Journal of Refractive Surgery

Original Article Supplemental Data

SMILE With Low Energy Levels: Assessment of Early Visual and Optical Quality Recovery

David Donate, MD; Rozenn Thaëron, MSc

Abstract

PURPOSE:

To assess early visual and optical quality recovery according to preoperative refraction after small incision lenticule extraction (SMILE) at low energy level for myopia and myopic astigmatism treatment.

METHODS:

This prospective study included 101 eyes of 101 patients separated into three groups: low myopia (30 eyes), with a mean spherical equivalent (SE) of −2.13 ± 0.73 diopters (D); moderate myopia (33 eyes), with a mean SE of −4.53 ± 0.86 D; and high myopia (38 eyes), with a mean SE of −6.54 ± 0.78 D. Visual acuity, contrast sensitivity, refraction, corneal higher order aberrations, modulation transfer function, Strehl ratio, and Objective Scatter Index (OSI) were measured.

RESULTS:

All eyes had a corrected distance visual acuity of 20/20 or better at 1 day in the low myopia group and 1 month in the moderate and high myopia groups. The contrast sensitivity preoperative values recovered at 8 days in the low myopia group and 1 month in the moderate and high myopia groups. At 1 month, the low myopia group recovered the OSI preoperative values (0.61 to 0.69) and the OSI values were slightly higher than the preoperative value in the moderate and high myopia groups (0.74 to 1.00 and 0.8 to 1.14, respectively).

CONCLUSIONS:

The recovery time of visual and optical quality in the first month after SMILE correlated with preoperative refraction and the amount of postoperative scattering. Patients with low myopia recovered visual acuity faster and with less ocular scattering than patients with moderate or high myopia.

[J Refract Surg. 2019;35(5):285–293.]

Abstract

PURPOSE:

To assess early visual and optical quality recovery according to preoperative refraction after small incision lenticule extraction (SMILE) at low energy level for myopia and myopic astigmatism treatment.

METHODS:

This prospective study included 101 eyes of 101 patients separated into three groups: low myopia (30 eyes), with a mean spherical equivalent (SE) of −2.13 ± 0.73 diopters (D); moderate myopia (33 eyes), with a mean SE of −4.53 ± 0.86 D; and high myopia (38 eyes), with a mean SE of −6.54 ± 0.78 D. Visual acuity, contrast sensitivity, refraction, corneal higher order aberrations, modulation transfer function, Strehl ratio, and Objective Scatter Index (OSI) were measured.

RESULTS:

All eyes had a corrected distance visual acuity of 20/20 or better at 1 day in the low myopia group and 1 month in the moderate and high myopia groups. The contrast sensitivity preoperative values recovered at 8 days in the low myopia group and 1 month in the moderate and high myopia groups. At 1 month, the low myopia group recovered the OSI preoperative values (0.61 to 0.69) and the OSI values were slightly higher than the preoperative value in the moderate and high myopia groups (0.74 to 1.00 and 0.8 to 1.14, respectively).

CONCLUSIONS:

The recovery time of visual and optical quality in the first month after SMILE correlated with preoperative refraction and the amount of postoperative scattering. Patients with low myopia recovered visual acuity faster and with less ocular scattering than patients with moderate or high myopia.

[J Refract Surg. 2019;35(5):285–293.]

The latest refractive surgical technique, small incision lenticule extraction (SMILE) is a flapless surgical approach for the correction of refractive errors.1 Since 2008, several studies have reported high efficacy, predictability, and safety outcomes after SMILE. However, studies have reported slow recovery of visual quality after SMILE in the early postoperative period.2,3 Some cases have reported a loss of corrected distance visual acuity (CDVA).4

This all-in-one procedure is achieved by using a VisuMax femtosecond laser platform (Carl Zeiss Meditec AG, Jena, Germany) and is based on material photodisruption. When a femtosecond laser is focused and the energy is high enough, it produces a photodisruption of the material. At the focal point, the material is transformed into a hot gas called plasma, taking the shape of a cavitation bubble.5 The plasma threshold is the minimum energy necessary to induce plasma. With the VisuMax laser, the plasma threshold is determined during calibration by minimum energy, inducing a cut on a glass sample. This energy ranges from 95 to 105 nJ for every device (data from the manufacturer). The use of laser energy near the plasma threshold increases the regularity of the cut compared to the use of higher energies.5

In 2016, we observed that an energy level close to the plasma threshold during SMILE provides swift recovery at 1 and 3 months postoperatively.4 Moreover, other recent studies have confirmed our results and concluded that lowering laser energy levels can improve the surface quality of the lenticule.6,7 Our experience shows overall recovery time improvements, although patient recovery continues to vary during the first month.

The difference between visual quality concept and optical quality concept has not been described clearly in the literature. In this study, we have distinguished the optical quality and the visual quality as follows. The optical quality refers only to the evaluation of the optical beam from the cornea to the retina. The objective refraction (low order aberration), higher order aberration (HOA) measurements, scattering, and more global point spread function analysis are objective measurements of this optical quality. The visual quality refers to a global analysis of the vision perceived by the patient with a neuronal interpretation. It is mainly evaluated by subjective tests (visual acuity and contrast sensitivity). It could be affected by neuronal or cerebral issues and of course by the degradation of the optical beam from the cornea to the retina.

To our knowledge, this is the first article to describe visual and optical quality depending on preoperative spherical equivalent (SE) after low energy SMILE during the first month postoperatively. Therefore, we studied visual quality with visual acuity and contrast sensitivity and optical quality with modulation transfer function (MTF) cut-off, Objective Scatter Index (OSI), Strehl ratio, and HOAs.

Patients and Methods

This prospective study included 101 eyes of 101 patients who underwent SMILE for myopia and myopic astigmatism correction from September to December 2017. One eye of each patient was chosen at random for inclusion in the statistical analysis. Three groups were formed: low myopia, with an SE from −0.00 to −3.00 diopters (D); moderate myopia, with an SE from −3.125 to −6.00 D; and high myopia, with an SE from −6.125 to −10.00 D. According to the tenets of the Declaration of Helsinki, all patients were informed of the study, with approval from the local ethics committee, and signed a written informed consent form.

All patients underwent a baseline preoperative assessment including anterior and posterior segment examinations. Inclusion criteria were: age 18 to 45 years, stable refractive error with less than a 0.50 D change in sphere and cylinder in the previous year, CDVA of 20/20 or better, preoperative manifest spherical equivalent refraction from −0.50 to −9.00 D, and preoperative manifest refractive cylinder between 0.00 and −2.00 D. Exclusion criteria were: severe ocular surface disease, corneal disease of any kind, suspicion of keratoconus on corneal topography, cataract, glaucoma, macular disease, pregnancy, breastfeeding, or previous history of intraocular or corneal surgery.

Preoperative and Postoperative Examinations

All patients were evaluated preoperatively and at 1 day, 8 days, and 1 month after surgery. Three specially trained optometrists performed all tests included in this study. The examinations included autorefractometry, intraocular pressure measurement, keratometry (Tonoref II; Nidek Co. Ltd., Gamagori, Japan), monocular and binocular uncorrected distance visual acuity (UDVA) and CDVA, manifest refraction (CS-1600 Pola; Nidek Co. Ltd.), corneal pachymetry, anterior segment analysis by optical coherence tomography (Optovue Inc., Fremont, CA), topography, aberrometry, and mesopic pupillometry (OPD-Scan II; Nidek Co. Ltd.), objective evaluation of quality of vision (HD Analyzer; Visiometrics, Barcelona, Spain) and contrast sensitivity (CSV-1000E contrast sensitivity chart test face; Vector Vision, Greenville, OH), slit-lamp examination, and funduscopy. All procedures were performed uneventfully and no intraoperative or postoperative complications were observed.

Visual Quality Measurement

Measuring visual quality via visual acuity and contrast sensitivity provides an analysis of the image perceived by the patient. This global analysis of visual quality is therefore dependent on cerebral interpretation. Visual acuity is defined as the ability to discern objects of different sizes with full contrast. The visual acuity test is not as sensitive as the contrast sensitivity test. Contrast sensitivity is a measure of the ability to discern objects of differing sizes and contrasts at a spatial frequency of 3, 6, 12, and 18 cycles per degree (cpd) monocularly at a distance of 2.5 m in the patient's CDVA conditions. All visual acuity and contrast sensitivity measurements were taken in 85 lux controlled ambient lighting.5,8

Optical Image Quality Evaluation

The evaluation of optical quality is the analysis of the retinal image quality before cerebral interpretation. Optical quality can be affected by different optical phenomena (diffraction, aberration, scattering, absorption, and reflection).8,9 Using the HD Analyzer, we measured the optical quality parameters of the eye (eg, MTF cut-off frequency, Strehl ratio, and OSI). All measurements were conducted in mesopic conditions with a 4-mm artificial pupil. The MTF cut-off represents the highest spatial frequency at the lowest contrast (1%), which corresponds to an MTF threshold of 0.01. The MTF cut-off value can be affected by different optical phenomena, which could degrade the focusing quality of the wavefront. An MTF cut-off value above 30 cpd is usually recorded in normal eyes with good optical quality.10,11 The Strehl ratio is an expression of the ratio of the central maximum of the illuminance of the point spread function in the eye to the central maximum that would be found in a corresponding perfect optical system.12 Strehl ratio values near 0.2 have been correlated with good optical quality.13,14 In general, the higher the MTF cut-off value and Strehl ratio, the better the ocular optical quality. The OSI is an index reflecting the intraocular scattered state of light. The index is calculated by evaluating the amount of light outside the double-pass retinal intensity point spread function image in relation to the amount of light on the center. OSI values around 1 are usually recorded in normal eyes with low scattering, values from 1 to 7 in eyes with moderate diffused light, and values above 7 in eyes with high scattering.11,15–17 Wavefront analyses were performed using the OPD-Scan II with Placido-based topography and principle of scanning retinoscopy/funduscopy, which calculates corneal aberrations. We analyzed corneal HOAs such as trefoil, coma, and spherical aberration in the current study. Root mean square values of the total HOAs were calculated, with analysis up to the 8th order by expanding the set of Zernike polynomials using a 5-mm analysis diameter.

Surgical Technique

The postoperative target refraction was emmetropia in all eyes with our own nomogram. The surgeries were performed by the same surgeon using the VisuMax laser platform. In our study, the surgical parameters used during SMILE were as follows: operating room temperature of 19°C to 21°C, hygrometry rate of 45% to 55%, repetition rate of 500 kHz, laser energy settings index of 20 (equivalent to approximately 100 nJ), spot spacing of 4 µm, cap thickness of 135 µm, cap diameter of 7.8 mm, lenticule diameter of 7 mm, spot distance of 1.50 µm, and side cut of 1.5 mm with an angle of 90°. Details of the SMILE procedure have been described previously.5,18 Postoperatively, two drops of antibiotic and steroid drops (Tobradex; Alcon Laboratories, Inc., Fort Worth, TX) were applied three times a day for 1 week, and if necessary artificial tears three to six times a day for up to 1 month.

Statistical Analysis

All statistical analyses were performed in Excel 2016 software (Microsoft Corporation, Redmond, WA). The Shapiro–Wilk normality test was used to confirm normality of the data. The Friedmann and Wilcoxon signed-rank tests with post-hoc analysis were used for comparison of clinical outcomes in each group. We used the Kruskal–Wallis test to statistically compare data between the three groups. Repeated measures analysis of variance with Bonferroni correction was used to assess the means of clinical outcomes at different time points between each group. Kendall's correlation test was used to analyze the correlation between the OSI with visual acuity and contrast sensitivity, preoperative SE with the OSI, and MTF cut-off. Its correlation coefficient (r) was −1 ≤ r ≤ 1. Multiple linear regression models were conducted to evaluate the factors associated with optical quality parameters at 1 day, 8 days, and 1 month postoperatively. In all statistical tests, a P value of less than .05 was considered statistically significant.

Results

Demographic Data

A total of 101 eyes of 101 patients were separated into three groups (low myopia = 30 eyes, moderate myopia = 33 eyes, and high myopia = 38 eyes) and underwent SMILE. Apart from preoperative refractive values, there were no significant differences between the three groups (Table 1).

Preoperative Demographics of Eyes Undergoing SMILE

Table 1:

Preoperative Demographics of Eyes Undergoing SMILE

Visual Quality Measurement

Efficacy and Safety. Visual quality and refractive outcomes at 1 day, 8 days, and 1 month after SMILE are summarized in Table 2 and Figures 12. During the first postoperative month, we observed progressive UDVA and CDVA improvements in all three groups. At 1 day, the low myopia group had significantly better UDVA than the two other groups. At 1 month, the low and moderate myopia groups had significantly better UDVA than the high myopia group. Mean logMAR CDVA regained preoperative values at 1 day in the low myopia group, 8 days in the moderate myopia group, and 1 month in the high myopia group. All surgical procedures were uneventful.

Visual and Refractive Outcomes of SMILE

Table 2:

Visual and Refractive Outcomes of SMILE

Efficacy. Cumulative percentage of eyes attaining specified cumulative levels of uncorrected distance visual acuity 1 day, 8 days, and 1 month after small incision lenticule extraction with low energy levels for (A) low myopia (L-Group), (B) moderate myopia (M-Group), and (C) high myopia (H-Group).

Figure 1.

Efficacy. Cumulative percentage of eyes attaining specified cumulative levels of uncorrected distance visual acuity 1 day, 8 days, and 1 month after small incision lenticule extraction with low energy levels for (A) low myopia (L-Group), (B) moderate myopia (M-Group), and (C) high myopia (H-Group).

Safety. Outcomes at 1 day, 8 days, and 1 month postoperatively in the (A) low myopia (L-Group), (B) moderate myopia (M-Group), and (C) high myopia (H-Group) groups showing the distribution of change in postoperative corrected distance visual acuity (CDVA) in each group after small incision lenticule extraction with low energy levels.

Figure 2.

Safety. Outcomes at 1 day, 8 days, and 1 month postoperatively in the (A) low myopia (L-Group), (B) moderate myopia (M-Group), and (C) high myopia (H-Group) groups showing the distribution of change in postoperative corrected distance visual acuity (CDVA) in each group after small incision lenticule extraction with low energy levels.

Contrast Sensitivity. During the first postoperative month, we observed progressive contrast sensitivity improvements in the three groups, with the low myopia group showing the fastest improvements. The low myopia group regained preoperative values at 8 days as opposed to the moderate and high myopia groups at 1 month. At 1 month, significantly better contrast sensitivity was measured in the low and high myopia groups compared to the preoperative values (Figure 3).

Change in photopic contrast sensitivity preoperatively and after 1 day, 8 days, and 1 month in the (A) low myopia (L-Group), (B) moderate myopia (M-Group), and (C) high myopia (H-Group) groups after small incision lenticule extraction with low energy levels. * = significant difference (P < .05) compared with preoperative value.

Figure 3.

Change in photopic contrast sensitivity preoperatively and after 1 day, 8 days, and 1 month in the (A) low myopia (L-Group), (B) moderate myopia (M-Group), and (C) high myopia (H-Group) groups after small incision lenticule extraction with low energy levels. * = significant difference (P < .05) compared with preoperative value.

Optical Image Quality Evaluation

Refractive Outcomes. The mean manifest SE refraction values showed a slight hyperopic increase in the low myopia group (P = .036) and no changes were found in the moderate (P = .60) or high (P = .88) myopia groups at 1 day, 8 days, or 1 month postoperatively (Table 2, Figure 4). Figure A (available in the online version of this article) shows scatterplot and linear regression analyses of the attempted SE refractive change plotted against the achieved SE refractive change 1 month after surgery.

Predictability. Spherical equivalent refractive accuracy 1 day, 8 days, and 1 month after small incision lenticule extraction with low energy levels in (A) low (L-Group), (B) moderate (M-Group), and (C) high (H-Group) myopia groups.

Figure 4.

Predictability. Spherical equivalent refractive accuracy 1 day, 8 days, and 1 month after small incision lenticule extraction with low energy levels in (A) low (L-Group), (B) moderate (M-Group), and (C) high (H-Group) myopia groups.

Predictability. Linear regression analysis of the attempted spherical equivalent refractive change plotted against spherical equivalent refractive change after 1 month in the (A) low (L-Group), (B) moderate (M-Group), and (C) high (H-Group) myopia groups after small incision lenticule extraction with low energy levels. D = diopters

Figure A.

Predictability. Linear regression analysis of the attempted spherical equivalent refractive change plotted against spherical equivalent refractive change after 1 month in the (A) low (L-Group), (B) moderate (M-Group), and (C) high (H-Group) myopia groups after small incision lenticule extraction with low energy levels. D = diopters

Wavefront Aberration. Postoperatively, little change was observed in corneal total HOAs and coma in the three groups and they remained stable during the first month. The low myopia group showed a slight decrease in spherical aberration, whereas the high myopia group showed a slight increase in trefoil (Figure B and Table A, available in the online version of this article).

Time course of corneal higher order aberrations (HOAs) at each time point for the low, moderate, and high myopia groups. RMS = root mean square; SA = spherical aberration; * = significant difference (P < .05) compared with preoperative value; b = significant difference (P < .05) between the low and high myopia groups; c = significant difference (P < .05) between the moderate and high myopia groups

Figure B.

Time course of corneal higher order aberrations (HOAs) at each time point for the low, moderate, and high myopia groups. RMS = root mean square; SA = spherical aberration; * = significant difference (P < .05) compared with preoperative value; b = significant difference (P < .05) between the low and high myopia groups; c = significant difference (P < .05) between the moderate and high myopia groups

Corneal HOAs After SMILE

Table A:

Corneal HOAs After SMILE

Analysis of HD Analyzer

Table 3 and Figure C (available in the online version of this article) show the objective evaluation of the visual quality after SMILE. An OSI increase was observed in all three groups at 1 day postoperatively. The higher the myopia, the more the OSI value increased. OSI values gradually decreased from 1 day to 1 month postoperatively. The decrease was faster in the low myopia group than in the moderate and high myopia groups. The low myopia group returned to preoperative OSI values, whereas the moderate and high myopia groups reached values near 1. The MTF cut-off values and the Strehl ratio were unchanged in the low myopia group, but decreased at 1 day and progressively rose to near preoperative values at 1 month in the moderate and high myopia groups.

Optical Quality Parameters of HD Analyzer After SMILE

Table 3:

Optical Quality Parameters of HD Analyzer After SMILE

Change in Objective Scatter Index (OSI) preoperatively and after 1 day, 8 days and 1 month in the low (L-Group), moderate (M-Group), and high (H-Group) myopia groups. * = significant difference (P < .05) compared with preoperative value; a = significant difference (P < .05) between the low and moderate myopia groups; b = significant difference (P < .05) between the low and high myopia groups; c = significant difference (P < .05) between the moderate and high myopia groups

Figure C.

Change in Objective Scatter Index (OSI) preoperatively and after 1 day, 8 days and 1 month in the low (L-Group), moderate (M-Group), and high (H-Group) myopia groups. * = significant difference (P < .05) compared with preoperative value; a = significant difference (P < .05) between the low and moderate myopia groups; b = significant difference (P < .05) between the low and high myopia groups; c = significant difference (P < .05) between the moderate and high myopia groups

Correlation Between OSI and Visual Quality

Table B (available in the online version of this article) shows a direct correlation between the OSI values, UDVA, and CDVA in all groups. There was a significant correlation of the OSI and contrast sensitivity at 18 cpd in all three groups. The higher the OSI values, the lower the contrast sensitivity at 18 cpd.

Correlation Between Postoperative Values of OSI and Visual Quality Parameters

Table B:

Correlation Between Postoperative Values of OSI and Visual Quality Parameters

Correlation Between Preoperative SE and Optical Quality

Table C (available in the online version of this article) shows the correlation between preoperative SE, postoperative OSI values, and MTF cut-off values. The postoperative MTF cut-off value at 1 day (r = 0.177), 8 days (r = 0.28), and 1 month (r = 0.207) correlated with preoperative SE (P < .01). The higher the preoperative SE, the lower the postoperative MTF cut-off value. The postoperative OSI values at 8 days (r = −0.26) and 1 month (r = −0.22) correlated with preoperative SE (P < .01). The higher the preoperative SE, the higher the postoperative OSI value.

Correlation Between OSI or MTF Cut-off Postoperative Values and Preoperative SE

Table C:

Correlation Between OSI or MTF Cut-off Postoperative Values and Preoperative SE

As shown in Table D (available in the online version of this article), multiple regression models were conducted with the MTF cut-off or OSI at 1 day, 8 days, and 1 month postoperatively. The higher the preoperative SE, the further the MTF cut-off value decreased and the OSI value increased postoperatively. Neither age nor preoperative optical quality parameters showed a significant association with postoperative MTF cut-off or OSI values in the regression models.

Preoperative Factors Associated With Postoperative Optical Quality or Intraocular Scattering in Regression Analysis

Table D:

Preoperative Factors Associated With Postoperative Optical Quality or Intraocular Scattering in Regression Analysis

Safety and Optical Image Quality Evaluation

At 1 month, no eyes lost one line of CDVA in the low myopia group. In the moderate myopia group, 12.12% (4 eyes) lost one line of CDVA and had an MTF cutoff value of 23.34 ± 4.95 cpd, an OSI value of 1.55 ± 0.66, and corneal HOAs of 0.30 ± 0.04. In eyes without a CDVA change in the moderate myopia group, there was an MTF cut-off value of 35.57 ± 9.43 cpd, an OSI value of 0.93 ± 0.37, and corneal HOAs of 0.27 ± 0.08. The eyes that gained one line of CDVA had an MTF cut-off value of 41.01 ± 7.33 cpd, an OSI value of 0.88 ± 0.70, and corneal HOAs of 0.26 ± 0.07. No difference in each optical quality value between each subgroup (loss, gain, or unchanged lines of CDVA) was found. In the high myopia group, 8.33% (3 eyes) lost one line of CDVA and had a mean MTF cut-off value of 24.90 ± 7.98 cpd, a mean OSI value of 1.50 ± 0.57, and corneal HOAs of 0.37 ± 0.05. In eyes without a CDVA change in lines in the high myopia group, there was an MTF cut-off value of 31.52 ± 8.09 cpd, an OSI value of 1.09 ± 0.69, and corneal HOAs of 0.37 ± 0.09. The eyes that gained one line of CDVA had an MTF cut-off value of 31.35 ± 9.35 cpd, an OSI value of 1.1 ± 0.61, and corneal HOAs of 0.31 ± 0.10. No difference was found in each optical quality value between each subgroup (loss, gain, or unchanged lines of CDVA).

Discussion

This study shows a difference in visual and optical quality recovery based on the level of preoperative myopia. Recovery is faster for low myopia. The amount of light scattering, correlated with the preoperative spherical equivalent, is a factor limiting the rate of recovery. Nevertheless, the three groups regained visual and optical quality identical or close to the preoperative values 1 month after SMILE.

Visual quality analysis (visual acuity and contrast sensitivity) recovered according to the preoperative SE. However, contrast sensitivity recovered more slowly than visual acuity. This observation was expected because contrast sensitivity is a more accurate test of visual quality compared to visual acuity.19

Visual quality depends on neuronal interpretation. The following results of this study allow for an analysis of retinal image optical quality disregarding analysis and transfer of the neuronal image.8 The MTF cutoff value allows for an objective evaluation of optical quality. The analysis of MTF cut-off values shows little change in the optical quality in the low myopia group. In the moderate and high myopia groups, MTF cut-off values decreased statistically from the first postoperative day, reaching preoperative values after 1 month, indicating the transient decline in optical quality in these two groups. The correlation test between the MTF cut-off values and the SE confirms that recovery of postoperative optical quality is related to preoperative refraction. Recovery is faster for low myopia.

Optical quality recovery could be affected by different optical phenomena: diffraction, aberration, scattering, absorption, and reflection.8,9 In this study, there was not significant diffraction because the pupil diameter was greater than 3 mm in each eye. Achieved postoperative refraction was attempted and stable at each check point, so low order aberrations did not affect the optical recovery. Little change in HOAs was observed, but this was not clinically significant. This study showed the amount of light scattering (measured by the OSI) correlated with the preoperative SE. The higher the preoperative SE, the higher the postoperative OSI value. The OSI is also correlated with the visual acuity and contrast sensitivity. The higher the OSI value, the lower the visual quality. No corneal opacities were observed postoperatively, which could have been a reason for absorption and reflection modification. Further studies could be useful to assess the unlikely role of absorption and reflection on the optical quality recovery. This study showed a strong correlation between optical scattering and the rate of recovery, suggesting that scattering is a factor limiting the recovery time of the visual and optical quality in the first month after SMILE.

This study confirmed that the use of an energy close to the plasma threshold in SMILE surgery provides better safety results than the use of a high energy level. No eyes lost two lines of CDVA at 1 month postoperatively. Only 6.9% of eyes in this study (3 eyes in the moderate myopia group and 4 in the high myopia group) lost one line of CDVA (from 20/16 to 20/20) at 1 month postoperatively. In our previous studies, at 1 month postoperatively, we observed 28% of eyes lost one line of CDVA in the group with a high energy level (180 nJ).4 In the current study, the eyes with a loss of one line of CDVA showed a higher OSI value, a lower MTF cut-off value, and equivalent total HOAs compared to the eyes without a CDVA change line. Due to the small number of eyes, the optical quality results of the eyes with a loss of one line of CDVA were not statistically different from those of the eyes without a CDVA change line. However, this emphasized the statistical correlation between the amount of scattering and the visual quality recovery.

This study also showed a correlation between the OSI values and preoperative SE values. There are many potential causes to this correlation. The preoperative SE is related to the thickness of the extracted lenticule and the residual posterior stromal thickness. From the thickness of the lenticule, the hypotheses of a postoperative mismatch between the cap interface and the lenticule interface can be discussed. More likely, we believe the scatter phenomenon is due to inflammation, the healing process,19,20 and/or lenticule interface irregularities. As the density of the corneal structure progressively lessens toward the posterior part of the cornea,21 laser cutting may be irregular, explaining the higher scatter values for the highest myopia. When using higher energy, the phenomena of lenticule irregularities6 and inflammation19,20 increased. They lead to a decrease in postoperative visual quality characterized by a decrease in MTF cut-off22 in connection with an increase in HOAs7 and an increase in light scattering characterized by the OSI.22 These optical phenomena are responsible for the longer recovery time described in the literature2–4,23–25 (Table E, available in the online version of this article). Using a confocal microscopy or rotating Scheimpflug camera with automated corneal densitometry software during the first month could help to correlate the laser energy used, the inflammatory responses, and the quality of vision.

Results of Previous Studies Comparing Visual and Optical Outcomes After SMILE

Table E:

Results of Previous Studies Comparing Visual and Optical Outcomes After SMILE

There were some limitations in this study. As suggested Ji et al.7, the results of low energy SMILE might depend on meticulous surgical manipulation. To minimize this factor, the same experienced surgeon performed each surgical procedure. Furthermore, our clinical outcomes may not be generalizable due to the variations from one femtosecond laser to another. Temperature or hygrometry rate within the surgical room could affect the precision of the laser, which varies from day to day. Confirmation of our results in a multicenter study would be ideal. The main limitation in this study was that these results are only valid for laser energy at 100 nJ. Other factors could affect the visual and optical recovery with higher energy levels.

This study confirmed the safety and efficacy of SMILE with a low energy level. Patients recovered their preoperative visual and optical quality within 7 to 30 days depending on the degree of preoperative myopia. Recovery was faster for those with low myopia than moderate and high myopia. Temporary light scattering was a factor limiting patient optical recovery.

References

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  17. Chen T, Yu F, Lin H, et al. Objective and subjective visual quality after implantation of all optic zone diffractive multifocal intraocular lenses: a prospective, case-control observational study. Br J Ophthalmol. 2016;100:1530–1535. doi:10.1136/bjophthalmol-2015-307135 [CrossRef]
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  19. Liu YC, Teo EP, Lwin NC, Yam GH, Mehta JS. Early corneal wound healing and inflammatory responses after SMILE: comparison of the effects on different refractive corrections and surgical experiences. J Refract Surg. 2016;32:346–353. doi:10.3928/1081597X-20160217-05 [CrossRef]
  20. Dong Z, Zhou X, Wu J, et al. Small incision lenticule extraction (SMILE) and femtosecond laser LASIK: comparison of corneal wound healing and inflammation. Br J Ophtalmol. 2014;98:263–269. doi:10.1136/bjophthalmol-2013-303415 [CrossRef]
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Preoperative Demographics of Eyes Undergoing SMILE

ParameterLow MyopiaModerate MyopiaHigh MyopiaPa
No. of eyes303338
Age (y)
  Mean ± SD31.37 ± 5.8431.94 ± 6.1331.82 ± 5.35.89
  Range20 to 4322 to 4323 to 39
Sphere (D)
  Mean ± SD−1.89 ± 0.72−4.25 ± 0.87b−6.54 ± 0.78c,d< .001
  Range−0.25 to −3.00−2.50 to −5.50−5.25 to −8.75
Cylinder (D)
  Mean ± SD−0.48 ± 0.45−0.56 ± 0.44−0.80 ± 0.52d.02
  Range0.00 to −1.750.00 to −1.750.00 to −2.00
Spherical equivalent (D)
  Mean ± SD−2.13 ± 0.73−4.53 ± 0.86b−6.94 ± 0.75c,d< .001
  Range−0.625 to −3.00−3.125 to −5.88−6.00 to −9.00
Average keratometry (D)
  Mean ± SD43.91 ± 1.6144.02 ± 1.3744.45 ± 1.59.35
  Range40.53 to 47.3841.18 to 46.8840.88 to 47.55
Central corneal thickness (µm)
  Mean ± SD560 ± 40549.60 ± 34.83552.30 ± 30.34.62
  Range493 to 637490 to 630495 to 619
Mesopic pupillometry (mm)
  Mean ± SD6.18 ± 0.806.22 ± 0.845.74 ± 0.80.76
  Range3.88 to 7.934.71 to 7.233.96 to 7.23

Visual and Refractive Outcomes of SMILE

ParameterPreoperativePostoperativePa

1 Day8 Days1 Month
UDVA (logMAR)
  Low myopia
    Mean ± SD0.82 ± 0.33−0.03 ± 0.05b−0.05 ± 0.05b−0.07 ± 0.00b< .001
    Range1.40 to 0.050.05 to −0.100.00 to −0.100.00 to −0.20
  Moderate myopia
    Mean ± SD1.34 ± 0.19c0.05 ± 0.12b,c−0.01 ± 0.11b−0.05 ± 0.00b< .001
    Range1.52 to 0.850.40 to −0.100.30 to −0.200.15 to −0.20
  High myopia
    Mean ± SD1.50 ± 0.08d,e0.09 ± 0.13b,d0.04 ± 0.11b,d−0.02 ± 0.07b,d,e
    Range1.52 to 1.100.40 to −0.100.40 to −0.100.20 to −0.10< .001
   P< .001< .001< .001.004
CDVA (logMAR)
  Low myopia
    Mean ± SD−0.07 ± 0.04−0.06 ± 0.05−0.07 ± 0.04−0.09 ± 0.04b.004
    Range−0.10 to 0.00−0.10 to 0.00−0.10 to 0.00−0.20 to 0.00
  Moderate myopia
    Mean ± SD−0.06 ± 0.05−0.01 ± 0.06b,c−0.05 ± 0.06−0.07 ± 0.05< .001
    Range−0.10 to 0.00−0.01 to 0.15−0.20 to 0.05−0.20 to 0.00
  High myopia
    Mean ± SD−0.05 ± 0.05−0.01 ± 0.09b,d−0.02 ± 0.07b,d,e−0.07 ± 0.04
    Range−0.10 to 0.05−0.10 to 0.30−0.20 to 0.20−0.10 to 0.00< .001
   P.16.008< .001.08
Efficacy index
  Low myopia, mean ± SD0.92 ± 0.150.97 ± 0.141.02 ± 0.13
  Moderate myopia, mean ± SD0.80 ± 0.190.82 ± 0.190.97 ± 0.15
  High myopia, mean ± SD0.76 ± 0.220.84 ± 0.200.94 ± 0.14
Safety index
  Low myopia, mean ± SD0.99 ± 0.151.02 ± 0.141.06 ± 0.11
  Moderate myopia, mean ± SD0.90 ± 0.160.97 ± 0.161.02 ± 0.13
  High myopia, mean ± SD0.90 ± 0.170.92 ± 0.190.99 ± 0.20
MRSE
  Low myopia
    Mean ± SD−2.13 ± 0.730.16 ± 0.23b0.20 ± 0.28b0.31 ± 0.17b< .001
    Range−0.63 to −3.00−0.25 to 0.63−0.38 to 0.88−0.13 to 0.75
  Moderate myopia
    Mean ± SD−4.53 ± 0.860.13 ± 0.40b0.17 ± 0.27b0.22 ± 0.26b< .001
    Range−3.13 to −5.88−0.88 to 1.13−0.50 to 0.63−0.38 to 0.63
  High myopia
    Mean ± SD−6.94 ± 0.750.20 ± 0.50b0.24 ± 0.37b0.25 ± 0.32b
    Range−6.00 to −9.00−1.25 to 1.25−0.63 to 0.88−0.50 to 0.88< .001
   P< .001.41.47.50

Optical Quality Parameters of HD Analyzer After SMILE

ParameterPreoperativePostoperativePa

1 Day8 Days1 Month
OSI
  Low myopia
    Mean ± SD0.61 ± 0.311.16 ± 0.42b0.97 ± 0.60b0.69 ± 0.21< .001
    Range0.30 to 1.600.40 to 2.200.40 to 3.500.20 to 1.20
  Moderate myopia
    Mean ± SD0.74 ± 0.531.84 ± 1.02b,d1.26 ± 0.68b,d1.00 ± 0.53b,d< .001
    Range0.30 to 2.600.50 to 4.600.40 to 3.100.40 to 2.60
  High myopia
    Mean ± SD0.78 ± 0.531.70 ± 1.101b,e1.59 ± 0.85b,e1.14 ± 0.62b,e< .001
    Range0.30 to 2.600.40 to 4.600.60 to 3.600.40 to 3.60
   Pc.46.03.02.001
MTF cut-off frequency (cpd)
  Low myopia
    Mean ± SD41.83 ± 8.8637.61 ± 10.8537.63 ± 10.4840.69 ± 7.83.28
    Range20.44 to 55.4716.26 to 54.2915.91 to 53.3824.43 to 54.69
  Moderate myopia
    Mean ± SD39.96 ± 7.8629.64 ± 12.91b,d31.69 ± 9.96b35.08 ± 9.84d.04
    Range18.08 to 54.757.58 to 53.0612.41 to 54.0717.11 to 54.32
  High myopia
    Mean ± SD38.87 ± 10.629.35 ± 11.56b,e26.46 ± 9.54b,c31.54 ± 8.91b,e< .001
    Range12.85 to 52.1010.39 to 51.108.67 to 47.0013.67 to 48.64
   Pc.47< .001< .001.001
Strehl ratio
  Low myopia
    Mean ± SD0.23 ± 0.060.22 ± 0.080.22 ± 0.080.22 ± 0.05.70
    Range0.13 to 0.410.09 to 0.370.10 to 0.360.14 to 0.34
  Moderate myopia
    Mean ± SD0.23 ± 0.050.17 ± 0.08b,d0.18 ± 0.06b0.18 ± 0.04b< .001
    Range0.13 to 0.370.07 to 0.340.09 to 0.320.12 to 0.27
  High myopia
    Mean ± SD0.23 ± 0.100.16 ± 0.06b,c0.16 ± 0.05b,e0.17 ± 0.04b,e.004
    Range0.10 to 0.300.07 to 0.300.07 to 0.290.09 to 0.24
   Pc.64.01.006< .001

Corneal HOAs After SMILE

ParameterPreoperativePostoperativePa

1 Day8 Days1 Month
No. of eyes (n)
  Low myopia23232323
  Moderate myopia25252525
  High myopia28282828
RMS, total HOAs (µm)
  Low myopia
    Mean ± SD0.231 ± 0.050.258 ± 0.096b0.309 ± 0.177b0.281 ± 0.065 b.023
    Range0.135 to 0.3560.150 to 0.5820.137 to 0.8830.173 to 0.411
  Moderate myopia
    Mean ± SD0.258 ± 0.1530.298 ± 0.100b0.335 ± 0.125b0.273 ± 0.079.002
    Range0.119 to 0.9570.140 to 0.6220.166 to 0.6620.117 to 0.417
  High myopia
    Mean ± SD0.219 ± 0.050.350 ± 0.174b0.371 ± 0.134b0.364 ± 0.101b,d,e< .001
    Range0.126 to 0.320.136 to 0.9160.145 to 0.6380.122 to 0.610
   Pc.88.013.150.004
RMS, SA (µm)
  Low myopia
    Mean ± SD0.125 ± 0.0270.087 ± 0.065b0.084 ± 0.053b0.117 ± 0.088b< .001
    Range0.051 to 0.1650.022 to 0.3410.027 to 0.2910.029 to 0.470
  Moderate myopia
    Mean ± SD0.125 ± 0.0440.102 ± 0.0600.093 ± 0.0430.128 ± 0.082.48
    Range0.008 to 0.2830.010 to 0.2260.026 to 0.1900.018 to 0.440
  High myopia
    Mean ± SD0.134 ± 0.0450.154 ± 0.060e,f0.163 ± 0.094e,f0.195 ± 0.115e,f.11
    Range0.055 to 0.2640.041 to 0.3040.036 to 0.4320.056 to 0.575
   Pc.680.009.0003.001
RMS, coma (µm)
  Low myopia
    Mean ± SD0.137 ± 0.0770.198 ± 0.101b0.206 ± 0.113b0.217 ± 0.101b.002
    Range0.021 to 0.3030.023 to 0.4620.024 to 0.4170.071 to 0.460
  Moderate myopia
    Mean ± SD0.151 ± 0.0830.204 ± 0.104b0.243 ± 0.116b0.202 ± 0.100b.009
    Range0.042 to 0.3780.026 to 0.4400.063 to 0.5560.071 to 0.460
  High myopia
    Mean ± SD0.139 ± 0.1040.186 ± 0.0990.232 ± 0.135b0.257 ± 0.105b
    Range0.020 to 0.6400.031 to 0.3950.062 to 0.5410.062 to 0.433< .001
   Pc.590.980.700.08
RMS, trefoil (µm)
  Low myopia
    Mean ± SD0.090 ± 0.0470.115 ± 0.1070.121 ± 0.0940.121 ± 0.083.08
    Range0.015 to 0.1870.013 to 0.5370.028 to 0.4480.015 to 0.408
  Moderate myopia
    Mean ± SD0.110 ± 0.0680.110 ± 0.0420.128 ± 0.0830.126 ± 0.150.093
    Range0.015 to 0.3500.043 to 0.2100.019 to 0.3030.024 to 0.825
  High myopia
    Mean ± SD0.123 ± 0.1420.151 ± 0.081b,e,f0.153 ± 0.074b0.155 ± 0.105b
    Range0.027 to 0.8700.024 to 0.3720.050 to 0.3080.038 to 0.551.002
   Pc.520.040.120.050

Correlation Between Postoperative Values of OSI and Visual Quality Parameters

ParameterUDVACDVAContrast Sensitivity

3 cpd6 cpd12 cpd18 cpd






rPrPrPrPrPrP
Low myopia
  D1−0.149.348−0.319.047a−0.089.557−0.203.174−0.434.003a−0.343.018a
  D8−0.296.066−0.161.322−0.008.970−0.045.766−0.211.150−0.411.005a
  M1−0.617.011a−0.422.287−0.531.045−0.452.167−0.408.287−0.526.048a
Moderate myopia
  D1−0.284.043a−0.378.009a−0.149.290−0.322.023a−0.427.002a−0.331.015a
  D8−0.305.029a−0.297.043a0.017.910−0.124.367−0.228.086−0.28.036a
  M1−0.331.025a−0.369.015a−0.139.346−0.238.098−0.139.323−0.276.046a
High myopia
  D1−0.31.017a−0.435.003a−0.226.096−0.354.008a−0.344.010a−0.29.029a
  D8−0.38.003a−0.46.001a0.026.853−0.384.003a−0.369.004a−0.237.060
  M10.038.793−0.166.2680.124.377−0.034.819−0.093.501−0.21.116

Correlation Between OSI or MTF Cut-off Postoperative Values and Preoperative SE

ParameterrP
MTF cut-off
  D10.177< .001a
  D80.280< .002a
  M10.207< .003a
OSI
  D1−0.134.06
  D8−0.26< .01a
  M1−0.22< .01a

Preoperative Factors Associated With Postoperative Optical Quality or Intraocular Scattering in Regression Analysis

ParameterMTF Cut-offOSI


D1D8M1D1M8M1






bPbPbPbPbPbP
SE0.271.010a0.393.000a0.321.003a−0.181.090−0.291.010a−0.326.004a
Age−0.120.263−0.095.5240.042.5180.094.3740.158.1910.009.789
OSI−0.050.946−0.168.306−0.107.7390.084.4930.217.1480.244.110
MTF cut-off (cpd)0.031.7450.103.7400.099.872−0.031.722−0.129.939−0.166.801
Strehl ratio0.014.5260.112.9480.111.7510.023.267−0.110.627−0.146.897

Results of Previous Studies Comparing Visual and Optical Outcomes After SMILE

StudyRepetition Rate (kHz)Cut Energy (nJ)No. of EyesAge (y)Cap Thickness (μm)Preop SE (D)Postoperative Outcomes

Follow–up (Days)Postop SE (D)UDVA (logMAR)Efficacy IndexSafety Index% of Eyes ≥ 20/20 UDVAMTF Cut–offSROSINo Difference With Preoperative CS Value
Shah et al.2200155 to 1705126100–4.87300.1356
Shah et al.320016016526100 to 130−4.62833
20016016428100 to 130−4.72856
Donate et al.450018016429.74130−3.99300.860.9457
50010032229.78130−3.92300.951.0487
Ji et al.6500104.915826.32120−4.688−0.07
500130.919325.96120−4.528−0.05
Miao et al.155001306628.67110−6.40200.1031.710.181.09
5001306628.67110−6.40400.0933.270.180.94
Liu et al.2050018019725120−5.22931 mo
Liu et al.22500150 to 1602022.9110134.510.211.11
500150 to 1602022.9150124.760.171.96
Sekundo et al.2350053−4.68360−0.19881 y
Verstergraad et al.2450035−7.60180−0.09−0.08606 mo
Hjortdal et al.2550017079038.3170−7.19900.91.0760
Authors

From OPHTEO, Lyon, France.

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

AUTHOR CONTRIBUTIONS

Study concept and design (RT); data collection (RT); analysis and interpretation of data (DD, RT); writing the manuscript (DD, RT); critical revision of the manuscript (DD, RT); statistical expertise (RT); administrative, technical, or material support (RT); supervision (DD, RT)

Correspondence: David Donate, MD, OPHTEO, 52 avenue Marechal de Saxe, Lyon 69006, France. E-mail: david.donate@yahoo.fr

Received: August 03, 2018
Accepted: April 15, 2019

10.3928/1081597X-20190416-01

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