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Aberrations and Visual Performance Following Standard Laser Vision Correction/MarcosFigure 4. Increment of total aberrations (third order and higher) withLASIK versus increment of corneal aberrations. The dotted line indicatesa linear function through the origin (y=x). Most data points areabove this line, indicating a higher increase of corneal aberrations.Coefficient of correlation r=0.73, P=.0024.RESULTSChange of Total Aberrations With LASIKAs has been reported, total aberrations increasefollowing standard LASIK for myopia. 3 Figure 3shows the root-mean-square (RMS) wavefront errorbefore and after LASIK for third order and higheraberrations (ie, excluding tilt, defocus, and astigmatism),for a 6.5-mm pupil. Eyes are sorted byincreasing preoperative refraction. The averageincrease was 1.9 times, and the effect was more pronouncedfor the highest preoperative myopes.Spherical aberration increased by a factor of 3.9. 3 Asshown in Figure 1, this was a common trend in mosteyes (central area of positive aberrations surroundedby a ring of negative aberration). Third orderterms (including coma) 1 increased by a factor of 2,and were likely associated with a decentration of theablation pattern (the central area appeared decenteredin some cases, as shown in Figure 1). 3Aberrations increased significantly in all eyesexcept for two (eyes #10 and #11).Do Corneal Aberrations Change Similiar to TotalAberrations After LASIK?Figure 1 shows that although corneal and totalaberrations are in general different in normal eyes(prior to surgery), they show a high degree of similarityafter surgery. 10 Although there is a good correlationbetween corneal and total aberrations aftersurgery (r=0.97, P < .0001), a direct comparisondemonstrates the following 10 : 1) The spherical aberrationinduced in the anterior surface of the corneasignificantly exceeds that induced in the whole eye.This attenuation is likely produced by a sphericalaberration of negative sign induced on the posteriorsurface of the cornea (related to the reported forwardshift of the posterior corneal surface followingrefractive surgery). 2) The correlation of the incrementof total aberration with the increment ofcorneal aberrations (Fig 4) is lower (r=0.73, P =.0024) than the correlation of postoperative totaland postoperative corneal aberrations. This suggestsa significant role of the interaction of cornealand internal aberrations prior to surgery, and insome cases relevant after surgery. For example, therelative amount of corneal and internal (probablycrystalline lens) aberrations prior to surgeryexplains the surprisingly good outcome encounteredin eyes #10 and #11. 10 In general, total aberrationsshould be better related to visual performance thanaberrations of the anterior corneal surface alone,since they take into account possible changes of theposterior corneal surface and interaction with otherocular components (crystalline lens and pupil).Predictions From Aberrometry and PsychophysicalMeasurements of Visual PerformanceContrast sensitivity function represents the contrastdegradation imposed by the optics and posteriorvisual processing as a function of spatial frequency.Since only the optics are modified with LASIK,one expects any change induced in contrast sensitivityfunction to be due to a change only in the opticalsystem, and more specifically in the modulationtransfer function (MTF) of the eye. The Strehl ratio(normalized volume under the MTF) is an alternativeglobal image quality parameter to the rootmean square error. Both metrics are, in general,well-correlated. The MTF can be obtained easilyfrom the wave aberration using Fourier optics. Forbetter comparison with the one-dimensional contrastsensitivity function (for vertical gratings), weused the horizontal section of the MTF. We computedthe MTF for a 3-mm diameter pupil, to simulatea closer condition to the contrast sensitivity functionmeasurement, performed with an undilated pupil.The area under these curves was computed, usinglinear units for both spatial frequency and contrastunits, a linear interpolation, and integratingJournal of Refractive Surgery Volume 17 September/October 2001 S599
Aberrations and Visual Performance Following Standard Laser Vision Correction/MarcosABCVA = 1BCVA = 0.9BCVA = 1BFigure 5. A) Modulation transferratio postoperative/preoperative(solid line) and contrast sensitivityratio postoperative/preoperative(circles and dashed line) as afunction of spatial frequency. Dataare average of 22 eyes. Pupildiameter = 3 mm for the modulationtransfer function (MTF), andundilated for the contrast sensitivityfunction (CSF). B) Simulation ofretinal image quality (obtained byconvolution of the 20/20 line on aSnellen chart and the correspondingpoint spread function) and clinicalmeasurements of best spectacle-correctedvisual acuity, forthree eyes (#14, #21, #11) withdifferent surgical outcomes.between 3 and 18 c/deg. The area under the MTF(average of 22 eyes) decreased by a factor of 1.38after LASIK, and the area under the contrastsensitivity function (average of 22 eyes) decreasedby a factor of 1.51 after LASIK. This indicates thatthe average image contrast degradation estimatedfrom wave aberration data accounts for most of thedecrease in contrast sensitivity in this spatial frequencyrange. Figure 5 shows the average contrastratio before and after surgery for both modulationtransfer and contrast sensitivity as a function ofspatial frequency. Again, both functions tend todecrease similarly with LASIK. The fact that contrastsensitivity function seems to suffer a slightlylarger degradation could be due to the fact thatpupils were larger than 3 mm during the contrastsensitivity function measurements, that the visualperformance and aberration measurements werenot collected on the same day, that MTF computationswere based on monochromatic aberrationswhereas contrast sensitivity function was measuredin polychromatic light, or that other factors apartfrom the aberrations (such as haze) may also contributeto contrast degradation. The eye in whichthe root mean square decreased (although not significantly)after LASIK experienced a slightincrease in the MTF postoperative/preoperativeratio, as well as a contrast sensitivity improvementat certain spatial frequencies (3 and 12 c/deg).We also compared visual performance (best spectacle-correctedvisual acuity [BSCVA]) with simulationsof retinal images of a Snellen chart. Thesewere generated by convolution of point-spread-functions(computed from the wave aberrations) withimages of the Snellen chart. These simulations donot represent vision of patients, but provide an ideaof the image quality on the retinal plane. Figure 5Bshows simulations of the 20/20 line of a Snellenchart for three eyes (#14, #21, #11) after LASIK.Clinical BSCVA measurements for these eyes arereported. In many cases, such as eye #14, a cleardegradation of retinal image quality is not associatedwith a line loss, indicating that unlike contrastsensitivity, high-contrast visual acuity is not a verysensitive measurement to evaluate the changesinduced by refractive surgery.From measurements of total and corneal aberrations,contrast sensitivity and visual acuity measuredin a group of eyes before and after surgery, weconclude that:1) Both total and corneal aberrations increasesignificantly following standard myopic LASIK.2) Total aberrations should be better correlated tovisual performance than corneal aberrations alone.Although most of the changes occur on the anteriorcorneal surface, in general corneal aberrations arenot sufficient to understand the changes induced byrefractive surgery. Preoperative interaction of thecorneal and internal optics, and possible changesinduced on the posterior corneal surface also contributeto the total aberration pattern.3) Most of the decrease in contrast sensitivity canbe accounted for by the decrease in the modulationtransfer function (computed from the total waveaberration)4) High contrast visual acuity is not a very sensitivemeasurement of image quality.REFERENCES1. Oshika T, Klyce SD, Applegate RA, Howland HC, ElDanasoury MA. Comparison of corneal wavefront aberrationsafter photorefractive keratectomy and laser in situkeratomileusis. Am J Ophthalmol 1999;127:1-7.S600 Journal of Refractive Surgery Volume 17 September/October 2001
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