Introduction
One of the most promising and exciting developments in the world of refractive surgery has been the advent of laser in situ keratomileusis (LASIK). The surgical technique involves the creation of a hinged lamellar corneal flap, after which an excimer laser is used to make a refractive cut on the underlying stromal bed. LASIK is a fusion of old and new technologies, with its roots in keratomileusis and automated lamellar keratectomy (ALK). However, as currently practiced, it is perhaps best thought of as photorefractive keratectomy (PRK) performed under a flap instead of on the corneal surface. About 9 million procedures have been performed in the United States since the approval of the excimer laser for refractive surgery in late 1995. About 1.4 million procedures were performed in 2007 alone (Marketscope).
Spherical aberration: a schematic diagram for the human eye.
History of the Procedure
The era of keratomileusis began in 1966 with Pureskin, who demonstrated that refractive changes could be achieved by creating a corneal flap and removing central tissue in a lamellar fashion under the flap. He found that the smaller the diameter of the resected disc, the greater the refractive change.
Jose Barraquer developed the idea of resecting a corneal disc and freezing it, followed by shaping the disc with a cryolathe. However, the technique was limited by complexity of the equipment and tissue damage to the resected corneal disc caused by freezing. Ruiz and Barraquer performed keratomileusis in situ in the late 1980s. Using principles developed by Krumeich, this technique involved first removing a corneal disc with a microkeratome. Refractive change was accomplished by performing a second plano cut with the microkeratome. The thickness and diameter of this second disc of tissue determined the end refractive result; then, the first disc was sutured back onto the cornea. Problems included complexity, poor predictability, and irregular astigmatism.
Burratto and Pallikaris were the first to combine the use of the excimer laser and microkeratome technology. Burratto's original work involved performing a corrective excimer laser ablation on the back of a resected disc of corneal tissue. This disc was replaced and sutured onto the cornea. Pallikaris developed the technique of performing the excimer laser corrective ablation in the corneal stromal bed under a hinged flap. He first studied the procedure in rabbits, followed by blind human eyes in 1989, and then sighted eyes in 1991.
In 1993, Steve Slade added the refinement of using an automated microkeratome to create the flap and was one of the first US surgeons to perform LASIK.
Indications
As of December 2008, LASIK has been approved by the Food and Drug Administration (FDA) for several different laser platforms, including the VISX STAR S4, Allegretto Wavelight, Alcon LADARVision 4000, and Technolas and NIDEK lasers. The approved range for myopic, hyperopic, and custom treatments varies slightly between platforms.
Table 1 summarizes these devices and their FDA status.
Table 1. Device Summary and FDA Status
Open table in new window
Table
| Myopia (MRSE) -Conventional LASIK | Wavefront Parameters | Hyperopia LASIK | PRK (Myopia) | |
| LADARVision 4000 (Alcon) Small diameter beam; infrared pupil tracker. Dilation required. | < -9.0 D sph;-0.50 D to -3.0 D cyl | ≤ -7.0 D sph with ≤ -4.0 D cyl | Up to +6.0 D with ≤ -6.0 D cyl | -1. 0 D to -10.0 D with ≤ 4.0 D cyl |
| NIDEK EC-5000 | -1.0 D to -14.0 D sph; ≤ 4.0 D cyl | N/A | N/A | -0.75 D to -13.0 D sph; -1.0 D to -8.0 D sph with -0.5 to 4.0 D cyl |
| VISX Star S4 | (S2, S3) < -14.0 D sph;-0.50 D to -5.0 D cyl | ≤ -6.0 D sph with ≤ 3.0 D cyl | +0.50 D to +5.0 D sph; ≤ +3.0 D cyl | ≤ -12.0 D sph with ≤ -4.0 D cyl |
| Technolas 217 (B&L) | < -11.0 D sph with ≤ -3.0 D cy | (217z): <-7.0 D sph with ≤ -3.0 D cyl | +1.0 D to +4.0 D sph; ≤ 2.0 D cyl | N/A |
| Wavelight Allegretto Wave® | < -12.0 D sph with < -6.0 D cyl | < -7.0 D sph with < -3.0 D cyl | <+6.0 D sph with <+5.0 cyl | N/A |
| Myopia (MRSE) -Conventional LASIK | Wavefront Parameters | Hyperopia LASIK | PRK (Myopia) | |
| LADARVision 4000 (Alcon) Small diameter beam; infrared pupil tracker. Dilation required. | < -9.0 D sph;-0.50 D to -3.0 D cyl | ≤ -7.0 D sph with ≤ -4.0 D cyl | Up to +6.0 D with ≤ -6.0 D cyl | -1. 0 D to -10.0 D with ≤ 4.0 D cyl |
| NIDEK EC-5000 | -1.0 D to -14.0 D sph; ≤ 4.0 D cyl | N/A | N/A | -0.75 D to -13.0 D sph; -1.0 D to -8.0 D sph with -0.5 to 4.0 D cyl |
| VISX Star S4 | (S2, S3) < -14.0 D sph;-0.50 D to -5.0 D cyl | ≤ -6.0 D sph with ≤ 3.0 D cyl | +0.50 D to +5.0 D sph; ≤ +3.0 D cyl | ≤ -12.0 D sph with ≤ -4.0 D cyl |
| Technolas 217 (B&L) | < -11.0 D sph with ≤ -3.0 D cy | (217z): <-7.0 D sph with ≤ -3.0 D cyl | +1.0 D to +4.0 D sph; ≤ 2.0 D cyl | N/A |
| Wavelight Allegretto Wave® | < -12.0 D sph with < -6.0 D cyl | < -7.0 D sph with < -3.0 D cyl | <+6.0 D sph with <+5.0 cyl | N/A |
Relevant Anatomy
The cornea is a thin layer of transparent tissue that protects the intraocular contents and refracts light. Average central corneal thickness is about 550 µm, increasing to about 700 µm in the periphery. The cornea has a diameter (from the front surface) of about 11 mm vertically and 12 mm horizontally. The air-tear interface is the first refractive surface that light encounters and accounts for about 80% of the eye's total refractive power; the average corneal curvature (K readings) in the adult cornea is approximately 44.00 diopters (D).
Anatomically, the cornea consists of 5 layers: epithelium, Bowman layer, stroma, Descemet membrane, and endothelium.
Three types of cells are present in the epithelium: (1) basal columnar cells attached to the epithelial basement membrane via hemidesmosomes, (2) wing cells noted for thin winglike projections, and (3) surface cells joined by connecting bridges and covered by microvilli. Mucin is attached strongly to the surface. Usually, 5-7 layers of cells are present. Unlike stratified squamous epithelium in other areas of the body, the epithelium in the eye has an exceptionally smooth and regular surface, contributing to the transparency and light transmission characteristics of the cornea.
The Bowman layer is not a membrane, but rather an acellular structure consisting of collagen and representing the most superficial layer of the stroma.
The stroma makes up about 90% of the corneal thickness and consists of regularly arrayed flattened bundles of collagen called lamellae. Approximately 200-250 lamellae are present in the human cornea. Each bundle extends the width of the cornea and is about 2 µm thick and up to 260 µm wide. The parallel arrangement of these bundles together with the uniform spacing between collagen fibrils helps explain corneal transparency. Although relatively acellular, stromal fibroblasts called keratocytes can be found scattered throughout the stroma between lamellae, and they are responsible for collagen production and wound healing.
The Descemet membrane is composed of a fine latticework of collagen fibers. It represents a true basement membrane, and it is produced by the corneal endothelium.
The endothelium is a single layer of hexagonal cells whose sole purpose is to act as a barrier to the influx of fluid into the cornea and to pump fluid out of the cornea keeping it deturgesced and clear. These cells are incapable of regeneration.
The cornea is richly innervated; myelin sheaths are present on the nerves as they traverse the superficial layers of the cornea. The nerve endings lose their sheath as they penetrate the epithelium. In terms of density, more nerve endings are present in the corneal epithelium than anywhere else in the human body.1,2
Contraindications
Contraindications include unstable refractive error, active collagen vascular disease (especially in the presence of iritis or scleritis), pregnancy, presence of a pacemaker, any ongoing active inflammation of the external eye (eg, conjunctivitis, severe dry eye), and a refractive error outside the range of laser correction (it is common to have patients treated slightly outside the approved range, but they must understand that it is an off-label use of the excimer laser).
Other contraindications include leaving less than a calculated residual bed of 250 µm of untouched cornea, as well as signs, symptoms, or topographic findings consistent with keratoconus.
Patients who are on Accutane (isotretinoin), Cordarone (amiodarone hydrochloride), and Imitrex (sumatriptan) should be treated with caution, and patient counseling should be provided because these medications may adversely affect corneal wound healing.
A history of herpetic keratitis is a relative contraindication. Although patients have been treated safely with a history of herpes simplex keratitis and the appropriate use of prophylactic antivirals, reactivation of the virus following treatment remains a concern.
Patients who cannot cooperate with procedures under a topical anesthetic and cannot accurately fixate or lay flat without difficulty are poor candidates for refractive surgery.
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References
Smolin G, Thoft RA. Basic Science: Anatomy of the Conjunctiva, Cornea and Limbus. The Cornea. 1994;3-12.
Kanski JJ. Disorders of the Cornea and Sclera. In: Clinical Ophthalmology: A Systematic Approach. 3rd ed. Revised. ISBN 0 7506 1886 8: 1994:100.
Durairaj VD, Balentine J, Kouyoumdjian G, et al. The predictability of corneal flap thickness and tissue laser ablation in laser in situ keratomileusis. Ophthalmology. Dec 2000;107(12):2140-3. [Medline].
Petrauskas J. Reinventing Refraction with Wave front Technology. EyeNet. 2000;4:33-35.
Chalita MR, Finkenthal J, Xu M, Krueger RR. LADARWave wavefront measurement in normal eyes. J Refract Surg. Mar-Apr 2004;20(2):132-8. [Medline].
Koffler MC, Burlew JA. Corneal Topography and the Corneal Modeling System. Mediguide Ophthalmology. 1991;6:1-5.
Wilson EW, Klyce SD. Screening for Corneal Topography Abnormalities before Refractive Surgery. Ophthalmology. 1994;101:147-52.
Rabinowitz YS. Videokeratographic indices to aid in screening for keratoconus. J Refract Surg. Sep-Oct 1995;11(5):371-9. [Medline].
Arevalo JF, Ramirez E, Suarez E, et al. Incidence of vitreoretinal pathologic conditions within 24 months after laser in situ keratomileusis. Ophthalmology. Feb 2000;107(2):258-62. [Medline].
Quinto GG, Camacho W, Behrens A. Postrefractive surgery dry eye. Curr Opin Ophthalmol. Jul 2008;19(4):335-41. [Medline].
Huppertz M, Schmidt E, Winfried T. Eye tracking and refractive surgery. In: Customized Corneal Ablation: The Quest for SuperVision. ed. 2001:149-160.
Perez-Santonja JJ, Ayala MJ, Sakla HF, Ruiz-Moreno JM, Alio JL. Retreatment after laser in situ keratomileusis. Ophthalmology. Jan 1999;106(1):21-8. [Medline].
Jacobs JM, Sanderson MC, Spivack LD, Wright JR, Roberts AD, Taravella MJ. Hyperopic laser in situ keratomileusis to treat overcorrected myopic LASIK. J Cataract Refract Surg. Mar 2001;27(3):389-95. [Medline].
Hsu SY, Chang MS, Lee CJ. Intraocular pressure assessment in both eyes of the same patient after laser in situ keratomileusis. J Cataract Refract Surg. Jan 2009;35(1):76-82. [Medline].
Gimbel HV, Penno EE, van Westenbrugge JA, Ferensowicz M, Furlong MT. Incidence and management of intraoperative and early postoperative complications in 1000 consecutive laser in situ keratomileusis cases. Ophthalmology. Oct 1998;105(10):1839-47; discussion 1847-8. [Medline].
Stulting RD, Carr JD, Thompson KP, Waring GO 3rd, Wiley WM, Walker JG. Complications of laser in situ keratomileusis for the correction of myopia. Ophthalmology. Jan 1999;106(1):13-20. [Medline].
Wilson SE. LASIK: management of common complications. Laser in situ keratomileusis. Cornea. Sep 1998;17(5):459-67. [Medline].
Joo CK, Kim TG. Corneal perforation during laser in situ keratomileusis. J Cataract Refract Surg. Aug 1999;25(8):1165-7. [Medline].
Jacobs JM, Taravella MJ. Incidence of intraoperative flap complications in laser in situ keratomileusis. J Cataract Refract Surg. Jan 2002;28(1):23-8. [Medline].
John T, Velotta E. Nontuberculous (atypical) mycobacterial keratitis after LASIK: current status and clinical implications. Cornea. Apr 2005;24(3):245-55. [Medline].
Chang MA, Jain S, Azar DT. Infections following laser in situ keratomileusis: an integration of the published literature. Surv Ophthalmol. May-Jun 2004;49(3):269-80. [Medline].
Webber SK, Lawless MA, Sutton GL, Rogers CM. Staphylococcal infection under a LASIK flap. Cornea. May 1999;18(3):361-5. [Medline].
Smith RJ, Maloney RK. Diffuse lamellar keratitis. A new syndrome in lamellar refractive surgery. Ophthalmology. Sep 1998;105(9):1721-6. [Medline].
Moshirfar M, Welling JD, Feiz V, Holz H, Clinch TE. Infectious and noninfectious keratitis after laser in situ keratomileusis Occurrence, management, and visual outcomes. J Cataract Refract Surg. Mar 2007;33(3):474-83. [Medline].
Wang MY, Maloney RK. Epithelial ingrowth after laser in situ keratomileusis. Am J Ophthalmol. Jun 2000;129(6):746-51. [Medline].
Muller LT, Candal EM, Epstein RJ, Dennis RF, Majmudar PA. Transepithelial phototherapeutic keratectomy/photorefractive keratectomy with adjunctive mitomycin-C for complicated LASIK flaps. J Cataract Refract Surg. Feb 2005;31(2):291-6. [Medline].
Solomon R, Donnenfeld ED, Perry HD. Photorefractive keratectomy with mitomycin C for the management of a LASIK flap complication following a penetrating keratoplasty. Cornea. May 2004;23(4):403-5. [Medline].
Pande M, Hillman JS. Optical zone centration in keratorefractive surgery. Entrance pupil center, visual axis, coaxially sighted corneal reflex, or geometric corneal center?. Ophthalmology. Aug 1993;100(8):1230-7. [Medline].
Lam DS, Leung AT, Wu JT, et al. Management of severe flap wrinkling or dislodgment after laser in situ keratomileusis. J Cataract Refract Surg. Nov 1999;25(11):1441-7. [Medline].
Donnenfeld ED, Perry HD, Doshi SJ, Biser SA, Solomon R. Hyperthermic treatment of post-LASIK corneal striae. J Cataract Refract Surg. Mar 2004;30(3):620-5. [Medline].
Narvaez J, Chakrabarty A, Chang K. Treatment of epithelial ingrowth after LASIK enhancement with a combined technique of mechanical debridement, flap suturing, and fibrin glue application. Cornea. Oct 2006;25(9):1115-7. [Medline].
Toda I. LASIK and the ocular surface. Cornea. Sep 2008;27 Suppl 1:S70-6. [Medline].
Johnson JD, Harissi-Dagher M, Pineda R, Yoo S, Azar DT. Diffuse lamellar keratitis: incidence, associations, outcomes, and a new classification system. J Cataract Refract Surg. Oct 2001;27(10):1560-6. [Medline].
Donnenfeld ED, Kim T, Holland EJ, et al. ASCRS White Paper: Management of infectious keratitis following laser in situ keratomileusis. J Cataract Refract Surg. Oct 2005;31(10):2008-11. [Medline].
Randleman JB, Russell B, Ward MA, Thompson KP, Stulting RD. Risk factors and prognosis for corneal ectasia after LASIK. Ophthalmology. Feb 2003;110(2):267-75. [Medline].
Binder PS. Analysis of ectasia after laser in situ keratomileusis: risk factors. J Cataract Refract Surg. Sep 2007;33(9):1530-8. [Medline].
Caster AI, Friess DW, Potvin RJ. Absence of keratectasia after LASIK in eyes with preoperative central corneal thickness of 450 to 500 microns. J Refract Surg. Oct 2007;23(8):782-8. [Medline].
Randleman JB, Trattler WB, Stulting RD. Validation of the Ectasia Risk Score System for preoperative laser in situ keratomileusis screening. Am J Ophthalmol. May 2008;145(5):813-8. [Medline].
Knorz MC, Wiesinger B, Liermann A, Seiberth V, Liesenhoff H. Laser in situ keratomileusis for moderate and high myopia and myopic astigmatism. Ophthalmology. May 1998;105(5):932-40. [Medline].
Maldonado-Bas A, Onnis R. Results of laser in situ keratomileusis in different degrees of myopia. Ophthalmology. Apr 1998;105(4):606-11. [Medline].
Montes M, Chayet A, Gomez L, Magallanes R, Robledo N. Laser in situ keratomileusis for myopia of -1.50 to -6.00 diopters. J Refract Surg. Mar-Apr 1999;15(2):106-10. [Medline].
Probst L, Hakim O, Nichols B, Baird M. LASIK Results from TLC, The London Laser Center. In: The Art of LASIK. 2nd ed. 1999:303-8.
Reviglio VE, Luna JD, Rodriguez ML, Garcia FE, Juarez CP. Laser in situ keratomileus using the LaserSight 200 laser: results of 950 consecutive cases. J Cataract Refract Surg. Aug 1999;25(8):1062-8. [Medline].
Kezirian G, Casebeer J. The CRS LASIK Study. In: The Art of LASIK. 2nd ed. 1999:293-302.
Carr J, Thompson K, Stulting R, Waring G. LASIK: Emory Vision Correction Center. In: The Art of LASIK. 2nd ed. 2000:281-92.
Kezirian GM, Stonecipher KG. Comparison of the IntraLase femtosecond laser and mechanical keratomes for laser in situ keratomileusis. J Cataract Refract Surg. Apr 2004;30(4):804-11. [Medline].
Arbelaez MC, Knorz MC. Laser in situ keratomileusis for hyeropia and hyperopic astigmatism. J Refract Surg. Jul-Aug 1999;15(4):406-14. [Medline].
Argento CJ, Cosentino MJ. Laser in situ keratomileusis for hyperopia. J Cataract Refract Surg. Aug 1998;24(8):1050-8. [Medline].
Tran DB, Sarayba MA, Bor Z, et al. Randomized prospective clinical study comparing induced aberrations with IntraLase and Hansatome flap creation in fellow eyes: potential impact on wavefront-guided laser in situ keratomileusis. J Cataract Refract Surg. Jan 2005;31(1):97-105. [Medline].
Chang JS. Complications of sub-Bowman's keratomileusis with a femtosecond laser in 3009 eyes. J Refract Surg. Jan 2008;24(1):S97-101. [Medline].
Chiang PK, Hersh PS. Comparing predictability between eyes after bilateral laser in situ keratomileusis: a theoretical analysis of simultaneous versus sequential procedures. Ophthalmology. Sep 1999;106(9):1684-91. [Medline].
Gimbel HV, van Westenbrugge JA, Penno EE, Ferensowicz M, Feinerman GA, Chen R. Simultaneous bilateral laser in situ keratomileusis: safety and efficacy. Ophthalmology. Aug 1999;106(8):1461-7; discussion 1467-8. [Medline].
Dawson DG, Grossniklaus HE, McCarey BE, Edelhauser HF. Biomechanical and wound healing characteristics of corneas after excimer laser keratorefractive surgery: is there a difference between advanced surface ablation and sub-Bowman's keratomileusis?. J Refract Surg. Jan 2008;24(1):S90-6. [Medline].
Randleman JB, Dawson DG, Grossniklaus HE, McCarey BE, Edelhauser HF. Depth-dependent cohesive tensile strength in human donor corneas: implications for refractive surgery. J Refract Surg. Jan 2008;24(1):S85-9. [Medline].
Durrie DS, Slade SG, Marshall J. Wavefront-guided excimer laser ablation using photorefractive keratectomy and sub-Bowman's keratomileusis: a contralateral eye study. J Refract Surg. Jan 2008;24(1):S77-84. [Medline].
Wang Z, Chen J, Yang B. Posterior corneal surface topographic changes after laser in situ keratomileusis are related to residual corneal bed thickness. Ophthalmology. Feb 1999;106(2):406-9; discussion 409-10. [Medline].
Ditzen K, Huschka H, Pieger S. Laser in situ keratomileusis for hyperopia. J Cataract Refract Surg. Jan 1998;24(1):42-7. [Medline].
Further Reading
Keywords
myopia, LASIK, laser in situ keratomileusis, shortsighted, vision loss, visual deficit
