1. Comparison of Wavefront and Corneal Aberration Changes after Advanced Corneal Surface Ablation and Femtosecond Thin Flap LASIK Tahra AlMahmoud, MBBS 1, Rejean Munger, PhD 2, W. Bruce Jackson, MD 2 1 Department of Surgery, Faculty of Medicine and Health Sciences, United Arab Emirates University, Al Ain, UAE 2 Department of Ophthalmology, University of Ottawa Eye Institute, Ottawa, Ontario, Canada Financial Disclosure: Dr. W. Bruce Jackson, MD, FRCSC is a consultant for VISX/AMO

2. Purpose To compare the ocular and corneal aberrations changes after myopic wavefront-guided advanced surface ablation (WF-ASA) techniques (AA-PRK, LASEK, Epi- PRK, and Epi-LASIK) and IntraLase iLASIK (IntraLase™ FS Laser) using STAR S4 IR Laser.

3. Introduction Advanced surface ablation (ASA) covers a range of techniques including: 1.Epithelial flap–on procedures such as Laser Epithelial Keratomileusis (LASEK), Epikeratome Laser Assisted In situ Keratomileusis (Epi-LASIK), 2.Epithelial flap–off such as Epikeratome Photorefractive Keratectomy (Epi-PRK), and Alcohol Assisted Photorefractive Keratectomy (AA-PRK). ASA is a term that was introduced in 2003 to highlight the beneficial aspects of new ablation patterns, different epithelial management techniques, better pain control, and use of MMC compared with earlier conventional Photorefractive Keratectomy (PRK).

4. Introduction Laser ablations considered the principle source of surgically induced HOA’s. Flap creation is also an important contributor to increased aberration. IntraLase LASIK (iLASIK) is known for it ability to create planar flap, astigmatically neutral as well as evidence that it reduces the incidence of flap-related complications. Spherical-like aberrations have been implicated as one of the most important HOA’s after wavefront-guided refractive surgery.

5. Materials and Methods Ocular wavefront aberration and corneal topography of 240 eyes in the WF-ASA techniques, and 138 eyes in the iLASIK group were obtained before and 3 months following treatment. Wavefront (ocular) aberrations were measured using the WaveScan, wavefront TM system. Corneal aberrations were obtained from elevation maps(Pentacam, Oculus (Oculus, Wetzlar, Germany)) and calculated by ray-tracing using custom software. Aberrations were described as Zernike polynomials. All eyes been treated with wavefront-guided VISX STAR S4 IR TM (Abbott Medical Optics Inc. (AMO), Santa Ana, USA) with an eye-tracking device and Fourier Wavefront-guided surface ablation with iris registration.

6. Materials and Methods The full treatment to achieve emmetropia was used and a spherical adjustment up to the manifest refraction was incorporated to the treatment plan. 0.02% Mitomycin C was applied to the stromal bed for 15 to 30 seconds for eyes undergoing ASA with spheres of –6.00D or more and cylinder of more than 2.00D. iLASIK flaps were set to 9 mm diameter and 110 µm Corneal and ocular aberrations were described as Zernike polynomials and analysis focused on total high order aberrations (HOAs) and spherical aberration (SA).

7. Results There was statistically significant (P<0.001) surgically induced increase in corneal SA, HOAs and coma in each of the techniques at 3 months (Table 2). Waverfront SA also increased significantly (P<0.001) in both groups but not Wavefront HOAs (Table 3). A significant difference (P 0.05) (Table 3).

8. Results Also a difference between the ASA and iLASIK group was identified for corneal HOAs and coma (P<0.05) but not for SA at 3 months postoperatively (Table 2). Ocular change in SA were weakly correlated to preoperative SA (-0.3, P<0.001) in both groups but very strongly correlated to applied spherical correction (-0.68, P<0.001) in ASA group. Surgically induced corneal SA was weakly correlated to preoperative corneal SA (-0.34, P<0.001) in ASA group and applied spherical correction for both groups.

9. Table (1): Patients Demographics Preoperative SE (D) Preoperative sphere (D) Preoperative cylinder (D) Age (years) Epi-LASIK Mean ± SD range -4.02 ± 1.93 -1.00 to -7.00 -4.28 ± 1.94 -1.00 to -7.25 0.53 ± 0.54 0.00 to 2.00 36 ± 8 24 to 53 Epi-PRK Mean ± SD range -3.98 ± 2.01 -1.13 to -7.75 -4.22 ± 2.03 -1.25 to -6.25 0.48 ± 0.43 0.00 to 1.55 32 ± 8 23 to 60 LASEK Mean ± SD range -4.11 ± 1.95 -1.25 to -7.50 -4.34 ± 1.87 -1.25 to -7.50 0.47 ± 0.54 0.00 to 2.00 32 ± 9 20 to 52 AA–PRK Mean ± SD range -4.07 ± 1.97 -1.00 to -7.75 -4.44 ± 1.99 -2.0 to -8.25 0.74 ± 0.55 0.00 to 2.0 36 ± 10 22 to 58 Total ASA Mean ± SD range -4.04±1.958 -1.00 to -7.75 -4.32±1.9524 -1.00 to -8.25 0.5546±0.526 0 to 2 34±8.9 20 to 60 iLASIK Mean ± SD range -3.79 ±1.95 -0.37 to -8.75 -4.14±1.98 -1.00to -9.50 0.69±0.66 0.00 to 3.50 40 ± 9.0 20 to 58 N = 240 for ASA \(60 per ASA treatment types), N=137 for iLASIK

10. Table (2): Corneal aberration with 5.00 mm pupil HOA (µm)SA (µm)Coma (µm) All ASA N= 240 Mean ± SD or Median (50% CI) Preop0.349 (0.297;0.435)0.175 (0.127;0.238)0.202 (0.130;0.291) 3 M postop0.539 (0.440;0.662)0.319 (0.223;0.424)0.246 (0.152;0.359) Change [P value] ¶ 0.178 (0.059;0.310) <0.001 0.142 (0.0570; 0.252) <0.001 0.036 (-0.058; 0.145) 0.001 iLASIK N = 137 Mean ± SD or Median (50% CI) Preop0.214 (0.164;0.275)0.223 (0.170;0.270)0.140 (0.083;0.212) 3 M postop0.316 (0.244;0.393)0.328 (0.242;0.441)0.200 (0.139;0.281) Change [P value] ¶ 0.102 (0.015;0.192) <0.001 0.118 (0.032; 0.209) <0.001 0.065 (-0.032;0.159) <0.001 ¶ - Paired t test or Signed Rank test, * - no significant change following surgery

11. Table (3): Ocular aberration with 5.00 mm pupil HOA (µm)SA (µm)Coma (µm) All ASA N = 240 Mean ± SD or Median (50% CI) Preop0.157 (0.120;0.193)0.038 (0.003;0.076)0.086 (0.054;0.128) 3 M postop0.225 (0.170;0.296)0.154 (0.087;0.209)0.099 (0.066;0.148) Change [P value] ¶ 0.072 (0.009; 0.139) <0.001 0.119 (0.048; 0.178) <0.001 0.008 (-0.034; 0.063) 0.010 iLASIK N = 137 Mean ± SD or Median (50% CI) Preop0.143 (0.102;0.192)0.039 (0.009; 0.082)0.092 (0.057;0.150) 3 M postop0.148 (0.103;0.222)0.154 (0.087; 0.209)0.113 (0.067;0.162) Change [P value] ¶ 0.017 (-0.040; 0.066) 0.227 * 0.048 (0.004; 0.085) <0.001 0.020 (-0.035;0.065) 0.052 * ¶ - Paired t test or Signed Rank test, * - no significant change following surgery

12. Discussion Our study is consistent with other reports showing that the WF-ASA and iLASIK procedure induces HOA’s. Placing the ablation under a thin planar LASIK flap may account for less induced aberrations. In addition, wound healing may not have stabilized at 3 months for ASA resulting in higher aberrations than might be present with a longer follow-up. At 3 months all procedures resulted in a significant increase in HOAs and SA. Femtosecond Thin Flap LASIK induced significantly less wavefront and corneal HOAs than ASA.

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