Journal of Refractive Surgery

Original Article 

Adjustment of Spherical Equivalent Correction According to Cap Thickness for Myopic Small Incision Lenticule Extraction

Hun Lee, MD; David Sung Yong Kang, MD; Dan Z. Reinstein, MD; Cynthia J. Roberts, PhD; Renato Ambrósio Jr, MD, PhD; Timothy J. Archer, PhD; Seung Ki Jean, MR; Eung Kweon Kim, MD, PhD; Kyoung Yul Seo, MD, PhD; Ikhyun Jun, MD, PhD; Tae-im Kim, MD, PhD

Abstract

PURPOSE:

To evaluate the amount of spherical equivalent correction for three different cap thicknesses (120, 130, and 140 µm) during myopic small incision lenticule extraction (SMILE) and determine the association between the amount of spherical equivalent correction and several variables in each cap thickness group.

METHODS:

In this retrospective, comparative, observational case series study, the authors compared refractive errors, keratometric values, laser setting (sphere correction, cylinder correction, spherical equivalent correction, optical zone, and cap diameter), and spherical aberration measured preoperatively and at 3 months postoperatively between three different cap thickness groups: 120 µm (n = 554), 130 µm (n = 377), and 140 µm (n = 90). Multiple linear regression analyses were used to determine the associations between the amount of spherical equivalent correction and several variables, including age, preoperative spherical equivalent, optical zone diameter, central corneal thickness, preoperative mean keratometric values, and preoperative corneal asphericity.

RESULTS:

According to cap thickness, attempted correction is adjusted to achieve the same refractive outcomes for different cap thicknesses. There were significant differences in the amount of sphere correction and spherical equivalent correction, as well as lenticule thickness, among subgroups. Changes in keratometric values, corneal asphericity, and spherical aberration were also significantly different among subgroups (all P < .001). Changes in keratometric values, corneal asphericity, and spherical aberration significantly increased as cap thickness increased. Preoperative spherical equivalent mainly influenced the amount of spherical equivalent correction in each group.

CONCLUSIONS:

Dioptric adjustment of spherical equivalent correction according to cap thickness is essential to obtain similar refractive outcomes in myopic SMILE procedures.

[J Refract Surg. 2019;35(3):153–160.]

Abstract

PURPOSE:

To evaluate the amount of spherical equivalent correction for three different cap thicknesses (120, 130, and 140 µm) during myopic small incision lenticule extraction (SMILE) and determine the association between the amount of spherical equivalent correction and several variables in each cap thickness group.

METHODS:

In this retrospective, comparative, observational case series study, the authors compared refractive errors, keratometric values, laser setting (sphere correction, cylinder correction, spherical equivalent correction, optical zone, and cap diameter), and spherical aberration measured preoperatively and at 3 months postoperatively between three different cap thickness groups: 120 µm (n = 554), 130 µm (n = 377), and 140 µm (n = 90). Multiple linear regression analyses were used to determine the associations between the amount of spherical equivalent correction and several variables, including age, preoperative spherical equivalent, optical zone diameter, central corneal thickness, preoperative mean keratometric values, and preoperative corneal asphericity.

RESULTS:

According to cap thickness, attempted correction is adjusted to achieve the same refractive outcomes for different cap thicknesses. There were significant differences in the amount of sphere correction and spherical equivalent correction, as well as lenticule thickness, among subgroups. Changes in keratometric values, corneal asphericity, and spherical aberration were also significantly different among subgroups (all P < .001). Changes in keratometric values, corneal asphericity, and spherical aberration significantly increased as cap thickness increased. Preoperative spherical equivalent mainly influenced the amount of spherical equivalent correction in each group.

CONCLUSIONS:

Dioptric adjustment of spherical equivalent correction according to cap thickness is essential to obtain similar refractive outcomes in myopic SMILE procedures.

[J Refract Surg. 2019;35(3):153–160.]

Femtosecond lasers that photodisrupt corneal tissue at different depths with high precision and minimal inflammation have become available for corneal refractive surgery. Small incision lenticule extraction (SMILE) has been developed as a technique involving extraction of the lenticule with the femtosecond laser for the correction of myopia and myopic astigmatism.1,2 This procedure extracts the refractive lenticule through a small corneal incision in the absence of a flap, and ensures preservation of the anterior corneal stromal layer.1,3 The surgeon can customize the femtosecond laser treatment parameters of SMILE, including the cap and lenticule characteristics, depending on the amount of myopic correction required.

Among the laser parameters, cap thickness can be set from 100 to 160 µm by the surgeon. Several studies have compared the use of thick and thin caps in SMILE.4,5 In particular, SMILE with a cap thickness of 140 µm showed less corneal wound-healing response, smoother refractive lenticules, and better recovery than that with a cap thickness of 120 µm.6 However, a recent prospective study yielded conflicting results (ie, that the surface regularity of lenticules decreased as cap thickness increased).7 Another study demonstrated a 10% increase in the amount of spherical equivalent correction with a cap thickness of 160 µm than with a cap thickness of 130 µm, based on a 3% increase in spherical equivalent correction for every 10-µm increase in cap thickness.5 It is also possible that thicker cap thickness results in lower precision of the laser cut, diminishing the quality of the cut and the ease of the dissection. Liu et al.6 reported contradictory findings that it was harder to dissect the cap in eyes with a cap thickness of 120 µm than in those with a cap thickness of 140 µm.6

To the best of our knowledge, there have been few studies on standardization of cap thickness for SMILE procedures and there is no available evidence in favor of a particular cap thickness for better results. Moreover, there have been no reports on adjustment of spherical equivalent correction according to cap thickness. Therefore, in the current study, we aimed to evaluate and compare the amount of spherical equivalent correction among three different cap thicknesses during SMILE.

Patients and Methods

This was a retrospective observational case series of consecutive patients undergoing SMILE between May 2016 and April 2017. The study 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. All patients provided written informed consent to allow their medical information to be included for analysis and publication.

Inclusion criteria for the current study were as follows: age 19 to 45 years; stable refraction at least 1 year prior to surgery with spherical myopia up to −6.25 diopters (D) and/or myopic astigmatism up to −4.50 D cylinder; preoperative corrected distance visual acuity (CDVA) of 20/25 or better; and normal preoperative corneal topography. Patients were excluded from the analyses if they had any optical opacities or pathology on slit-lamp examination, previous corneal surgeries, ocular trauma, intraocular surgery, severe dry eye, corneal disease, ocular infection, or collagen vascular/autoimmune diseases. One eye from each patient was included in the analysis via randomization between the two eyes, regardless of ocular dominance, refraction, or presence of aberrations.

The patients underwent a complete preoperative ophthalmic examination that included measurement of the uncorrected distance visual acuity (UDVA) (logMAR) and CDVA (logMAR), refraction (objective, subjective, and cycloplegic), slit-lamp biomicroscopy, measurement of intraocular pressure (software version 1.05, NT-530; Nidek Co., Ltd., Aichi, Japan) and central corneal thickness (software version 1.04, POKET-II; Quantel Medical, Clermont Ferrand, France), Pentacam corneal tomography (software version 1.20.r.127; Oculus Optikgeräte GmbH, Wetzlar, Germany) for the measurement of keratometric values and corneal asphericity (Q values), and measurement of spherical aberration (Keratron Scout; Optikon, Rome, Italy). Follow-up examination was performed at 3 months postoperatively for measurement of the same parameters as before surgery. All measurements were performed by a single, experienced technician (SKJ).

SMILE

All patients underwent the SMILE procedure under topical anesthesia using the 500-kHz VisuMax femtosecond laser (software version 2.4.0; Carl Zeiss Meditec, Jena, Germany). The intended depth of the superior cap was set between 120 and 140 µm and the length of the incision was set to 2 mm at 130° (right eye) and 30° (left eye) of the superotemporal cornea. The consecutive surgical procedures were performed at the same surgical site in all eyes by one surgeon (DSYK) using different cap thicknesses. A 120-µm cap thickness was used for consecutive patients between May and August 2016, a 130-µm cap thickness was used between September and December 2016, and a 140-µm cap thickness was used between January and April 2017. All procedures were carried out with a spot spacing of 4.5 µm. The optical zone diameter ranged from 6.3 to 7.2 mm and the cap diameter ranged from 7.28 to 8 mm. The target postoperative refraction was emmetropia in all eyes. To achieve the same refractive outcomes for different cap thicknesses, all patients underwent the SMILE procedure using dioptric adjustment of spherical equivalent correction according to cap thickness.

During the SMILE procedure, optical zone centration is targeted to the coaxial corneal light reflex. 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 with the vertex of the contact glass, which is centered to the laser system and the center of the lenticule to be created. After successful femtosecond laser cutting, the refractive lenticule of the intrastromal corneal tissue was extracted through the side-cut opening using forceps. All surgeries were uneventful and no complications such as suction loss, black spots, difficult dissection, or incomplete separation of lenticule occurred in any of the eyes.

Statistical Analysis

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. The Kolmogorov–Smirnov test was used to confirm data normality. The paired t test was used to evaluate the differences between preoperative and 3-month postoperative measurements. For subgroup analysis according to cap thickness, one-way analysis of variance (ANOVA) with Bonferroni correction was used to compare the three subgroups. Multiple linear regression analyses were used to determine the associations between the amount of manifest refraction spherical equivalent (MRSE) correction and the following variables in each cap thickness group: age, preoperative MRSE, optical zone diameter, central corneal thickness, preoperative mean keratometric values, and preoperative corneal asphericity. Variables that were easily obtained from routine preoperative examination data and were likely to influence the refractive results independent of the attempted MRSE were selected.8 Polar and vector terms (eg, cylinder and non-axially symmetric higher order aberrations) were not included. Variables with obvious collinearity were not modeled together. Simple regression analysis was used to determine the association between the amount of MRSE correction and preoperative MRSE according to the degree of myopia (mild, moderate, and high).

Results

A total of 1,021 eyes from 1,021 patients met the inclusion criteria for this study, including 547 (54%) eyes from men and 474 (46%) from women. Baseline characteristics and cap and lenticule parameters are summarized in Table A (available in the online version of this article). At 3 months postoperatively, there were significant improvements in mean UDVA (from 1.17 ± 0.26 to −0.02 ± 0.03 logMAR) (Figure A, available in the online version of this article). The mean efficacy index at 3 months postoperatively was 1.01 ± 0.05. A total of 1,019 (99%) eyes exhibited a postoperative UDVA of 20/20 or better. At 3 months postoperatively, the CDVA was unchanged in 796 (78.0%) eyes, 220 (21.5%) eyes had gained one line, and 5 (0.5%) eyes had gained two or more lines. No patient had lost any lines of CDVA at 3 months postoperatively. The mean safety index at 3 months postoperatively was 1.05 ± 0.09. The linear regression model of attempted versus achieved spherical equivalent had a slope and coefficient (R2) of 0.991 and 0.977, respectively (y = 0.9913x – 0.0768; R2 = 0.9772). The mean MRSE had significantly improved from −3.98 ± 1.01 to 0.04 ± 0.15 D. A total of 1,021 (100%) eyes were within ±0.50 D of the target refraction. In terms of refractive astigmatism, 100% of the eyes were within ±0.50 D of astigmatism.

Characteristics of the Study Eyes (N = 1,021)

Table A:

Characteristics of the Study Eyes (N = 1,021)

Visual outcomes after small incision lenticule extraction. (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; SEQ = spherical equivalent refraction

Figure A.

Visual outcomes after small incision lenticule extraction. (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; SEQ = spherical equivalent refraction

Table 1 shows the results for subgroup analysis according to cap thickness. There were significant differences in sphere correction, spherical equivalent correction, lenticule thickness, and residual stromal bed thickness among subgroups (all P < .001; Table 1 and Figure 1). There was a significant difference in lenticule thickness between the groups with 120- and 130-µm cap thicknesses (P = .003), and between the groups with 120- and 140-µm cap thicknesses (P < .001).

Subgroup Analysis According to Cap Thickness

Table 1:

Subgroup Analysis According to Cap Thickness

Amounts of sphere and manifest refraction spherical equivalent correction in the 120, 130, and 140 µm cap thickness groups. D = diopters; SE = spherical equivalent. Error bars represent ±2 standard deviation of the mean. *P < .05 and ***P < .001.

Figure 1.

Amounts of sphere and manifest refraction spherical equivalent correction in the 120, 130, and 140 µm cap thickness groups. D = diopters; SE = spherical equivalent. Error bars represent ±2 standard deviation of the mean. *P < .05 and ***P < .001.

There were significant differences in terms of changes in keratometric values, corneal asphericity, and spherical aberration among the subgroups (all P < .001; Table 2 and Figure 2). Changes in keratometric values increased as cap thickness increased from 120 to 140 µm. Changes in corneal asphericity and corneal spherical aberration also significantly increased as cap thickness increased.

Changes in Keratometry Values and Corneal Asphericity Among Subgroups According to Cap Thickness

Table 2:

Changes in Keratometry Values and Corneal Asphericity Among Subgroups According to Cap Thickness

Changes in keratometric values, corneal asphericity, and spherical aberration between the 120, 130, and 140 µm cap thickness groups. D = diopters. Error bars represent ±2 standard deviation of the mean. *P < .05 and ***P < .001.

Figure 2.

Changes in keratometric values, corneal asphericity, and spherical aberration between the 120, 130, and 140 µm cap thickness groups. D = diopters. Error bars represent ±2 standard deviation of the mean. *P < .05 and ***P < .001.

The multiple regression equation was expressed as follows: amount of MRSE correction (D) = −0.588 + (1.019 × preoperative MRSE) + (0.003 × age) for a 120-µm cap thickness; amount of MRSE correction (D) = −0.929 + (0.997 × preoperative MRSE) + (0.113 × corneal asphericity) for a 130-µm cap thickness; and amount of MRSE correction (D) = −0.986 + (1.015 × preoperative MRSE) for a 140-µm cap thickness (Table 3).

Results of Multiple Regression Analysis to Evaluate the Impact of Clinical Variables on Laser Setting of Spherical Equivalent Correction in Each Cap Thickness Group

Table 3:

Results of Multiple Regression Analysis to Evaluate the Impact of Clinical Variables on Laser Setting of Spherical Equivalent Correction in Each Cap Thickness Group

Discussion

In the current study, we demonstrated that the amount of MRSE correction and changes in keratometric values, corneal sphericity, and corneal spherical aberration varied significantly with variation in cap thickness. Furthermore, based on the current results, dioptric adjustment of MRSE correction according to cap thickness is essential to obtain similar refractive outcomes among three different cap thicknesses during myopic SMILE procedures.

Although there is no fixed standard regarding the ideal cap thickness for SMILE procedures, a few studies investigating visual outcomes and morphologic changes in the cornea with different cap thicknesses have yielded conflicting results.4–7,9 In particular, in one study evaluating the effects of different cap thicknesses (130, 140, 150, or 160 µm) on refractive outcomes, four different cap thicknesses were found to offer comparable efficiency, predictability, visual quality, and safety, as well as optimal refractive outcomes.5 The final postoperative MRSE was −0.10 ± 0.60 D for 130-µm cap thickness, −0.15 ± 0.27 D for 140-µm, −0.12 ± 0.23 D for 150-µm, and −0.17 ± 0.25 D for 160-µm cap thickness, without any statistically significant difference between the groups.5 In a prospective, paired-eye study comparing 120- and 140-µm cap thicknesses, both groups showed excellent safety, efficacy, and predictability after surgery.6 However, the corneal wound-healing response was less and the recovery better with a cap thickness of 140 mm, indicated by less persistence of brightly reflective particles in the interface layer on in vivo confocal microscopy, disappearance of the hyperreflectivity line at the interface layer on three-dimensional optical coherence tomography, and smoother anterior surfaces of lenticules on scanning electron microscopy.6 Guell et al.5 suggested that application of an additional 10% spherical equivalent correction is necessary to obtain the same refractive outcomes with a cap thickness of 160 µm as with a cap thickness of 130 µm. In accordance with this, dioptric adjustment of spherical equivalent correction according to cap thickness was applied for all patients during the SMILE procedure in our study. Consequently, the amount of spherical equivalent correction in our study increased significantly with increase in cap thickness.

Our study shows that similar results in terms of visual acuity and postoperative refractive outcomes can be obtained even with variation in spherical equivalent correction with variation in cap thickness. Specifically, additional correction of spherical equivalent was performed in the thicker cap thickness group when compared with the thinner cap thickness group and, consequently, changes in keratometric values significantly increased as cap thickness increased. It is obvious that the amount of flattening effects would also vary accordingly; this was demonstrated in our study by the significant difference in keratometry changes between groups with significantly different amounts of spherical equivalent correction.

Considering the importance of sufficient residual stromal bed thickness to prevent iatrogenic keratectasia, greater residual bed thickness is desirable during the SMILE procedures.10 In addition, an anterior cornea with integrated collagen fibers and compact Bowman membrane may provide greater resistance. In the current study, we hypothesized that a thicker cap would require greater flattening to achieve the same effects as those with a thinner cap. Thus, a thicker cap needs greater tissue removal to achieve the same flattening effects. Although this would, in essence, leave behind less residual stromal bed, the biomechanical effects would have to be looked into in a further study. In addition, by evaluating corneal shape changes described as corneal asphericity, we found that the anterior corneal surface shifted from a prolate shape to an oblate shape following SMILE, which is in line with previous reports.11,12 The thinner cap thickness group tended to induce fewer Q value increases than the thicker cap thickness group, which is in accordance with changes in mean keratometric values.

We found that dioptric adjustment of MRSE correction according to cap thickness is necessary to obtain similar refractive correction effects and postoperative visual acuity recovery. When investigating factors that could affect the amount of spherical equivalent correction with each cap thickness, preoperative MRSE could mainly influence the amount of spherical equivalent correction. Additionally, the patient's age was one of the variables affecting the amount of spherical equivalent correction in the 120-µm cap thickness group. Specifically, the amount of spherical equivalent correction was significantly influenced by preoperative MRSE (P < .001, β = 0.990) followed by age (P = .003, β = 0.017) in the 120-µm cap thickness group. The effect of patient age amounted to 0.003 D of overcorrection per increasing year of age, which means that amount of laser setting for MRSE decreased as the patient's age increased. The standardized partial regression coefficient (β) was calculated to determine the magnitude of the influence of each variable. Factors found to have a statistically significant influence on the amount of spherical equivalent correction in regression modeling were (in weight order) preoperative MRSE and age. Hjortdal et al.13 reported that although patient age was one of the predictors resulting in an undercorrection of 0.012 D per increasing year of age, the treatment nomogram may need only minor corrections based on the findings of a 0.25 D undercorrection and safety and efficacy were not influenced by age. In our study, for a total of 1,021 eyes, patient age was one of the variables affecting the amount of spherical equivalent correction. The multiple regression equation was expressed as follows: amount of MRSE correction (D) = 2.381+ (1.003 × preoperative MRSE) + (−0.029 × cap thickness) + (0.002 × age) + (0.060 × optical zone diameter) (R2 = 0.983, P < .001). However, age variable was controlled for subgroup analysis in our study, which was evidenced by no difference in patient age among subgroups according to one-way ANOVA with Bonferroni correction. That might indicate that the patient's age affected the amount of spherical equivalent correction in the 120-µm cap thickness group only. Furthermore, considering that the effect of age is minimal (0.003 vs 0.012 D per increasing year of age) and mean error in spherical equivalent refraction at 3 months postoperatively was only 0.04 D in our study, the effects of age seem clinically insignificant and the amount of MRSE correction does not need to be adjusted in terms of the patient's age. Further studies that investigate the relationships between age, refractive outcomes, and laser setting including cap and lenticule parameters following myopic SMILE were needed.

In the 130-µm cap thickness group, preoperative MRSE (P < .001, β = 0.993) and preoperative corneal asphericity (P = .041, β = 0.012) were explanatory variables. Preoperative MRSE was the only explanatory variable relevant to the amount of spherical equivalent correction (P < 0.001, β = 0.996) in the 140-µm cap thickness group. Accordingly, significant relationships were noted between the amount of spherical equivalent correction and preoperative MRSE in all three groups regardless of the degree of myopia (Figure B, available in the online version of this article). In one study reporting the visual and refractive outcomes of SMILE for low myopia, the target postoperative sphere was hyperopic for all patients younger than 42 years. A linear function was used for the target hyper-opia with a target sphere of +0.75 D for a 21-year-old patient, decreasing linearly to plano for a 42-year-old patient.14 Another study investigating early visual and refractive outcomes after SMILE for treating myopia and myopic astigmatism over −10.00 D demonstrated that, based on the surgeon's experience to compensate for regression, 0.25 to 0.75 D overcorrection should be added to the target correction according to preoperative manifest refraction and age.15

Relationships between the amount of manifest refraction spherical equivalent (MRSE) and preoperative MRSE in the 120, 130, and 140 μm cap thickness groups according to the degree of myopia. D = diopters

Figure B.

Relationships between the amount of manifest refraction spherical equivalent (MRSE) and preoperative MRSE in the 120, 130, and 140 μm cap thickness groups according to the degree of myopia. D = diopters

The current study had several limitations, including its retrospective design and relatively short period of follow-up. Specifically, eyes were not randomly assigned to a different cap thickness group due to the retrospective nature of the study. Given that cap thickness was chosen from 100 to 160 µm by surgeons and there was no standardization for choosing the proper cap thickness, we analyzed clinical data before and after the SMILE procedure consecutively performed by one surgeon with different cap thicknesses. Obviously, it is ideal to compare the refractive outcomes after applying same nomogram among three different cap thicknesses, without any additional correction of spherical equivalent in the thicker cap thickness group. However, based on the surgeon's experience, achieved refractive correction during the myopic SMILE procedure was reduced when using a thicker cap thickness without dioptric adjustment of spherical equivalent correction, instead of thinner cap thickness, which is the main reason we applied the dioptric adjustment of spherical equivalent correction according to cap thickness. When considering our goal in the current study, it is important to note that the amount of spherical equivalent correction can vary with cap thickness. More advanced research using a prospective randomized controlled design evaluating any differences in corneal biomechanics and corneal healing process between various cap thicknesses is needed to validate our results.

We demonstrated that different flattening effects accompanying variation in the amount of spherical equivalent correction with regard to cap thickness could introduce the same refractive outcomes between the three different cap thickness groups during myopic SMILE. Thus, dioptric adjustment of spherical equivalent correction according to cap thickness is of utmost importance when considering different cap thickness in myopic SMILE procedures.

References

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  4. El-Massry AA, Goweida MB, Shama Ael-S, Elkhawaga MH, Abdalla MF. Contralateral eye comparison between femtosecond small incision intrastromal lenticule extraction at depths of 100 and 160 µm. Cornea. 2015;34:1272–1275. doi:10.1097/ICO.0000000000000571 [CrossRef]
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  7. Weng S, Liu M, Yang X, et al. evaluation of human corneal lenticule quality after SMILE with different cap thicknesses using scanning electron microscopy. Cornea. 2018;37:59–65. doi:10.1097/ICO.0000000000001404 [CrossRef]
  8. Liyanage SE, Allan BD. Multiple regression analysis in myopic wavefront laser in situ keratomileusis nomogram development. J Cataract Refract Surg. 2012;38:1232–1239. doi:10.1016/j.jcrs.2012.02.043 [CrossRef]
  9. He M, Wang W, Ding H, Zhong X. comparison of two cap thickness in small incision lenticule extraction: 100 µm versus 160 µm. PLoS One. 2016;11:e0163259. doi:10.1371/journal.pone.0163259 [CrossRef]
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  11. Zhang H, Wang Y, Li H. Corneal spherical aberration and corneal asphericity after small incision lenticule extraction and femtosecond laser-assisted LASIK. J Ophthalmol. 2017;2017:4921090.
  12. Gyldenkerne A, Ivarsen A, Hjortdal JO. Comparison of corneal shape changes and aberrations induced By FS-LASIK and SMILE for myopia. J Refract Surg. 2015;31:223–229.
  13. Hjortdal JO, Vestergaard AH, Ivarsen A, Ragunathan S, Asp S. Predictors for the outcome of small-incision lenticule extraction for myopia. J Refract Surg. 2012;28:865–871. doi:10.3928/1081597X-20121115-01 [CrossRef]
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  15. Qin B, Li M, Chen X, Sekundo W, Zhou X. Early visual outcomes and optical quality after femtosecond laser small-incision lenticule extraction for myopia and myopic astigmatism correction of over -10 dioptres. Acta Ophthalmol. 2018;96:e341–e346. doi:10.1111/aos.13609 [CrossRef]

Subgroup Analysis According to Cap Thickness

Characteristic120 µm (n = 554)130 µm (n = 377)140 µm (n = 90)P
Age (y)28.7 ± 5.828.0 ± 5.627.2 ± 6.2.036
Preoperative sphere (D)−3.50 ± 0.97−3.50 ± 0.97−3.68 ± 1.30.257
Preoperative cylinder (D)−0.94 ± 0.72−0.91 ± 0.73−0.93 ± 0.82.821
Preoperative MRSE (D)−3.97 ± 0.98−3.95 ± 0.96−4.15 ± 1.36.251
Preoperative mean K values42.79 ± 1.2242.83 ± 1.2642.82 ± 1.40.870
Preoperative corneal asphericity−0.25 ± 0.11−0.26 ± 0.10−0.25 ± 0.13.399
Preoperative Pentacam CCT557.6 ± 23.0559.3 ± 23.7563.6 ± 25.3.063
Laser setting
Sphere correction (D)−4.08 ± 1.00−4.45 ± 0.97−4.73 ± 1.33< .001
Cylinder correction (D)−0.93 ± 0.71−0.91 ± 0.73−0.93 ± 0.82.890
Spherical equivalent correction (D)−4.54 ± 1.01−4.90 ± 0.96−5.19 ± 1.39< .001
Optical zone (mm)6.76 ± 0.166.77 ± 0.196.74 ± 0.18.361
Cap diameter (mm)7.76 ± 0.157.75 ± 0.177.72 ± 0.15.120
Lenticule thickness (µm)101.0 ± 16.5104.5 ± 14.6108.7 ± 19.8< .001
Residual stromal bed thickness (µm)331.7 ± 27.3321.4 ± 26.7314.5 ± 25.6< .001
3-month logMAR UDVA−0.02 ± 0.03−0.02 ± 0.03−0.02 ± 0.04.283
3-month logMAR CDVA−0.03 ± 0.04−0.03 ± 0.06−0.03 ± 0.06.708
3-month sphere (D)0.10 ± 0.140.11 ± 0.180.10 ± 0.16.478
3-month cylinder (D)−0.13 ± 0.14−0.13 ± 0.14−0.12 ± 0.14.776
3-month MRSE (D)0.04 ± 0.140.05 ± 0.170.04 ± 0.16.620

Changes in Keratometry Values and Corneal Asphericity Among Subgroups According to Cap Thickness

Characteristic120 µm (n = 554)130 µm (n = 377)140 µm (n = 90)P
Keratometry
Preoperative42.79 ± 1.2242.83 ± 1.2642.82 ± 1.40.870
Postoperative38.84 ± 1.4838.73 ± 1.4938.46 ± 1.67.074
Change−3.95 ± 0.89−4.11 ± 0.86−4.37 ± 1.14< .001
Corneal asphericity
Preoperative−0.25 ± 0.11−0.26 ± 0.10−0.25 ± 0.13.399
Postoperative0.19 ± 0.270.28 ± 0.300.38 ± 0.36< .001
Change0.45 ± 0.250.54 ± 0.290.63 ± 0.31< .001
Corneal spherical aberration
Preoperative0.29 ± 0.100.29 ± 0.100.29 ± 0.11.602
Postoperative0.27 ± 0.110.31 ± 0.110.35 ± 0.13< .001
Change−0.02 ± 0.110.02 ± 0.110.05 ± 0.11< .001

Results of Multiple Regression Analysis to Evaluate the Impact of Clinical Variables on Laser Setting of Spherical Equivalent Correction in Each Cap Thickness Group

VariablePartial Regression Coefficient (B)Standardized Partial Regression Coefficient (β)PR2
120-µm cap thickness (n = 560)< .001.982
  Preoperative MRSE (D)1.0190.990< .001
  Age0.0030.017.003
  Constant−0.588
130-µm cap thickness (n = 378)< .001.987
  Preoperative MRSE (D)0.9970.993< .001
  Preoperative corneal asphericity0.1130.012.041
  Constant−0.929
140-µm cap thickness (n = 94)< .001.993
  Preoperative MRSE (D)1.0150.996< .001
  Constant−0.986

Characteristics of the Study Eyes (N = 1,021)

CharacteristicsMean ± SDRange
Age (y)28.3 ± 5.819 to 45
Preoperative sphere (D)−3.51 ± 1.00−6.12 to −1.00
Preoperative cylinder (D)−0.93 ± 0.73−4.50 to 0.00
Preoperative MRSE refraction (D)−3.98 ± 1.01−7.13 to −1.50
Preoperative mean K values42.81 ± 1.2538.39 to 46.63
Preoperative corneal asphericity−0.26 ± 0.11−0.71 to 0.17
Preoperative Pentacam CCT558.7 ± 23.5519.0 to 651.0
Laser setting
Sphere correction (D)−4.27 ± 1.05−7.25 to −1.35
Cylinder correction (D)−0.92 ± 0.73−4.50 to 0.00
Spherical equivalent correction (D)−4.73 ± 1.05−8.15 to −2.35
Optical zone (mm)6.76 ± 0.176.30 to 7.20
Cap diameter (mm)7.75 ± 0.167.28 to 8.00
Cap thickness (µm)125.5 ± 6.5120.0 to 140.0
Lenticule thickness (µm)103.0 ± 16.359.00 to 154.00
3-month sphere (D)0.11 ± 0.16−0.25 to 0.75
3-month cylinder (D)−0.13 ± 0.14−0.50 to 0.00
3-month MRSE (D)0.04 ± 0.15−0.25 to 0.56
Authors

From the Department of Ophthalmology, International St. Mary's Hospital, Catholic Kwandong University College of Medicine, Incheon, South Korea (HL); The Institute of Vision Research, Department of Ophthalmology, Yonsei University College of Medicine, Seoul, South Korea (HL, SKJ, EKK, KYS, IJ, TK); Eyereum Eye Clinic, Seoul, South Korea (DSYK, SKJ); London Vision Clinic, London, United Kingdom (DZR, TJA); the Department of Ophthalmology, Columbia University Medical Center, New York (DZR); Sorbonne Université, Paris, France (DZR); Biomedical Science Research Institute, Ulster University, Coleraine, Northern Ireland (DZR, TJA); the Department of Ophthalmology & Visual Science and Department of Biomedical Engineering, The Ohio State University, Columbus, Ohio (CJR); Rio de Janeiro Corneal Tomography and Biomechanics Study Group, Rio de Janeiro, Brazil (RA); and Federal University of the State of Rio de Janeiro, Rio de Janeiro, Brazil (RA).

Supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2016R1A2B4009626) and the research fund of Catholic Kwandong University International St. Mary's Hospital (CKU-201706040001).

Dr. Kang is a consultant for Avedro Inc., SCHWIND eye-tech-solutions, and Carl Zeiss Meditec AG. Dr. Reinstein is a consultant for Carl Zeiss Meditec AG and has a proprietary interest in the Artemis technology through patents administered by the Center for Technology Licensing at Cornell University, Ithaca, New York. Drs. Ambrósio and Roberts are consultants for Oculus Optikgeräte GmbH. The remaining authors have no proprietary or financial interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (HL, DSYK, DZR, CJR, RA, EKK, KYS, TK); data collection (HL, DSYK, SKJ); analysis and interpretation of data (HL, DSYK, DZR, CJR, RA, TJA, EKK, KYS, IJ, TK); writing the manuscript (HL, DSYK, TK); critical revision of the manuscript (HL, DSYK, DZR, CJR, RA, TJA, SKJ, EKK, KYS, IJ, TK); statistical expertise (HL, CJR, TK); administrative, technical, or material support (SKJ, TK); supervision (EKK, TK)

Correspondence: Tae-im Kim, MD, PhD, Department of Ophthalmology, Yonsei University College of Medicine, 50 Yonseiro, Seodaemungu, Seoul 120-752, South Korea. E-mail: tikim@yuhs.ac

Received: September 06, 2018
Accepted: February 05, 2019

10.3928/1081597X-20190205-01

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