Journal of Refractive Surgery

Review 

Femtosecond Laser Technology in Corneal Refractive Surgery: A Review

George D. Kymionis, MD, PhD; Vardhaman P. Kankariya, MD; Argyro D. Plaka, MD; Dan Z. Reinstein, MD, MA(Cantab), FRCSC, FRCOphth, FEBO

Abstract

PURPOSE:

To discuss current applications and advantages of femtosecond laser technology over traditional manual techniques and related unique complications in corneal refractive surgical procedures, including LASIK flap creation, intracorneal ring segment implantation, astigmatic keratotomy, presbyopic treatments, and intrastromal lenticule procedures.

METHODS:

Literature review.

RESULTS:

From its first clinical use in 2001 for LASIK flap creation, femtosecond lasers have steadily made a place as the dominant flap-making technology worldwide. Newer applications are being evaluated and are increasing in their frequency of use.

CONCLUSIONS:

Femtosecond laser technology is rapidly becoming a heavily utilized tool in corneal refractive surgical procedures due to its reproducibility, safety, precision, and versatility.

From the Department of Ophthalmology, University of Crete, Medical School, Heraklion, Greece (Kymionis, Kankariya, Plaka); London Vision Clinic, London, United Kingdom (Reinstein); and the Department of Ophthalmology, Columbia University Medical Center, New York, New York (Reinstein).

Dr Reinstein is a consultant for Carl Zeiss Meditec, has a proprietary interest in the Artemis technology (ArcScan Inc), and is an author of patents related to VHF digital ultrasound administered by the Cornell Center for Technology Enterprise and Commercialization, Ithaca, New York. The remaining authors have no financial or proprietary interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (G.D.K., D.Z.R.); data collection (G.D.K., V.P.K., A.D.P.); analysis and interpretation of data (G.D.K., V.P.K., A.D.P.); drafting of the manuscript (G.D.K., V.P.K., A.D.P., D.Z.R.); critical revision of the manuscript (G.D.K., V.P.K., D.Z.R.); supervision (G.D.K.)

Correspondence: George D. Kymionis, MD, PhD, University of Crete, Medical School, 71003 Heraklion, Crete, Greece. Tel: 30 28 1039 4656; Fax: 30 28 1039 4653; E-mail: kymionis@med.uoc.gr

Received: July 19, 2012
Accepted: October 18, 2012

Abstract

PURPOSE:

To discuss current applications and advantages of femtosecond laser technology over traditional manual techniques and related unique complications in corneal refractive surgical procedures, including LASIK flap creation, intracorneal ring segment implantation, astigmatic keratotomy, presbyopic treatments, and intrastromal lenticule procedures.

METHODS:

Literature review.

RESULTS:

From its first clinical use in 2001 for LASIK flap creation, femtosecond lasers have steadily made a place as the dominant flap-making technology worldwide. Newer applications are being evaluated and are increasing in their frequency of use.

CONCLUSIONS:

Femtosecond laser technology is rapidly becoming a heavily utilized tool in corneal refractive surgical procedures due to its reproducibility, safety, precision, and versatility.

From the Department of Ophthalmology, University of Crete, Medical School, Heraklion, Greece (Kymionis, Kankariya, Plaka); London Vision Clinic, London, United Kingdom (Reinstein); and the Department of Ophthalmology, Columbia University Medical Center, New York, New York (Reinstein).

Dr Reinstein is a consultant for Carl Zeiss Meditec, has a proprietary interest in the Artemis technology (ArcScan Inc), and is an author of patents related to VHF digital ultrasound administered by the Cornell Center for Technology Enterprise and Commercialization, Ithaca, New York. The remaining authors have no financial or proprietary interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (G.D.K., D.Z.R.); data collection (G.D.K., V.P.K., A.D.P.); analysis and interpretation of data (G.D.K., V.P.K., A.D.P.); drafting of the manuscript (G.D.K., V.P.K., A.D.P., D.Z.R.); critical revision of the manuscript (G.D.K., V.P.K., D.Z.R.); supervision (G.D.K.)

Correspondence: George D. Kymionis, MD, PhD, University of Crete, Medical School, 71003 Heraklion, Crete, Greece. Tel: 30 28 1039 4656; Fax: 30 28 1039 4653; E-mail: kymionis@med.uoc.gr

Received: July 19, 2012
Accepted: October 18, 2012

Erratum
This article has been amended to include a factual correction. An error was identified subsequent to its original printing. In Table 1 on page 913 of the article “Femtosecond Laser Technology in Corneal Refractive Surgery: A Review” by Kymionis et al., which was published in the December 2012 issue of the Journal of Refractive Surgery, the Additional Procedures listed for Zeiss VisuMax should be “FLEx, SMILE, ICRS, LK, PKP” instead of “FLEx, SMILE.” This error was acknowledged on page 72, volume 29, issue 1. The online article and its erratum are considered the version of record. 

The femtosecond laser is a focused infrared laser with a wavelength of 1053 nm that uses ultrafast pulses with a duration of 100 fs (100×10−15 seconds). It is a solid-state Nd:Glass laser similar to an Nd:YAG laser, which operates on the principle of photoionization (laser-induced optical breakdown), producing photodisruption at its focal point, resulting in a rapidly expanding cloud of free electrons and ionized molecules (plasma). Small volumes of tissue are vaporized with the formation of cavitation gas bubbles consisting of carbon dioxide and water, which eventually dissipate into the surrounding tissues.1 In this process, collateral damage seen with a femtosecond laser is 106 times less than an Nd:YAG laser, thus demonstrating its precision and safety when used in corneal surgeries.2

The technology of the femtosecond laser was first introduced in late 2001 and technological evolution has resulted in a gradual increase in its higher laser firing frequency, which recently reached 500 kHz from its original 6 kHz.3–5 The higher laser frequency permits lower energy per pulse and tighter line separation, which leads to smoother corneal stromal bed creation. Currently, five femtosecond laser systems are commercially available: 1) IntraLase (Abbott Medical Optics Inc, Santa Ana, California); 2) Femtec (20/10 Perfect Vision, Heidelberg, Germany); 3) Femto LDV (Ziemer Ophthalmic Systems, Port, Switzerland); 4) VisuMax (Carl Zeiss Meditec AG, Jena, Germany); and 5) WaveLight FS200 (Alcon Laboratories Inc, Ft Worth, Texas). Current laser platforms differ in pulse energy and frequency, applanation surface (flat or curved), laser delivery (raster or spiral pattern), available applications, and mobility (Table 1).4

Comparison of Commercially Available Femtosecond Lasers

Table 1: Comparison of Commercially Available Femtosecond Lasers

There are a wide range of available and evolving femtosecond laser applications in the field of refractive surgery, with LASIK flap creation being the most utilized. Additional procedures include astigmatic keratotomy (AK), channel creation for implantation of intrastromal corneal ring segments (ICRS), intrastromal lamellar pocket creation for the insertion of intracorneal inlays for the treatment of presbyopia, femtosecond lenticule extraction (FLEx), small-incision lenticule extraction (SMILE), and intrastromal presbyopia correction (INTRACOR), and these are likely to expand in the future. In addition to expansion of femtosecond laser applications in corneal refractive surgery, technological advances should also lead to an improvement in the safety and efficacy of the procedures.

LASIK Flap Creation

Corneal flap creation in LASIK is the most common application of the femtosecond laser in corneal refractive surgery. More than 55% of all LASIK procedures in the United States were performed with femtosecond lasers in 2009.4 The proportion is even greater in high-volume practices, and femtosecond lasers are gaining more acceptance worldwide. When using femtosecond technology for flap creation, each pulse of the laser causes the generation of a small amount of microplasma at its focal point in the corneal tissue leading to formation of microscopic gas bubbles, which then dissipate into surrounding tissue. These pulses, when applied adjacent to each other in a raster pattern, result in a cleavage plane to create the lamellar cut. More pulses are then applied in a peripheral circular pattern to create the vertical side cuts, thus creating a LASIK flap. The flap can then be lifted for excimer laser ablation. Recently introduced higher laser firing speeds (eg, IntraLase FS 150, WaveLight FS200, VisuMax 500) have reduced the energy requirements, thus reducing the cavitation bubble size and duration, tissue inflammation, time of flap creation, and ease of flap lifting.5

The major advantages of femtosecond laser flap creation compared to mechanical microkeratomes (Table 2) are reduced incidence of flap complications (eg, buttonholes, epithelial abrasions, short and irregular flaps), greater surgeon choice of flap diameter, thickness, side-cut angle, hinge position and length, increased precision with improved flap safety and thickness predictability, and capability of cutting thinner flaps (Fig 1) to accommodate thin corneas and high refractive errors.6–10 Additional advantages include stronger flap adherence and therefore less influence by trauma, fewer induced higher order aberrations, better contrast sensitivity, lesser need for retreatment, lesser rate of epithelial ingrowth, and lesser incidence of dry eye.10–14 Femtosecond laser–created flaps are planar in architecture as opposed to most microkeratome flaps,8 which have been shown to possess significant variability in the thickness profile.15 This is important as uniformity of flap thickness may also affect the predictability of excimer laser stromal photoablation as stromal anatomy, hydration, and ultraviolet absorbance varies with corneal depth.6,16 Visual and refractive outcomes of femtosecond laser–assisted LASIK demonstrate excellent safety and efficacy, with most studies reporting equivalence with microkeratome LASIK.17 With the current acceptance and future promise of femtosecond lasers, it will more than likely be the dominant technology used globally for flap creation in LASIK.

Comparison of Microkeratomes and Femtosecond Lasers for LASIK Flap Creation

Table 2: Comparison of Microkeratomes and Femtosecond Lasers for LASIK Flap Creation

Horizontal nongeometrically corrected Artemis very high-frequency digital ultrasound (ArcScan Inc, Morrison, Colorado) B-scan of a cornea after the creation of a 95-μm LASIK flap using the VisuMax femtosecond laser.

Figure 1. Horizontal nongeometrically corrected Artemis very high-frequency digital ultrasound (ArcScan Inc, Morrison, Colorado) B-scan of a cornea after the creation of a 95-μm LASIK flap using the VisuMax femtosecond laser.

Femtosecond lasers promise excellent reproducibility and versatility in applications, but at the same time they demonstrate a unique set of complications.18 Confluent cavitation bubbles during intrastromal treatment (opaque bubble layer) may confound the ability of the surgeon and the excimer laser eye tracker device to locate the pupil for centration purposes. Gas bubbles routinely accumulate in the flap interface during femtosecond laser treatment, but occasionally they may dissect into the deep stromal bed, resulting in a posterior stromal opaque bubble layer that does not escape when the flap is lifted.19 In rare instances, bubbles may escape into the corneal subepithelial space and larger central vertical gas breakthrough may potentially result in a flap buttonhole.20,21 Posterior vertical gas breakthrough may result in air bubbles in the anterior chamber.22

Another complication of femtosecond laser–assisted LASIK is transient light sensitivity syndrome,23,24 which may occur days to weeks after the procedure and is characterized by extreme photophobia with good visual acuity and absence of clinical findings on examination. It resolves within a few weeks after being treated with an aggressive course of topical corticosteroids. After femtosecond laser–assisted LASIK, “rainbow glare” is an optical phenomenon experienced as colored bands of light radiating from a white light source when viewed in a dark environment.25 The proposed mechanism is diffraction of light by micro-irregularities on the back surface of the femtosecond laser–created LASIK flap.

Diffuse lamellar keratitis (DLK) after LASIK femtosecond laser–enabled flap creation tends to have little effect on visual acuity and is associated with a higher energy level for flap creation and larger flap diameter.26 Mild transient lamellar keratitis limited to the periphery is still encountered occasionally, perhaps causally related to the higher energies used for making the vertical side cuts. This type of DLK is likely attributable to photodisruption-induced microscopic tissue injury aggravated by ocular surface inflammatory mediators.27,28 Another likely etiology is damage to the epithelium at the site of the side cut and release of proinflammatory cytokines from the damaged epithelial cells. Hainline et al29 reported central lamellar flap necrosis following the use of a femtosecond laser. It appears to differ from DLK because the location of stromal inflammation is in the flap anterior stroma and corticosteroid treatment seemed to have little effect on outcomes.

Loss of suction during femtosecond laser flap creation is usually not as serious a complication as with a mechanical microkeratome, and the suction ring may be reapplied and treatment resumed immediately in many cases. If suction is lost during the side-cut phase, a new side cut is made just inside the diameter of the interface cut.30 It is important to note flaps created with a femtosecond laser may be harder to lift at the time of retreatment, and therefore if retreatment is necessary, it should be attempted early after the initial procedure.31 Other reported complications include gas bubble under the conjunctiva, unintended epithelial flap, interface haze, interface stromal irregularities, and macular hemorrhage.32–36

Intrastromal Corneal Ring Segment Implantation

Intrastromal corneal ring segments are crescent-shaped polymethylmethacrylate implants originally designed to correct low to moderate myopia.37 Currently, they are used to treat postoperative LASIK corneal ectasia,38 pellucid marginal degeneration,39 and keratoconus.40

Intrastromal corneal ring segments are inserted in intrastromal channels (created either manually or using a femtosecond laser) at 75% depth of the thinnest pachymetry. This results in an arc shortening effect and redistribution of corneal peripheral lamellae to produce flattening of the central cornea.41 Their effect is proportional to the thickness of the implant and inversely proportional to the implant diameter.42 Compared to the manual technique, a femtosecond laser makes tunnel creation faster, easier, and more reproducible and offers accurate tunnel dimensions (width, diameter, and depth).43 With mechanical dissectors, segment depth may be shallower at positions further from the incision but depth is consistent throughout when using a femtosecond laser.44

The surgical procedure with a femtosecond laser, as in the mechanical procedure, is typically performed under topical anesthesia.45 After marking a reference point (pupil center or first Purkinje reflex) on the cornea and measuring the corneal thickness by ultrasonic pachymetry at the area of implantation (5-mm diameter), the disposable suction ring of the femtosecond laser system is centered. The disposable glass lens is applanated to the cornea to fixate the eye and help maintain the precise distance from the laser head to the focal point. An entry cut with the femtosecond laser is created with the aim of allowing access for ring placement in the tunnel. The tunnel is then created at approximately 70% to 80% of the corneal thickness. Intrastromal corneal ring segments are inserted in the created tunnels (Fig 2).

Slit-lamp photograph demonstrating two intrastromal corneal ring segments (arrows) placed in a channel created with a femtosecond laser for the treatment of keratoconus.

Figure 2. Slit-lamp photograph demonstrating two intrastromal corneal ring segments (arrows) placed in a channel created with a femtosecond laser for the treatment of keratoconus.

Theoretically, compared with mechanical tunnel creation, which is based on surgeon skill, the femtosecond laser–assisted procedure should generate a more accurate stromal dissection, leading to better visual and refractive results. However, similar visual and refractive outcomes with both procedures were reported over short-term follow-up in eyes with keratoconus and postoperative LASIK ectasia.46,47 Kubaloglu et al43 compared the clinical outcomes of keratoconic patients treated with two types of ICRS. Both implants were safe and effective and no difference was observed in visual or refractive outcomes when comparing mechanical microkeratome– and femtosecond laser–created channels. They also reported that the use of the femtosecond laser made the procedure faster, easier, and more comfortable for the patient. Further experience and the development of more accurate nomograms should improve clinical outcomes.

Complications associated with the mechanical technique include epithelial defects, anterior or posterior perforation with the mechanical spreader, shallow or uneven placement of the ICRS, decentration, extension of the incision towards the central cornea or limbus, and corneal stromal edema around the incision and channel from surgical manipulation.48,49 Most cases of extrusion have been observed in eyes implanted using mechanical dissection; however, three cases of ring extrusion in advanced keratoconus and one case of segment migration to the incision site have been reported with femtosecond laser channel creation.45

Coskunseven et al50 reported complications after implantation of ICRS in keratoconic patients using the IntraLase femtosecond laser stating that intraoperative incomplete channel creation (2.7%) and postoperative segment migration (1.3%) were the most common complications. The study also demonstrated intraoperative adverse events such as galvanometer lag error (0.6%), endothelial perforation (0.6%), and vacuum loss (0.1%) and postoperative complications such as superficial movement of the segments (0.1%), corneal melting (0.2%), and infection (0.1%).

Astigmatic Keratotomy

Femtosecond lasers have recently been used for the correction of natural or postoperative lamellar/penetrating keratoplasty (PK) corneal astigmatism.51–56 Corneal astigmatism is a common finding after PK and may cause significant visual impairment. Possible causes are scar formation, corneal thickness mismatch between the graft and recipient tissue and irregular forces created by sutures.57 Astigmatic errors after PK may be corrected with several surgical techniques such as refractive procedures (LASIK, photorefractive keratectomy), relaxing incisions, compression sutures, and wedge resections.58,59 Astigmatic keratotomy is a simple, safe, and minimally invasive technique. Therefore, AK is the most commonly used method for the reduction of high amounts of astigmatism in postoperative PK patients.60–63 The technique is similar to limbal relaxing incisions, with incisions placed inside the donor-recipient junction as it behaves like a new limbus due to a fibrotic ring formed as part of the healing response. Astigmatic keratotomy should only be performed after all corneal sutures are removed.

Astigmatic keratotomy may be performed manually with a diamond knife as well as with a femtosecond laser. Major limitations of manual AK are technical difficulties (especially in nonorthogonal astigmatism), compromised reproducibility, unpredictability, and complications such as wound dehiscence, epithelial abrasions, and perforation.60–64 Compared with the mechanical method, the use of a femtosecond laser offers the advantages of higher precision and stability as well as more accurate planning of the length, depth, and optical zone of the cuts (Fig 3).52 Femtosecond laser AK has been reported to be effective in reducing astigmatism and improving uncorrected (UDVA) and corrected distance visual acuity (CDVA).64 Yoo et al65 reported the use of anterior segment optical coherence tomography (AS-OCT) as a guide for the planned incision depth. In particular, in postoperative Descemet stripping endothelial keratoplasty (DSEK) patients, it is important to evaluate AS-OCT so as not to include the DSEK donor lenticular thickness for pachymetry measurements as this will lead to inadvertent recipient full-thickness incision and significant overcorrection. In postoperative DSEK eyes, the aim should be for the incisions to be up to 90% of the recipient corneal thickness only.

Slit-lamp photograph demonstrating astigmatic keratotomy incisions (arrows) performed with a femtosecond laser for naturally occurring astigmatism.

Figure 3. Slit-lamp photograph demonstrating astigmatic keratotomy incisions (arrows) performed with a femtosecond laser for naturally occurring astigmatism.

Presbyopia Treatment

Femtosecond lasers are now also used for the creation of intrastromal pockets to insert biocompatible intracorneal inlays for the treatment of presbyopia.66 Intracorneal inlays are available with different mechanisms including refractive intracorneal inlays, which have an annular refractive zone for near vision, whereas other intracorneal inlays have no refractive power and work by increasing the curvature in the center of the pupil; in addition, pinhole intracorneal inlays enable near vision by taking advantage of the pinhole effect. Intracorneal inlays are inserted in the nondominant eye either under a LASIK flap or into a stromal pocket created by a femtosecond laser.

Implantation of intracorneal inlays has been described for the treatment of presbyopia and hyperopia using corneal flaps created by mechanical microkeratomes66,67 or femtosecond lasers68,69 or corneal pockets created by mechanical microketatomes.70 Femtosecond laser–assisted intracorneal pocket creation could increase the precision of the inlay position by customization of depth and length of the tunnel, which could enhance the predictability, resulting in better final outcomes and improving the safety of the procedure (Fig 4). The development of special software for customized pockets could further simplify and increase the efficacy of the procedure. Prospective comparative studies are needed to evaluate the long-term results of the technique and optimize the laser parameters. In addition, there is no need to change or add new equipment in a modern refractive surgery center, except to obtain the special injector and mask.70,71

Slit-lamp photograph demonstrating intracorneal inlay (Flexivue Microlens; Presbia Cooperatief UA, Amsterdam, The Netherlands) implantation (arrow) in a pocket created with a femtosecond laser.

Figure 4. Slit-lamp photograph demonstrating intracorneal inlay (Flexivue Microlens; Presbia Cooperatief UA, Amsterdam, The Netherlands) implantation (arrow) in a pocket created with a femtosecond laser.

In addition to pocket creation for inlay implantation, femtosecond lasers are being used in INTRACOR as first described by Ruiz et al.72 The INTRACOR procedure is a femtosecond laser–based incisional method for intrastromal correction of presbyopia using the Technolas 520FS (previously Femtec [Technolas Perfect Vision GmbH, Munich, Germany]) femtosecond laser platform. During INTRACOR, two to four cylindrical ring incisions are created in the corneal stroma aiming to change its biomechanical properties and induce a central hyperprolate region for the treatment of presbyopia. Studies have shown improved uncorrected near visual acuity (range: J1 to J2) with minimal or no change in UDVA.71 These early results show reasonable efficacy but current safety is a concern as studies have reported a loss of 2 lines of CDVA in 2.1%,72 7%,73 8%,74 and 26% of eyes75 as well as a loss of contrast sensitivity.76 Further, it remains to be determined whether these multifocal ablation patterns are functionally reversible.

Intrastromal Keratomileusis

Since femtosecond lasers were first introduced into refractive surgery, the ultimate goal has been to create an intrastromal lenticule that can be removed manually (refractive lenticule extraction [ReLEx]), thereby circumventing the need for an excimer laser. This was first described in 1996 using a picosecond laser to generate an intrastromal lenticule that was removed manually after lifting the flap77,78; however, significant manual dissection was required, leading to an irregular surface. The switch to femtosecond laser improved the precision79 and studies were performed in rabbit eyes in 199880 and partially sighted eyes in 200381; however, these initial studies were not followed with further clinical trials.

Following the introduction of the VisuMax femtosecond laser in 2007,82 the intrastromal lenticule method was reintroduced in a procedure called femtosecond lenticule extraction (FLEx). The 6-month results of the first 10 fully seeing eyes treated were published in 200883 and results of a larger population have since been reported.84 The results were similar to LASIK except for a slower visual recovery time, however, adjustments in energy settings and scan patterns have improved the visual recovery time.85,86 Intrastromal lenticule procedures may have advantages over LASIK as all of the potential variables associated with excimer laser ablation are avoided, such as stromal hydration,87 laser fluence,88,89 and other environmental factors.90

Following the successful implementation of FLEx, a new procedure called small-incision lenticule extraction (SMILE) was developed (Fig 5). This procedure involves creating one or two small incisions through which the lenticule interfaces can be separated, allowing the lenticule to be removed, thus eliminating the need to create a flap. Therefore, this procedure will only cut a small proportion of anterior corneal nerves (only in the location of the incisions) meaning that postoperative dry eye should be significantly less than that experienced after the creation of a flap or after surface ablation. Another potential benefit of SMILE is increased biomechanical stability as the anterior stromal lamellae, known to be the strongest region of the stroma,91 remain intact and therefore continue to contribute to the corneal biomechanics. The results of the first prospective trials of SMILE have been reported92,93 and now more than 50 surgeons routinely perform this procedure worldwide. The VisuMax is currently the only femtosecond laser being used for intrastromal lenticular surgery.

Retroillumination photograph of the cornea of a left eye 1 day after small-incision lenticular extraction. The 2.25-mm small incision can be seen superotemporally through which the refractive stromal lenticule was extracted. The edge of the lenticule can also be clearly seen. The image on the right shows a magnified view of the area indicated by the yellow square. In this magnified image, the edge of the cap—the anterior interface of the lenticule—can also be seen outside the edge of the lenticule.

Figure 5. Retroillumination photograph of the cornea of a left eye 1 day after small-incision lenticular extraction. The 2.25-mm small incision can be seen superotemporally through which the refractive stromal lenticule was extracted. The edge of the lenticule can also be clearly seen. The image on the right shows a magnified view of the area indicated by the yellow square. In this magnified image, the edge of the cap—the anterior interface of the lenticule—can also be seen outside the edge of the lenticule.

Conclusions

Femtosecond laser technology has gained widespread acceptance in the field of corneal refractive surgery due to its versatility, precision, and reproducibility. Femtosecond laser technology is now being used successfully in performing various steps (capsulorrhexis, clear corneal incisions, and phacolysis) of cataract surgery precisely and reproducibly.94 Additionally, femtosecond laser technology is utilized for donor and recipient preparation in penetrating keratoplasty as well as lamellar keratoplasty.95 Femtosecond lasers hold great promise and will continue to provide more applications in ophthalmic surgery, ultimately contributing to the goal of emmetropia.

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Comparison of Commercially Available Femtosecond Lasers

Parameter IntraLase iFS 150 Femto LDV Zeiss VisuMax FemTec 2010 WaveLight FS200
Laser type Amplifier Oscillator Fiber optic amplifier Amplifier Oscillator-amplifier
Wavelength (nm) 1053 1045 1043 1053 1045
Laser pattern Raster Segmental Spiral Spiral Raster
Centration Computer Mechanical Mechanical Mechanical Computer
Visualization of surgery Visual and virtual Virtual Visual Visual Visual and virtual
Mobile No Yes No No No
Suction Single syringe Single built in Single built in on limbus Single built in Dual built in
Applanation surface Planar Planar Curved Curved Modified planar
Additional procedures AK, Wedge, LK, PKP, ICRS, Biopsy, Pocket LK, PKP, Pocket, ICRS FLEx, SMILE, ICRS, LK, PKP AK, LK, PKP, ICRS, INTRACOR AK, LK, PKP, ICRS

Comparison of Microkeratomes and Femtosecond Lasers for LASIK Flap Creation

Parameter Microkeratome Femtosecond Laser
Flap shape Meniscus Planar
Flap/hinge diameter Keratometry dependent Computer control
Flap thickness Dependent on pachymetry, keratometry, IOP, blade quality and translational speed Computer control
Thickness predictability Moderate High
Side cut Shallow angled Computer control
Epithelial ingrowth More than femtosecond laser flaps Less
Unique complications Flap buttonhole Opaque bubble layer, vertical gas break-through, transient light sensitivity syndrome, rainbow glare
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