Laser Education, Science and Safety by Peter Vitruk, PhD, MInstP, CPhys, DABLS

Introduction
The "sound scientific basis and proven efficacy in order to ensure public safety" is one of the main eligibility requirements of the ADA CERP recognition standards and procedures.¹ The scientific foundation for understanding soft-tissue laser ablation and coagulation is based on the soft tissue's light scattering and absorption spectra.^2–7

Unfortunately, some laser dentistry educational programs and publications include misinterpretations about soft- and hard-tissue laser science and safety. Such misrepresentations partially take their origin in the laser dentistry curriculum guidelines, which date back to the early 1990s.⁸

In this article, I'll discuss some important laser-tissue interaction concepts—ones that are missing from the vocabulary of the Laser Dentistry Curriculum Guidelines and Standards,⁸ namely absorption spectra,^2–7 hot glass tip^9–11 and plasma plume.^12,13

Absorption spectra and soft-tissue laser ablation
A chromophore is defined as a molecule or substance capable of absorbing specific laser wavelengths. The main chromophores for ablation and coagulation of oral soft tissue are known to be hemoglobin, oxyhemoglobin, melanin and water.^2–7 These four chromophores are distributed unevenly within oral tissue. Water and melanin, for example, reside in the 100–300-µm-thick epithelium;¹⁴ water, hemoglobin and oxyhemoglobin reside in the subepithelium (lamina propria and submucosa).^15,16

Each of the oral soft tissue's four main chromophores has a known optical absorption coefficient spectrum. Fig. 1 presents absorption spectra for the different chromophore concentrations of water, melanin, hemoglobin (Hb) and oxyhemoglobin (HbO2). Light scattering by the soft tissue is insignificant at erbium and CO2 laser wavelengths. Light scattering by the soft tissue dominates over absorption at near-IR diode and Nd:YAG laser wavelengths, and facilitates a wider-spread coagulation and thermal damage. ^2–7

Fig. 1 illustrates how the oral epithelium (e.g., at 75 percent water and 2 percent melanin) absorbs the Nd:YAG and diode laser wavelengths in the 800–1,100nm range 100–1,000 times less efficiently than the CO2 and erbium laser wavelengths. Fig. 1 also illustrates that the near-infrared Nd:YAG and diode laser wavelengths in the 800–1,100nm range are absorbed by the oral subepithelial soft tissue (e.g., at 75 percent water and 10 percent blood) approximately 1,000–10,000 times less efficiently than the CO2 and erbium laser wavelengths.

The shallower the absorption depth (i.e., stronger absorption), the less energy is required to ablate the tissue within the exposed volume. Therefore, the mid-infrared erbium and infrared CO2 laser wavelengths are highly efficient and spatially accurate laser ablation tools because of their very strong absorption by the soft tissue. The deeper the absorption depth (i.e., weaker absorption) and the stronger the scattering, the more energy is required to ablate the tissue. Therefore, the near-IR diode and Nd:YAG laser wavelengths are highly inefficient and spatially inaccurate photothermal laser ablation tools¹⁷ because of their weak absorption by the soft tissue.

Hot glass tip
The near-infrared wavelengths of dental diode lasers cannot photothermally ablate soft tissue, except for high-melanin-content epithelium.^4–7 Instead, the near-infrared diode laser beam heats the charred distal end of its fiber optic glass tip to 500–900 degrees Celsius.^9–11 The glowing hot glass tip, then, conducts heat to the soft tissue.^{9–11, 18}

Soft tissue is burned on contact with the hot glass tip. The efficacy of this device-tissue interface (charred hot glass surface) is highly dependent on multiple factors:

• The user's technique and skill in charring the glass tip.
• The user's hand speed and tip-tissue contact duration.^9-11
• Degradation of the glass tip's char, which reduces tip temperature and increases the near-infrared-induced subsurface thermal-induced tissue necrosis, and leads to mechanical tearing of the tissue by the glass tip's edges.
• Biocompatibility and sterility of the char that's produced by burned ink or corkwood when applying the hot tip to the soft tissue.^19–20
• Biocompatibility of the hot glass and its cladding materials at 500–900 degrees Celsius operating temperatures when applying the hot tip to the soft tissue.¹⁹
• Biocompatibility of fractured glass produced by the thermal gradient- induced fractures of the hot glass tip at 500–900 degrees Celsius operating temperatures.²¹

Plasma plume
The ease of the soft-tissue CO2 laser surgery (Fig. 2a) is largely based upon the low-temperature water vaporization at 100 degrees Celsius. The collateral damage in the heat-affected zone is much appreciated coagulation and hemostasis. In some hard-tissue cutting applications, however, a very high ablation temperature (approximately 5,000 degrees Celsius) could result in extremely bright thermal radiation (Figs. 2b and 2c).

The hard-tissue laser's "beam interactions with the hard tissue can generate intense plasma emissions … requiring suitable optical filtering for direct viewing," while "plasma emissions … may contain sufficient UV," requiring the UV exposure limits to be addressed. Also, the high brightness of the hydroxyapatite plasma in the visible spectrum (Figs. 2b and 2c) may interfere with the laser's pointing accuracy by affecting the target's visibility, because of the enamel's high translucence and light scattering.^{12, 13, 22, 23, 24}

Summary
The science-based absorption properties of the soft tissue can adequately explain the different ablative and coagulative properties of practical lasers at different wavelengths. Such explanation depends critically on the concentration of the chromophores in the tissue. Similarly, the science-based absorption properties of the soft tissue explains that the only practical modality of soft-tissue cutting with practical diode and Nd:YAG lasers is the "hot tip."

Last, but not least, the nonbeam laser hazards and respective safety measures need to be addressed in view of the optical emission spectrum of the plasma plume created during the hard-tissue cutting with the 9,300nm lasers. All the above are absent from the laser dentistry education guidelines and standards,8 but are critically important to be a part of the CERP-approved laser dentistry education in compliance with the ADA CERP standards.¹

References
1. Recognition Standards and Procedures. Commission for Continuing Education Provider Recognition. Chicago, IL. March 2015, ADA.org/CERP.
2. Jacques SL. Optical properties of biological tissues: a review. Phys Med Biol. 2013;58(11):R37-61.
3. Jacques SL. Origins of tissue optical properties in the UVA, visible, and NIR regions. In: Alfano RR, Fujimoto JG, ed. OSA TOPS on Advances in Optical Imaging Photon Migration. Optical Society of America 1996;2:364–69.
4. Fisher JC. Basic laser physics and interaction of laser light with soft tissue. In: Shapshay SM. ed. Endoscopic laser Surgery Handbook, New York, NY: Marcel Dekker; 1987:96-125.
5. Fisher JC. Qualitative and quantitative tissue effects of light from important surgical lasers. In: Wright CV, Fisher JC, ed. Laser surgery in gynecology: a clinical guide. Philadelphia, PA: Saunders; 1993:58-81.
6. Vogel A, Venugopalan V. Mechanisms of pulsed laser ablation of biological tissues. Chem Rev. 2003;103(2):577-644.
7. Vitruk P. Oral Soft Tissue Laser Ablative & Coagulative Efficiencies Spectra. Implant Practice US, 2014;7(6):22-27.
8. White JM, Barr RE, Bawden B, Cecchini SCM, Coluzzi DJ, Gianni W, Gilio DA, Goldstein AJ, Gregg RH, Martin E, McCarthy DK, Monzon G, Park JS, Parkins FM, Pietrini DR, Rechmann P, Rice JH, Roshkind DM, Siminovsky WA, Sulewski JG, Sun GS, Trevino E. Curriculum guidelines and standards for dental laser education. University of California, San Francisco School of Dentistry. SPIE'1993 Symposium Proceedings, Vol. 1880.
9. Levine R, Vitruk P. Hemostasis and Coagulation with Ablative Soft-Tissue Dental Lasers and Hot-Tip Devices. Inside Dentistry. 2016 Aug;12(8):37-42.
10. Romanos GE. Diode laser soft-tissue surgery; Advancements aimed at consistent cutting, improved clinical outcomes. Compendium Contin Educ Dent 2013;34(10):752-758.
11. Romanos GE, Belikov AV, Skrypnik AV, et al. Uncovering dental implants using a new thermo-optically powered (TOP) technology with tissue air-cooling. Lasers Surg Med. 2015:47(5):411-420.
12. ANSI Z136.3 Safe Use Of Lasers in Health Care. 2011.
13. ANSI Z136.1 Safe Use Of Lasers. 2014.
14. Prestin S, Rothschild SI, Betz CS, Kraft M. Measurement of epithelial thickness within the oral cavity using optical coherence tomography. Head Neck 2012;34(12):1777-1781.
15. Squier CA, Brogden KA, editors. Human oral mucosa: Development, structure, and function. Chichester, West Sussex, U.K.: Wiley-Blackwell, 2011:14-16.
16. Nanci A. Oral mucosa. Chapter 12 in: Nanci A. Ten Cate's oral histology: Development, structure, and function. (7tth edition.) St. Louis, Mo.: Mosby Elsevier, 2008:319-357.
17. Willems PWA, Vandertop WP, Verdaasdonk RM, van Swol CFP, Jansen GH. Contact laser-assisted neuroendoscopy can be performed safely by using pretreated 'black' fibre tips. Lasers Surg Med 2001;28(4):324-329.
18. van As G. The diode laser – Tip selection and initiation. http://www.amdlasers.com/pdf/Tip_Selection_and_Initiation.pdf. Accessed September 10, 2015.
19. ISO. Biological evaluation of medical devices – Part 1: Evaluation and testing within a risk management process. ISO 10993-1:2009, Fourth Edition. October 13, 2009.
20. Updated 510(k) sterility review guidance K90-1; Final guidance for industry and FDA. U.S. Department of Health and Human Services, Food and Drug Administration, Center for Devices and Radiological Health, Office of Device Evaluation. August 30, 2002:1-6.
21. Medical devices with sharps injury prevention features. Guidance for industry and FDA staff. U.S. Department of Health and Human Services, Food and Drug Administration, Center for Devices and Radiological Health, General Hospital Devices Branch, Division of Anesthesiology, General Hospital, Infection Control, and Dental Devices, Office of Device Evaluation. August 9, 2005:1-17.
22. Ultraviolet Radiation: TLV Physical Agents. 7th Edition. ACGIH, Cincinnati, OH. 2010.
23. Brodbelt RHW, O'Brien WJ, Fan PL, Frazer-Dib JG, Yu R. Translucency of Human Dental Enamel. J Dent Res. 1981; 60(10):1749-51.
24. Fried D, Glena RE, Featherstone JD, Seka W. Nature of light scattering in dental enamel and dentin at visible and near-infrared wavelengths. Appl Opt. 1995;34(7):1278-85.

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