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Aberrations and Visual Performance Following Standard Laser Vision Correction/MarcosFigure 1. Total and corneal wave aberrationmaps, before and after standard LASIK formyopia for A) eye #14, B) eye #16, C) eye#18, and D) eye #19. Total aberrations measuredwith laser ray tracing. Corneal aberrationmeasured using a commercial cornealvideokeratoscope and custom software. Tilts,defocus, and astigmatism were cancelled.Pupil diameter = 6.5 mm.This technique provides similar results to Shack-Hartmann and the Spatially Resolved Refractometerin normal eyes, and is appropriate in highlyaberrated eyes. 8 In this technique, a set of parallelrays is projected sequentially through 37 differentpupil positions (forming a hexagonal array thatsamples a 6.5-mm pupil). The corresponding aerialretinal images are collected on a high-resolutionCCD camera. The deviation of each centroid fromthe principal ray is proportional to the local slope ofthe wave aberration. The wave aberration isobtained from the derivatives using a least-meansquaresprocedure and described as a seventh orderZernike polynomial expansion. Measurements wereperformed on dilated pupils. The patient fixatedfoveally and the eye’s pupil was aligned to the opticalaxis of the instrument. Figure 1 (A, B, C, and D,left panels) shows examples of total wave aberrationsfor four patients before and after LASIK. Rootmean-square(RMS) wavefront error is typicallyused as an optical quality metric.Corneal Aberrations From Corneal TopographyCorneal aberrations were estimated using aplacido-disk-based videokeratoscope (Atlas MastervueCorneal Topography System, HumphreyInstruments-Zeiss, San Leandro, CA) and customsoftware developed by Sergio Barbero 9 (Instituto deOptica, CSIC, Madrid, Spain) in Matlab(Mathworks, Nathick, MA) and Zemax (FocusSoftware, Tucson, AZ). In brief, the slopes of thecorneal wave aberration were calculated by virtualray tracing on the corneal elevation, and cornealaberrations were then obtained using a similar procedureto that described for the total aberrations.Since corneal aberrations use the corneal reflex as areference and total aberrations are measured withrespect to the pupil center, a realignment algorithmwas applied to allow a direct comparison of cornealand total aberration maps. The small tilt betweenthe videokeratographic axis and the line of sightwas not considered. A series of corneal aberrationmaps were obtained over 6.5 mm, shifting the centerat 0.1-mm steps. The difference, total – cornealRMS, was then computed as a function of pupil location.These maps are smooth and show a welldefined minimum, slightly decentered from thecorneal reflex, which was used as a common axis.Figure 1 (A, B, C and D, right panels) shows examplesof corneal wave aberrations for four patientsbefore and after LASIK.Journal of Refractive Surgery Volume 17 September/October 2001 S597
Aberrations and Visual Performance Following Standard Laser Vision Correction/MarcosVisual Performance: Contrast Sensitivity Function andVisual AcuityVisual performance was assessed by means ofcontrast sensitivity function (with best spectaclecorrection) and best spectacle-corrected high-contrastvisual acuity. Contrast sensitivity was measuredusing a standard CVS-1000E chart (VectorVision, Arcanum, OH). This chart uses vertical sinusoidalgrids (at 3, 6, 12, and 18 c/deg), a forceddouble-alternative choice paradigm, and a calibratedluminance of 85 cd/m 2 . Figure 2 shows the averagecontrast sensitivity function (22 patients) beforeand after LASIK. High contrast visual acuity wasmeasured using a conventional Snellen chart.Figure 2. Contrast sensitivity function (average of 22 eyes) beforeand after LASIK, measured with the CSV-1000E chart. Lines arethird order polynomial fits to the data, error bars represent standarderror of the mean, with best spectacle-correction and undilutedpupil.Patients and ProceduresTwenty-two eyes from 12 patients (mean age,28 ± 5 yr; preoperative spherical error, -2.50 to-13.00 diopters [D]) participated in the study.LASIK was performed using a narrow-beam, flyingspotexcimer laser (Chiron Technolas 217-C; Bausch& Lomb, Surgical, Dornach, Germany), assisted byan eye-tracker. The flap diameter (performed with aHansatome microkeratome) was 8.5-mm and theintended depth 180 µm. Photoablation was appliedto a 6-mm optical zone, with a transition zone of 9mm. The LASIK procedures were conducted at theInstituto de Oftalmobiología Aplicada, Universidadde Valladolid, Spain, by Dr. Jesús Merayo-Lloves.Total aberrations were measured about 1 monthbefore and between 1 and 3 months after LASIK atthe Instituto de Optica, Madrid, Spain. Data weretypically the average of five sets of measurements.Corneal topography was performed during the sameexamination session. Visual performance measurementswere conducted at the Instituto deOftalmobiología Aplicada, Universidad deValladolid, by Raúl Martín and GuadalupeRodríguez, before and between 6 months and 1 yearafter LASIK.Figure 3. Root-mean-square (RMS) wavefronterror before (gray bars) and after (black bars)LASIK, for total third and higher order aberrations.Pupil diameter = 6.5 mm. Eyes are sorted byincreasing preoperative spherical error. Adaptedfrom Moreno-Barriuso et al. 3S598 Journal of Refractive Surgery Volume 17 September/October 2001
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