Advances in refractive surgery: microkeratome and femtosecond laser flap creation in relation to safety, efficacy, predictability, and biomechanical stability Karl Stonecipher a, Teresa S. Ignacio b and Megan Stonecipher Purpose of review Methods of flap creation have changed over the years from the evolution of the mechanical microkeratome to the introduction of the IntraLase femtosecond laser keratome, both of which have different mechanisms of action to create corneal resections. Previous studies report the advantages and disadvantages of the mechanical microkeratome and the IntraLase femtosecond laser. The critical components in laser in-situ keratomileusis surgery remain the same, however: safety, efficiency, predictability, and biomechanical stability. Recent findings Keratoectasia and flap efficiency remain a constant safety concern in laser in-situ keratomileusis surgery. Unexpectedly thick flaps as well as variable thickness continue to be a concern with safety in relation to microkeratome technology. Epithelial preservation, flap complications, and newer issues such as Transient Light Sensitivity Syndrome are safety concerns of flap creation. Improved outcomes with regards to vision, induced astigmatism, induced higher-order aberrations, and enhancement rates are seen to favor predictability of femtosecond technologies over the microkeratome. Recent biomechanical studies show improved healing with femtosecond laser flap creation compared with bladeassisted flap creation. Summary The aim of this review is to summarize the key components for both the microkeratome and the femtosecond laser and to update on the recent advances reported on these topics. Keywords biomechanical stability, efficacy, femtosecond laser, microkeratome, predictability, safety Curr Opin Ophthalmol 17:368 372. ß 2006 Lippincott Williams & Wilkins. a The Laser Center, Greensboro, North Carolina, USA and b Department of Ophthalmology, University of California Irvine, USA Correspondence to Karl G. Stonecipher, MD, Southeastern Eye Center, 3312 Battleground Avenue, Greensboro, NC 27410, USA E-mail: StoneNC@aol.com Current Opinion in Ophthalmology 2006, 17:368 372 Abbreviation LASIK laser in-situ keratomileusis ß 2006 Lippincott Williams & Wilkins 1040-8738 Introduction Laser in-situ keratomileusis (LASIK) has become the most popular corneal refractive surgery. One of the critical steps in this procedure is the creation of the corneal flap. The IntraLase femtosecond laser microkeratome (IntraLase Corp., Irvine, California, USA) and the mechanical microkeratome platforms have different mechanisms of action to create corneal resections. The mechanical microkeratome uses shear force through the use of an oscillating blade, traveling across the cornea in a torsional or translational approach. The femtosecond laser creates a corneal resection by delivering laser pulses at a predetermined depth in the cornea of 1 mm diameter, which expands to 2 3 mm. These pulses create microphotodisruption or an expanding bubble of gas (CO 2 ) and water that in turn cleave the tissue and create a plane of separation. Pulses are scanned in a spiral or raster pattern and placed next to each other to create a planar LASIK flap [1]. Previous studies report the advantages and disadvantages of mechanical microkeratome and the Intra- Lase femtosecond laser [2 4], but the critical components in LASIK surgery remain the same: safety, efficiency, predictability, and biomechanical wound healing. The aim of this review is to summarize these components for both the microkeratome and the femtosecond laser and to update the reader in the recent advances reported on these topics. Safety Epithelial preservation is a key factor in healing and subsequent avoidance of postoperative complications. Significant variability favoring the laser keratome has been reported with regard to loose epithelium or epithelial slides and epithelial defects during flap creation [2]. Newer mechanical keratome designs have reduced the incidence of these epithelial-related issues, but even with newer designs epithelial slides have been reported in as many as 2.6% of those flaps created with newer mechanical microkeratomes designs [5]. There is a growing concern about iatrogenic corneal ectasia after LASIK even though it is relatively uncommon, with a reported incidence of 0.66% in a study by Pallikaris et al. [6]. Strategies to prevent ectasia include preserving a minimum residual stromal bed of 250 300 mm, making flap thickness predictability a critical factor to LASIK safety [7 9]. With the microkeratome, multiple factors determine the corneal thickness profile, such as the quality of 368
Refractive surgery Stonecipher and Ignacio 369 the blade s cutting edge, speed of the microkeratome pass, speed of blade oscillation, ease of pass on the cornea, and advancement of the microkeratome along the track of the suction ring. Clinical studies have shown the inaccuracies of different microkeratomes [10 11]. Unexpectedly thick flaps have been reported as well as variable thickness, with the flaps created with a mechanical microkeratome being thinner in the center and thicker in the periphery [12]. Femtosecond laser flaps have been shown to be of more uniform thickness, as seen in Fig. 1. Other issues that have plagued microkeratome technology such as epithelial defects, epithelial sloughing, incomplete flaps, irregular flaps, and buttonhole flaps are exceptionally rare with the IntraLase. If an unexpected thickness is suspected, the surgeon has the ability to measure corneal thickness with newer pachymetric technologies [13]. With real-time pachymetric measurements, the surgeon may choose not to elevate the flap and to create a different flap at a later date. A flap buttonhole is therefore a rare occurrence with the IntraLase unless the surgeon chooses to lift a visibly irregular flap. Certain side effects unique to the femtosecond laser, such as Transient Light Sensitivity Syndrome, have been reported [14]. Transient Light Sensitivity Syndrome was initially reported in 2001 when higher laser pulse energy was used to create the flaps; it presents in patients after surgery as good visual acuity with delayed onset of photophobia, which is transient and resolves spontaneously as the name implies. On slit lamp examination, there are no signs of an inflammatory process. Patients usually respond well with steroid drops. It was postulated that higher energies activated the keratocytes and cause the delayed recovery of the keratocytes to its normal state. The incidence significantly decreased after surgeons began using lower energies. The 30 khz version allows even lower energy settings with closer spot separation than the previous 15 khz or 10 khz platforms. With the recent release of the 60 khz platform, even faster rates of flap creation and lower raster bed energies are possible. Efficiency Refractive surgeons have assumed that the smoother the optical surfaces, the better the visual and refractive outcomes following laser refractive surgery [15 18]. Recent studies have suggested that microkeratomes can induce optical aberrations just by the creation of the flap prior to the laser ablation [16,19]. Morphologic studies of the appearance of the stromal surface following LASIK flap creation have documented the abnormalities that can occur when a metal blade is used [20 22]. Assumptions have been made about the stromal bed produced by the IntraLase. The IntraLase femtosecond laser creates flaps by placing laser pulses contiguously and eventually producing a cleavage plane [23]. Others liken this to creating perforations on a postage stamp to assist in tearing off each stamp. Similar to the microkeratome, the femtosecond laser technology has evolved through the years. The femtosecond laser currently scans with a repetition rate of 30 khz in most practices utilizing this Figure 1 Zeiss Visante imaging system and flap uniformity Using a Zeiss Visante imaging system it is evident that the flap uniformity favours the planar IntraLase femtosecond laser keratome in comparison to the meniscus shaped mechanical microkeratome created flap.
370 Refractive surgery technology. In January 2006 the 60 khz platform was introduced, but no clinical data are available at time of printing this article. Higher repetition rates allow the use of lower surgical energy settings and closer spot/line separation with an increase in the speed of the procedure. Flap creation can occur in as short a time as 17 s. Closer spot/line separation makes the cleavage plane easier to separate. Additionally, the smaller the perforation (which is adjustable on the laser by energy control), the smoother or finer is the edge of the cut [23]. The presence of residual adhesions that necessitate a manual dissection to elevate the flap created with the femtosecond laser has caused surgeons concern that the potential for a rougher surface exists and could lead to undesirable optical and hence refractive effects. Recent reports document that the femtosecond laser in the 15 khz mode operating in clinical settings can create a smooth stromal surface indistinguishable from that created with the newer mechanical microkeratome technology at depths less than 130 mm [24]. The 30 khz femtosecond laser permits a tighter spot/line separation and a lower energy per pulse, which creates even smoother corneal stromal beds (unpublished data submitted for publication to Cornea). Recent data presented with the 30 khz platform show improved flap predictability when compared with other published mechanical keratome platforms [5,25 27] (Fig. 2). Predictability Previous reports compared the refractive, visual, and best corrected acuity outcomes of the mechanical keratomes and the IntraLase. These studies showed significantly better predictability of the Manifest Refraction Spherical Equivalent at the 0.50 Diopters level and a reduction in the overall induced astigmatism in spherical treatments with the IntraLase [2,4]. Recent Figure 2 A comparison of several microkeratome and IntraLase laser keratome mean flap efficiencies with standard deviations 200 180 160 140 120 100 80 60 40 20 0 156 ± 29 µ Hansa- 180 116 ± 16.1 µ Zyoptix XP 107 ± 14 µ LSK 1-100 107 ± 14 µ M2 130 114 ± 14 µ IL-130 15 khz 124 ± 12.3 µ IL-120 30 khz Other issues related to efficiency that have been shown to affect the mechanical keratome and not the femtosecond laser keratome include preoperative pachymetry, preoperative keratotomy, age, intraocular pressure, and refractive error [2,28 30]. reports from comparative United States Food and Drug Administration trials showed surgically induced refractive change in spherical patients with a mechanical keratome was 0.42 0.28 D, whereas with the IntraLase laser keratome it was only 0.05 0.11 D [31]. Better postoperative early visual outcomes and contrast sensitivity data have favored the femtosecond laser over the mechanical keratome as well [32]. One possible explanation for these findings is the dry bed produced when an IntraLase femtosecond flap is lifted. Laser ablation rateshavebeenshowntovarywithtissuehydration [33,34]. The creation of the IntraLase flap is a relatively dry procedure, unlike the microkeratome platform where the cornea is routinely irrigated before the microkeratome is passed. This results in a more standardized tissue hydration with the IntraLase laser. A second possible explanation for these findings may lie in the morphology of the flap (Fig. 1). The IntraLase creates uniformly planar flaps as opposed to meniscus-shaped flaps created by mechanical keratomes. These features have been reported to make the IntraLase laser a better option than the microkeratome in the treatment and prevention of higher-order aberrations after LASIK surgery [4,32]. Better predictability results in better outcomes, which mean fewer enhancements. In a recent evaluation of 18 471 eyes collected in a prospective fashion, using the same surgeon and controlling population sample refractive errors, the enhancement rates seen in bladeassisted LASIK was 4.2% versus 1.6% with IntraLase LASIK [25]. Biomechanics The impact of corneal biomechanics is rapidly being recognized as an integral part of refractive surgery outcomes. It is thought that the corneal stroma consists of lamellae that run from limbus to limbus across the corneal arc. The lamellae consist of organized collagen fibers and relax toward the periphery when severed by a central ablative process [35,36 ]. It has been shown that a significant number of collagen fibers are severed in LASIK. This led to the conclusion that LASIK causes significant reduction in corneal biomechanics and longterm instability because there is minimal biomechanical loading distributed in the flap, and therefore the flap does not contribute to the biomechanical stability of the cornea [36 ]. Reports of re-lifting a microkeratome flap 11 years after initial surgery supported this theory. Data published more recently suggest that the human corneal stroma heals in an incomplete fashion after LASIK; resulting in a weak, central, and paracentral hypocellular primitive stromal scar that averages 2.4% as strong as normal corneal stroma. The peripheral scar averages 28.1% as strong as normal corneal stroma but displays variability. The IntraLase flaps heal similarly centrally
Refractive surgery Stonecipher and Ignacio 371 but stronger than a conventional microkeratome peripherally, yet still below the tensile strength of normal corneal tissue. In general, the IntraLase flap seems to have a stronger biomechanical stability [37 ]. Jorge Alio and colleagues recently compared the biomechanical response of the cornea to laser vision correction including LASIK with the IntraLase; blade-initiated LASIK with the Moria M2 microkeratome; and LASEK. All of the refractive settings for flap thickness, diameter, and hinge position were identical in the LASIK patients. Alio et al. [38] noted that the differences were highly significant between blade-initiated LASIK and IntraLase LASIK (P < 0.001) and not statistically significant between surface ablation and IntraLase LASIK (P < 0.645). In addition to improved safety, efficiency, and predictability with recent advances in keratome technology, initial biomechanical reports have been promising. Further studies are needed, however, in order to add a new dimension to customization of not only the ablation procedure, but of flap creation as well. Conclusion The IntraLase femtosecond laser significantly reduces the complications that have plagued the mechanical microkeratome technology. With the rare incidence of flap-related complications, the IntraLase femtosecond laser is becoming recognized as a safer method of flap creation. Its excellent efficiency and predictability with reduced incidence of induced surgical astigmatism and induced higher order aberration makes it a beneficial tool in refractive surgery. References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: of special interest of outstanding interest Additional references related to this topic can also be found in the Current World Literature section in this issue (p. 419). 1 Kurtz RM, Liu X, Elner VM, et al. 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