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Proper Use of Er:YAG Lasers and Contact Sapphire Tips When Cutting Teeth and Bone: Scientific Principles and Clinical Application

Dentistry Today August 1, 200416 Mins read2.1k Views

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It is interesting to consider that dental education relies upon the concepts of classical Newtonian mechanics. Dentists are taught how mechanical work is produced from the force of a drill on a tooth, and that the heat that results (via friction) is a byproduct of the drill’s action on the tooth.^1,2 Dentists are generally familiar with how things move, the forces that move them, and the thermodynamic consequences of these forces. These are accurate explanations for everyday clinical experiences in dentistry.³

In the science of laser-tissue interactions, however, the clinical consequences are the result of an entirely different set of laws called quantum mechanics. For example, with Er:YAG lasers, the thermodynamic equations that dentists are accustomed to (work producing heat) are in fact reversed, and heat (vaporization of water) is observed to produce work.⁴ Dentists using lasers need to be aware of these different quantum rules, the reversed thermodynamic equations, and the logic behind them so that they can make proper decisions when treating patients with lasers (Figure 1).

Figure 1. Difference between the mechanics and thermodynamics of drills and Er:YAG lasers.

When performing a procedure with an Er:YAG laser and a contact sapphire tip with the intention of cutting calcified biologic tissues (teeth and bone), a clinician has entered the extraordinary quantum world of very small distances between atoms and molecules. Er:YAG laser photons (traveling at 186,000 miles/sec) uniquely target the chromophore of molecular water within calcified tissue, but not the calcified structure itself.⁵ Photon absorption by selected molecular chromophores in a biologic tissue (in this case water), with the concomitant quantum interactions, is referred to as the science of photobiology and is a discipline that does not behave in a classical Newtonian manner. Photobiology is largely unfamiliar to dentists and focuses on the interactions of the nearly instantaneous transfer of quantized packets of energy (laser photons) to selected chromophores (absorptive molecules) in vital tissues.⁵

Figure 2. Tetrahedral geometry of a water molecule.

Figure 3. Hydrogen bonds between water molecules.

Figure 4. Water freely entering the ablation area.

Figure 5. Tip embedded in ablation crater without room for removal of ablation products or water cooling spray.

Figure 6. Thermal animation of continually firing contact tip embedded in tissue resulting in a quantum heat trap.

Figure 7. Up-and-down motion used for a retrograde endodontic preparation.

Figure 8. Back off from the ablation crater, allow the crater to cool, flush, and start again.

Figure 9. Alveoloplasty at an angle of 30° with copious water spray on the ablation site.

Figure 10. Up-and-down handpiece motion.

Photobiology and its quantum consequences rely upon a set of laws with which dental practitioners need to be familiar, and hence a dentist using a laser in clinical practice should have a perfunctory understanding of governing principles when cutting (ablating) calcified tissues (teeth and vital bone) with Er:YAG lasers. As noted, the mechanics, interactions, and logic of cutting teeth and bone with lasers is fundamentally different than that of using a conventional dental drill. With dental Er:YAG laser devices and contact tips, the concept that exerting more force—or pushing the tip harder against the tooth—will result in faster cutting does not apply. With Er:YAG lasers and contact tips, a dentist must use an up-and-down motion and back off from a growing ablation crater (laser-drilled hole) to accomplish safe and predictable cuts in calcified tissues. This is related to certain quantum interactions. Only with an understanding of these quantum interactions can a dentist be effective and make correct clinical decisions.

The purpose of this paper is to define and characterize clinical ablation (cutting) protocols with contact sapphire tips and Er:YAG lasers on calcified tissues. This paper provides an explanation of some aspects of quantum physics involved when lasers are used to cut teeth and bone and provides a practical approach for avoiding what this author describes as the “quantum heat trap” effect (a thermodynamic event that produces a decrease in ablation efficiency). This can occur from excessive heat generation in a growing ablation crater due to incorrect use of Er:YAG irradiation and contact sapphire tips.

ER:YAG LASERS AND WATER ABSORPTION
Atoms in a molecule are never at rest, and for each type of molecule there are standard vibration modes. For water molecules, there are 3 normal modes of vibration, referred to as symmetric stretching, bending, and assymmetric stretching. These vibration modes depend entirely on the atomic geometry of the covalent bonds (–OH) in the water molecules and carry specific vibrational frequencies.⁶ The Er:YAG laser at a wavelength of 2.94 µm has the highest specificity for water absorption of all the mid infrared lasers.⁷ This is because one of the specific molecular vibration modes in the water molecule generates a frequency that corresponds directly to the mid infrared wavelength of of the Er:YAG laser (2.94 µm).⁶ It is for this reason that the absorption peak in water for Er:YAG energy is so high (Figures 2 and 3).

Importance of hydrogen bonds
The covalent (–OH) bonds in water molecules will bend and stretch depending on the state of the hydrogen bonds between water molecules. Hydrogen bonds are weak forces that attract water molecules to each other and hence give water its property of cohesion. These facts are important because, as noted above, it is the vibrational modes in water molecules that are directly targeted by 2.94-µm Er:YAG optical energy. Further, these vibrational modes are directly influenced by changes in the hydrogen bonds between the water molecules. For example, if the temperature of water is raised significantly, the hydrogen bonds attracting the water molecules to each other will weaken (and conversely the covalent –OH bonds will strengthen), changing the atomic geometry of the water molecules. Any geometric change in the molecule will then negatively alter the absorption peak for water (because the vibrational frequencies are also altered), and this will make Er:YAG lasers less efficient.⁶

Therefore, dental practitioners using Er:YAG lasers are dealing fully and entirely with the quantum interactions of 2.94-µm optical energy and the thermal and quantum changes that can occur to the target chromophore (molecular water). These quantum realities have enormous significance for the ablation (laser cutting) protocols used by dentists. Specifically, an attempt must be made to keep heat production at a minimum during ablation procedures and hence prevent a negative shift in the absorption peak for water.

NONCONTACT HARD-TISSUE ABLATION WITH ER:YAG LASERS
One review of the literature indicated that a major clinical advantage of an Er:YAG laser for dental applications is its ability to ablate teeth and bone with minimal thermal damage.⁸ The photobiology of the Er:YAG-enamel interaction has been defined.^9-11 Water molecules within the prismatic enamel layer of a tooth make up only 4% of its chemical composition but represent 11% of enamel’s total volume. As the Er:YAG laser energy interacts with the enamel matrix, water absorbs the laser energy and rapid vaporization occurs. As this photothermal reaction takes place, the steam generated within the enamel is associated with a volumetric expansion and greatly increased pressure within the enamel matrix. This in turn produces microevaporative explosions that result in a thermally driven, mechanical ablation of the tooth structure.¹¹

A focus has been on the effect of water spray and thermal load on the ablation of calcified tissues. In reference to teeth, Wigdor and Visuri^12,13 found that as long as enamel is continually hydrated with a water spray during laser exposure, the temperature of the enamel and deeper tissues will increase only minimally as the ablation reaction continues. Here the water supply serves as a heat sink. If quantum mechanics is applied to the above experimental findings with the laser cutting of calcified tissue, Er:YAG laser photons are in effect packets of optical energy striking a tooth. These quantized packets do not behave in a classical Newtonian sense (ie, photons are not akin to colliding billiard balls or bullets ballistically imparting a force, thereby fracturing or abrading a tooth).¹⁴ As the energy of the Er:YAG photons is absorbed by the chromophore (molecular water), it is converted to vibrational and rotational energy within the target molecules, which is the molecular basis for heat. It is this transfer of energy (to water) that will cause expansion and pressure in a confined environment (the tooth), leading to the explosive “thermal-mechanical ablation” of tooth structure.⁶

With this understanding, it has become widely accepted that efficient ablation of biological tissues with pulsed Er:YAG lasers is always associated with a concomitant ejection of material. This material ejection occurs as a result of a phase transition in the water that is embedded within the tissue matrix, and the speed of material ejection and depth of the ablation crater appear to depend on the mechanical confinement strength of the extracellular tissue matrix (ie, soft tissue or hard tissue, with hard tissue requiring more energy for adequate ablation and tissue ejection).^15-19

Using the above reasoning to explain Wigdor and Visuri’s data, by continually hydrating the enamel of the irradiated teeth with a water spray at the irradiation site, any significant temperature increase is kept to a minimum (ie, the water spray acts as a heat sink), the negative shift in the absorption coefficient is prevented, and the spray aids in washing away the boiling ejected material (products of ablation), enabling the reaction (thermal mechanical ablation) to continue smoothly.

Reviewing the medical literature, the same logic holds true for bone ablation with Er:YAG lasers. As many dental practitioners are currently using different commercially available erbium laser systems to cut bone during oral surgery, the results by Truong et al²⁰ investigating Er:YAG ablation of the osseous nasal dorsum should also be considered. Truong established that irradiating at 300-microsecond pulses and 10Hz, plus a continuous water spray on the area of irradiation during Er:YAG ablation, resulted in the lowest level of thermal damage. These are the same quantum heat considerations a dental clinician must be aware of when ablating teeth, and are of equal or greater importance when cutting bone, as bone is vital tissue. To prevent a shift towards greater heat and a negative absorption shift for water, the data suggest that a continuous water spray on the osseous site being irradiated (acting as a heat sink) is the best way to avoid unwanted thermal sequelae.

Two experiments by Vodopyanov^21,22 confirm that a cooling water spray directly on the ablation site prevents poor ablation and heat buildup. He verified the effect of temperature on the absorption of Er:YAG laser radiation in water and presented data showing that the peak 2.94-µm absorption coefficient of water will decrease as energy deposition (ie, heat) from the laser increases, increasing the kinetic (vibrational) energy of the water and hence weakening the hydrogen bonds. More recent data²³ present essentially the same picture for the Er:YAG-water interactions. This study by Shori, et al reported the following:

(1) As water absorbs more of the incident energy of an Er:YAG laser and the temperature of the water increases, the length and strength of the OH bond in the water molecule changes because of the large increase in kinetic energy.
(2) When this important phenomenon occurs, the absorption peak for the water molecule shifts to wavelengths that are significantly shorter than 2.94 µm.
(3) The peak water absorption coefficient for Er:YAG photons at high laser intensities (more energy and heat) drops to the equivalent of only 25% of the water molecules’ original absorption ability at room temperature. This can occur almost instantaneously as a lazed area heats up. This negative shift in the absorption peak for water greatly diminishes the ability of the beam to perform thermal mechanical ablation of tissues.

These findings describing Er:YAG ablation dynamics have profound clinical implications for the interaction of these lasers with mineralized dental tissues, particularly with the use of contact energy delivery tips. For example, as the Shori conclusions are directly applicable to nonmineralized (soft) tissue, energy needed for hard-tissue ablation will be greater, meaning the heat produced by the laser will be greater, and residual heat from the ablation will remain in the mineralized tooth or bone longer than what would occur in soft tissue. If a clinician is using a small-diameter contact tip in an incorrect manner and happens to embed the tip within calcified tissue (tooth or bone), a second negative quantum occurrence will transpire that will fuel a marked heat increase in the system and greatly inhibit further ablation. This second occurrence is based on the Pauli exclusion principle of quantum mechanics (described below) and should be avoided.^3,14

CONTACT CUTTING OF CALCIFIED TISSUES WITH ER:YAG LASERS AND THE PAULI EXCLUSION PRINCIPLE
It has been shown that an adequate supply of water directed to the target tissue and the explosive ejection of boiling products of ablation are of paramount importance for efficient Er:YAG ablation. If properly performed, overheating, cracking, and melting of calcified biologic structures during the ablation (which can occur with either contact or noncontact energy delivery systems) will not occur. However, as ablation (laser drilling) occurs in these tissues and a crater forms, the quantum rules that have been described begin to change and need to be considered.

First, if an adequate water spray is not directed into the crater, a heat buildup at the tooth-beam interface will occur, and the products of ablation will start to accumulate. As the ablation crater becomes deeper and less water is reaching the reaction, the temperature in the deepest part of the crater will rise. This will occur almost immediately if a contact tip is being used by a practitioner in the same manner as a drill bit (ie, the Newtonian concept of pushing it into a tooth like a rotating handpiece) and it becomes embedded in the ablation crater. To prevent this from occurring, a second quantum principle of contact laser ablation must be understood. If improperly performed, it is at this point that the Pauli exclusion principle will present itself and must be considered.^3,14

The Pauli exclusion principle is a fundamental rule of quantum mechanics that states that 2 things cannot occupy the same space at the same time. In other words, solid matter will keep out even the tiniest pieces of other matter. If we apply this quantum principle to Er:YAG contact-tip ablation of calcified tissues, the application of the law becomes clear. For example, if a small-diameter (600 µm or less) contact sapphire tip (ie, a solid piece of matter) becomes embedded within the calcified tissue being ablated, an impermeable barrier will be formed around the tip. This occurs because the diameter of the tip increases in a cone-like pattern from the apex of the tip (600 µm) to where it is attached to the laser handpiece (1,000 µm). Once this happens, irrigating water will not be allowed into the ablation crater because the tip is occupying the volume of the crater. This is the Pauli exclusion principle. Also, the boiling material and ablation products from the ablation reaction at the apex of the crater cannot exit and be ejected from the crater for the same reason, ie, the solid sapphire tip is now in the way and filling the volume of the crater (Figures 4 and 5).

Concurrently, it is important to remember that all lasers deliver optical energy in one direction. In other words, they are end-cutting devices with ablation occurring in a straight directional vector from the end of the tip. When using a small-diameter contact tip, the laser energy is more focused with a smaller spot size, and the laser beam will be intensified without increasing the power setting of the laser or the duration of application. This phenomenon can be explained by the following equation: power density (W/cm²) = total power (W)/spot size (cm²).

This equation indicates that the power density (brilliance) of the beam increases as the spot size of the beam becomes smaller. Therefore, precise and controlled ablation of teeth or bone in extremely small areas can be accomplished with the Er:YAG laser. However, as a smaller ablation crater becomes deeper, it becomes increasingly difficult to inhibit negative thermal sequelae. A dental laser practitioner must be aware of this, because if one of these solid sapphire tips becomes embedded within calcified tissue being cut, the following will occur: (1) water is not entering the crater, (2) boiling ejected material is not leaving the creater (dissipating heat), and (3) the laser is still firing into the tissue, directly in a straight vector toward the apex of the crater.

If this series of events occurs, there will be a profound negative thermodynamic shift to the reaction, and ablation will stall. This phenomenon is referred to by this author as the “quantum heat trap.”

THE QUANTUM HEAT TRAP AND THE ER:YAG LASER WITH A CONTACT TIP
The quantum heat trap occurs as a consequence of 2 phenomena. The first is the clinical manifestation of the Pauli exclusion principle. This occurs because the addition of exogenous water to the ablation site is completely blocked. The heat from the laser is trapped as the boiling material ejected from the ablation area is prevented from leaving the ablation crater. As previously described, this occurs because the physical volume of the solid sapphire contact tip is filling the crater. The second is the continuous deposition and “stacking” of pulses from the laser into the apex of the ablation crater, triggering a negative absorption shift for water to lower wavelengths. This is represented in Figure 6.

Another way of describing the quantum heat trap can be found in the work of Majaron, et al.²⁴ They performed a threshold and efficiency analysis of Er:YAG laser ablation of hard dental tissue. An interesting model of heat diffusion and ablation dynamics was described, and they entered the term “ablation front” into the dental laser literature. The ablation front is the physical distance, or depth into a solid structure, at which the laser pulse will cause ablation. If the thermal front of the reaction (heat diffusion into the system from the laser) overtakes the ablation front of the reaction (as could immediately happen with a continuously firing embedded solid sapphire tip for the reasons previously discussed), then a stall-out effect in the ablation reaction will occur. Therefore, with any delivery system (contact or noncontact) the operator is constantly attempting to keep the ablation front of the laser-tissue interaction ahead of the thermal front. However, with a contact tip delivery system, greater care is required.

METHOD TO AVOID THE QUANTUM HEAT TRAP
A practitioner using an erbium dental laser on hard tissue should never keep the contact energy delivery tip in one place on the tooth for more than 2 to 4 seconds at a time. Within the area the laser will cut, the clinician should at all times combine an up-and-down motion with a side-to-side motion. Use of these motions (instead of pushing the tip into the solid tissue like a bur) will prevent negative quantum interactions from occurring. These recommendations will effectively allow the following:
(1) the boiling products of ablation to explosively exit from a growing crater, and (2) allow more water into the crater to act as a heat sink and flushing mechanism (Figures 7 and 8).

Without this approach, the thermal front of the Er:YAG-tooth ablation reaction will overtake the ablation front of the reaction. The result will be a negative heat buildup and a fall into the quantum heat trap with ablation “stall out.”

Equivalence in morphological features and chemical composition of bone was documented experimentally by Sasaki and coworkers.²⁵ They compared contact bone cutting with an Er:YAG laser to a dental bur. In this study, Sasaki et al examined the bony surface after Er:YAG laser ablation at an output energy of 100 mJ/pulse in contact mode with an incident angle of approximately 30º to the moistened surface of the bone under constant saline irrigation at a pulse rate of 10 Hz.

Sites drilled using a conventional micromotor were used as controls at 10,000 rpm also with an angle of approximately 30º and constant saline irrigation. Analysis involved the use of a scanning electron microscopy (SEM) and Fourier transformed infrared (FTIR) spectroscopy.

It was found that Er:YAG laser ablation (in the contact mode) produced a groove with similar dimensions to that produced by bur drilling. SEM observations revealed that the groove produced by the Er:YAG laser had well-defined edges and a smear layer-free surface with a characteristically rough appearance and entrapped fibrin-like tissue. Additionally, the FTIR spectroscopy revealed that the chemical composition of the bone surface after contact Er:YAG laser ablation was much the same as that following bur drilling. The authors were able to show that Er:YAG contact laser irradiation with irrigation at a 30º angle (presumably to avoid embedding the contact tip in the tooth) and bur drilling were essentially equivalent (Figures 9 and 10).

These results were essentially duplicated by Aoki, et al²⁶ observing substantial equivalence in bony cuts, morphology, and chemical composition of bone when the cuts were made with a contact Er:YAG laser or a rotating bur.

CONCLUSIONS
The following conclusions can be drawn:

The conventional New-tonian mechanics associated with drilling hard dental tissues, including bone, must be superseded with nontraditional quantum mechanics when drilling calcified structures with a Er:YAG laser in the contact mode.
The target chromophore of water undergoes significant changes in its ability to further absorb Er:YAG laser radiation if there is a large temperature change in the system being irradiated.
By changing the approach and protocol when cutting with Er:YAG lasers and contact tips, the quantum heat trap leading to “stall out” can be avoided.

For the Er:YAG dental laser systems, a practitioner must make maximum use of the water spray focused on the ablation area. All available data would preclude the use of Er:YAG lasers for “closed” or “flapless” surgical procedures for many of the reasons discussed above. If these systems are used with side-to-side and up-and-down motions as well as copious irrigation at the site of ablation, it will promote the most efficient and least damaging ablation of tooth and bone.

References

1. Baum L. Textbook of Operative Dentistry. 2nd ed. Philadelphia, Pa: WB Saunders; 1985.
2. Stannard JG. Materials in Dentistry. 2nd ed. Hanover, Mass: Denali Publishing; 1988.
3. Serway R. Physics for Scientists and Engineers With Modern Physics. 2nd ed. Philadelphia, Pa: Saunders College Publishing; 1986:93-95, 991.
4. Hewitt PG. Conceptual Physics. 9th ed. San Francisco, Calif: Addison Wesley Pub Co; 2001:496-635.
5. Niemz MH. Laser-Tissue Interactions: Fundamentals and Applications. 2nd rev ed. Berlin, Germany: Springer-Verlag; 2002.
6. Welch AJ, Van Gemert MJC. Optical-Thermal Response of Laser-Irradiated Tissue. New York, NY: Plenum Pub Corp; 1995:865-902.
7. Hale GM, Querry MR. Optical constants of water in the 200 nm to 200 µm wavelength region. Appl Opt. 1973;12:555-563.
8. Bornstein ES, Lomke MA. The safety and effectiveness of dental Er:YAG lasers: a literature review with specific reference to bone. Dent Today. Oct 2003;22:129-133.
9. Walsh JT Jr, Flotte TJ, Deutsch TF. Er:YAG laser ablation of tissue: effect of pulse duration and tissue type on thermal damage. Lasers Surg Med. 1989;9:314-326.
10. Walsh JT Jr, Deutsch TF. Er:YAG ablation of tissue: measurement of ablation rates. Lasers Surg Med. 1989;9:327-337.
11. Moshonov J, Stabholz A, Leopold Y, et al. Lasers in dentistry. Part B—Interaction with biological tissues and the effect on the soft tissues of the oral cavity, the hard tissues of the tooth and the dental pulp [in Hebrew]. Refuat Hapeh Vehashinayim. 2001;18(3-4):21-28, 107-108.
12. Wigdor HA, Visuri SR, Walsh JT Jr. Effect of water on dental material ablation of the Er:YAG laser. Proc SPIE. 1994;2128:267-272.
13. Visuri SR, Walsh JT, Wigdor HA. Erbium laser ablation of hard tissue: Control of the thermal load. Proc SPIE. 1994;2134A;130-133.
14. Hey T. The New Quantum Universe. 2nd ed. Cambridge, England: Cambridge University Press; 2003:1-15, 107-111.
15. Nahen K, Vogel A. Shielding by the ablation plume during Er:YAG laser ablation. Proc SPIE. 2001;4257:282-297.
16. Hibst R, Keller U. Experimental studies of the application of the Er:YAG laser on dental hard substances: I. Measurement of the ablation rate. Lasers Surg Med. 1989;9:338-344.
17. Walsh JT, Deutsch TF. Measurement of Er:YAG laser ablation plume dynamics. Appl Phys B. 1991;52:217-224.
18. Venugopalan V, Nishioka NS, Mikic BB. The thermodynamic response of soft biological tissues to pulsed infrared-laser irradiation [published correction appears in Biophys J. 1996;71:3530]. Biophys J. 1996;70:2981-2993.
19. Majaron B, Plestenjak P, Lukac M. Thermo-mechanical laser ablation of soft biological tissue: modeling the micro-explosions. Appl Phys B. 1999;69:71-80.
20. Truong MT, Majaron B, Pandoh NS, et al. Erbium:YAG laser contouring of the nasal dorsum: a preliminary investigation. Proc SPIE. 2001;4244:113-120.
21. Vodopyanov KL. Bleaching of water by intense light at the maximum of the lambda equals 3 µm absorption band. Sov Phys JETP. 1990;70:114-121.
22. Vodopyanov KL. Saturation studies of H2O and HDO near 3400 cm-1 using intense picosecond laser pulses. J Chem Phys. 1991;94:5389-5393.
23. Shori RK, Walston AA, Stafsudd OM, et al. Quantification and modeling of the dynamic changes in the absorption coefficient of water at lambda equals 2.94 µm. IEEE Journal of Selected Topics in Quantum Electronics. 2001;7:959-970.
24. Majaron B, Lukac M, Sustercic D, et al. Threshold and efficiency analysis in Er:YAG laser ablation of hard dental tissue. Proc SPIE. 1996;2922:233-242.
25. Sasaki KM, Aoki A, Ichinose S, et al. Scanning electron microscopy and Fourier transformed infrared spectroscopy analysis of bone removal using Er:YAG and CO2 Lasers. J Periodontol. 2002;73:643-652.
26. Aoki A, Yoshino T, Akiyama F, et al. Comparative study of Er:YAG laser and rotating bur for bone ablation. Int Congr Ser. 2003;1248:389-391.

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 practices general, implant, and laser dentistry in Natick, Mass. Dr. Bornstein can be reached at drericdmd@mindspring.com.

Disclosure: Dr. Bornstein is the chief science officer for NOMIR Medical Technologies, a compa