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The Femtosecond Laser: a New Tool for Refractive and Corneal Surgery

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The Femtosecond Laser: a New Tool for Refractive and Corneal Surgery Chapter 7 The Femtosecond Laser: a New Tool for Refractive 7 and Corneal Surgery Mitchell P. Weikert, Anne Bottros Core Messages efits, such as optical precision, liberation from cornea–blade contact, and flexibility in ablation ■ The short pulse width of the femtosecond pattern design. The femtosecond (FS) laser, with laser permits photodisruption of corneal its low energy levels and ultrashort pulse widths, tissue at lower energy levels, allowing for has emerged as the dominant laser platform used tighter spot placement and smoother ab- for LASIK flap creation. While mechanical mi- lation patterns. crokeratomes continue to be more common, use ■ The femtosecond laser creates flaps with of the FS laser has grown exponentially over the more predictable and flexible dimen- last few years. In addition to refractive applica- sions than mechanical microkeratomes. tions, the flexibility of the laser platform has seen ■ IntraLASIK achieves visual and refrac- its use extend beyond LASIK, into other forms tive results equivalent or slightly superior of corneal surgery. This chapter will discuss the to those of mechanical microkeratomes. fundamentals of FS laser mechanics and review ■ Although the femtosecond laser has an its clinical performance with regard to LASIK, excellent safety profile, it still carries the intracorneal ring segment implantation, and risk of complications, such as decentra- penetrating/lamellar keratoplasty. tion, diffuse lamellar keratitis, flap stria, and transient light sensitivity. ■ The femtosecond laser can be pro- 7.2 Mechanism of Action grammed to create channels for in- trastromal corneal ring segments, to The femtosecond laser achieves its surgical ef- produce more stable trephination pat- fect through a process termed “photodisruption”. terns in penetrating keratoplasty, and to Normally, the cornea is transparent to visible and prepare the donor and host for lamellar near-infrared (IR) wavelengths, with minimal to keratoplasties. no absorption of incident light. However, when the intensity (power per unit of area) of the fo- cused laser beam exceeds a certain threshold (approximately 1011–1012 W/cm2), pronounced 7.1 Introduction changes occur in the absorption characteristics of the tissue [14, 30]. Electrons within the tissue Laser-assisted in situ keratomileusis (LASIK) absorb incident photons and become liberated continues to be one of the dominant surgical when the added energy exceeds their ionization procedures employed to correct refractive errors. potential. These free electrons impact adjacent Until recently, flap creation in LASIK was entirely molecules, creating new ions to feed the pro- dependent upon blade-based, mechanical mi- cess. The resulting cascade converts the matter crokeratomes with their inherent disadvantages. to a collection of freely moving electrons and Although microkeratomes produce excellent re- ions, otherwise known as plasma. When energy sults, laser-based technologies offer certain ben- is removed from the system (i.e., the laser pulse is 84 The Femtosecond Laser: a New Tool for Refractive and Corneal Surgery completed), the electrons and ions recombine to 7.3 Clinical Applications form a gas, which rapidly expands within the tis- of the FS laser sue, creating both a cavitation bubble and shock wave. These forces produce cleavage of the tissue The IntraLase FS15 and FS30 (IntraLase, Irvine, surrounding the focal point of the laser, resulting CA, USA) are the only commercially available in photodisruption. versions of the femtosecond laser approved for As stated above, the laser intensity at its focal use in the United States. The FEMTEC (20/10 point must exceed a certain threshold to create Perfect Vision, Heidelberg, Germany) is a ver- photodisruption. Since power is defined as energy sion of the FS laser that is available in Europe. per unit of time, these high intensity levels can be Currently, the primary clinical application for achieved over a given focal area by increasing the the FS laser is the creation of the lamellar corneal laser’s energy or by decreasing its pulse duration. flap during laser in situ keratomileusis (LASIK). Near-IR lasers are available with pulse widths on However, given the flexibility of the laser plat- 7 the orders of nanoseconds (10–9 s), picoseconds form, surgical indications are rapidly expanding (10–12 s), and femtoseconds (10–15 s). Although to include such procedures as channel creation photodisruption can be achieved with each of for intracorneal ring segments, as well as various these lasers, the fluence (energy per unit of area) forms of penetrating and lamellar keratoplasty. necessary to produce optical breakdown will de- crease along with the pulse width. In addition, the predictability of the breakdown threshold 7.3.1 LASIK Using also increases with shorter pulse duration [11]. the Femtosecond Laser Lower energy thresholds and higher predict- ability translate into smaller cavitation bubbles, The term “IntraLASIK” has been coined to de- decreased shock waves, and improved control scribe laser in situ keratomileusis performed of tissue disruption [14, 20]. Laser spots can be with the IntraLase femtosecond laser. To create placed closer together, with reduced disruption the LASIK flap, the cornea is applanated with a of surrounding tissue, thinner intervening tissue flat glass lens. The laser energy is focused at a pre- septa, and smoother interfaces [12]. Because of cise distance from the applanation surface, which these factors and its poor clinical performance, determines the thickness of the flap. Laser spots the picosecond laser was abandoned in favor can be placed in a spiral pattern that progresses of the femtosecond laser. The FS laser’s narrow from the central cornea to the mid-periphery, pulse width and lower energy make it an ideal producing a lamellar cut. More commonly, they instrument for corneal surgery. Laser spots can are placed in successive lines that progress from be placed side by side to produce lamellar cuts one side of the cornea to the other in a raster pat- or stacked on top of each other to make vertical tern. After the flap thickness has been defined, incisions. the spots are stacked vertically in a circular pat- tern to create the side cut. The flap hinge is de- fined by a portion of the flap circumference that Summary for the Clinician is spared from the side cut. As discussed above, photodisruption produces gas in the interface. If ■ The short pulse width of the femtosec- the gas cannot escape from the interface, it will ond laser permits photodisruption of often penetrate the stroma causing temporary corneal tissue at lower energy levels. opacification of the cornea. To reduce this risk, ■ The lower energy, smaller cavitation a lamellar pocket can be produced by the laser bubbles and reduced shock waves allow adjacent to the hinge. This deeper opening can for tighter spot placement with smooth- provide a place for gas to vent, increasing the er ablation patterns. likelihood of a clear stromal bed and timely pro- gression of the LASIK procedure. After the laser dissection has been completed, the remaining microadhesions are bluntly dissected with a spat- 7.3 Clinical Applications of the FS laser 85 ula when the flap is lifted. The FS laser creates in separate suites, requiring the patient to move a square-edged, planar flap with a uniform thick- from one room to another between procedures. ness across its entire diameter. This is in contrast However, when real estate and laser design per- to a flap produced by a mechanical microkera- mit, the two can be placed in the same room with tome, which has a tapered edge and a meniscus a patient bed that pivots between them. profile (i.e., thinner centrally and thicker periph- erally). The even thickness and vertical edges of the FS laser flap may provide more stability, with 7.3.1.1 Laser Settings increased resistance to displacement or stria for- mation. The IntraLase permits adjustment of multiple Since the FS laser can only be used to create surgical parameters, including the LASIK flap di- the LASIK flap, an excimer laser is still required mensions, pocket design, and the treatment pat- to perform the refractive portion of the corneal terns for both the interface and side cuts. A list ablation. Spacing issues and excimer platform of the adjustable parameters, their ranges, and designs often dictate that the two lasers must be typical settings are shown in Table 7.1. As experi- Table 7.1 Surgical parameters available for adjustment with the IntraLase FS laser. The ranges and typical settings are given. Note: energy settings and spot/line separations are typically lower for the FS30 than for the FS15. N nasal, S superior, T temporal Category Parameter Range Flap dimensions Thickness 100–400 µm Diameter 5.0–9.0 µm Lamellar cut Treatment pattern Raster/spiral Hinge location Raster 0, 90, 180° (N/S/T) Spiral 0–359° Hinge angle Raster 45–90° Spiral 0–359° Raster spot separation Spot 6–14 µm Line 6–14 µm Spiral spot separation Tangential 6–14 µm Radial 6–14 µm Energy 15 kHz 1–6 µJ 30 kHz 1–4 µJ Side cut Angle 30–90° Energy 15 kHz 1–6 µJ 30 kHz 1–4 µJ Pocket Width 0.100–0.500 mm Start depth 100–300 µm Energy 15 kHz 1–6 µJ 30 kHz 1–4 µJ Spot separation Tangential 4–14 µm Radial 4–14 µm 86 The Femtosecond Laser: a New Tool for Refractive and Corneal Surgery 7 Fig. 7.1 Stromal interface following flap creation ration of 9 mm. b shows a rougher stromal interface with the femtosecond laser. a demonstrates a smooth- when the spot energy is raised to 3.8 mJ, while the spot er stromal interface produced with a spot energy of and line separations are unchanged. 1.8 mJ, a spot separation of 11 mm, and a line sepa- ence with each laser accrues, the parameters can while the surgeon supports the suction ring.
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