Department of Ophthalmology, Siriraj Hospital, Mahidol University, Bangkok, Thailand (SS), TRSC International LASIK Center, Bangkok, Thailand (SS, EC), London Vision Clinic, London, United Kingdom (DZR, TJA), Department of Ophthalmology, Columbia University Medical Center, New York, (DZR), and Centre Hospitalier National d'Ophtalmologie, Paris, France (DZR).
Received August 19, 2011; accepted April 9, 2012.
Sabong Srivannaboon Dept of Ophthalmology Siriraj Hospital Mahidol University 2 Prannok Rd., Bangkoknoi Bangkok, 10700 Thailand e-mail: firstname.lastname@example.org
Myopic laser eye surgery has been shown to induce higher-order aberrations (HOAs),1,2 primarily spherical aberration (SA) due to the changes in central corneal asphericity.3–8 SA is the major cause of night vision disturbances such as halos, glare, and starbursts, which have been reported after myopic laser refractive surgery.5,9 Barraquer was first to introduce the concept of aspheric resection of cornea in the form of parabolic keratomileusis.10 The induction of SA has also been shown to be reduced by the use of larger optical zones with conventional profiles11 but the introduction of aspheric ablation profiles demonstrated an advantage with respect to the subjective induction of night vision disturbances.12 The majority of modern excimer laser platforms have now incorporated an aspheric ablation profile to reduce the amount of induced SA.13
To evaluate the risk of inducing night vision disturbances, it is important to measure the induction of SA for the ablation profile that is being used. The amount of induced SA is proportional to the level of myopia corrected.14 Therefore, the risk of night vision disturbances is greater for higher myopia.15 Also, a patient with high SA preoperatively is at greater risk of night vision disturbances. Therefore, the risk of night vision disturbances is a function of the preoperative SA and the myopic refraction to be treated and our group has previously suggested that there appears to be a tolerable level of SA.16
The current excimer laser has implemented an aspheric ablation profile as a standard treatment (some platform referred to as the Aberration Smart Ablation (ASA) profile).17 This profile has been optimized using laser fluence control for both beam projection and reflection due to curved corneal surface. The target postoperative cornea contour is set to conform to a flattening ellipse with a Q factor of −0.35 to −0.15.17 However, the disadvantage of aspheric ablation profiles is that the ablation depth is greater than spherical ablation profiles. Some lasers also provide the option of using a non-aspheric ablation profile (referred to as the Tissue Saving Ablation [TSA] profile) where the ablation depth is lower due to the lack of asphericity. The TSA profile can then be used for low myopia where there is less concern about SA induction and myopic retreatments or patients with thin corneas where conservation of corneal tissue is a concern.
In the present study, we set out to measure and compare the induction of SA for an aspheric ablation profile (ASA) and a non-aspheric ablation profile (TSA) using the MEL80 excimer laser (Carl Zeiss Meditec, Jena, Germany).
This study was performed under the approval of ethics committee of Faculty of Medicine, Siriraj Hospital, Mahidol University. Informed consent was provided to all patients. A prospective randomized paired-eye controlled study of 30 eyes of 15 myopic patients was conducted. All patients received a routine preoperative evaluation for LASIK surgery. Patients with ocular abnormalities or corrected distance visual acuity (CDVA) worse than 20/20 were excluded. Patients in whom there was a difference in spherical equivalent refraction between eyes was >1.00 D were also excluded.
All LASIK procedures were performed using a Hansatome microkeratome (Bausch & Lomb, Salt Lake City, UT) with a 160-μm head and 8.5 mm ring, and the MEL80 excimer laser (Carl Zeiss Meditec, Jena, Germany). For each patient, one eye was treated with the MEL80 standard aspheric profile (ASA) and the other eye was treated with the MEL80 non-aspheric profile (TSA), assigned at random using Microsoft Excel's random number function (Microsoft Excel 2003, Microsoft Corporation, Seattle, Washington). The target postoperative refraction was plano in all eyes. A 6 mm optical zone was used for all ablations.
Manifest refraction, uncorrected distance visual acuity and CDVA were measured before and 3 months after surgery. Refractive outcomes including accuracy, efficacy, and safety were analyzed according to the protocol described by Reinstein and Waring.18,19 Ocular aberration was measured before and 3 months after surgery using the wavefront supported custom ablation aberrometer (Carl Zeiss Meditec, Jena, Germany), with cycloplegia using 1 drop of Tropicamide 0.5% (Mydriacyl, Alcon Laboratories (Thailand) Ltd., Bangkok). Wavefront exams were repeated until a 7 mm pupil diameter was obtained. Wavefront examinations were analyzed using Optical Society of America notation.20
The wavefront was recalculated with an analysis zone ranging from 3 to 7 mm in 0.5 mm intervals, and the SA was recorded for each diameter. The mean pre and postoperative SA was calculated and plotted against the analysis zone for both the ASA and TSA groups. The change in SA was calculated for each analysis zone as the difference between the pre and postoperative SA at the respective analysis zone. The induction of SA per diopter was calculated as the change in SA at a 6 mm analysis zone divided by the maximum myopic meridian treated.
The area under the curve of SA plotted against analysis zone diameter was calculated using the trapezium rule applied to each 0.5 mm interval. This has been previously described by Reinstein et al. and is known as the RAWS parameter.16 The RAWS parameter was calculated for pre and postoperative SA for both groups, including the percentage change in RAWS parameter. The change in coma and total higher-order aberration (root mean square [RMS]) was also calculated and compared in each group.
Analysis of variance test with Bonferroni adjustment (SPSS software) was performed within groups to test for statistically significant differences in SA at different pupil diameters (pre and post). Student's paired t-tests were performed between groups to test for statistically significant differences in preoperative spherical equivalent refraction and the change in RAWS parameter. Comparison of postoperative SA at different pupil diameters was also done using Analysis of variance test with Bonferroni correction for multiple comparison (SPSS software). A p < 0.05 was considered statistically significant. All statistics were calculated using Microsoft Excel 2003 and SPSS version 18.0. The sample size was calculated using G*Power software for Mac version 3.1.2. A priori compute required sample size-given alpha 0.05 and power of 0.80. The effect size was calculated using the mean difference of the pilot group (0.069 + 0.104). The total sample size of at least 15 in each group was achieved.
The mean age was 29.9 ± 3.5 years (range: 24 to 34 years). The mean preoperative spherical equivalent refraction was −2.43 ± 1.53 D (range: −0.50 to −5.00 D) in the ASA group and −2.54 ± 1.47 D (range: −0.50 to −5.00 D) in the TSA group. The mean preoperative spherical equivalent difference between eyes was 0.208 D (range: 0.00 to 1.00 D). There was no statistically significant difference in preoperative spherical equivalent refraction between groups (p = 0.87).
Fig. 1 shows the attempted vs. achieved spherical equivalent refraction for both groups. Fig. 2 shows a spherical equivalent refractive outcome bar graph for both groups; all eyes were within ±1.00 D. Fig. 3 shows the accuracy in terms of defocus equivalent refraction for both groups. Fig. 4 shows the efficacy for both groups; 100% of eyes were 20/25 or better. Fig. 5 shows the safety in terms of change in Snellen lines of CDVA for both groups; no eyes lost two or more lines CDVA in either group. Fig. 6 shows the stability of spherical equivalent refraction for both groups.
The normality of the aberration data was test using Kolmogorov–Smirnov Test and the Shapiro–Wilk Test.
Table 1 shows the mean pre- and postoperative SA at 1.0 mm intervals for a 3 to 7 mm analysis zone for the ASA group. There was a statistically significant increase in SA for a 6.0 to 7.0 mm analysis zone. The SA increased by 33% at the 6 mm analysis zone. By comparing postoperative SA at different pupil diameters, the statistically significant differences between pupil diameters were shown in Table 2 (with standard error and 95% CI). The pre and postoperative coma (RMS) at 6 mm zone did not show a statistically significant difference (Table 3).
Table 1 shows the mean pre- and postoperative SA at 1.0 mm intervals for a 3 to 7 mm analysis zone for the TSA group. There was a statistically significant increase in SA for a 5 to 7 mm analysis zone. The SA increased by 44% at the 6 mm analysis zone. By comparing postoperative SA at different pupil diameters, the statistically significant differences between pupil diameters were shown in Table 2 (with standard error and 95% CI). The pre and postoperative coma (RMS) at 6 mm zone did not show a statistically significant difference (Table 3).
ASA vs. TSA Group
Fig. 7 shows the mean change in SA plotted at 0.5 mm intervals for a 3 to 7 mm analysis zone for both ASA and TSA groups. The table below the graph shows the mean values and the p value at each analysis zone. There was less induction of SA in the ASA group, which was statistically significant for 4.5 to 7 mm analysis zone. The induction of SA per diopter was 0.024 ± 0.015 μm/D in the ASA group and 0.035 ± 0.021 μm/D in the TSA group.
Fig. 8 shows the mean pre- and postoperative RAWS parameter (area under the SA curve for a 3 to 7 mm analysis zone). There was a 34.8% increase in RAWS parameter in the ASA group (changing from 158 to 213 μm/mm2, p < 0.05) compared with a 74.4% increase in the TSA group (changing from 141 to 246 μm/mm2, p < 0.05).
Due to a rather small sample size in this study, a power calculation was recalculated using protocol of power analysis (Post hoc) and the power (1-β error probability) of 0.60 to 0.85 was achieved.
The induction of SA after myopic ablations is associated with increased side effects related to optical quality degradation. This is the first prospective randomized and intraindividual controlled study to demonstrate that aspheric profiles induce less SA over a range of pupil sizes compared with non-aspheric ablation profiles, which confirmed the previous reports using aspheric ablation profiles.10–12 However, there was still a statistically significant increase in SA for both profiles at 6 mm, the optical zone of the ablation.
This study demonstrates the way in which SA increases with pupil size in normal myopic eyes, and shows that the relationship between pupil diameter and SA becomes more significant after the central flattening of a myopic ablation. The induction of SA per diopter was found to be 0.024 μm/D for the aspheric profile and 0.035 μm/D for the non-aspheric profile. When evaluating a patient for refractive surgery, the postoperative SA can be estimated by the following formula:
Reinstein et al., have previously suggested that the tolerable level of SA to be 0.384 μm at 6 mm.16 Therefore, for an ASA ablation profile, if the intended treatment is >11.00 D, the patient should be counseled that there is a greater risk of night vision disturbances, assuming preoperative SA is 0.12 μm16 and the amount of the induction is linear. Similarly, for a TSA ablation profile, there is a greater risk of night vision disturbances if the intended treatment is >7.50 D. However, this estimation of the induction of SA might not be completely valid because this study has limited the amount of myopic treatment up to only −5.00 D. If the induction of SA is greater in higher myopia, the tolerable level of the treatment in both ablation profiles will be less than the calculation. Furthermore, the interindividual tolerance to SA might be different. Also, this analysis is based on the average SA induction, but there is a large variation between eyes as has been reported previously21 and demonstrated by the standard deviation in the present study. Moreover, the SA induction will be different for each laser platform,21 and therefore, the user should be aware of the SA induction for the laser platform that they use. Many different groups are working on developing new ablation profiles to further reduce the induction of SA with encouraging results. For example, McAlinden et al. have shown no statistically significant difference in the amount of induction of all higher-order aberrations in one laser platform.22
Similarly, hyperopic ablations are known to induce negative SA as opposed to positive SA after myopic ablation.23 It is worth measuring the SA induction in hyperopic ablations so that the postoperative SA can be predicted in a similar manner to that described earlier for myopic profiles. However, there is less concern about SA in hyperopia because the magnitude of the treatments is lower than in myopia and the majority of eyes have some positive SA naturally, which is offset against the induction of negative SA by a hyperopic ablation.
This study demonstrates how knowledge of the SA induction of ablation profiles for a particular laser can help to decide the best treatment option to account for the refraction to be treated, the corneal thickness, the preoperative SA, and the patient expectations. However, this study is limited by a small number of eyes. In addition, although a subjective assessment of psychophysical parameters such as contrast sensitivity or quality of vision questionnaire would be of interest,24 it was considered out of the scope of the present study that aimed to compare the induction of SA between two ablation profiles in particular at different pupil diameter. Furthermore, the subjective assessment can vary due to interindividual tolerance, and the sample size needs to be large to be able to distinguish a significant difference.25
Other aberrations, such as coma, also play some important role in night vision—in particular, coma due to decentration.21 This study found no statistically significant change in the coma aberration RMS in either group.
This study provides the evidence to support the benefit of less SA induction provided by an aspheric ablation profile over a non-aspheric ablation profile. Although aspheric ablation profiles remove slightly more tissue, they may be a preferred mode of treatment for higher myopia with larger pupils within constraints of flap thickness. Further study is currently being carried out to continue the development of ablation profiles to continue to reduce the induction of SA.
Dept of Ophthalmology
2 Prannok Rd., Bangkoknoi
Dr. Reinstein is a consultant for Carl Zeiss Meditec. The remaining authors have no proprietary or financial interest in the materials presented herein.
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spherical aberration; myopic LASIK; laser ablation profile; excimer laser; night vision disturbance