Accurate centration of the treatment zone during corneal refractive procedures plays an important role in achieving good visual outcomes.1–3 Visual axis, pupil center, and corneal vertex have been the suggested treatment centers for refractive procedures. The corneal intercept of the visual axis would be the most ideal reference, but it is difficult to center the crossing point. Corneal vertex centration has been reported to be better than entrance pupil centration regarding the induced ocular aberrations and asphericity.4–6 Moreover, it is well known that the pupil center shifts with changes in pupil size.7
Small incision lenticule extraction (SMILE) using only a femtosecond laser is a new technique for the correction of myopia and myopic astigmatism.8 This procedure extracts the refractive lenticule through a small corneal incision ranging from 2 to 5 mm, with the absence of a flap and the preservation of the anterior-most stromal lamellae and Bowman's layer.8,9 Thus, SMILE provides many benefits for patients because it has reduced risk for traumatic flap displacement and reduces postoperative dry eye symptoms due to reduced damage to corneal nerves while providing excellent clinical outcomes.10,11
The SMILE procedure is performed under mild suction and does not involve an eye tracking system. The alignment of the refractive lenticule relies entirely on the patient's support, which enables a coaxial fixation to the light at the moment of application of suction, resulting in lenticule formation centered on the corneal vertex of the coaxially fixating eye. For better postoperative visual quality after SMILE, the need for developing a sophisticated centration technique for SMILE has increased. We propose a triple marking centration method and compare this new method with the subjective patient fixation method with regard to clinical results and induction of corneal higher order aberrations (HOAs).
In the current study, we investigated the lenticule decentration following the SMILE procedure via the subjective patient fixation method or triple marking centration method using the Scout tangential topography difference map. We compared visual and refractive outcomes and induction of corneal HOAs between the two methods and further investigated the relationship between the magnitudes of total decentration and induction of corneal HOAs in each method.
Patients and Methods
This retrospective, comparative observational case series was performed with the approval of the Institutional Review Board of Yonsei University College of Medicine (Seoul, South Korea). All study conduct adhered to the tenets of the Declaration of Helsinki and followed good clinical practices.
Patients were included in the analyses if they had stable refraction at least 1 year prior to surgery with spherical myopia up to −9.00 diopters (D) and/or myopic astigmatism up to −3.50 D cylinder, preoperative corrected distance visual acuity (CDVA) of 20/25 or better, normal preoperative corneal topography, age younger than 45 years, and available preoperative and postoperative data. The exclusion criteria were presence of severe ocular surface diseases, ocular trauma, keratoconus, or cataract and history of intraocular or corneal surgery. Only the left eye of each patient was included.12,13 The subjective fixation method was used in consecutive patients between October 2014 and September 2015 and the triple marking centration method was used between October 2015 and November 2016. Finally, a total of 110 eyes of 110 patients who met the inclusion and exclusion criteria were retrospectively analyzed.
All patients underwent a preoperative ophthalmic examination that included evaluation of logMAR uncorrected distance visual acuity (UDVA) and CDVA, manifest refraction, slit-lamp examination (Haag-Streit, Köniz, Switzerland), intraocular pressure (IOP) measurement (non-contact tonometer; NT-530, NCT Nidek Co., Ltd., Aichi, Japan), dilated fundus examination, corneal thickness assessment using a Scheimpflug camera (Pentacam HR; Oculus Optikgeräte, Wetzlar, Germany), and corneal topography including corneal wavefront aberrations derived using the Keratron Scout topographer (Optikon, Rome, Italy). These examinations were all repeated at 3 months postoperatively.
For the measurement of changes in corneal aberrations, corneal wavefront analysis was implemented based on corneal topographic data obtained with the Keratron Scout topographer. The root mean square (RMS) values of the 3rd order coma aberration, 4th order spherical aberration, and total HOAs were calculated. The RMS value of total HOAs was analyzed up to the 7th order by expanding the set of Zernike polynomials. The individual coefficients of vertical and horizontal coma were also analyzed.
All SMILE treatments were performed using the 500-kHz VisuMax femtosecond laser (software version 2.4.0; Carl Zeiss Meditec, Jena, Germany) by one experienced surgeon (DSYK).14 Target refraction was emmetropia in all cases. For patients using the subjective patient fixation method (subjective centration group), alignment was subjective and relied entirely on the patient's fixation to the target light. At the moment of contact between the individually calibrated curved contact glass and the cornea, a meniscus tear film appears, at which point the patient is able to see the fixation target clearly because the vergence of the fixation beam is adjusted according to the individual eye's refraction. At this point, the surgeon instructs the patient to look directly at the green light and, once in position, the corneal suction ports are activated to fixate the eye in this position. In this way, the patient aligns the visual axis and hence the corneal vertex to the vertex of the contact glass, which is centered to the laser system and the lenticule formation centered on the corneal vertex of the coaxially fixating eye to be created.
In the triple marking centration method (triple centration group, Figure 1), three centration points were marked preoperatively at the slit lamp while the patient, in sitting position, fixated on the center of the slit-lamp beam, which was narrowed as much as possible until all three markings were completed. The first two markings were made 7 mm apart at the horizontal meridian with guidance of the 7-mm long slit beam. These two markings were made by appropriately bisecting the first Purkinje reflex or the coaxially sighted corneal light reflex. The third marking was made on the inferior cornea by vertically (90°) rotating the slit-lamp beam by bisecting the first Purkinje reflex or the coaxially sighted corneal light reflex. It is recommended that a small spot be used for the marking process so that the points are as small as possible. They also need to be large enough to be seen during the docking process. These three markings aided the surgeon to guide proper centration as seen in the surgical monitor. In addition, during the surgery, the surgeon gently rotates the contact glass to align the horizontal marks on the eye to the horizontal axis of the reticule and the third inferior mark to the vertical axis of the reticule to compensate for any cyclotorsion.13
Triple marking centration method. (A) Surgeon's view after docking of operating eye. Triple markings were made at the horizontal meridian and the inferior cornea by bisecting the coaxially sighted corneal light reflex. During the docking procedure, decentration and static cyclotorsion were corrected via triple marking centration method. (B) The eye was centered at the coaxially sighted corneal light reflex and cyclotorsion was corrected after the docking procedure. yellow circle = marking; arrowhead = pupil center; arrow = coaxially sighted corneal light reflex
The subsequent surgical procedure was the same for both groups. After successful femtosecond laser cutting, the refractive lenticule of the intrastromal corneal tissue was extracted through the side cut opening using forceps.
A difference map of the tangential curvature was generated for each eye using the preoperative and 3-month postoperative Keratron Scout scans. The difference map images were imported into Microsoft PowerPoint 2010 software (Microsoft Corporation, Redmond, WA), and a previously prepared grid and set of concentric circles was overlaid.15 The lines of the grid were distributed equally with 0.1-mm steps and the concentric circles had radii increasing in 0.1-mm steps, between 2.5 and 3.5 mm radii. The difference map was magnified on the screen and made as large as possible so that the center of the optical zone could be confidently visualized within half a step. The optical zone was defined on the tangential topography difference map as the central zone up to the mid-peripheral power inflection point. The best-fitting circle and central grid were superimposed on the optical zone to determine the location of the optical zone center with reference to the corneal vertex. The topographic graph displayed the (0, 0) point as the corneal vertex, and a coordinate (x, y) in millimeters for any point relative to the (0, 0) point. Positive x values indicated temporal decentration and negative x values indicated nasal decentration. The x and y coordinates of the optical zone centration were plotted on a 360° polar plot to show the location of the center of the optical zone relative to the corneal vertex. Centration was analyzed in terms of horizontal, vertical, and total decentration. Further, we compared angle kappa (preoperative offset), which was generated by subtracting the pupil center coordinate from the corneal vertex.
Statistical analysis was performed using SPSS software (version 22.0; IBM Corporation, Armonk, NY). Differences were considered statistically significant when the P values were less than .05. After confirming data normality distribution, we used the independent t test for continuous variables and the chi-square or Fisher's exact test for categorical variables to determine significant differences between the two groups. We performed the paired t test to evaluate the differences between preoperative and 3-month postoperative parameters. Simple linear regression analysis was used to determine the potential associations.
A post-hoc power analysis using the G*Power program (Franz Faul, Kiel University, Kiel, Germany) indicated that we had power of 0.93 to detect a significant association (α< .05) when an effect size index of 0.66 calculated from mean values of total decentered displacement for both groups was used.
There were 55 eyes from 55 patients in each group, with no differences in demographic data and ocular characteristics between the two groups (Table A, available in the online version of this article). All surgical procedures were uneventful and no postoperative complications were observed during the study period.
Baseline Characteristics of Eyes Undergoing SMILE by Subjective Patient Fixation Method or Triple Marking Centration Method
Table B (available in the online version of this article) and Figure 2 show visual and refractive outcomes for both groups. The mean efficacy index at 3 months postoperatively was 1.27 ± 0.21 in the subjective centration group and 1.31 ± 0.20 in the triple centration group (P = .288). The mean safety index at 3 months postoperatively was 1.28 ± 0.18 in the subjective centration group and 1.31 ± 0.19 in the triple centration group (P =.374).
Changes in Visual Acuity and Refractive Errors Before and After SMILE by Subjective Patient Fixation Method or Triple Marking Centration Method
Visual outcomes after small incision lenticule extraction with the subjective patient fixation method or triple marking centration method. (A) Uncorrected distance visual acuity (UDVA) outcomes, (B) change in corrected distance visual acuity (CDVA), (C) distribution of achieved spherical equivalent outcomes, (D) spherical equivalent refractive accuracy, and (E) refractive astigmatism at 3 months postoperatively. D = diopters
Table 1 shows the pupillary offset and distance of the achieved centration from the attempted target (corneal vertex) in both groups. There were no significant differences in pupillary offset including x and y axis between the two groups. There were significant differences in horizontal decentered displacement, vertical decentered displacement, and total decentered displacement between the two groups.
Comparison of Preoperative Offset and Decentered Displacement Relative to the Corneal Vertex Between Eyes Undergoing SMILE by Subjective Patient Fixation Method or Triple Marking Centration Method
The achieved centration of each eye is depicted in Figure 3. The points corresponding to the centers of the lenticules were more accumulated around the target point (0,0) in the triple centration group compared to the subjective centration group. Decentration occurred along the vertical axis mainly in the superior direction in the subjective centration group. A total of 42 of 55 treated eyes (76%) were within 0.50 mm in the subjective centration group and 54 of 55 treated eyes (98%) in the triple centration group (P = .001; Figure A, available in the online version of this article).
Scatterplot showing distribution of treatment centers with respect to the corneal vertex via the subjective patient fixation method or triple marking centration method. Positive vertical coordinates stand for superior displacements and negative for inferior ones. Positive horizontal coordinates stand for temporal displacements and negative for nasal ones.
Distribution of total decentered displacements after subjective small incision lenticule extraction with the subjective patient fixation method or triple marking centration method.
Total HOAs, coma, vertical coma, and spherical aberration increased in the subjective centration group, whereas total HOAs and vertical coma increased in the triple centration group at 3 months postoperatively (Table C, available in the online version of this article). When comparing the magnitude of differences, changes in total HOAs, coma, and spherical aberration were significantly greater in the subjective centration group than in the triple centration group (P = .026 for total HOAs, P = .032 for coma, and P = .013 for spherical aberration; Figure 4).
Changes in Corneal HOAs of Eyes Undergoing SMILE by Subjective Patient Fixation Method or Triple Marking Centration Method
Changes in root mean square higher order aberrations (RMS HOAs), coma, horizontal coma, vertical coma, and spherical aberration after small incision lenticule extraction with the subjective patient fixation method or triple marking centration method. Error bars represent 95% confidence interval of the mean. *P < .05.
There was a significant relationship between the pupillary offset (y axis) or optical zone × pupillary offset (y axis) and vertical decentered displacement in the subjective centration group, whereas there was no relationship in the triple centration group (Figure B, available in the online version of this article). There was a trend showing a negative relationship between the pupillary offset (x axis) or optical zone × pupillary offset (x axis) and horizontal decentered displacement in the triple centration group.
Relationship between the preoperative pupillary offset and decentered displacement after small incision lenticule extraction with the subjective patient fixation method or triple marking centration method.
Regarding the total decentration, the association between the total decentered displacement and induced vertical coma or induced spherical aberration was significant in the subjective centration group (Figure C, available in the online version of this article). In the triple centration group, no associations were found between the total decentered displacement and induced HOAs. There was a significant relationship between the horizontal decentered displacement and induced horizontal coma in the subjective centration group only (Figure D, available in the online version of this article). Additionally, there were significant relationships between the magnitude of the horizontal decentration × optical zone or the magnitude of the horizontal decentration × change in manifest refraction spherical equivalent (ΔMRSE) and induced horizontal coma in the subjective centration group, but not in the triple centration group. The association between the vertical decentered displacement and induced vertical coma was significant in both groups. Additionally, there was a significant relationship between the magnitude of the vertical decentration × optical zone or the magnitude of the vertical decentration × ΔMRSE and induced vertical coma in both groups (Figure D).
Relationship between the total decentered displacement and induced corneal higher order aberrations (HOAs) after subjective small incision lenticule extraction with the patient fixation method or triple marking centration method.
Relationship between horizontal/vertical decentered displacement and induced horizontal/vertical coma after small incision lenticule extraction with the subjective patient fixation method or triple marking centration method. ΔMRSE = change (postoperative–preoperative) in manifest refraction rical equivalent
Induction of total HOAs, coma, and spherical aberration was more significant in the subjective centration group than in the triple centration group. Moreover, the total decentration showed significant correlation with induced vertical coma or induced spherical aberration in the subjective centration group, whereas no associations were found between the total decentration and induced corneal HOAs in the triple centration group.
More specifically, the magnitude of horizontal decentration was found to be associated with induced horizontal coma in the subjective centration group, which was in line with the results of Li et al.12 Horizontal coma induction is also associated with the values produced by multiplying the magnitude of horizontal decentration by optical zone or ΔMRSE in the subjective centration group but not in the triple centration group. Interestingly, vertical coma induction was associated with the magnitude of vertical decentration and the values produced by multiplying the magnitude of vertical decentration by optical zone or ΔMRSE in both groups.
By means of the triple centration technique, we expect that static cyclotorsion control in SMILE was achieved by providing a reference in both the horizontal and vertical meridians. Cyclotorsion correction with two corneal markings would theoretically be sufficient for cyclotorsion control. Furthermore, by providing the third reference point, the first Purkinje reflex or the coaxially sighted corneal light reflex that serves to identify the corneal vertex helps the surgeon achieve better centration. Therefore, we believe that better centration and horizontal cyclotorsion control together helped to achieve less induction of corneal HOAs after SMILE. Moreover, based on the results from regression analysis between the vertical decentration and vertical coma induction, more efforts to minimize vertical decentration are advisable even when applying the triple marking centration method.
Several authors reported the results of the distribution of lenticule decentration following SMILE using different instruments.12,15,16 However, the method used to determine the magnitude of decentration varies among studies and affects the magnitude of decentration reported. For instance, Li et al.12 used postoperative anterior elevation maps of the Scheimpflug camera to analyze the centers of the optical zone. They defined optical zone as the area below the sphere on the postoperative map, when the preoperative best-fit sphere was used. This approach has serious limitations. First, the postoperative elevation is no longer best fit, which uses an error minimization approach for fitting the surface. Second, the area below the reference sphere is easily altered by changing the region over which the sphere is fit, sacrificing robustness.
Lazaridis et al.16 used an objective measurement method where the optical zone centration was obtained as the maximum value of the pachymetric difference map of the Scheimpflug camera. Decentration reported by Lazaridis et al. was up to 1.13 mm from the corneal vertex. Larger decentration values in that study might be attributed to the centration analysis method. Corneal thickness difference map might not be an appropriate method to determine treatment centration because the anterior surface has been substantially altered after refractive surgery and thus the normal-to-the-surface calculation and direction in which corneal thickness is calculated are not comparable between preoperative and postoperative displays.17 We believe that analyzing curvature difference maps is a more appropriate approach to determine treatment centration because the direction of the subtraction is parallel to the reference axis.
Reinstein et al.15 used the difference map of the tangential curvature for each eye with the preoperative and 3-month postoperative Atlas topography (Carl Zeiss Meditec). In that study, the authors used a method similar to the one described in our study to measure the distance between the corneal vertex and center of the optical zone. One difference was that they used the Atlas device instead of the Keratron Scout device used in our study. The Atlas device has large mires and the Keratron Scout device has small mires. This makes the Keratron Scout device more sensitive to changes in curvature because small mires have greater spatial resolution to detect changes in curvature. Taken together, we believe that the tangential topography difference map of the Keratron Scout device is the more appropriate method to determine the magnitude of decentration over elevation, pachymetry, or large mire Placido topography.
In the current study, corneal vertex centration in myopic SMILE using the triple marking centration method yielded improved centering results, accompanied by smaller induction of RMS total HOAs, coma, and spherical aberrations. These favorable results can be attributed to the nature of the triple marking centration method. Induction of fewer corneal HOAs in the triple centration group can be attributed to cyclotorsion adjustments and better centration via the triple marking centration method when considering that cyclotorsion during laser refractive surgery could lead to a decrease in the refractive outcomes due to induction of astigmatism and HOAs.18
The current study had several limitations, including its retrospective design. A prospective, randomized, masked, paired-eye study with an appropriate sample size would allow a more thorough comparison with regard to decentered displacement and induction of HOAs between the centration methods.
We demonstrated that SMILE with the triple marking centration method can achieve more accurate centration compared with the subjective patient fixation method, resulting in less induction of RMS total HOAs, coma, and spherical aberrations. Further improvements in the centration method during the SMILE procedure will improve centration accuracy, consequently resulting in minimal induction of corneal HOAs and improvement of visual quality.
- Bueeler M, Mrochen M, Seiler T. Maximum permissible lateral decentration in aberration-sensing and wavefront-guided corneal ablation. J Cataract Refract Surg. 2003;29:257–263. doi:10.1016/S0886-3350(02)01638-3 [CrossRef]
- Mrochen M, Kaemmerer M, Mierdel P, Seiler T. Increased higher-order optical aberrations after laser refractive surgery: a problem of subclinical decentration. J Cataract Refract Surg. 2001;27:362–369. doi:10.1016/S0886-3350(00)00806-3 [CrossRef]
- Wang L, Koch DD. Residual higher-order aberrations caused by clinically measured cyclotorsional misalignment or decentration during wavefront-guided excimer laser corneal ablation. J Cataract Refract Surg. 2008;34:2057–2062. doi:10.1016/j.jcrs.2008.08.015 [CrossRef]
- Reinstein DZ, Archer TJ, Gobbe M. Is topography-guided ablation profile centered on the corneal vertex better than wavefront-guided ablation profile centered on the entrance pupil?J Refract Surg. 2012;28:139–143. doi:10.3928/1081597X-20111115-01 [CrossRef]
- Arbelaez MC, Vidal C, Arba-Mosquera S. Clinical outcomes of corneal vertex versus central pupil references with aberration-free ablation strategies and LASIK. Invest Ophthalmol Vis Sci. 2008;49:5287–5294. doi:10.1167/iovs.08-2176 [CrossRef]
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- Yang Y, Thompson K, Burns SA. Pupil location under mesopic, photopic, and pharmacologically dilated conditions. Invest Ophthalmol Vis Sci. 2002;43:2508–2512.
- Sekundo W, Kunert KS, Blum M. Small incision corneal refractive surgery using the small incision lenticule extraction (SMILE) procedure for the correction of myopia and myopic astigmatism: results of a 6 month prospective study. Br J Ophthalmol. 2011;95:335–339. doi:10.1136/bjo.2009.174284 [CrossRef]
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- Li M, Zhao J, Miao H, Shen Y, Sun L, Tian M, et al. Mild decentration measured by a Scheimpflug camera and its impact on visual quality following SMILE in the early learning curve. Invest Ophthalmol Vis Sci. 2014;55:3886–3892. doi:10.1167/iovs.13-13714 [CrossRef]
- Ganesh S, Brar S, Pawar A. Results of intraoperative manual cyclotorsion compensation for myopic astigmatism in patients undergoing small incision lenticule extraction (SMILE). J Refract Surg. 2017;33:506–512. doi:10.3928/1081597X-20170328-01 [CrossRef]
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Comparison of Preoperative Offset and Decentered Displacement Relative to the Corneal Vertex Between Eyes Undergoing SMILE by Subjective Patient Fixation Method or Triple Marking Centration Methoda
|Characteristic||Subjective Patient Fixation Method||Triple Marking Centration Method||P|
|Preoperative pupillary offset (mm)|
| Pupillary offset||0.20 ± 0.12 (0.01 to 0.46)||0.20 ± 0.09 (0.06 to 0.47)||.689|
| x axis||−0.08 ± 0.16 (−0.43 to 0.32)||−0.04 ± 0.13 (−0.29 to 0.35)||.099|
| y axis||0.10 ± 0.11 (−0.12 to 0.35)||0.12 ± 0.12 (−0.15 to 0.41)||.412|
|Decentered displacement (mm)|
| Horizontal decentered displacement||−0.05 ± 0.17 (−0.55 to 0.27)||0.01 ± 0.15 (−0.39 to 0.34)||.048|
| Vertical decentered displacement||0.28 ± 0.23 (−0.40 to 0.95)||0.16 ± 0.18 (−0.44 to 0.53)||.002|
| Total decentered displacement||0.36 ± 0.19 (0.02 to 0.95)||0.25 ± 0.14 (0.02 to 0.59)||.001|
Baseline Characteristics of Eyes Undergoing SMILE by Subjective Patient Fixation Method or Triple Marking Centration Methoda
|Characteristic||Subjective Patient Fixation Method||Triple Marking Centration Method||P|
|No. of patients||55||55|
|Age (y)||28.6 ± 6.4 (20 to 46)||27.5 ± 6.2 (20 to 46)||.358|
|Gender (% women)||47.3%||48.1%||.927|
|Refractive errors (D)|
| Spherical||−4.41 ± 1.74 (−8.75 to −1.50)||−3.79 ± 1.52 (−7.00 to −1.12)||.051|
| Cylindrical||−0.90 ± 0.66 (−2.75 to 0.00)||−1.09 ± 0.97 (−3.37 to 0.00)||.231|
| MRSE||−4.86 ± 1.80 (−9.19 to −1.56)||−4.34 ± 1.62 (−8.06 to −1.31)||.114|
|CCT (μm)||553.3 ± 26.1 (515.0 to 614.0)||552.6 ± 31.5 (496.0 to 675.0)||.895|
|Optical zone (mm)||6.7 ± 0.2 (6.2 to 7.2)||6.7 ± 0.2 (6.3 to 7.2)||.737|
|Ablation depth (μm)||−93.7 ± 24.8 (−148.0 to −40.0)||−88.2 ± 22.6 (−132.0 to −37.0)||.228|
Changes in Visual Acuity and Refractive Errors Before and After SMILE by Subjective Patient Fixation Method or Triple Marking Centration Methoda
|Characteristic||Subjective Patient Fixation Method||Triple Marking Centration Method|
|UDVA (logMAR)||1.27 ± 0.33||−0.10 ± 0.08||< .001||1.24 ± 0.36||−0.11 ± 0.07||< .001||.314||.762|
|CDVA (logMAR)||0.01 ± 0.01||−0.10 ± 0.06||< .001||0.01 ± 0.01||−0.11 ± 0.07||< .001||.459||.410|
|Refractive errors (D)|
| Spherical||−4.41 ± 1.74||0.12 ± 0.31||< .001||−3.79 ± 1.52||0.20 ± 0.29||< .001||.164||.080|
| Cylindrical||−0.90 ± 0.66||−0.32 ± 0.24||< .001||−1.09 ± 0.97||−0.33 ± 0.24||< .001||.928||.223|
| MRSE||−4.86 ± 1.80||−0.04 ± 0.33||< .001||−4.34 ± 1.62||0.04 ± 0.29||< .001||.196||.165|
Changes in Corneal HOAs of Eyes Undergoing SMILE by Subjective Patient Fixation Method or Triple Marking Centration Methoda
|Characteristic||Subjective Patient Fixation Method||Triple Marking Centration Method|
|Total HOAs||0.44 ± 0.08||0.57 ± 0.18||0.13 ± 0.17||< .001||0.48 ± 0.11||0.55 ± 0.14||0.07 ± 0.15||.002||.381||.026|
|Coma (μm)||0.25 ± 0.11||0.35 ± 0.18||0.09 ± 0.16||< .001||0.28 ± 0.11||0.31 ± 0.16||0.03 ± 0.16||.170||.179||.032|
|Horizontal coma (μm)||0.14 ± 0.16||0.15 ± 0.17||0.01 ± 0.15||.658||0.15 ± 0.15||0.18 ± 0.16||0.02 ± 0.10||.134||.361||.619|
|Vertical coma (μm)||−0.07 ± 0.16||−0.19 ± 0.23||−0.12 ± 0.20||< .001||−0.04 ± 0.21||−0.11 ± 0.23||−0.07 ± 0.18||.005||.088||.219|
|Spherical aberration (μm)||0.25 ± 0.09||0.34 ± 0.14||0.08 ± 0.13||< .001||0.28 ± 0.10||0.30 ± 0.13||0.02 ± 0.11||.107||.230||.013|