Refractive surgery for myopia and myopic astigmatism induces an increase in higher-order aberrations (HOAs) and may lead to visual losses that are detected under scotopic (low light) conditions and by low-contrast visual acuity testing. The increases in HOAs are known to be responsible for patients' complaints of glare, halo, and disturbances in night vision.1–4 Laser in situ keratomileusis (LASIK) is known to induce more HOAs than photorefractive keratectomy (PRK) in the scotopic condition.5
Many authors suggest that treatment with a larger ablation zone may reduce the increase in optical aberrations.5–7 In a recent report, Endl et al.6 demonstrate that the use of larger ablation zones and peripheral blend zones in PRK decreased HOAs in photopic conditions and increased them in scotopic conditions. Larger and blend zone treatments are recognized to be more physiologic and may contribute to minimizing the deterioration in optical performance. This modality, however, has not shown comparative benefits over the conventional treatment in higher-order corneal aberrations following surgery.
We designed a randomized prospective study to measure the difference in HOAs under scotopic conditions after laser-assisted subepithelial keratectomy (LASEK) using a conventional (6.0 mm) optical zone in 1 eye and a larger (6.5 mm) optical zone with blend (8.0 mm) zone in the other eye to determine the effect of the ablation pattern on HOAs.
Patients and Methods
In this prospective study, 19 of 26 consecutive patients with myopia and myopic astigmatism were enrolled from September 2001 to February 2002 (mean patient age 29.4 years [range 24 to 42 years]). Patients with diabetes mellitus, connective tissue disease, amblyopia, corneal disease, cataract, glaucoma, and retinal disease were excluded from the study. All patients received full explanations of the procedures, and informed consent was obtained before surgery in accordance with the Declaration of Helsinki.
Laser-assisted subepithelial keratectomy was performed using a conventional (6.0 mm) optical zone in 1 eye and a larger (6.5 mm) optical zone with 8.0 mm blend zone in the other eye of the same patient. The choice of treatment was randomized by the use of each patient's chart number. The mean preoperative spherical equivalent (SE) refraction was −4.86 diopter (D) ± 1.70 (SD) (range −3.00 to −8.25 D) in eyes treated conventionally and −4.77 ± 1.62 D (range −3.13 to −7.75 D) in those treated with a larger optical zone. There was no statistically significant difference in the baseline manifest refraction between the eyes assigned for conventional or larger zone treatment (P = .41). The interval between the procedures in both eyes was 1 week in all patients.
The preoperative ophthalmic examination in all patients included slitlamp microscopy, fundus examination, cycloplegic and manifest refractions, corneal keratometry, corneal topography, corneal pachymetry, and Goldmann tonometry. The pupil diameter was measured under photopic and scotopic illumination using a Rosenbaum near-card scale. The HOAs were measured by WaveScan® (Visx) in the natural scotopic condition after 10 minutes of dark adaptation. The HOAs were presented as root mean square (RMS [μm]) in Belle aberration maps, which displayed HOAs only after sphere and cylinder had been removed. All the RMS values were obtained from the 6.0 mm pupil measurement condition. At 3 months, all patients were asked whether they had complaints of glare or halo.
The LASEK treatment was performed as described.8–11 Briefly, after pre-incision of the corneal epithelium with a special microtrephine with an 8.0 mm diameter blade (J 2900S, Janach), an alcohol solution cone (J 2905, Janach) with an 8.5 mm diameter was placed on the cornea. Then, 0.3 cc of a 20% alcohol solution was instilled in the cone and left for 30 seconds. After the epithelial flap was gently detached, an excimer laser (Star S3, Visx) with the following operative parameters was used: emission wavelength 193 nm, energy fluence 160 mJ/cm2, repetition rate 10 Hz. To avoid ablation decentration, an eye-tracking device (ActiveTrack®, Visx) was applied in all patients. After laser ablation, the stromal surface was irrigated with a balanced salt solution and the epithelial flap was repositioned using a spatula (J 2920A Janach).
All patients were examined 30 minutes and 1 day after surgery. One drop of ofloxacin 0.3% (Ofloxacine®) and fluorometholone 0.1% (Fluorometholon®) were prescribed 4 times daily for the first postoperative month, 3 times daily for the second month, twice daily for the third month, and then once a day for the fourth month. Uncorrected visual acuity (UCVA), best corrected visual acuity (BCVA), and refractive errors were measured at 1 week and 1 and 3 months. The HOAs were measured by aberrometry at 1 and 3 months.
Paired t tests were used to compare the preoperative pupil size, preoperative and postoperative visual acuities, mean SE refractions, and RMS values of HOAs in both groups of eyes. Normalized polar Zernike coefficients obtained from WaveScan were compared between eyes treated with larger zones and with conventional zones and preoperatively and postoperatively. Group comparisons were made using the t test with Bonferroni correction to eliminate statistically significant findings resulting from chance. A P value less than 0.05 was considered significant.
There were no differences between the 2 groups in preoperative and postoperative refractions and visual acuities (Tables 1 and 2). Preoperatively, the mean RMS wavefront error of the HOAs was 0.26 ± 0.09 in the conventional ablation group and 0.27 ± 0.09 in the larger zone with blend zone ablation group. The between-group difference was not statistically significant (P =. 74). The mean preoperative pupil sizes in photopic and scotopic conditions were 3.29 ± 0.71 mm and 6.51 ± 0.94 mm, respectively, in the conventional zone group, and 3.32 ± 0.71 mm and 6.53 ± 0.98 mm, respectively, in the larger zone group. The between-group difference in pupil size was not statistically significant (P = .57 and P = .33, respectively).
At 1 and 3 months, there was no difference between the 2 groups in UCVA and postoperative SE refraction (Tables 1 and 2). At 3 months, the mean RMS wavefront error of the HOAs in the larger zone group (0.41 ± 0.14) was significantly smaller than that in the conventional group (0.61 ± 0.28) (P = .006). At 1 month, the RMS in the larger zone group was also smaller than that in the conventional group. This difference was not statistically significant (P = .19) (Table 3).
On questioning at 3 months, 2 patients in the conventional group and 1 patient in the larger zone group complained of glare. Because these numbers were small, they were not included in the statistical analysis.
Normalized polar Zernike coefficients showed statistically significant differences between the preoperative and postoperative coma and spherical aberration terms in the conventional zone group and in the spherical aberration term in the larger zone group (Figure 1). At 3 months, the Zernike coefficients were significantly different between the 2 groups in coma only (P = .03). Although 12 terms of Zernike coefficients (Z31, Z33, Z40, Z42, Z44, Z51, Z53, Z55, Z60, Z62, Z64, and Z66) were compared, for simplicity, Figure 1 presents only 3 terms (Z31, Z33, and Z40).
Changes in the induced RMS in HOAs at 3 months relative to the attempted dioptric correction were compared. Patients were divided into 2 groups by the mean preoperative SE (less than and greater than −5.0 D). In the group with a mean SE less than −5.0 D, the detected aberrations were larger in patients in the conventional zone group than in those in the larger zone group, but the difference was not statistically significant (P = .09). In the group with a mean SE greater than −5.0 D, the difference between the 2 groups was statistically significant (Figure 2).
Our data show that at 3 months, increases in HOAs were smaller after treatment with a larger zone with blend zone than after treatment with the conventional zone. In several studies, aberrations induced by PRK or LASIK decreased continuously over 18 months, with most occurring in the first 2 months. After this initial period, the observed changes in aberration were minimal.5,6,12 This change in optical aberration has been attributed to the wound-healing reaction that occurs during the first 2 months.
The difference in HOAs by ablation zone diameter is implicated by the relationship between pupil size and optic zone size. In this study, the mean pupil size in the 2 groups was 6.51 ± 0.94 mm and 6.53 ± 0.98 mm in the natural scotopic condition. The aberrometer protocol used in the study involved measurements performed in the natural scotopic condition after a 10-minute dark adaptation period. The use of a 6.5 mm ablation zone with 8.0 mm transition zone appeared to be sufficient to cover the range of patients' pupils in low-light conditions and minimize an abrupt change in the shape of the corneal surface within the scotopic pupils.
Higher-order aberrations after refractive surgery are known to be 4th-order dominance (spherical aberration) in larger pupils.3,5,6 Our study showed that significant increases in spherical aberration were observed after surgery in both treatment groups (Figure 1). However, coma increased significantly only in the conventional treatment group. In his eye model with a subclinical decentration setting, Mihashi13 shows that small ablation areas could cause more significant amount of wavefront aberrations in coma and spherical aberrations than larger ablation areas in mesopic vision. In the clinical setting, since subclinical decentration may occur because of eye movement during refractive laser surgery, an increase in coma seems to be 1 of the factors that increase postoperative HOAs with a smaller ablation zone.
The difference in RMS values was more evident in patients whose attempted correction was greater than −5.0 D. Reports by Endl et al.,6 Applegate et al.,7 and Martinez et al.3 show that the magnitude of the induced aberration appears to be correlated with the magnitude of the attempted correction. Martínez et al.3 propose that this might be due to the deeper ablations and greater changes in corneal power between treated and untreated zones that are seen with larger attempted corrections. We hypothesized that if the increase in aberration was caused by this power shift, the increase would be greater in patients with deeper ablations, and the difference between conventional and large ablation zone treatments was more obvious in the group with larger attempted corrections.
The main limitation of larger zone treatment is the increased ablation depth in proportion to the square of the ablation diameter. In LASIK, it is more difficult to secure sufficient residual bed thickness and sufficient optical zone when the cornea is thin or correction of a high refractive error is required. The corneal flap does not contribute to the stability. Constructing the flap decreases the ablative bed thickness, and the flap size also limits the larger zone treatment. With PRK, there is less limitation in the size of optical zone and more ablation is possible. However, the larger amount of ablation in PRK is known to cause more corneal haziness. Laser-assisted subepithelial keratectomy, an alternative procedure modified from PRK, is known to decrease postoperative haze,8,11 TGF-βb levels in tears, and possibly wound healing.8–10 Thus, we believe that LASEK may be particularly well suited for the alternative technique using a larger zone with a blend zone. If one were to apply this technique to LASIK, producing a well-centered flap with a large diameter would be essential.
In this study, we sought to compare the amount of HOA between eyes treated with a conventional (6.0 mm) optical zone and those treated with a 6.5 mm optical zone with 8.0 mm blend zone. To differentiate the contribution of the 6.5 mm zone and the 8.0 mm blend zone to the reduction in HOAs, the 2 procedures would have to be performed separately. However, as many surgeons already use this combined modality in clinical practice, this study sought to evaluate its usefulness as an alternative for minimizing the visual losses that are detected under scotopic conditions. Therefore, the exact contribution of each modification, larger zone or blend zone, remains unclear and awaits further studies.
In conclusion, larger ablation zone treatment with a blend zone decreased the surgically induced HOA compared to the conventional zone treatment. Larger zone treatment may be useful in patients who have large pupils and considered an alternative surgical technique for preventing a deterioration of optical performance in low-light conditions after surgery.
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