Journal of Refractive Surgery

Original Article Supplemental Data

Comparison of Visual Outcomes After Variable Spot Scanning Ablation Versus Wavefront-Optimized Myopic LASIK

Mounir A. Khalifa, MD, PhD; Ehab F. Mossallam, MD, PhD; Tamer H. Massoud, MD, PhD; Mohamed Shafik Shaheen, MD, PhD

Abstract

PURPOSE:

To compare the refractive and visual outcomes of variable spot scanning ablation versus the wavefront-optimized ablation for myopia and myopic astigmatism.

METHODS:

Fifty patients with myopia who underwent LASIK (100 eyes) were divided into two equal groups. Myopic correction performed using the variable spot scanning (VSS) ablation with the VISX STAR S4/IR platform (Abbott Medical Optics, Inc., Santa Ana, CA) (VSS group) and wavefront-optimized (WFO) ablation profile with the WaveLight Allegretto Wave Eye-Q platform (Alcon Laboratories, Inc., Fort Worth, TX) (WFO group). Manifest refraction, uncorrected distance visual acuity, and corrected distance visual acuity were obtained preoperatively and 1 day and 1 and 3 months postoperatively. Wavefront measurement and contrast sensitivity testing were done preoperatively and 3 months postoperatively.

RESULTS:

Postoperative mean refractive spherical equivalent was −0.14 ± 0.2 diopters for the VSS group and −0.15 ± 0.28 diopters for the WFO group. Forty-eight eyes of the VSS group and 47 eyes of the WFO group were within ±0.5 diopters. Postoperative mean corrected distance visual acuity was 1.05 ± 0.13 for the VSS group and 1.06 ± 0.12 for the WFO group. The postoperative uncorrected distance visual acuity was 1.01 ± 0.16 for the VSS group and 1.01 ± 0.11 for the WFO group. The safety index was 1.12 for the VSS group and 1.06 for the WFO group, whereas the efficacy index was 1.07 for the VSS group and 1.01 for the WFO group. The mean induced positive spherical aberration was 0.041 ± 0.046 µm for the VSS group and 0.195 ± 0.171 µm for the WFO group (P < .001). Mesopic contrast sensitivity testing showed no statistically significant differences between groups at all tested spatial frequencies.

CONCLUSIONS:

Both VSS and WFO treatments showed similar refractive and visual outcomes. Both induced significant positive spherical aberration, significantly more with WFO.

[J Refract Surg. 2015;31(1):22–28.]

From the Department of Ophthalmology, Tanta University, Tanta, Egypt (MAK); and the Department of Ophthalmology, Alexandria University, Alexandria, Egypt (EFM, THM, MSS).

Supported by a grant from Abbott Medical Optics, Inc.

The authors have no financial or proprietary interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (MAK); data collection (MAK, EFM, MSS); analysis and interpretation of data (MAK, EFM, THM); drafting of the manuscript (THM); critical revision of the manuscript (MAK, EFM, THM, MSS); statistical expertise (THM); administrative, technical, or material support (MAK, THM); supervision (MK)

Correspondence: Mounir A. Khalifa, MD, PhD, 559 Al-Horeya Avenue, Gleem, Alexandria, Egypt. E-mail: mounir.khalifa100@gmail.com

Received: September 02, 2014
Accepted: October 29, 2014

Abstract

PURPOSE:

To compare the refractive and visual outcomes of variable spot scanning ablation versus the wavefront-optimized ablation for myopia and myopic astigmatism.

METHODS:

Fifty patients with myopia who underwent LASIK (100 eyes) were divided into two equal groups. Myopic correction performed using the variable spot scanning (VSS) ablation with the VISX STAR S4/IR platform (Abbott Medical Optics, Inc., Santa Ana, CA) (VSS group) and wavefront-optimized (WFO) ablation profile with the WaveLight Allegretto Wave Eye-Q platform (Alcon Laboratories, Inc., Fort Worth, TX) (WFO group). Manifest refraction, uncorrected distance visual acuity, and corrected distance visual acuity were obtained preoperatively and 1 day and 1 and 3 months postoperatively. Wavefront measurement and contrast sensitivity testing were done preoperatively and 3 months postoperatively.

RESULTS:

Postoperative mean refractive spherical equivalent was −0.14 ± 0.2 diopters for the VSS group and −0.15 ± 0.28 diopters for the WFO group. Forty-eight eyes of the VSS group and 47 eyes of the WFO group were within ±0.5 diopters. Postoperative mean corrected distance visual acuity was 1.05 ± 0.13 for the VSS group and 1.06 ± 0.12 for the WFO group. The postoperative uncorrected distance visual acuity was 1.01 ± 0.16 for the VSS group and 1.01 ± 0.11 for the WFO group. The safety index was 1.12 for the VSS group and 1.06 for the WFO group, whereas the efficacy index was 1.07 for the VSS group and 1.01 for the WFO group. The mean induced positive spherical aberration was 0.041 ± 0.046 µm for the VSS group and 0.195 ± 0.171 µm for the WFO group (P < .001). Mesopic contrast sensitivity testing showed no statistically significant differences between groups at all tested spatial frequencies.

CONCLUSIONS:

Both VSS and WFO treatments showed similar refractive and visual outcomes. Both induced significant positive spherical aberration, significantly more with WFO.

[J Refract Surg. 2015;31(1):22–28.]

From the Department of Ophthalmology, Tanta University, Tanta, Egypt (MAK); and the Department of Ophthalmology, Alexandria University, Alexandria, Egypt (EFM, THM, MSS).

Supported by a grant from Abbott Medical Optics, Inc.

The authors have no financial or proprietary interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (MAK); data collection (MAK, EFM, MSS); analysis and interpretation of data (MAK, EFM, THM); drafting of the manuscript (THM); critical revision of the manuscript (MAK, EFM, THM, MSS); statistical expertise (THM); administrative, technical, or material support (MAK, THM); supervision (MK)

Correspondence: Mounir A. Khalifa, MD, PhD, 559 Al-Horeya Avenue, Gleem, Alexandria, Egypt. E-mail: mounir.khalifa100@gmail.com

Received: September 02, 2014
Accepted: October 29, 2014

The most commonly observed vision degrading problem following laser refractive correction is induced higher-order aberrations, most notably spherical aberration. This is especially important after standard ablation procedures, which constitute the majority of treatments performed worldwide.1,2 The induction of this rotationally symmetric aberration was hypothesized to be due to reflection of a part of the incident energy off the surface when moving from the corneal apex to the periphery due to changes in the angle of the incident beam, the so-called “cosine effect.” This leads to loss of a part of the photoablative energy of the excimer laser, resulting in variation of the ablation depth per pulse in the center compared to the periphery with significant undercorrection in the periphery and, consequently, spherical aberration induction.3–6

Aspheric ablation profiles were designed to solve this problem, mainly by increasing the ablation depth toward the periphery of the optical zone while keeping the central ablation depth unchanged. Theoretically, this would result in pre-compensation for the expected fourth-order spherical aberration in the average eye. This technology has been adopted by several platforms and has been given different names.6–10

Several studies compared the outcomes of the aspheric, manifest refraction-based, wavefront-optimized (WFO) ablation profile of the WaveLight Allegretto Wave Eye-Q platform (Alcon Laboratories, Inc., Fort Worth, TX) to those of wavefront-guided treatments of other platforms.11–14 However, to our knowledge, no study has compared any of the aspheric ablation profiles to their counterparts on different platforms.

The aim of this study was to compare the refractive and visual outcomes after correcting myopia and myopic astigmatism using two different aspheric, manifest refraction-based ablation profiles: the variable spot scanning (VSS) ablation (VISX STAR S4/IR platform; Abbott Medical Optics, Inc., Santa Ana, CA) versus the WFO procedure (WaveLight Allegretto Wave Eye-Q platform; Alcon Laboratories, Inc.).

Patients and Methods

Patients

This prospective, non-randomized, controlled clinical trial included a total of 100 eyes of 50 patients who underwent LASIK. They were divided into two groups (the VSS and WFO groups), each consisting of 25 patients (50 eyes). For the VSS group, bilateral LASIK was performed with the VISX STAR S4/IR platform using the VSS ablation, whereas for the WFO group bilateral WFO-LASIK was performed with the WaveLight Allegretto Wave Eye-Q platform.

Inclusion criteria consisted of preoperative myopic refractive errors ranging from −1.0 to −10.0 diopters (D) of spherical equivalent with astigmatism up to 2.75 D at the spectacle plane, an age of 18 years or older, and a stable refraction. A corrected distance visual acuity (CDVA) of 0.9 or better for each eye included in the study was a must. Normal corneal topography, normal anterior and posterior surface elevation maps, and a central corneal thickness of 500 µm or greater were also mandatory. All patients with dry eye, abnormal corneal topography, thin corneas, corneal opacities, or other ocular or recorded systemic illnesses were excluded. Patients experiencing intraoperative or postoperative complications were also excluded.

The study was conducted between 2012 and 2013. The study protocol was approved by the ethics committee of the Faculty of Medicine of Alexandria University. The risks and advantages of the procedure were explained and an informed consent was obtained from all patients.

Preoperative and Postoperative Examination

Preoperatively, all patients received a detailed ocular and medical history, full ophthalmic examination, measurement of uncorrected distance visual acuity (UDVA), manifest refraction, and CDVA. Keratometric data, corneal topography, thickness data, and height maps of the anterior and posterior corneal surfaces were obtained from the Oculus Pentacam HR (Oculus Optikgeräte GmbH, Wetzlar, Germany). Evaluation of the higher-order aberration (HOA) status of the enrolled eyes was performed using the VISX WaveScan aberrometer (Abbott Medical Optics, Inc.) under mesopic conditions for both groups with undilated pupils. Mesopic contrast sensitivity testing using the CVS-1000 chart (Vector-Vision, Inc., Greenville, OH) with the patient wearing his or her manifest refraction was also done.

Postoperatively, patients were examined at 1 day, 1 week, and 1 and 3 months. Biomicroscopic examination, UDVA, manifest refraction, and CDVA testing were performed in these visits. Safety and efficacy were expressed in terms of the safety and efficacy indices with the former calculated as the ratio of the 3-month postoperative CDVA to the preoperative CDVA and the latter as the ratio of the postoperative UDVA to the preoperative CDVA. Aberrometric evaluation of the postoperative HOA and mesopic contrast sensitivity testing were done at the 3-month visit to check for the visual quality experienced by the patients. The largest possible wavefront diameter was used for the recording and comparison of preoperative and postoperative wavefront data.

Surgery

VISX STAR platforms produce a broad excimer laser beam, which becomes resized and redirected through an iris-diaphragm (variable aperture) with a beam homogenizer to produce seven variable spot sizes ranging from 0.65 to 6.5 mm that overlap to smooth the ablation near the edge of the beam.15

The original standard manifest refraction-based ablation profile in these platforms involves the ablation of a 6- (default) or 6.5-mm (optional) optical zone with concentric, pupil-centered, decreasing diameter laser spots fired at a rate of 10 Hz. Cosine effect compensation with the standard ablation profile is performed by (optionally) adding a peripheral blend zone to the treated optical zone during refractive data entry, resulting in increasing the number of pulses fired in the periphery (compensating for energy loss and reflection). The addition of this blend zone also increases the diameter of the total treatment area to 8 mm. The design for such treatments is performed at the computer linked directly to the laser platform.8,15

The VSS procedure (VSS refractive) with variable repetition rate is a manifest refraction-based aspheric ablation profile that has recently become available for standard refractive corrections in VISX STAR platforms (previously used only with wavefront-guided treatments). The use of the VSS technology helps provide a higher and variable laser repetition rate (up to 20 Hz). The variability of the repetition rate helps avoid thermal damage to the corneal stroma. The spot scanning technology together with the variability of the spot size also improve the match to the target ablation shape.8,15

In addition to the previous advantages over standard profile, the use of the VSS ablation for standard manifest refraction-based correction allows the ablation of larger diameter optical (up to 8.5 mm) and total treatment (up to 9.5 mm) zones. Also, aspheric compensation in the VSS ablation has become integral to the treatment design and is achieved through the same concept of increasing the number of pulses fired in the periphery.

The planning for the VSS ablation is done by special software loaded to a computer. Data exported to the laser consist only of basic refractive data (refraction, keratometric readings, and corneal thickness) without any aberrometric data.

The WFO ablation profile of the WaveLight Allegretto Wave Eye-Q platform is an aspheric ablation profile that is planned to compensate for the cosine effect through an algorithm that increases the ablation depth in the mid-periphery of the optical zone (up to 35%). It is a manifest refraction-based treatment that uses a fixed diameter (0.95 mm) flying spot laser with a repetition rate of 400 Hz. The default optical zone for such treatments is 6.5 mm (it can be manipulated from 5 to 7 mm in 0.5-mm steps) with a maximum total treatment zone of 9 mm.7

For the VSS group, manifest refraction and other basic refractive data (keratometric readings and corneal thickness) were entered on the VSS software. They were revised, approved, and transmitted to the VISX STAR S4/IR. Exported data consisted of manifest refraction, basic refractive data, and the treatment design (no wavefront measurements included). For the WFO group, refractive data were directly entered to the lap-top linked to the WaveLight Allegretto Wave Eye-Q laser system. They were revised and approved prior to commencing surgery.

Surgeries were performed by one surgeon (MAK) at Horus Vision Correction Center (Alexandria, Egypt) for the VSS group and at Alex LASIK Center (Alexandria, Egypt) by another surgeon (EFM) for the WFO group.

For both groups, the optical zone was fixed at 6.5 mm. The surgical technique included the creation of a corneal flap by the Moria M2 mechanical microkeratome (Moria, Antony, France) with the 130-µm single use head. The choice of the microkeratome ring was based on recommendations of the nomogram provided by the manufacturer. Laser ablation for both groups was centered on the line of sight (center of the entrance pupil) and centration was controlled by the pupil tracking system of each platform.

The same postoperative treatment regimen of combined steroid antibiotic eye drops together with non-preserved artificial tears was followed for all patients.

Statistical Analysis

Data analysis was performed using SPSS software (version 20.0; SPSS, Inc., Chicago, IL) for Windows. Normality of data samples was evaluated by the Kolmogorov–Smirnov test. When parametric analysis was possible, the paired t test was used for comparisons between the preoperative and postoperative data and the Student’s t test for comparison between the postoperative data of both groups. The Wilcoxon rank sum test (for comparisons between the preoperative and postoperative data) and Mann–Whitney U test (for comparison between the postoperative data of both groups) were applied to assess the significance of such differences when parametric analysis was not possible. A P value of less than .05 was considered statistically significant. Correlation coefficients (Pearson or Spearman, depending on whether normal conditions could be assumed) were used to assess the correlation between different variables. Standard graphs for reporting the outcomes in refractive surgery according to the Waring protocol and its modifications16–18 were used for displaying and summarizing the main outcomes of this study.

Results

The mean age was 27.4 ± 9.6 years (range: 18 to 37 years) for the VSS group and 29.2 ± 18.8 years (range: 19 to 51 years) for the WFO group. The VSS group included 10 males (40%) and 15 females (60%), whereas the WFO group included 9 males (36%) and 16 females (64%).

Refractive Outcomes

The preoperative mean refractive spherical equivalent (MRSE) was −3.8 ± 1.98 D for the VSS group and −4.03 ± 2.25 D for the WFO group (P = .271). At the 1-month follow-up visit, the MRSE was −0.32 ± 0.41 D for the VSS group and −0.30 ± 0.38 D for the WFO group (P = .282), whereas the 3-month postoperative MRSE was −0.14 ± 0.2 D for the VSS group and −0.15 ± 0.28 D for the WFO group (P = .303). No statistically significant difference existed on comparing any of the preoperative or postoperative values between both groups.

Figure 1 shows the attempted versus the achieved MRSE with strongly positive correlation for both groups (r = 0.966 for the VSS group and 0.9656 for the WFO group).

Attempted versus achieved mean refractive spherical equivalent for the variable spot scanning (VSS) and wavefront optimized (WFO) group.

Figure 1.

Attempted versus achieved mean refractive spherical equivalent for the variable spot scanning (VSS) and wavefront optimized (WFO) group.

Both techniques showed high predictability with 48 (96%) eyes of the VSS group and 47 (94%) eyes of the WFO group within ±0.5 D of emmetropia. One hundred percent of the eyes of both groups were within ±1.0 D of emmetropia (Figure 2).

Predictability of both techniques at 3 months postoperatively. VSS = variable spot scanning; WFO = wavefront optimized; D = diopter

Figure 2.

Predictability of both techniques at 3 months postoperatively. VSS = variable spot scanning; WFO = wavefront optimized; D = diopter

Regarding stability at 3 months postoperatively, both techniques were stable with no statistically significant differences among the measured MRSEs as mentioned previously (Figure A, available in the online version of this article).

Visual Outcomes

Preoperatively, the VSS group had a mean CDVA of 0.94 ± 0.1 and the WFO group had a mean CDVA of 1.0 ± 0.14. Three months postoperatively, the VSS group had a mean CDVA of 1.05 ± 0.13 and the WFO group had a mean CDVA of 1.06 ± 0.12 (P = .416). The UDVA at 3 months postoperatively was 1.01 ± 0.16 for the VSS group and 1.01 ± 0.11 for the WFO group (P = .465).

The safety index was 1.12 for the VSS group and 1.06 for the WFO group, whereas the efficacy index was 1.07 for the VSS group and 1.01 for the WFO group (Figure 3).

(A) Safety and (B) efficacy 3 months postoperatively. CDVA = corrected distance visual acuity; VSS = variable spot scanning; WFO = wavefront optimized

Figure 3.

(A) Safety and (B) efficacy 3 months postoperatively. CDVA = corrected distance visual acuity; VSS = variable spot scanning; WFO = wavefront optimized

HOAs

No statistically significant differences existed between the measured wavefront diameters for either group preoperatively (VSS = 6.371 ± 0.65 mm, WFO = 6.53 ± 0.59 mm, P = .173) or at 3 months postoperatively (VSS = 6.171 ± 0.611 mm, WFO = 6.267 ± 0.572 mm, P = .262). Also, no statistically significant differences were found on comparing the measured wavefront diameter for each group preoperatively and postoperatively (VSS: P = .152, WFO: P = .231).

Data of preoperative values of HOAs root mean square and measured aberrations’ coefficients are summarized in Table A (available in the online version of this article), showing no statistically significant differences between both groups.

Table 1 shows induced aberrations for both groups 3 months postoperatively. The 3-month mean induced HOA root mean square value was 0.135 ± 0.257 µm for the VSS group and 0.178 ± 0.223 µm for the WFO group. No statistically significant difference was found (P = .236).

Comparison of Induced HOA for Both Groups 3 Months Postoperatively

Table 1:

Comparison of Induced HOA for Both Groups 3 Months Postoperatively

For third-order aberrations, a statistically significant difference was found between the induced coma for the VSS group (−0.066 ± 0.105 µm) and that of the WFO group (−0.169 ± 0.197 µm) (P = .007), whereas no statistically significant difference was found between the induced trefoil for the VSS group (−0.014 ± 0.065 µm) and that of the WFO group (−0.002 ± 0.121 µm) (P = .323).

The mean induced fourth-order positive spherical aberration was 0.041 ± 0.046 µm for the VSS group and 0.195 ± 0.171 µm for the WFO group. A statistically significant difference was found on comparing both groups (P = .000) (Figure 4).

Induced higher-order aberration (HOA) for both techniques 3 months postoperatively. WFO = wavefront optimized; VSS = variable spot scanning

Figure 4.

Induced higher-order aberration (HOA) for both techniques 3 months postoperatively. WFO = wavefront optimized; VSS = variable spot scanning

Mesopic Contrast Sensitivity

Table B (available in the online version of this article) summarizes monocular contrast sensitivity outcomes obtained in both groups under mesopic conditions 3 months postoperatively. No statistically significant differences were found between both techniques for all tested spatial frequencies (Figure B, available in the online version of this article).

Discussion

Studies based on corneal aberrations, wavefront aberrations, and visual performance demonstrated that the retinal image quality degrades after “classic” (conventional) photorefractive treatments.19–22 This occurrence has been attributed to the induction of HOA, especially the fourth-order, rotationally symmetric, spherical aberration.4,5,23 The main indictment for this aberration induction included, in addition to flap-related factors and biomechanical changes,24,25 the loss of energy of the laser beam on ablating the more peripheral areas of the cornea. The last factor was considered the only modifiable among the other causes. The introduction of aspheric ablations heralded a new era of low level customization that is tailored for every laser platform, each on its own, compensating for the expected induced spherical aberration.6,7

This study was performed with the aim of comparing the visual and refractive outcomes of two aspheric ablation techniques: the VSS of the VISX STAR S4/IR platform and the WFO of the WaveLight Allegretto Wave Eye-Q platform.

Both techniques showed comparable accuracy, predictability, efficacy, safety, and stability. WFO ablation has been reported before, either alone26–28 or in comparison to other wavefront-guided ablation profiles,11–14 to yield good refractive and visual outcomes. To our knowledge, this is the first report on the manifest refraction-based VSS ablation.

Both ablation profiles induced postoperative HOAs with no statistically significant differences between either group in regard to the induced total HOA root mean square value. WFO treatments have been reported by Moshirfar et al.,12 Padmanabhan et al.,28 and Smadja et al.29 to result in increased postoperative HOA root mean square with variable amounts.

In both procedures, a statistically significant induction of positive spherical aberration was noticed postoperatively. Moreno-Barriuso et al.1 and Waheed et al.2 reported greater values of induced spherical aberration after conventional LASIK. Comparable results, with values closer to those reported in this study for the induced spherical aberration, were reported by Padmanabhan et al.28 and Smadja et al.29 after WFO-LASIK. They also reported that the magnitude of the induced spherical aberration was directly proportional to the amount of myopic ablation performed. Changes in corneal asphericity,1,7,28 posterior corneal shape,30 and biomechanical responses to myopic ablation25,31 have all been suggested as causes explaining the induction of spherical aberration postoperatively.

Yet, the VSS ablation induced significantly less positive spherical aberration than the WFO procedure. The explanation of this finding might lie in the understanding of the nature of the laser beam produced by each platform and the theoretical background for performing aspheric ablation by each of them. The WaveLight Allegretto Wave Eye-Q platform produces a fixed size (0.95 mm), lower energy, flying spot laser beam, and performs aspheric ablations through an algorithm that increases the ablation depth (up to 35%) in the mid-periphery of the optical zone to compensate for energy loss in this area.7,26 On the other hand, the VISX STAR S4/IR platform generates a broad, higher energy laser beam that becomes resized and redirected through an iris-diaphragm (variable aperture) with beam homogenizer to produce the seven variable spot sizes that overlap to smooth the ablation near the edge of the beam. It uses algorithms to compensate for energy loss and reflection by increasing the number of pulses fired in the periphery (not the mid-periphery) of the optical zone.8,15 Theoretically, because the peripheral laser pulses delivered during the VSS ablation have higher energy (due to their original broad beam nature) than the flying laser spots of the WFO procedure, it can be deduced that more effective ablation and, therefore, slightly better cosine-effect compensation takes place at the periphery of the optical zone with the resultant less induction of positive spherical aberration by the this procedure.

In this study, we also reported significantly increased coma aberration in both groups postoperatively. The induction of the third-order coma aberration after LASIK has been attributed to a variety of factors. Mrochen et al.32 reported that induction of coma aberration after refractive excimer laser photoablation can be attributed to even minute amounts of decentration (less than 1 mm away from the line of sight). Tsai and Lin33 argued that the use of an eye tracker can help to avoid severe decentrations, but does not ensure perfect centration. Another explanation of induced coma following corneal photorefractive surgery, suggested by Pallikaris et al.,24 is the creation of a corneal flap. They concluded that the use of a microkeratome induces coma aberration along the direction of the hinge. On the other hand, Waheed et al.2 reported no predictable trends in flap-induced aberrations and no significant association between the hinge position and the pattern of the induced coma aberration. Porter et al.34 demonstrated that despite the initial significant increase in vertical coma 20 minutes after flap creation, this effect subsided within 24 hours, suggesting that the postoperatively induced coma aberration may be more attributable to the laser ablation itself rather than the microkeratome incision. A third, but less recognized, cause of coma induction is asymmetrical wound healing, as suggested by immunohistological studies.35 Due to the difficulty in perfectly assigning the real cause of induced coma, it became more unlikely for us to logically explain the statistically significant higher values of coma induced in both groups.

A statistically significant higher amount of the induced coma in the WFO group compared to the VSS group was also reported in this study despite the same centration protocol for both platforms (on the line of site). A factor that may help explain this difference is that each group had procedures performed by a different surgeon (MAK and EFM), each with his own head positioning and flap centration preferences.

Contrast sensitivity testing for both groups showed no statistically significant differences for all tested spatial frequencies, suggesting that despite the differences reported for the induced aberrations between both groups, no major effect on the retinal image quality and visual performance took place.

The drawbacks of this study include, in addition to non-randomization and the different surgeons for each group, the short-term follow-up and the lack of correlation between the amount of induced aberrations and the depth of ablation. However, patients are still being observed and more are being recruited. We hope to report longer term results for 6 months and 1 year in the near future. Also, dividing cases into mild, moderate, and high according to the MRSE and reporting results accordingly will be taken into consideration. Other studies comparing either technique to other aspheric or wavefront-guided treatments should also be performed.

VSS refractive and WFO ablations are equally effective, safe, predictable, and stable for treating myopia with or without astigmatism. Both induced minimal but significant positive spherical aberration, which was significantly higher with WFO ablation than with VSS ablation. However, the same visual performance of the eyes of either group was recorded as evidenced by the nonsignificant postoperative differences in visual acuity and mesopic contrast sensitivity testing.

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Stability of both techniques 3 months postoperatively. VSS = variable spot scanning; WFO = wavefront optimized

Figure A. Stability of both techniques 3 months postoperatively. VSS = variable spot scanning; WFO = wavefront optimized

Mean monocular contrast sensitivity function measured under mesopic conditions 3 months postoperatively. VSS = variable spot scanning; WFO = wavefront optimized; c/d = cycles per degree

Figure B. Mean monocular contrast sensitivity function measured under mesopic conditions 3 months postoperatively. VSS = variable spot scanning; WFO = wavefront optimized; c/d = cycles per degree




Comparison of Induced HOA for Both Groups 3 Months Postoperatively

Parameter VSS (µm) WFO (µm) Pa
Δ HOA RMS (SD) 0.135 ± 0.257 0.178 ± 0.223 .236
Δ Spherical aberration (SD) 0.041 ± 0.046b 0.195 ± 0.171b .001b
Δ Trefoil (SD) 0.014 ± 0.065 0.002 ± 0.121 .323
Δ Coma (SD) 0.066 ± 0.105b 0.169 ± 0.197b .007b

10.3928/1081597X-20141218-03

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