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Clarification of Femtosecond Laser Terminology: Energy, Fluence, Dose

2025; Slack Incorporated (United States); Volume: 41; Issue: 1 Linguagem: Inglês

10.3928/1081597x-20241120-02

1938-2391

Holger Lubatschowski, Jodhbir S. Mehta,

Intraocular Surgery and Lenses

Guest Editorial freeClarification of Femtosecond Laser Terminology: Energy, Fluence, Dose Holger Lubatschowski, PhD, ; , PhD Jodhbir S. Mehta, PhD, FRCS(Ed), , PhD, FRCS(Ed) Holger Lubatschowski, PhD and Jodhbir S. Mehta, PhD, FRCS(Ed) Address correspondence to Jodhbir S. Mehta, PhD, FRCS(Ed), Singapore National Eye Center, 11 Third Hospital Avenue, Singapore 168751; email: E-mail Address: [email protected]. Journal of Refractive Surgery, 2025;41(1):e2–e4Cite this articlePublished Online:January 01, 2025https://doi.org/10.3928/1081597X-20241120-02 PDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookxLinkedInRedditEmail SectionsMoreIntroductionThe application of femtosecond laser technology in the field of refractive corneal surgery has become a well-established practice, as evidenced by the growing number of keratotorefractive lenticule extraction (KLEx)1 procedures. Currently, more than 10 million procedures have been performed, available on four clinical platforms.2 Over time, the procedures have become increasingly predictable and, most importantly, more precise. The accuracy of femtosecond laser photodisruption has improved in conjunction with a more nuanced comprehension of laser–tissue interactions. Some authors have begun to use the term dissection instead of disruption, emphasizing the smoother, more controlled tissue separation of modern femtosecond lasers that leads to a more predictable outcome. At least seven parameters influence the quality of femtosecond laser dissection and, consequently, the clinical outcome. These parameters include pulse energy, focal diameter, pulse repetition rate, pulse duration, the arrangement of pulses (spot spacing), temporal and spatial intensity distribution in the focus (spot shape), and wavelength (Figure 1).3Figure 1. At least seven laser parameters influence the incision quality and thus the clinical outcome in corneal surgery. Created using data from Rathjen C, Steinlechner M. Lenticule and interface properties in CLEAR®. In: Sekundo W, Wagner FM. Femtosecond Laser Assisted Lenticule Extraction. Springer Nature Switzerland AG; 2024:373–382.In the past, two parameters have been identified as contributing to a measurable improvement in the procedure: pulse energy and repetition rate. Although an increase in repetition rate primarily results in an increase in cutting speed and a reduction in cutting time, a reduction in pulse energy has been demonstrated to contribute significantly to the precision of the cuts and reduced tissue reactions.4–6 In the context of KLEx, cutting accuracy is of paramount importance, because two cuts are made on top of each other, and any unevenness in both cuts can contribute to aberrations. This has prompted a trend among manufacturers to reduce the pulse energy.In recent times, laser systems have been compared on the basis of individual laser parameters. This approach to comparison is fundamentally sound and important for user orientation. However, it is also important to clarify that the totality of the parameters determines the dissection process and not just a few isolated ones. In this context, the term "dose" has recently caused confusion in connection with the application of laser pulses. Indeed, the term "overdose" has even been used in this context.In addition to its use in toxicology and pharmacy, the term "dose" is employed in radiation physics, wherein it is expressed in energy per mass (J/kg). One Gray is defined as one joule of absorbed ionizing radiation per kilogram of body tissue. The term is not limited to the effects of ionizing radiation (radioactivity, x-rays), but is also frequently used in conjunction with ultraviolet radiation, particularly when photochemical processes or mutagenic effects are of consequence.The term "dose" is not commonly used in the context of laser applications. Alternative terms such as "energy," "fluence," or "energy density" are more prevalent and offer a more meaningful understanding (Table 1).Table 1 Comparison of Different Energy-related Physical Parameters in the Context of Their UseTermUnitCommentEnergyJoule (J)Can refer to the emitted or absorbed energy of a single pulse or to the total energy of several pulses.Fluence, radiant exposureJ/cm2The irradiated energy per area determines the type of interaction. Particularly in non-linear processes such as ablation or dissection. This physical quantity indicates the threshold above which a desired process takes place (eg, optical breakdown) and a corresponding effect occurs (eg, dissection).Energy densityJ/cm3It results from the fluence in conjunction with the penetration depth of the radiation and describes the energy required to vaporize or ionize a certain volume. In photoablation with excimer lasers, this is the optical penetration depth of the radiation (fractions of µm); in photodissection with infrared fs pulses, it is independent of the material/tissue.DoseGray (J/kg)Describes the absorbed energy per mass.The cumulative effect of individual radiation exposures over a longer period of time is a function of the dose. The cumulative effect of ultraviolet or x-ray radiation applied at different times can be significant. For example, radiation applied today can have a damaging effect that is additive to the damaging effect of radiation applied a day or a month earlier or later. During anterior segment surgery with a near infrared femtosecond laser, the temperature in the immediate vicinity of the interaction site exhibits a brief increase of less than 0.4 degrees Celsius, followed by a rapid decline within milliseconds. The energy of infrared light does not lead to accumulated effects over a longer period of time. Although one may enjoy the warmth of a fireplace for several days in a row, it is important to consider the ultraviolet dose absorbed when sunbathing over several hours or the x-ray radiation absorbed during x-ray diagnostics even over months and years.Furthermore, in the context of the laser effect of femtosecond laser pulses on the cornea, the term "dose" is sometimes incorrectly used with the unit J/cm2. Apart from the unusual unit, this concept of dose is a combination of only two (pulse energy and spot spacing) out of the seven fundamental process parameters (Figure 1) and is therefore inherently ambiguous and lacks meaning. The application of a few pulses with high pulse energy and large spot spacing can result in the same dose value as that achieved with many pulses with low pulse energy and small spot spacing. However, the resulting surgical outcome will be different. It is instructive to recall the early days of femtosecond lasers, when several microjoules per pulse were used in the cornea with spot spacings corresponding to a multiple of the focal diameter.Furthermore, the emitted pulse energy of a laser cannot be definitively attributed to the absorbed pulse energy at the laser focus. Different laser processes convert different amounts of photon energy into mechanical and thermal energy. In ultraviolet photoablation (193-nm excimer laser radiation), for example, the pulse energy is totally absorbed within fractions of a micrometer of the tissue and converted into vaporization energy. In the case of photodissection with near-infrared light, on the other hand, a significant portion of the light is transmitted.7 In addition, when higher pulse energies are used, a considerable part of the light energy is converted into mechanical energy (cavitation) and used to separate the tissue. The stroma is cleaved beyond the focal diameter along the lamellar structure of the cornea and thus does not always follow the intended course of the dissection line. At lower pulse energies, just above the threshold for optical breakdown, this mechanical cleaving is less pronounced. The tissue separation (photodissection) occurs almost exclusively within the laser focus. Because many more pulses are required, the process is less energy efficient, but significantly more precise.All commercially available femtosecond lasers have been tested and have been demonstrated to be safe. This is backed up by more than 20 years of experience and evidence. In this context, the introduction of the term "dose" is not incorrect. However, it is not useful and there is a risk that the term "dose," which often has negative connotations (ultraviolet radiation, x-rays, radioactive radiation), may raise fears in patients and perhaps also in some physicians or even authorities that are completely unfounded. This could unnecessarily jeopardize the reputation of an extraordinarily safe technology, prevent new applications with potentially higher exposure, and ultimately harm the entire industry.1.Dupps WJ, Randleman JB, Kohnen T, Srinivasan S, Werner L; Scientific Nomenclature for Keratorefractive Lenticule Extraction. Scientific nomenclature for keratorefractive lenticule extraction (KLEx) procedures: a joint editorial statement. J Refract Surg. 2023; 39(11):726–727. 10.3928/1081597X-20231010-0 PMID:37937757 > LinkGoogle Scholar2.Carl Zeiss Meditec. ZEISS expands ophthalmic offerings to improve patient care with new digital AI tools and revolutionary surgical solutions [press release]. October 4, 2024. Accessed October 15, 2024. https://www.zeiss.com/meditec-ag/en/media-news/press-releases/2024/zeiss-at-aao.html#contact; latest access > Google Scholar3.Rathjen C, Steinlechner M. Lenticule and interface properties in CLEAR®. In: Sekundo W, Wagner FM. Femtosecond Laser Assisted Lenticule Extraction. Springer Nature Switzerland AG; 2024:373–382. > CrossrefGoogle Scholar4.Lubatschowski H. Overview of commercially available femtosecond lasers in refractive surgery. J Refract Surg. 2008; 24(1):S102–S107. 10.3928/1081597X-20080101-18 PMID:18269159 > LinkGoogle Scholar5.Riau AK, Angunawela RI, Chaurasia SS, Tan DT, Mehta JS. Effect of different femtosecond laser-firing patterns on collagen disruption during refractive lenticule extraction. J Cataract Refract Surg. 2012; 38(8):1467–1475. 10.1016/j.jcrs.2012.03.037 PMID:22814054 > Crossref MedlineGoogle Scholar6.Riau AK, Liu YC, Lwin NCet al.. Comparative study of nJ- and µJ-energy level femtosecond lasers: evaluation of flap adhesion strength, stromal bed quality, and tissue responses. Invest Ophthalmol Vis Sci. 2014; 55(5):3186–3194. 10.1167/iovs.14-14434 PMID:24764066 > Crossref MedlineGoogle Scholar7.Vogel A, Noack J, Nahen Ket al.. Energy balance of optical breakdown in water at nanosecond to femtosecond time scales. Appl Phys B. 1999; 68(2):271–280. 10.1007/s003400050617 > CrossrefGoogle Scholar Previous article Next article FiguresReferencesRelatedDetails Request Permissions InformationCopyright 2025, SLACK IncorporatedPDF download • 188.6 KBAddress correspondence to Jodhbir S. Mehta, PhD, FRCS(Ed), Singapore National Eye Center, 11 Third Hospital Avenue, Singapore 168751; email: jodhbir.s.mehta@singhealth.com.sg.From Independent Consultant, Hanover, Germany (HL); Gottfried Wilhelm Leibniz Universität Hannover, Hanover, Germany (HL); Singapore National Eye Center, Singapore (JSM); Singapore Eye Research Institute, Singapore (JSM); and Ophthalmology and Visual Sciences Academic Clinical Programme, Duke-NUS Medical School, Singapore (JSM).Disclosure: JSM has received honoraria from Carl Zeiss Meditec, Moria, Teleon, and Ziemer. HL is a consultant for SCHWIND eye-tech-solutions GmbH and Ziemer Ophthalmic Systems AG, and has other financial or non-financial interests in Ziemer Ophthalmic Systems AG. Published online1/01/25