and dental applications. Current diode technology can emit photons over a wide range of the electromagnetic spectrum, ranging from the red visible (600 to 700 nm) to the near infrared (800 to 1000 nm). The wavelength generated by the diode laser is related to the diode formed from different concentrations and layers of materials, such as gallium-arsenide and gallium-aluminum-arsenide.1 Currently available, FDA-approved diode lasers for dental applications have a photobiology that is always based on thermal tissue interactions, but is fundamentally different from other dental hard- and soft-tissue lasers by the degree of thermal interaction based on energy penetration in the tissues.4
If the wavelength of the Er:YAG (2.94 um) is compared to conventional dental diode soft-tissue lasers (810 nm to 980 nm wavelength), the depth of penetration per pulse with diodes is estimated to be greater by a factor of 104 (10,000x) or 4 cm.4 A diode laser is a solid-state laser with completely different chromophore targeting and absorption characteristics than the Er:YAG. Diode lasers have shorter wavelengths (800+ nm vs. 2.94 um), and thus have high absorption peaks in melanin and hemoglobin. This will cause the laser energy to essentially pass through the interstitial and intracellular water, and produce thermal hemostatic and necrotic effects much deeper in the tissues as the photons are absorbed by the tissue pigments. Hence, the Er:YAG laser is safer and more controlled for purposes of cutting tissue like bone, but does not have good hemostatic properties like the diode.5
|Figure 1. Laser energy absorption graph.|
Before using a diode laser, a clinician should understand the energy being delivered, the target of the laser energy (darker pigments), and how much laser energy is needed to achieve a specific task in a controlled fashion. In addition, the clinician needs to understand how the energy changes once the laser delivery fiber comes in contact with oral tissues and/or fluids, including blood. This knowledge is necessary to achieve the desired effect, while minimizing collateral damage to otherwise healthy tissue (Figure 1).
PHOTOBIOLOGY OF NEAR-INFRARED LASER ENERGY
Niemz4 has determined that all effects with near-infrared laser wavelengths at pulse durations of 1 microsecond or greater are thermal in nature. There are 5 factors to consider regarding heat generation by these lasers:
(1) wavelength and optical penetration depth of the laser;
(2) absorption characteristics of exposed tissue;
(3) temporal mode (pulsed or continuous);
(4) exposure time; and
(5) power density of the laser beam.
|Figure 2. Optical fiber.||Figure 3. "Cone" dispersion pattern of laser energy.|
The first parameter of near-infrared diode lasers that must be understood is the penetration depth of the optical energy. Diode lasers in the near infrared range have a very low absorption coefficient in water, hence they achieve deep optical penetration in tissues that contain 80% water (including the oral mucosa and gingiva). This means that for a conventional dental diode soft-tissue laser (as stated earlier), the depth of penetration per pulse is estimated to be greater than the Er:YAG hard-tissue laser by a factor of 104.4 The shorter wavelengths of the near-infrared diode and Nd:YAG lasers have very high absorption peaks in molecules (chromophores) such as melanin and hemoglobin. This will allow the laser energy to pass with minimal absorption through water, producing thermal effects much deeper in the tissue (up to 4 cm) as the photons are absorbed by the deeper tissue pigments. This photobiology allows for controlled, deeper soft-tissue coagulation, as the photons that emerge (in a cone pattern of energy) from the distal end of a near-infrared diode laser fiber are absorbed by blood and other tissue pigments (Figures 2 and 3). It should be noted that respective absorption, tissue temperature, and thermal effect gradients are within this illuminated volume or cone of tissue.
The next parameter to bear in mind is the heat effects on the tissue being irradiated, based on the pulse mode of currently available near-infrared systems. Presently, for dental treatment, near-infrared lasers emit photons in the continuous wave (CW) or gated CW pulsed mode for diode systems, and free running pulsed (FRP) mode for Nd:YAGs. This fact is important because the length (duration) of the tissue exposure to the photon energy of the laser will govern the thermal tissue interaction that is achieved.
In the CW or gated CW mode, laser photons are emitted at one single power level in a continuous stream. When the stream is gated, there is an intermittent shuttering of the beam, as a mechanical gate is positioned in the path of the beam, essentially turning the laser energy on and off. The duration of on and off times of this type of laser system is generally on the order of milliseconds (1 millisecond = 1/1000th of a second), and the "power-per-pulse" stays at the average power of the CW beam. Nd:YAG lasers (in the FRP mode) can produce very large peak energies of laser energy, for extremely short time intervals on the order of microseconds (1 microsecond = 1/1,000,000th second). As an example, one of these lasers with a temporal pulse duration of 100 microseconds, with pulses delivered at 10 per second (10 Hz), would mean that the laser photons are hitting the tissue for only 1/1000th of a second (total time) and the laser is "off" for the remainder of that second. This will give the tissue significant time to cool before the next pulse of laser energy is emitted. These longer intervals between pulses will benefit the thermal relaxation time of the tissue. The CW mode of operation always will generate more heat than a pulsed energy application.4
If the temporal pulses are too long (or the exposure in CW is too long), the thermal relaxation effect in the tissues is overcome, and irreversible damage to nontarget areas can occur. If adequate cooling and appropriate exposure times are practiced, these problems will be prevented. Therefore, not only the ultimate temperature reached in the tissue interaction with the laser energy is of concern, but the temporal duration of this temperature increase also plays a significant role for the induction of desired tissued effects, and the inhibition of irreversible tissue damage. For nano- and picosecond pulses (that today's dental lasers cannot achieve), heat diffusion during the laser pulse would be negligible.4
|Figure 4. "Hot tip" in a periodontal pocket.|
The power density of the beam is determined by the peak power generated by the laser, divided by the area of the focused beam. This means that the smaller the diameter of the fiber used to deliver the energy (200 um, 400 um, 600 um), and the closer the fiber is to the tissue (ie, a smaller "spot size," not touching the tissue), the greater the power density (amount of emitted photons per square millimeter of the beam) and the greater the thermal interaction. With a noncontact "clean" fiber tip, the 2 most important considerations are the spot size of the beam and the distance of the fiber tip to the tissue. If the dental near-infrared lasers are used in the "contact mode" with a "hot tip" fiber, the energy delivery, and hence the photobiology, will substantially change. These changes need to be understood by the practitioner, and are discussed in the next section.6,7 Yet another parameter that Niemz did not examine, but is commonly employed during dental procedures because it is readily available in the operatory, is the effect that a cooling water spray has on the tissue effects and photobiology. A "hot tip" laser procedure can be modified with successful results and minimal damage to the adjacent healthy tissue by applying a water spray to the surgical site from an air/water syringe. The water cools the tissue to help control the heat buildup in the tip, and also flushes some of the coagulum away from the tip so that it is a modified "hot tip." Whether the procedure is being performed in a dry field or with a water spray has significant effects on how the procedure is performed. If the variations are understood, successful results can be accomplished with both techniques. However, this irrigation and cooling cannot be accomplished if the tip is placed within a periodontal or periimplant pocket for sulcular curettage. Hence, during these "closed procedures" the power must not exceed 1 to 1.4 W, with the practitioner never staying in one spot for more than a second or 2 during treatment (Figure 4).
QUANTUM CHANGES IN THE FIBER TIP WITH CONTACT "HOT TIP" TECHNIQUES
|Figure 5. Tissue and coagulum detritus on fiber tip.||Figure 6. "Hot tip" blackbody radiator.|
In addition to being the means to deliver laser photons to a target tissue, the silica fibers at the tip of the diode laser device can act as a "hot tip" cutting device. When an activated, unclad fiber tip comes in contact with tissue and fluid, debris will immediately accumulate on the tip. This debris will absorb the intense infrared laser energy propagating through the fiber, which will cause the tip to heat and immediately carbonize the detritus. As the energy from the infrared laser photons continues to be absorbed by this newly carbonized tip, the tip will become red hot (above 7260C).8 Once this occurs, the tip of the fiber (becoming a "blackbody radiator") will generate a secondary visible optical emission as it becomes incandescent and glows (Figures 5 and 6). As more photons from the near infrared dental laser continue to bombard the black, carbonized tip and are absorbed by the organic debris, there is a rapid increase in temperature at the tip.9,10
It is this intense heat of the carbonized and glowing fiber tip that is known as the "hot tip" for diode laser cutting procedures. With this "hot tip" technique, the penetrating laser energy is substantially reduced, and the photobiology and laser-tissue interaction is profoundly different from what is found when using a noncarbonized fiber that emits only the primary emission, near-infrared photons. These realities need to be understood clearly by the practitioner so that safe and predictable surgical procedures can be realized with these lasers.7
"HOT TIPS" AND BLACKBODY RADIATORS
To understand the thermodynamic and photobiologic ramifications of the intense heat and subsequent carbonization of the fiber tip, a short review of how black solids absorb and then reemit electromagnetic energy is needed. In the mid 1800s Gustav Kirchoff proposed a rule for radiant energy that stated "a hot opaque substance emits a continuous spectrum of radiation." He clarified that black solid objects "glow" and emit light when heated. This phenomenon is referred to as "blackbody emission."9 In October 1900, Max Plank, by examining the available experimental data concerning the emission of heat and light from high-temperature solids, began our modern understanding of quantum mechanics.9,10 Plank's analysis revealed some fundamental rules of quantum mechanics (blackbody energy release arises from thermal radiation and thermal excitation of atoms). Dentists using diode lasers in the contact mode can appreciate a few of the quantum realities about the "hot tips" (blackbodies) that are part of the cutting process.9,10
|Figure 7. CIE diagram depicting blackbody source temperatures in degrees Kelvin and subsequent visible wavelength emissions.|
|Figure 8. Incandescent blackbody radiator—an incision in gingival hyperplastic tissue.|
(1) Theoretically, a blackbody is an object that absorbs all light (eg, the carbonized tip absorbs a large percentage of the infrared photons being emitted from the laser).
(2) As the carbonized tip continues to absorb laser photons, it heats up (ie, the longer the laser is firing into the "hot tip," or the higher the output energy, the hotter the tip will be).
(3) The energy and peak wavelength of emitted photons depends on the temperature of the tip (ie, the hotter the tip becomes, the more total light—infrared, visible, and ultraviolet—will be emitted from the tip).
(4) The heated tip emits light (photons) in a continuous spectrum at infrared, visible, and ultraviolet wavelengths (ie, no longer just the single infrared wavelength from the primary emission of the laser).
(5) Hotter objects are brighter at all wavelengths9 (Figures 7 and 8).
Although a true blackbody radiator is only theoretical, many materials behave in a similar manner when they are heated by an external energy source. An example of another common "blackbody radiator" is the tungsten filament in a conventional light bulb. The filament glows and becomes incandescent as it is heated by an electrical current, and becomes brighter as it gets hotter.11,12
Tissue Thermodynamics of "Hot Tip" Emissions
To accomplish safe and predictable surgical procedures with a "hot tip" and a near infrared dental diode laser, the clinician must be familiar with the very narrow therapeutic window afforded by the tip's thermal interactions with tissue. When radiant optical and thermal energy is applied to biological tissues with a hot tip, the temperature of the contact area rises immediately. At 450C, the tissue becomes hyperthermic. At 500C, there is reduction in cellular enzyme activity and some cell immobility. At 600C, proteins denature, and there is evidence of coagulation. At 800C, cell membranes are permeable, and at 1000C, water and tissue begin to vaporize.13
If the temperature increases for 2 to 5 seconds beyond 800C, there will be irreversible damage to the mucosa, bone, periodontal, and dental structures.4 These considerations are of direct importance for contact tip procedures such as a gingivectomy, gingivoplasty, frenectomy, incision and drainage, removal of a fibroma, and sulcular currettage.13
SURGICAL OBJECTIVES WITH "HOT TIPS"
|Figure 9. Contact vaporization with a "hot tip" around an ailing implant to remove tissue.||Figure 10. Superficial cautery of tissue after "hot tip" vaporization around ailing implant.|
The objective when using a diode laser with a "hot tip" is to generate sufficient thermal energy at the tip to cause immediate tissue vaporization and ablation limited to the line of the incision. To accomplish this properly, the tissue must be heated rapidly to several hundred degrees Celsius at the contact point of the tip. A diode or Nd:YAG laser can readily accomplish this if properly used in a contact mode. As the optical and thermal energy (of the secondary blackbody emission) is directly transferred to the tissue in the vicinity of the tip, a controlled vaporization ensues (Figures 9 and 10).
During these procedures, it is imperative to keep treatment contact intervals in any one spot relatively short (1 to 2 seconds), since limited exposure to the tip will minimize damage to the peripheral tissues. The heat will be transferred deeper into the tissues via heat conduction, and can be rapidly dissipated by the tissues if contact periods are very short. If the contact exposure time is too long (more than 2 to 3 seconds in one area), the ability of the tissues to dissipate heat is overcome, and irreversible damage occurs to nontarget tissues.
PHOTOBIOLOGY DIFFERENCES WITH CONTACT "HOT TIPS"
|Figure 11. Clean, "uncarbonized" fiber: note unattenuated forward power transmission.||Figure 12. Carbonized fiber tip: note significantly attenuated forward power transmission.|
As stated, in contact mode a large percentage of the near-infrared photons (the primary emission of the laser) are absorbed by the blackbody tip and carbonized coagulum. As a result, the emission—and hence penetration and absorption of these primary (single wavelength) infrared photons generated from the laser—are greatly decreased. Consequently, the ability of the laser to coagulate in the deeper tissues is now greatly attenuated. Therefore, the size of the resulting coagulation zone associated with an incision with the tip is dependent on the exposure time of the "hot tip" to the tissue and the heat conduction from the tip to the tissue. These greatly decreased primary emissions of the laser through a carbonized tip were studied in detail by Grant et al,8 as they specifically looked at the "fiber interaction" during contact laser surgery. Grant showed that with tissue deposits at the tip of the fiber absorbing larger amounts of laser light, immediate carbonization occurs. The carbonization of the fiber tip leads to an increase in temperature, and this can result in significant damage to the optical quality of the fiber (the temperature spikes to greater than 9000C). They also found that once the carbonization of the tip occurs, the tip no longer functions as an adequate light guide. The laser will no longer adequately photocoagulate, but rather it incises and cauterizes the tissue because of the intense heat at the tip8 (Figures 11 and 12).
It is important to remember that the glass portion of an optical transmission fiber consists of 2 regions—the core that runs through the center of the strand and the cladding that surrounds the core. The cladding has a different refractive index than the core and acts as a mirror that causes the laser light to reflect back into the core during its transmission through the fiber.12 Furthermore, longer lasing times and higher power drastically reduces the forward power transmission of the laser light, as the fiber tip sustains more and more heat induced damage.8 When testing a 360-um fiber with an 830-nm diode laser at 3 watts CW, I have found (testing with a laser power meter) that an immediate 30% loss of forward power transmission is observed with fiber carbonization from tissue detritus. Further loss was observed as lasing time continued and tissue debris accumulated.
This phenomenon was examined in vivo by Willems and Vandertop.14 Using diode and Nd:YAG lasers, conventional fiber tips and coated fiber tips were compared for ablation efficiency in rabbit cerebral tissue. With the conventional fiber tips, histology and thermal imaging demonstrated deleterious effects deep into the tissue. When using the coated fiber tip, they reported that almost all laser light was transformed into thermal energy (as the tip carbonized), and instantly produced ablative temperatures at the tip itself. Further, they reported that ablation was observed at low energy and power (1 W for 1 second) with thermal effects restricted only to the superficial structures.14 This restriction of thermal effects to superficial structures can be explained, as the forward power transmission of the laser light is attenuated when a larger percentage of the primary emissions of the laser are absorbed by the tip. As a result, the optical transmission qualities are damaged.8 Also of significance, as the quality of the fiber transmission diminishes as a result of damage to the tip, the energy, focus, and homogeneity of the energy being transmitted from the tip is affected. The primary energy that is still available for forward power transmission out of the tip is far less efficient for tissue penetration and photocoagulation. These are the important and fundamentally different biological consequences associated with diode lasers in the contact or noncontact modes.
MEDICAL/CLINICAL APPLICATIONS AND SAFETY OF "HOT TIPS" WITH NEAR-INFRARED ENERGY SOURCES
In 1987, Shapshay stated that the Nd:YAG laser contact probe was capable of precise soft-tissue ablation with minimal surrounding tissue damage, and was associated with rapid healing of the tissues (trachea and bronchi).15 In 1991, Absten described the use of hot contact tips with Nd:YAG lasers in gynecology for endometrial vaporization and ablation.16 Recently, in a study evaluating the thermal damage to the interior walls of veins with 600-um fibers in endovenous laser treatment, no major differences could be detected between the 3 diode laser wavelengths of 810 nm, 940 nm, and 980 nm. The laser wavelength interaction with the blood was strong enough to transfer the optical energy completely into heat at all wavelengths, even with new, uncarbonized fibers. If carbonization occurred, the tip preferentially absorbed the laser energy, causing extremely high temperature generation and a "hot tip." These studies strongly suggest that once a fiber tip is carbonized (and becomes a secondary blackbody emitter), subtle wavelength differences are not critical.17
The optical fiber tips used with near infrared lasers (800 nm to 1064 nm) experience heat induced carbonization almost immediately upon contact with oral tissues and/or blood. The carbonization is thermally driven and causes a dramatic degradation of the forward power transmission potential from the tip, as the tip absorbs the primary infrared photons from the laser and becomes red hot and incandescent. Upon carbonization, this tip can be referred to as a blackbody emitter of secondary radiation (ultraviolet, visible, and infrared light) and has a thermal interaction and photobiology distinctly different from what occurs with clean, uncarbonized noncontact fibers. It is no longer a deep photocoagulator.
This "hot tip" offers the clinician certain advantages over other surgical modalities, as it will adequately cauterize tissue within its immediate periphery if used correctly. This author would recommend not performing procedures over 3 watts in the CW mode with diode lasers unless the surgeon is very skilled and has adequate assistance with cooling irrigation. Also, for closed procedures (in the sulcus), 1 to 1.5 W is the maximum energy suggested with rapid movement of the fiber so as not to generate excessive heat in a particular area.
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17. Proebstle TM, Sandhofer M, Kargl A, et al. Thermal damage of the inner vein wall during endovenous laser treatment: key role of energy absorption by intravascular blood. Dermatol Surg. 2002;28:596-600.
Dr. Bornstein graduated from Tufts University School of Dental Medicine in 1992 and the Maimonides Medical Center General Practice Residency program in Brooklyn, NY, in 1993. He has been using lasers in his dental practice since 1995, and practi