O R I G I N A L P A P E R
Higher-order aberrations and visual acuity after LASEK
Berrak UrganciogluÆ Kamil Bilgihan ÆSertac Ozturk
Received: 23 December 2006 / Accepted: 13 June 2007 / Published online: 31 August 2007
Ó Springer Science+Business Media B.V. 2007
Abstract Background To determine ocular higher-order aberrations (HOAs) in eyes with supernormal vision after myopic astigmatic laser subepithelial keratomileusis (LASEK) and to compare the findings with those in eyes with natural supernormal vision. Methods Ocular HOAs were measured after LASEK in 20 eyes of 12 myopic astigmatic patients with postoperative uncorrected visual acuity (UCVA) of >20/16 (group 1). Patients who were included in the study had no visual symptoms like glare, halo or double vision. The measurements were taken 8.3 ± 3 months after LASEK surgery. In group 2 ocular HOAs were examined in 20 eyes of 10 subjects with natural UCVA of >20/16 as a control. Measurements were taken across a pupil with a diameter of 4.0 mm and 6.0 mm. Root-mean-square (RMS) values of HOAs, Z3 1, Z31, Z40, Z5 1, Z51and Z60were analyzed.
Results The mean RMS values for each order were higher in group 1 when compared with group 2 at 4.0 mm and 6.0 mm pupil diameters. There was no
statistically significant difference between groups in spherical and coma aberrations (P > 0.05). Mean RMS values for total HOAs were 0.187 ± 0.09 mm at 4.0 mm and 0.438 ± 0.178 mm at 6.0 mm pupil in group 1 and 0.120 ± 0.049 mm at 4.0 mm and 0.344 ± 0.083 mm at 6.0 mm pupil in group 2. The difference between groups in total HOAs was statis-tically significant at 4.0 mm and 6.0 mm pupil diameters (P < 0.05). Conclusion Ocular HOAs exist in eyes with supernormal vision. After LASEK, the amount of HOAs of the eye increases under both mesopic and photopic conditions. However the amount of HOA increase does not seem to be consistent with visual symptoms.
Keywords Higher-order aberrations LASEK Visual acuity
Introduction
Wavefront technology can detect the complete refrac-tive status of the eye and permit correction of ocular higher-order aberrations(HOAs). The third- and high-er-order Zernicke polynomials are named as HOAs. The relation between HOAs and visual performance is not clear. It has been assumed that as aberration increases, visual performance decreases. With the idea that correcting optical aberrations of normal human eyes results in obtaining supernormal vision, custom-ized corneal ablation was proposed to correct or at least
B. Urgancioglu K. Bilgihan
Department of Ophthalmology, Gazi University Medical School, Ankara, Turkey
B. Urgancioglu (&)
Kermes Sitesi 5.blok No:17, Umitkoy, Ankara, Turkey e-mail: bsekeryapan@yahoo.com
S. Ozturk
Department of Ophthalmology, Ufuk University Medical School, Ankara, Turkey
minimize the HOAs [1, 2]. Hence, minimizing the HOAs by wavefront guided refractive surgery was suggested to improve visual acuity [3]. However, Applegate et al. showed that for low levels of aberra-tions the RMS wavefront error was not a good predictor of visual acuity [4]. Also Levy et al. demonstrated the amount of ocular HOAs in eyes with natural super-normal vision is similar to reported amount of the HOAs in refractive surgery candidates [5].
Corneal topographic studies show that corneal aberrations increase after refractive surgery. In the process of correcting lower-order aberrations, higher-order ocular aberrations increase [6–8]. Also refractive surgery shifted the distribution of aberrations from coma-like aberrations to spherical-like aberrations.
This study was designed to assess the HOAs in eyes with uncorrected visual acuity (UCVA) of >20/ 16 after myopic laser-assisted subepithelial keratom-ileusis (LASEK). Additionally, we aimed to compare the postoperative HOAs with those in eyes with natural UCVA of >20/16.
Methods Instrument
Total ocular monochromatic aberrations were ob-tained using a Hartmann-Shack wavefront sensor, complete ophthalmic analysis system (COAS) aber-rometer (Wavefront Sciences Inc., Albuquerque, NM, USA) that is available within the Schwind Esiris excimer laser system. According to the manufacturer data, the COAS aberrometer has a 210 mm resolution in the pupil plane providing high-resolution sampling of aberrations. Detailed information and validity of the COAS aberrometer have been described else-where [9, 10]. The measurements with this device were centered on the center of the entrance pupil and taken across a naturally dilated pupil with diameters of 6.0 mm and 4.0 mm. A minimum of five mea-surements were taken for each eye. The most concordant measurement with subjective refraction and the best alignment was chosen for analysis. Patients
This study included 40 eyes of 22 individuals, who were divided into two groups. In group 1, we
measured the postoperative ocular aberrations of 20 eyes from 12 patients with UCVA of >20/16 after LASEK. Excimer laser procedures were performed with a flying spot excimer laser Schwind Esiris (Schwind GmbH, Kleinostheim, Germany). Preoper-ative refraction ranged from 1.5 to 6.0 D spheric and 0.25 to 3.0 D astigmatism. Postoperative videokeratographic examinations showed no sign of decentration or any irregularity other than the central flattening due to laser surgery. Optical symptoms like halo, glare and double vision were also asked about individually and separately for each eye. Patients were asked whether they had the symptoms and whether they considered them significant. Patients who had visual symptoms were excluded. Twenty eyes of 10 subjects with natural UCVA of >20/16 used as a control and formed the second group. All volunteers were informed and they gave written informed consent in accordance with institutional guidelines, according to the Declaration of Helsinki. All possible consequences of the study had been explained. Visual acuity was measured using the Snellen acuity chart under normal room illumination. The examination time ranged from 6 to 10 months (mean ± SD, 8.3 ± 3) after surgery.
LASEK technique
All operations were carried out by the same surgeon (KB) and assistant (BU). Surgical gloves were worn routinely. Topical proparacaine 0.5% was used to anesthetize the eyes. A drape and a lid speculum were inserted following the treatment of eyelids with 10% povidone-iodine. The epithelium was incised with a 7.5 trephine placed centrally, and 20% alcohol was applied for 20 s. The epithelium was detached and gathered at 12 O’clock. Spherical and cylindrical ablation was performed according to manifest refrac-tion, without any reduction with the ESIRIS excimer laser (Schwind, Germany). The ablation diameter was 6.5 mm in all eyes. Following the ablation, the cornea was irrigated with chilled balanced salt solution (BSS). After the epithelium was rolled to its original position, a drop of tobramycin 0.3% and diclofenac 0.1% fixed combination (Ocubrax, Alcon) was instilled, and a cooled lotrafilcon a soft contact lens (Focus Night & Day, Ciba Vision) was placed over the cornea with sterile forceps. The eyelid speculum and drape were removed.
Analysis of wavefront aberrations
The parameters analyzed included (1) Zernike coef-ficients from Z3 1, Z31, Z40, Z5 1, Z51 and Z60; (2)
Root-mean-square (RMS) values of HOA from Z3 1,
Z31, Z40, Z5 1, Z51 and Z60. Statistical analysis was
carried out using independent Student T test. A probability less than 5% (P < 0.05) was considered statistically significant.
Results
The mean RMS value for total HOA at 6 mm pupil was 0.438 ± 0.178 mm (range 0.20–0.88 mm) and 0.344 ± 0.083 mm (range 0.27–0.88 mm) in group 1 and group 2. When the measurements were taken across a pupil with a diameter of 4.0 mm the mean RMS value for total HOA was 0.180 ± 0.111 mm (range 0.08–0.49 mm) and 0.120 ± 0.049 mm (range 0.00–0.25 mm) in group 1 and group 2, respectively. Fourth-order SA (Z40) RMS value at 6 mm pupil was
calculated as 0.103 ± 0.055 mm (range 0.013– 0.308 mm) and 0.101 ± 0.084 mm (range 0.003– 0.193 mm) in group 1 and group 2. Analysis at 4.0 mm diameter revealed mean Z40 RMS values of
0.032 ± 0.028 mm (range 0.004–0.125 mm) for the postoperative group, and 0.027 ± 0.022 mm (range 0.001–0.079 mm) in the control group.
The mean RMS values for each order were higher in the refractive surgery group when compared with the control group at 6.0 mm (Table 1) and 4.0 mm (Table 2) pupil diameter. There was no statistically significant difference between groups in spherical and coma aberrations (P > 0.05). The difference between groups in total HOAs was statistically significant at 6.0 mm and 4.0 mm pupil diameters (P < 0.05). The mean RMS values for HOAs in the control group at 6.0 mm pupil were similar with those in refractive surgery candidates demonstrated by Wang et al. [11] (Table 3).
Discussion
Wavefront technology is capable of measuring an eye’s complete wavefront refractive aberration. HOAs are third- and higher-order Zernike polynomi-als and determine the amount of irregular astigma-tism of the eye. Correction of such optical aberrations may increase retinal image resolution and contrast, resulting in better than normal vision. Theoretically,
Table 1 Mean RMS values (mm) and range of HOA, Z3
Z40 1, Z31, Z51, Z51and Z60 of group 1 and group 2 at 6.0 mm pupil diameter
HOA—higher-order aberration
a Values are mean ± SD b Values are range
Zernicke coefficient (mm)
Group 1 Group 2 P value
HOA 0.438 ± 0.178a(0.20–0.88)b 0.344 ± 0.083 (0.22–0.51) 0.039 Z31 0.167 ± 0.127 (0.003–0.453) 0.114 ± 0.076 (0.003–0.252) 0.116 Z31 0.085 ± 0.072 (0.005–0.254) 0.067 ± 0.057 (0.002–0.211) 0.381 Z40 0.103 ± 0.055 (0.013–0.308) 0.101 ± 0.084 (0.003–0.193) 0.918 Z51 0.037 ± 0.025 (0.007–0.171) 0.023 ± 0.011 (0.004–0.044) 0.072 Z51 0.039 ± 0.021 (0.000–0.068) 0.022 ± 0.012 (0.005–0.057) 0.372 Z60 0.037 ± 0.032 (0.002–0.125) 0.024 ± 0.015 (0.002–0.056) 0.100
Table 2 Mean RMS values (mm) and range of HOA, Z3
Z40 1, Z31, Z5 1, Z51 and Z60 of group 1 and group 2 at 4.0 mm pupil diameter
HOA—higher-order aberration
a Values are mean ± SD b Values are range
Zernicke coefficient (mm)
Group 1 Group 2 P value
HOA 0.180 ± 0.111a(0.08–0.49)b 0.120 ± 0.049 (0.00–0.25) 0.36 Z31 0.067 ± 0.056 (0.010–0.210) 0.044 ± 0.030 (0.007–0.121) 0.121 Z31 0.045 ± 0.031 (0.002–0.106) 0.029 ± 0.022 (0.001–0.088) 0.069 Z40 0.032 ± 0.028 (0.004–0.125) 0.027 ± 0.022 (0.001–0.079) 0.481 Z51 0.017 ± 0.009 (0,001–0.105) 0.013 ± 0.008 (0,001–0.043) 0.420 Z51 0.022 ± 0.015 (0.000–0.064) 0.019 ± 0.017 (0.001–0.067) 0.629 Z60 0.025 ± 0.023 (0.000–0.116) 0.019 ± 0.012 (0.002–0.045) 0.494
application of wavefront-guided refractive surgery corrects irregular astigmatism and prevents iatrogenic irregular astigmatism induced by conventional refrac-tive surgery. Studies showed that wavefront-guided customized ablation reduces the increase of HOAs compared with the conventional ablation treatments [12,13]. However, in a different study, Phusitphoykai et al. showed that postoperative visual outcome with conventional LASIK and wavefront-guided custom-ized ablation was not significantly different, and HOAs did not significantly increase postoperatively in either group [14]. Recently, Kohnen et al. applied wavefront-guided LASIK for the correction of com-pound myopic astigmatism and observed no change or reduction in the total HOA RMS in 20.6% of the eyes for 6-mm [15].
We know that corneal ablative techniques add HOAs to the postsurgical eye. Even simply flattening the cornea in a myopia treatment increases oblateness and subsequently increases spherical aberration. Oshika et al. found that both PRK and LASIK increase the wavefront aberrations of the cornea and change the relative contribution of coma and spher-ical aberrations. Seiler et al. showed that PRK postoperatively increases ocular aberrations and impairs visual performance in a study of 15 eyes. The surgically-induced increase in optical aberration after myopic LASIK was studied by Moreno-Barri-uso et al. They found that LASIK induces significant numbers of third and HOAs [6–8]. Following these studies, it can be argued that both conventional and wavefront-guided refractive surgery induce HOAs
because of the ablation of the cornea. However, wavefront-guided refractive surgery is an effective method to correct residual refractive error and HOAs after refractive surgery.
In this study, the RMS values for Z40were
insignif-icantly higher in the refractive surgery group than the refractive surgery candidates group. It is known that the amount of the increase at Z40levels after refractive
surgery depends on the ablation depth. In the refractive surgery group preoperative spherical equivalent values were between 1.5 D from 3.5 D, except three of the eyes examined. In other words, for low levels of ablation, Z40levels may not be induced.
It is an important question whether changes in higher-order aberrations after refractive surgery are really a significant problem for postoperative visual acuity. Kohnen et al. proposed that HOA could be induced while UCVA was 20/20 or better in eyes which underwent wavefront-guided LASIK [15]. Du et al. compared HOAs and visual acuity after conventional and wavefront guided LASIK, and found that the increase of HOAs was higher in conventional treat-ment group even though visual acuity was similar between two groups [16]. Conversely, Sharma et al. suggested that patients with lower visual acuity after LASIK had significantly higher HOAs [17]. Conse-quently, it is controversial whether HOAs are respon-sible for the visual acuity after excimer laser refractive surgery. However, Applegate et al. showed that for low levels of aberrations the RMS wavefront error is not a good predictor of visual acuity [4]. Levy et al. studied the clinical significance of HOAs and their relation to visual acuity. They found that the amount of ocular HOAs in eyes with natural supernormal vision is similar to the reported amount of HOAs in refractive surgery candidates [5]. In this study it was found that the mean RMS values for HOAs in the control group at 6.0 mm pupil diameter were similar with those in the refractive surgery candidates demonstrated by Wang et al. [11].
It is important to remember that the effects of wavefront aberrations generally increase with increasing pupil size [18]. In this study, it was shown that there was an increase in the total HOAs of the eye after refractive surgery under mesopic conditions. However, visual acuity was measured under normal room illumination. Additionally, with the 4.0 mm pupil, the amount of total HOAs was greater at the refractive surgery group than the control group. In
Table 3 Mean root-mean-square values (mm) of HOA, Z3
Z40 1, Z31, Z51, Z51 and Z60 of group 2 and myopic eyes including those prior to refractive surgery reported in recent studies
Zernicke coefficient (mm)
Group 1 Wang et al. [11]
HOA 0.344 ± 0.083a 0.305 ± 0.095 Z31 0.114 ± 0.076 0.121 ± 0.093 Z31 0.067 ± 0.057 0.082 ± 0.067 Z40 0.101 ± 0.084 0.122 ± 0.077 Z51 0.021 ± 0.011 0.028 ± 0.023 Z51 0.022 ± 0.012 0.026 ± 0.021 Z60 0.024 ± 0.015 0.024 ± 0.020 HOA—higher-order aberration
other words, the increase of RMS wavefront error at low levels does not predict the photopic high contrast vision acuity after refractive surgery.
In conclusion, this study showed that the mean RMS value for total HOA was higher in the refractive surgery group when compared with the control group under mesopic and photopic conditions. This increase implies that excimer laser refractive surgery induces higher-order aberrations of the eye. However, the increase in the amount of HOA does not seem to have any negative effects on photopic high contrast visual acuity and it is not consistent with visual symptoms.
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