To compare corneal high-order aberrations and visual acuity after laser in situ keratomileusis (LASIK) with the flap created by a femtosecond laser (bladeless) to LASIK with the flap created by a mechanical microkeratome.
Prospective, randomized, paired-eye study.
Fellow eyes of 21 patients with myopia or myopic astigmatism were randomized by ocular dominance. Corneal topography and visual acuity were measured before and at 1, 3, 6, 12, and 36 months after LASIK. Wavefront errors from the anterior corneal surface were calculated from the topography data over 4- and 6-mm-diameter pupils and decomposed into Zernike polynomials to the 6th order.
There were no differences in corneal total high-order aberrations, spherical aberration, coma, or trefoil between methods of flap creation at any examination over 4- and 6-mm-diameter pupils. Over a 6-mm pupil, total high-order aberrations increased by 1 month after LASIK with both treatments ( P ≤ .001) and remained increased through 36 months ( P ≤ .001). Uncorrected and best-corrected visual acuity did not differ between methods at any examination and remained stable postoperatively through 3 years; the minimum detectable difference in visual acuity between treatments was ≤0.1 logMAR (≤1 line of vision, α = 0.05/6, β = 0.20, n = 21).
The planar configuration of the femtosecond laser flap did not offer any advantage in corneal high-order aberrations or visual acuity through 3 years after LASIK. Corneal high-order aberrations remain stable through 3 years after LASIK.
Laser in situ keratomileusis (LASIK) is the most common corneal refractive surgery for the correction of myopia, and involves photoablation of the corneal stroma deep to an anterior corneal flap. Flap creation is the main surgical step of this procedure and can result in complications. Flaps have traditionally been created with mechanical microkeratomes, but femtosecond laser technology has emerged as an alternative for flap creation. Femtosecond lasers use ultrafast pulses to induce photodisruption of tissue with minimal surrounding tissue damage. LASIK flaps created with the femtosecond laser have a planar configuration; flap thickness is uniform, in contrast to flaps created by a microkeratome, in which the center of the flap is thinner than the periphery. The planar flap configuration has been suggested to confer an optical advantage, possibly reducing high-order aberrations. Several studies have described the short-term clinical outcomes of patients who underwent LASIK with the flap created by a femtosecond laser versus a mechanical microkeratome; some have found little difference in outcome between the techniques, whereas others have suggested more favorable outcomes with the femtosecond laser.
LASIK has also been associated with long-term deficits of keratocytes in the flap stroma. The clinical effects of anterior keratocyte deficits have not been studied, but one consequence might be instability of postoperative corneal wavefront errors resulting from changes in the anterior corneal surface. In this randomized, contralateral eye study, we investigated differences between, and the stability of, corneal high-order aberrations and visual acuity through 3 years after LASIK with the flap created by either a femtosecond laser (bladeless) or a mechanical microkeratome. We have previously reported short-term visual outcomes for this trial.
Materials and Methods
Twenty-one subjects were recruited from patients attending the refractive surgery service at Mayo Clinic. All patients had myopia or myopic astigmatism, were >21 years old, and were determined to be suitable candidates for LASIK after a rigorous screening examination. Subjects were excluded if they had any corneal abnormalities; a history of ocular disease, trauma, or surgery; or diabetes mellitus or other systemic disease known to affect the eye; or if they used ocular medications. Systemic medications were permitted unless they were known to affect the cornea or anterior segment. Subject age at surgery was 38 ± 10 years (mean ± standard deviation; range, 22-54 years). Patients who developed new ocular conditions during the follow-up period were excluded from subsequent analysis if it was determined that the new condition would interfere with the outcomes. Fellow eyes of unoperated normal myopic controls were examined concurrently; age of controls was 43 ± 7 years (range, 29-55 years). This study complied with the Health Insurance Portability and Accountability Act and was approved by the Mayo Clinic Institutional Review Board. Informed consent was obtained from all subjects after explanation of the nature and possible consequences of the study.
Patients were stratified by ocular dominance and then 1 eye of each patient was randomized to LASIK with the flap created by a femtosecond laser, and the other eye to LASIK with the flap created by a mechanical microkeratome. Ocular dominance was tested by asking patients to use both hands to frame a distant target while an observer determined with which eye the target was aligned.
Laser In Situ Keratomileusis Procedure
Bladeless flaps were created with a 15-kHz femtosecond laser (IntraLase FS; IntraLase Corp, Irvine, California, USA). All flaps had a superior hinge and intended thickness of 120 μm. Raster line and spot separation were 9 and 11 μm, respectively; raster energy was 2.3 μJ, and side-cut energy was 2.5 μJ. Flaps created by the mechanical microkeratome (Hansatome; Bausch & Lomb, Rochester, New York, USA) had a superior hinge with intended thickness of 180 μm. Non-wavefront-guided ablation of the stromal bed was performed with a VISX Star S4 excimer laser (VISX, Santa Ana, California, USA) with radiant exposure of 160 mJ/cm 2 . Emmetropia was attempted in all cases by using an ablation zone that ranged from 6.5 × 6.5 mm for spherical corrections to 6.5 × 5.0 mm for astigmatic corrections. Both eyes of each patient were treated on the same day. All procedures followed a standard protocol: the bladeless flap was created first on the eye randomized to the femtosecond laser; the fellow eye then received a full LASIK procedure with the flap created by the mechanical microkeratome; finally, LASIK was completed on the first eye by separating and lifting the flap created by the femtosecond laser and ablating the stroma. It was not possible to mask patients as to which treatment was received in each eye. Postoperative topical medication regimens were identical for each eye and consisted of ciprofloxacin ophthalmic solution 4 times per day for 5 days, and fluorometholone 0.1% 4 to 8 times daily with a taper over 3 weeks.
Patients were examined before LASIK and at 1, 3, 6, 12, and 36 months after LASIK. At each examination, corneal topography, high-contrast visual acuity, low-contrast visual acuity, and manifest refraction were recorded. Whole-eye aberrometry was introduced to the study protocol during the enrollment phase and thus data were not available for all eyes at every examination; low-contrast visual acuity data were only available at 12 and 36 months after LASIK. Measurements were made by observers masked as to which treatment was received in each eye.
Corneal topography was recorded using a Humphrey Atlas corneal topography system (Humphrey Systems, Pleasanton, California, USA). Two to 4 topographic maps were recorded for each eye, centered over the line of sight. The videokeratography maps of each cornea were examined by 1 masked observer, and the map with the most complete image and the smallest nondigitized areas was selected for assessment of wavefront errors from the anterior corneal surface. Aberrations for the whole eye were measured with a Hartman-Shack aberrometer (VISX Wavescan, Santa Ana, California, USA). Two to 4 whole-eye wavefront analyses were acquired for each eye, and the examination with the highest quality and largest pupil diameter, as indicated by the aberrometry software, was selected by a masked observer.
High-contrast visual acuity was measured using the electronic Early Treatment of Diabetic Retinopathy Study testing protocol. Uncorrected visual acuity (UCVA) and best spectacle-corrected visual acuity (BCVA) were recorded as letter scores, which were converted to logarithm of the minimal angle of resolution (logMAR).
Best spectacle-corrected low-contrast visual acuity (LCVA) was measured by using a backlit 10% Sloan Translucent Low Contrast Chart (Precision Vision, La Salle, Illinois, USA) with a testing distance of 4 meters. LCVA was measured in a darkened room and recorded as letter scores under photopic (screen brightness, 139 cd/m 2 ) and mesopic (screen brightness, 1.1 cd/m 2 ) conditions; mesopic conditions were achieved by placing a neutral density (2 ND) filter in front of the low-contrast chart. ETDRS letter scores were converted to logMAR and Snellen equivalent.
From the corneal topography (anterior corneal surface) and whole-eye aberrometry data, the wavefront errors over 4-mm and 6-mm pupils were calculated using VOLCT (Sarver and Associates, Inc, Carbondale, Illinois, USA) and decomposed into Zernike polynomials to the 6th order. All high-order aberrations were summarized for Zernike orders 3-6 as:
Z n m
is the Zernike coefficient of radial order n and angular frequency m. Spherical aberration was expressed as |Z04|∣∣Z04∣∣
| Z 4 0 |
, coma as √(Z−13)2+(Z13)2
( Z 3 − 1 ) 2 + ( Z 3 1 ) 2
, and trefoil as √(Z−33)2+(Z33)2
( Z 3 − 3 ) 2 + ( Z 3 3 ) 2
. Anterior corneal surface wavefront errors were calculated by assuming that the keratometric refractive index was 1.3375 at a wavelength of 555 nm. For whole-eye aberrometry, wavefront errors were calculated over 4-mm and 6-mm wavefront diameters; data were excluded if the measured pupil was less than these wavefront diameters.
The study was powered a priori to detect a difference of 0. 15 logMAR in UCVA or BCVA at 3 years after LASIK by assuming that the standard deviation of the difference in visual acuity would be 0.15 logMAR. This required a minimum sample size of 16 subjects (α = 0.05/6, β = 0.20, paired test). Differences between treatments at each examination, and differences between preoperative and postoperative examinations for each treatment, were assessed by using 2-tailed paired t tests if the data were distributed normally and Wilcoxon signed rank tests if they were not. P values were adjusted for multiple comparisons by using the Bonferroni method, and P < .05 was considered statistically significant. All statistical analyses were performed with Statistical Analysis System Version 9.1.3 (SAS Institute Inc, Cary, North Carolina, USA). Minimum detectable differences were calculated post hoc for nonsignificant differences assuming α = 0.05/5 or 0.05/6 (depending on the comparison) and β = 0.20.
Incomplete whole-eye wavefront error data and no LCVA data were available in patients before LASIK. To determine if differences in whole-eye wavefront errors and LCVA exist between fellow unoperated eyes, we compared these between fellow eyes of the normal myopic controls.