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Separate Effects of the Microkeratome Incision and Laser Ablation on the Eye’S Wave Aberration

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Separate Effects of the Microkeratome Incision and Laser Ablation on the Eye’S Wave Aberration Separate Effects of the Microkeratome Incision and Laser Ablation on the Eye’s Wave Aberration JASON PORTER, MS, SCOTT MACRAE, MD, GEUNYOUNG YOON, PHD, CYNTHIA ROBERTS, PHD, IAN G. COX, PHD, AND DAVID R. WILLIAMS, PHD ● PURPOSE: To study the optical changes induced by the tion, the total and higher order root mean square of microkeratome cut, the subsequent laser ablation, and wavefront errors were nearly identical for both the study the biomechanical healing response of the cornea in and control eyes 3-months after the laser ablation, normal laser in situ keratomileusis (LASIK) eyes. indicating that a procedure in which the incision and the ● DESIGN: Prospective randomized clinical trial. ablation are separated in time to better control aberra- ● METHODS: A Hansatome microkeratome was used to tions does not compromise the outcome of a conventional cut a corneal flap in one eye (study eye) of 17 normal LASIK treatment. (Am J Ophthalmol 2003;136: myopic patients and a subsequent laser ablation was 327–337. © 2003 by Elsevier Inc. All rights reserved.) performed 2 months after this initial microkeratome incision. Control eyes received conventional LASIK VEN THOUGH EARLY CUSTOMIZED ABLATION RESULTS treatments at the latter time point. The wave aberration have been encouraging, further refinements of wave- of both the study and contralateral control eyes were E front-guided laser in situ keratomileusis (LASIK) measured over a 6-mm pupil with a Shack–Hartmann procedures are needed to obtain an aberration-free ablative wavefront sensor for all preoperative, postflap cut, and correction.1–4 It has been well established that conven- postablation visits. tional photorefractive keratectomy (PRK) and LASIK ● RESULTS: The eye’s higher order aberrations had a procedures induce higher order aberrations, including in- of approximately creased amounts of spherical aberration, despite their (03. ؍ small, but significant increase (P 30% 2 months after cutting a flap. No systematic changes accurate correction of spherocylindrical errors.5–10 To date, were observed in nearly all Zernike coefficients from little evidence has been presented to determine the origins their preoperative levels at 2 months postflap cut. A of these induced higher order aberrations. Consequently, significant difference between the study and control eyes not much is known about the importance or relative ؍ 3 was observed for one trefoil mode, Z3 (P .04). contribution of the two separate effects of creating a ● CONCLUSIONS: There was a wide variation in the microkeratome flap and performing a subsequent laser response of individual Zernike modes across patients ablation on the overall optical quality of an eye receiving after cutting a flap. The majority of spherical aberration a LASIK treatment. Developing a firm understanding of induced by the LASIK procedure seems to be due to the the aberrations induced by these two processes is crucial for laser ablation and not the microkeratome cut. In addi- achieving an optimal, customized correction of the eye’s higher order aberrations with laser refractive surgery. Accepted for publication Feb 12, 2003. Studies have been conducted to investigate structural InternetAdvance publication at ajo.com Feb 19, 2003. From The Institute of Optics (J.P.), Department of Ophthalmology changes in corneal shape due to the laser ablation in (S.M., G.Y.), and Center for Visual Science (D.R.W.), University of refractive surgery procedures. Howland and coworkers11 Rochester, Rochester, New York; Department of Ophthalmology (C.R.), found that changes made in the midperipheral and periph- Ohio State University, Columbus, Ohio; and Bausch & Lomb (I.G.C.), Rochester, New York. eral cornea could create biomechanical changes in the Supported by the National Institutes of Health grants EY01319 and central cornea in radial keratotomy. In PRK and LASIK EY07125, and a research grant from Bausch & Lomb. Jason Porter, procedures, central corneal shape can be altered by shape Geunyoung Yoon, Cynthia Roberts, David Williams, and Scott MacRae have served as consultants for Bausch & Lomb. In addition, the changes occurring outside of the ablation zone. The laser University of Rochester has a licensing agreement with Bausch & Lomb ablation itself creates a thickening of the peripheral stroma for the customized correction of higher order aberrations. (that is, an increase in corneal elevation outside of the Inquiries to Scott MacRae, MD, StrongVision, 100 Meridian Centre, Suite 125, Rochester, NY 14618; fax: (585) 756-1975; e-mail: scott_ ablation zone) and a secondary steepening of the midpe- [email protected] ripheral cornea. This peripheral steepening results in a 0002-9394/03/$30.00 © 2003 BY ELSEVIER INC.ALL RIGHTS RESERVED. 327 doi:10.1016/S0002-9394(03)00222-8 flattening of the central cornea and consequently a shift toward hyperopia.12,13 These peripheral and central shape TABLE 1. Average Preoperative Manifest Refractions and changes, along with the unintended hyperopic shift, can Attempted Corrections for Study and Control Eyes occur even for a PTK type ablation, which is not designed Preoperative Manifest Attempted Laser 14–17 to change corneal curvature. As a result, the biome- Refraction Correction chanical response of the cornea to the laser ablation will Eyes Sphere Cylinder Sphere Cylinder yield central flattening and hyperopic shifts for both myopic and hyperopic procedures. Few reports have dis- Study Ϫ2.62 D Ϫ0.50 D Ϫ2.55 D Ϫ0.67 D Ϫ Ϫ Ϫ Ϫ cussed the effects of these biomechanical changes on Control 2.66 D 0.65 D 2.61 D 0.70 D aberrations other than defocus, or sphere. D ϭ diopters. We sought to determine how the eye’s aberrations changed as a result of the microkeratome flap-cut and the laser ablation. The first experimental stage provided infor- mation on the degree of aberration change resulting only the contralateral control eye was treated with a conven- from the corneal flap-cut and the subsequent readhesion of tional LASIK treatment based on the most recent manifest the flap to the cornea. The second stage offered further refraction. The average preoperative manifest refractions information on the amount of aberrations that are induced and attempted corrections for both the study and control in LASIK solely due to the laser ablation. Comparing post- eyes are presented in Table 1. ablation aberration data from the study eyes with data from Manifest refractions and uncorrected and best-corrected the normal, one-step LASIK-treated, contralateral control visual acuities were performed preoperatively and at each eyes might provide preliminary insight for whether conven- postoperative examination. A Shack–Hartmann wave- tional LASIK yields better optical performance when per- front sensor (Zywave; Bausch & Lomb) was used to formed as a one- or two-step procedure. Finally, this study measure the monochromatic wave aberration of both the could also provide insight as to whether the majority of study and control eyes under scotopic conditions with spherical aberration induced after a LASIK procedure is a dilation (2.5% neosynephrine). The wave aberration was result of the microkeratome incision or the laser ablation. expressed as a Zernike polynomial up to and including fifth order aberrations (18 Zernike terms) over a 6-mm pupil. The Zernike polynomials were expressed according to the METHODS standard normalized polynomials and representation estab- lished by the VSIA Standards Taskforce team.18 Wave- SEVENTEEN HEALTHY MYOPIC PATIENTS BETWEEN 30 AND front analysis was made comparable for right and left eyes 50 years of age were enrolled in an institutional review by creating the mirror image of the wavefronts from all left board-monitored study. A full informed consent was ob- eyes (that is, flipping the signs of all appropriate Zernike tained for each patient. The myopia ranged between coefficients that exhibit odd symmetry about the vertical Ϫ0.75 and Ϫ6.75 diopters (D) with less than Ϫ2.50 D of axis) and performing the analysis. All measurements were astigmatism. made in the study and the contralateral control eyes The study was divided into two stages: a postmicrokera- preoperatively (Pre), at 20 minutes (20 min), 1 day (1D), tome incision (or postflap cut) stage and a postlaser ablation 1 week (1W), 1 month (1M), and 2 months (2M) stage. In the first stage, patients received a microkeratome postmicrokeratome incision. The same measurements were incision with a superior hinge in only one eye (study eye) also performed 1 day (1DLSK), 1 week (1WLSK), 1 month while the contralateral eye (control eye) was left untreated. A (1 MLSK), and 3 months (3 MLSK) postlaser ablation. Hansatome Microkeratome (Bausch & Lomb, Rochester, New York, USA) was used to perform the cut. The study eye ● STATISTICAL ANALYSIS: To determine which aberra- for each patient was chosen at random. All study eyes tions significantly changed due to the flap cut, two-tailed t received a 180-␮m microkeratome plate depth incision with tests were used to test for significance when comparing a flap diameter of 9.5 mm or 8.5 mm, and no subsequent laser postoperative differences in aberrations (relative to preop) treatment was performed after the incision. between the study and control eyes 2 months postflap cut. The second stage of the study was conducted at least 2 The same tests were used to investigate potential signifi- months after the initial microkeratome incision was made cant changes between the study and control eyes 3 months in the study eye. In the second stage, the microkeratome after the laser ablation. Finally, paired, two-tailed t tests flap was lifted in the study eyes and a conventional myopic were used to determine significant changes in aberrations excimer ablation (Technolas 217c; Bausch & Lomb) was from preoperative levels within the study eyes and the performed based on the preoperative and 2-month post- control eyes 3 months postablation. Values of P Ͻ .05 were microkeratome incision manifest refractions. At that time, considered to be significant.
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