Abstract
purpose. To investigate prospectively the relation between induced changes in higher-order aberrations of the eye and changes in contrast sensitivity by conventional laser in situ keratomileusis (LASIK) for myopia.
methods. In 200 eyes of 110 consecutive patients (mean age, 32.7 ± 8.4 years) undergoing LASIK, ocular aberrations and contrast sensitivity function were determined before and 1 month after surgery. The amount of myopic correction was 5.2 ± 2.8 D (range, 1.0–13.0). Ocular higher-order aberrations were measured for a 4-mm pupil using the Hartmann-Shack wavefront analyzer (KR-9000PW; Topcon, Tokyo, Japan). The root mean square (RMS) of the third- and fourth-order Zernike coefficients was used to represent coma- and spherical-like aberrations, respectively. Total higher-order aberrations were calculated as the RMS of the third- and fourth-order coefficients. Contrast sensitivity and low-contrast visual acuity were measured. From the contrast sensitivity data, the area under the log contrast sensitivity function (AULCSF) was calculated.
results. LASIK significantly improved logMAR best corrected visual acuity (Wilcoxon signed-rank test, P < 0.001), but significantly reduced AULCSF (P < 0.001) and low-contrast visual acuity (P = 0.007). Total higher-order (P < 0.001), coma-like (P < 0.001), and spherical-like (P < 0.001) aberrations were significantly increased after LASIK. The greater the amount of achieved myopia correction was, the more the changes in contrast sensitivity function and ocular higher-order aberrations were. The induced changes in AULCSF by LASIK showed significant correlations with changes in total higher-order (Pearson r = −0.221, P = 0.003), coma-like (r = −0.205, P = 0.006), and spherical-like (r = −0.171, P = 0.022) aberrations. The changes in logMAR low-contrast visual acuity by surgery significantly correlated with changes in total higher-order (r = 0.222, P = 0.003), coma-like (r = 0.201, P = 0.007), and spherical-like (r = 0.207, P = 0.005) aberrations.
conclusions. Conventional LASIK significantly increases ocular higher-order aberrations, which compromise the postoperative contrast sensitivity function.
As refractive surgery evolves, laser in situ keratomileusis (LASIK) has gained widespread popularity as the procedure of choice to correct refractive errors. LASIK can reduce refractive error and improve uncorrected visual acuity, but several problems still must be resolved regarding postoperative visual function. Previous studies have demonstrated that LASIK compromises contrast sensitivity after surgery,
1 2 3 4 5 6 7 8 as does radial keratotomy (RK)
9 10 and photorefractive keratectomy (PRK).
10 11 12 Others have reported that higher-order aberrations increase after LASIK,
13 14 15 16 17 18 RK,
9 19 20 and PRK.
13 21 22 Applegate et al.
20 reported that the area under the contrast sensitivity function decreases as the wavefront variances increase after RK.
9 A study using aberroscopy demonstrated that low-contrast visual acuity and glare visual acuity were adversely influenced by the increases in total ocular aberrations after PRK.
21 There has been no report, however, on the relation between changes in contrast sensitivity and changes in ocular aberrations after LASIK. In this prospective study, we investigated the relation between changes in higher-order aberrations of the eye and changes in contrast sensitivity caused by conventional LASIK.
We studied 200 eyes of 110 consecutive patients (69 males, 41 females) undergoing LASIK for myopia. Their ages ranged from 17 to 52 years (mean, 32.7 ± 8.4 [SD]), and preoperative refraction was −1.25 to −13.25 D (−5.34 ± 2.64 D). These were consecutive patients who were operated on between February 2001 and October 2002. The research adhered to the tenets of the Declaration of Helsinki, and informed consent was obtained from all subjects after explanation of the nature and possible consequences of the study.
All surgery was performed by one surgeon (KM) with an excimer laser system (Star S2; VISX Inc., Santa Clara, CA). Laser parameters included the following: wavelength, 193 nm; radiant exposure (fluence), 160 mJ/cm2; pulse repetition rate, 10 Hz; average ablation depth per pulse, 0.23 μm on the cornea; ablation zone diameter, 6.0 mm; and transition zone, 0.35 mm. Aspiration air flow was used for debris removal. An automated microkeratome (MK-2000; Nidek Ltd., Gamagori, Japan) was used to create a hinged corneal flap of 160-μm thickness (nominal value).
LogMAR (logarithm of the minimum angle of resolution) uncorrected visual acuity (UCVA), logMAR best spectacle-corrected visual acuity (BSCVA), contrast sensitivity function, and wavefront aberrations were evaluated before and 1 month after surgery. We tested two indices of contrast sensitivity function: contrast sensitivity on one system (CSV-1000E) and low-contrast visual acuity on a second system (CSV-1000LanC10%; both from Vector Vision Co., Greenville, OH). These tests were performed with best spectacle correction both before and after surgery. Ocular higher-order aberrations for a 4-mm pupil were measured with the Hartmann-Shack wavefront analyzer (KR-9000PW; Topcon, Tokyo, Japan).
The CSV-1000E provides a fluorescent luminance source that retroilluminates a translucent chart and automatically calibrates to 85 cd/m
2. Four spatial frequencies—3, 6, 12, and 18 cyc/deg—are present, and each spatial frequency has eight different levels of contrast. The test was performed monocularly with the eye in the undilated state at 2.5 m. With the patient’s manifest refraction in place, the patient identified the rows and eight columns of patches. The patient was asked to identify the grating pattern in each column. The contrast level of the last correct response was recorded as the contrast threshold in logarithmic values.
23 From the data obtained with the system, the area under the log contrast sensitivity function (AULCSF) was calculated according to the method of Applegate et al.
9 The log of contrast sensitivity was plotted as a function of log spatial frequency, and third-order polynomials were fitted to the data. The fitted function was integrated between the fixed log spatial frequency limits of 0.48 (corresponding to 3 cyc/deg) and 1.26 (18 cyc/deg), and the resultant value was defined as the AULCSF.
The CSV-1000LanC10% is based on the ETDRS chart (The Lighthouse, New York, NY). Although the original ETDRS chart uses alphabets as optotypes under 100% high contrast, this system uses the Landolt ring as the optotype under 10% low contrast. This chart displays five letters per line, and the entire chart is constantly visible. A 1-line step represents a change of 0.1 logMAR unit. Low-contrast visual acuity was scored by giving a credit of 0.02 logMAR units for each letter correctly identified under full spectacle correction. This test was performed monocularly in eyes with undilated pupils at 2.5 m.
The Hartmann-Shack wavefront analyzer (KR-9000 PW; Topcon), composed of an array of apertures in front of a charge-coupled device (CCD) camera, was used to measure ocular wavefront aberrations for a 4-mm pupil. A very narrow spot of light is projected onto the retina, and the light reflected from the fovea passes through the crystalline lens and cornea and exits the eye. The light emerging from the eye is focused on a CCD camera by each lenslet to form a spot pattern. Because the wavefront of each lenslet is perpendicular to the direction of the ray, the wavefront of the measured subjects can be reconstructed from the displacement of their focusing spots. The wavefront aberration was expanded with the normalized Zernike polynomials. The Zernike polynomials are a combination of trigonometric and radial functions. A detailed description of Zernike polynomials is found elsewhere.
24
The magnitudes of the coefficients of the Zernike polynomials are represented as the root mean square (RMS; in micrometers) and are used to show the wavefront aberrations. The 0-order component in the Zernike polynomials has one term that represents a piston. The first order represents tilt with two terms. The second order includes three terms that represent defocus and astigmatism. The third order has four terms that represent coma and trefoil astigmatism. The fourth order has five terms that include the spherical aberrations. Spectacles can correct only the second-order aberrations. The RMS of the third-order Zernike coefficients was used to represent the coma-like aberration, and the RMS of the fourth-order coefficients was used to denote the spherical-like aberration. Total higher-order aberrations were calculated as the RMS of the third- and fourth-order coefficients.
Measurement results are summarized in
Table 1 . LASIK significantly improved UCVA (Wilcoxon signed-rank test,
P < 0.001) and BSCVA (
P < 0.001). After surgery, contrast sensitivity significantly decreased at all spatial frequencies from 3 to 18 cyc/deg
(Fig. 1) . There were significant reductions in AULCSF calculated from the measurements (
P < 0.001) and low-contrast visual acuity (
P = 0.007). As for ocular aberrations, total higher-order (
P < 0.001), coma-like (
P < 0.001), and spherical-like (
P < 0.001) aberrations were significantly increased after LASIK.
The magnitude of induced changes in contrast sensitivity and ocular aberrations were evaluated in relation to the amount of achieved myopia correction. The amount of changes in total higher-order (Pearson correlation coefficient r = 0.463, P < 0.001), coma-like (r = 0.453, P < 0.001), and spherical-like (r = 0.475, P < 0.001) aberrations had a significant positive correlation with the amount of myopia correction. The magnitude of deterioration in AULCSF (r = 0.322, P < 0.001) and low-contrast visual acuity (r = −0.165, P < 0.05) had significant correlations with the amount of achieved correction.
The changes in AULCSF by LASIK showed significant correlations with changes in total higher-order (
r = −0.221,
P = 0.003,
Fig. 2 ), coma-like (
r = −0.205,
P = 0.006,
Fig. 3 ), and spherical-like (
r = −0.171,
P = 0.022,
Fig. 4 ) aberrations. The changes in logMAR low-contrast visual acuity by surgery significantly correlated with changes in total higher-order (
r = 0
.222,
P = 0.003,
Fig. 5 ), coma-like (
r = 0.201,
P = 0.007,
Fig. 6 ), and spherical-like (
r = 0.207,
P = 0.005,
Fig. 7 ) aberrations.