Journal of Refractive Surgery

Original Article Supplemental Data

Corneal Remodeling After Myopic SMILE: An Optical Coherence Tomography and In Vivo Confocal Microscopy Study

Norman Romito, MD; Liem Trinh, MD; Isabelle Goemaere, MS; Vincent Borderie, MD, PhD; Laurent Laroche, MD; Nacim Bouheraoua, MD, PhD

Abstract

PURPOSE:

To study corneal remodeling during the first 6 months after myopic small incision lenticule extraction (SMILE) with a 10% overcorrection nomogram, by spectral-domain optical coherence tomography (SD-OCT) and in vivo confocal microscopy (IVCM).

METHODS:

This prospective non-randomized observational study included 60 eyes from 30 patients treated by SMILE for low to moderate myopia. A 10% overcorrection nomogram was applied for all eyes. Epithelial and corneal thickness maps were obtained within the central 6 mm, by SD-OCT, at each visit. Lenticule thickness was calculated by subtracting the postoperative central stromal thickness from the preoperative central stromal thickness. IVCM was performed at each visit.

RESULTS:

The mean surgical refractive correction was −3.99 ± 1.50 diopters (D) before and −0.09 ± 0.37 D after surgery. Central epithelial thickness increased from 53.7 ± 4.0 to 57.1 ± 4.1 µm at 6 months after SMILE (P < .001). The measured lenticule thickness was 16 ± 6.1 µm less than the programmed lenticule thickness (P < .001). Both central epithelial hyperplasia and the mismatch between measured and programmed lenticule thickness were positively correlated to the degree of myopia (r2 = 0.60, P < .001 and r2 = 0.47, P < .001, respectively). Fibrosis at the interface was not correlated with epithelial thickening (r2 = 0.06, P = .29) or lenticule thickness error (r2 = 0.07, P = .22).

CONCLUSIONS:

Both epithelial thickening and the mismatch between the targeted and achieved lenticule thickness resulted in a slight undercorrection with a 10% overcorrection nomogram in low and moderate myopia. Fibrosis at the interface was not responsible for lenticule thickness error. Additional overcorrection is required to increase accuracy.

[J Refract Surg. 2020;36(9):597–605.]

Abstract

PURPOSE:

To study corneal remodeling during the first 6 months after myopic small incision lenticule extraction (SMILE) with a 10% overcorrection nomogram, by spectral-domain optical coherence tomography (SD-OCT) and in vivo confocal microscopy (IVCM).

METHODS:

This prospective non-randomized observational study included 60 eyes from 30 patients treated by SMILE for low to moderate myopia. A 10% overcorrection nomogram was applied for all eyes. Epithelial and corneal thickness maps were obtained within the central 6 mm, by SD-OCT, at each visit. Lenticule thickness was calculated by subtracting the postoperative central stromal thickness from the preoperative central stromal thickness. IVCM was performed at each visit.

RESULTS:

The mean surgical refractive correction was −3.99 ± 1.50 diopters (D) before and −0.09 ± 0.37 D after surgery. Central epithelial thickness increased from 53.7 ± 4.0 to 57.1 ± 4.1 µm at 6 months after SMILE (P < .001). The measured lenticule thickness was 16 ± 6.1 µm less than the programmed lenticule thickness (P < .001). Both central epithelial hyperplasia and the mismatch between measured and programmed lenticule thickness were positively correlated to the degree of myopia (r2 = 0.60, P < .001 and r2 = 0.47, P < .001, respectively). Fibrosis at the interface was not correlated with epithelial thickening (r2 = 0.06, P = .29) or lenticule thickness error (r2 = 0.07, P = .22).

CONCLUSIONS:

Both epithelial thickening and the mismatch between the targeted and achieved lenticule thickness resulted in a slight undercorrection with a 10% overcorrection nomogram in low and moderate myopia. Fibrosis at the interface was not responsible for lenticule thickness error. Additional overcorrection is required to increase accuracy.

[J Refract Surg. 2020;36(9):597–605.]

Excimer laser refractive surgery for myopia involves a flattening of the central surface by ablation of the corneal stroma. The corneal epithelium is known to be able to remodel itself or compensate for the irregularities of the underlying stromal surface by altering its thickness profile, potentially leading to refractive regression.1,2 Many studies have reported an increase in epithelial thickness after excimer laser ablation for the treatment of myopia and hyperopia, which were shown to be responsible for the regression effect seen after photorefractive keratectomy3 and laser in situ keratomileusis (LASIK).4 Epithelial hyperplasia after small incision lenticule extraction (SMILE) has already been reported.5–8 Following procedures to correct myopia, the epithelium appears to be thicker after SMILE, as reported for LASIK,7 and epithelial hyperplasia is highly dependent on the magnitude of the induced refractive correction.5,8

Recent studies have reported a discrepancy between the estimated and measured lenticule thickness. Using very high frequency ultrasound, Reinstein et al9 found a systematic discrepancy in lenticule thickness between the target and achieved central stromal thickness, of 8 µm on average, 3 months after surgery. Luft et al10 showed, by spectral-domain optical coherence tomography (SD-OCT), that the mean observed decrease in central stromal thickness was 18.7 ± 5.7 µm smaller than the planned lenticule, and that this mismatch increased with lenticule thickness. The authors concluded that this discrepancy might be caused by biomechanical changes in the cornea, but several additional factors, such as keratocyte-mediated wound healing in the laser cut interface, were also thought possible.

Nevertheless, in neither epithelial remodeling studies nor the accuracy of lenticular thickness studies was the overcorrection factor applied during the SMILE procedure indicated. A knowledge of epithelial remodeling after SMILE and actual lenticule thickness would be useful to improve refractive outcomes with a more precise nomogram applied.

Three-dimensional corneal epithelial mapping in vivo by SD-OCT has recently become available in clinical practice, facilitating the capture of optical images and high-speed measurements of epithelial and corneal thicknesses.11 Moreover in vivo confocal microscopy (IVCM) makes it possible to evaluate the cellular structure of the cornea.12 Cell proliferation events are clearly important mediators of optical regression, although the exact mechanisms involved remain unknown.13

A 10% nomogram is generally proposed by the manufacturer. We therefore studied corneal remodeling for 6 months after SMILE for low to moderate myopia with a 10% overcorrection nomogram, using SD-OCT and IVCM.

Patients and Methods

Patients

This prospective observational study included 60 eyes treated by myopic SMILE at Centre Hospitalier National d'Ophtalmologie des Quinze-Vingts between September 2016 and June 2018. The tenets of the Declaration of Helsinki were respected, and the study was approved by the Ethics Committee of the French Society of Ophthalmology (Institutional Review Board 00008855). Informed consent was obtained from patients before surgery.

The inclusion criteria were: age 20 to 50 years, spherical equivalent refraction of −1.50 to −8.00 diopters (D), healthy ocular surface, adequate tear film, and stable refraction for at least 1 year. Eyes with thin corneas (central corneal thickness < 480 µm) and eyes affected by ocular diseases such as corneal dystrophy, keratoconus, corneal opacity, or severe dry eye syndrome were excluded.

All patients underwent a thorough preoperative clinical examination including medical, ocular, and family history, slit-lamp biomicroscopy, dilated fundus examination, manifest and cycloplegic refraction, dry eye assessment, and tear film break-up time. Other pre-operative examinations included topography (Orbscan II; Bausch and Lomb Surgical), stromal and epithelial thickness measurement by SD-OCT with RTVue (Optovue, Inc), and in vivo confocal microscopy (IVCM) (Heidelberg Engineering). Uncorrected (UDVA) and corrected (CDVA) distance visual acuity values were obtained by measuring Snellen visual acuity.

Patients were followed up at 1 week, 1 month, 3 months, and 6 months after surgery. At each visit, UDVA and CDVA were assessed, and slit-lamp examination, topography, SD-OCT, and IVCM were performed.

Surgical Technique

All SMILE procedures were performed under topical anesthesia, by a standard surgical technique, with a VisuMax 500-kHz femtosecond laser (Carl Zeiss Meditec). The optical zone was 6.5 mm and the cap thickness was 120 µm. The lenticule edge thickness used was 15 µm for all patients. In all cases, a 4.5-µm laser spot spacing and a laser cut energy of level 32 (corresponding to 160 nanojoules) was applied. A superior incision of 2.4 mm was created by the laser at the 10-o'clock position to extract the lenticule. The treatment was centered on the corneal vertex. After the cutting procedure with the femtosecond laser had been completed, the refractive lenticule of intrastromal corneal tissue was dissected and separated through the incision, and manually removed. The interface was washed with balanced salt solution. For all eyes, a 10% overcorrection nomogram was applied to both spherical and cylindrical components of the refractive error.

After surgery, patients were instructed to instill 0.1% dexamethasone and 0.3% tobramycin eye drops (Tobradex; Novartis Pharma SAS) into the eyes four times daily for 4 weeks, and lubricants four times daily for at least 4 weeks.

Measurement of Corneal Epithelial and Lenticule Thicknesses

SD-OCT was used for epithelial mapping with the corneal adaptor module, which produces telecentric scanning for anterior segment imaging. A wide-angle long lens was used with a scan width of 6 mm and a transverse resolution of 15 µm. The epithelial thickness maps were generated with an automatic algorithm and were divided into a total of 17 sectors: a central zone with a diameter of 2 mm, eight mid-peripheral zones within an annulus between the 2- and 5-mm diameter rings, and eight peripheral zones within an annulus between the 5- and 6-mm diameter rings. We calculated stromal thickness by subtracting the epithelial thickness from the corneal thickness and then calculated lenticule thickness by subtracting the postoperative central stromal thickness from the preoperative central stromal thickness. Two consecutive acquisitions were obtained to ensure data validity, and the mean of the two values was used for analysis.

Confocal Microscopy

A cornea-specific in vivo laser scanning confocal microscope was used for this study. After topical anesthesia with 1% tetracaine eye drops (Novartis Laboratories, Inc) and the instillation of high-viscosity eye gel (2.0 mg/g carbomer, Lacrigel; Europhta Laboratories), patients were asked to focus on an external target. All corneas were examined centrally with at least three z-axis scans from the epithelium to the endothelium. For each time point, we selected three of the clearest images from each layer. Superficial epithelial cells are characterized by a polygonal cell pattern, bright illuminated cytoplasm, reflecting nucleus, and a dark perinuclear halo. Intermediate epithelial cells appear as a regular mosaic with sharp reflective borders. The cell bodies have a reflectivity similar to that of superficial cells. Basal epithelial cells have a smaller diameter and appear cylindrical in section, with nuclei not defined by a reflective border.14

The subbasal nerve plexus images were considered to be the first clear images of the nerves at the level of Bowman's layer. Dendritic cell images were acquired at a depth of 35 to 70 µm, just beneath the basal epithelial layers. Dendritic cells were identified morphologically as bright individual dendritiform structures differentiated from corneal nerves.14 The anterior stromal images were considered to be the first clear images immediately posterior to Bowman's layer, whereas the posterior stromal images were considered to be the first clear images immediately anterior to the endothelium. The reflectivity of the images at the level of the cutting interface, corresponding to fibrosis, was analyzed by counting the white pixels. We used several built-in functions available in ImageJ software (National Institutes of Health), such as “make binary,” to convert each image to white or black pixels. Then we counted the number of white pixels corresponding to fibrosis using the automatic counting function of ImageJ software. We repeated this analysis on three consecutive images to approach three-dimensional results.

All selected images were identified and randomized by an examiner (NB). Quantitative analysis was then performed by a single masked examiner (NR), with ImageJ software for keratocyte density, dendritic cells and epithelial cells counting, and NeuronJ software (National Institutes of Health) for corneal subbasal nerve density. Subbasal nerve density was calculated by measuring total nerve length per image. Keratocyte density was determined from the layer in the cap 10 µm anterior to the interface (Anterior IF), at the interface, and 10 µm posterior to the interface (Posterior IF).

Safety and Efficacy

Efficacy was assessed by dividing the mean postoperative UDVA by the mean preoperative CDVA. Safety was assessed by dividing the mean postoperative CDVA by the mean preoperative CDVA.

Statistical Analysis

Results are presented as the mean ± standard deviation for continuous variables and as proportions (%) for categorical variables. The Snellen CDVA was converted into logarithm of the minimum angle of resolution (logMAR) units for analysis. We used the d'Agostino-Pearson test to assess the normality of our data distribution, and we then applied parametric statistics. We used t tests for paired data to compare preoperative and postoperative continuous data. The standard t test was used to compare continuous data, as appropriate.

Spearman's rank correlation analysis was used to determine the relationship between values. Corrected P values less than .05 were considered statistically significant. Statistical analysis was performed with SPSS for Windows software version 20.0 (SPSS, Inc).

Results

In total, 60 myopic eyes from 30 patients with plano target refraction were included. The mean age of the patients was 33.2 ± 7.1 years (range: 21 to 50 years). The mean surgical refractive correction was −3.99 ± 1.50 D (range: −8.00 to −1.75 D) before and −0.09 ± 0.37 D (range: −1.00 to +0.50 D) after surgery. Because only 5 eyes had myopia greater than −6.00 D, we considered our sample as low to moderate myopia. The preoperative data for all patients are presented in Table 1.

Patient Demographics and Ocular Characteristics at Inclusion (N = 60)

Table 1:

Patient Demographics and Ocular Characteristics at Inclusion (N = 60)

Visual Acuity and Refraction

Figure A (available in the online version of this article) shows the standard graphs for reporting refractive surgery results. UDVA was 0.0 logMAR (Snellen 20/20) or better in 56 patients (93%) and 0.1 logMAR (Snellen 20/25) or better in 60 patients (100%) (Figure AA). Mean UDVA was −0.08 ± 0.77 logMAR (Snellen 20/16). The efficacy index was 0.92 and the safety index was 1.0 for all eyes. One patient (1.6%) lost one line of CDVA and none of the eyes presented a loss of two lines (Figure AB).

Standard graphs for reporting refractive surgery, showing the visual and refractive outcomes for 60 myopic eyes after small incision lenticule extraction (SMILE). (A) Uncorrected distance visual acuity (UDVA) before and 6 months after SMILE. (B) Changes in the number of Snellen lines of corrected distance visual acuity (CDVA). (C) Target and achieved spherical equivalent (SE). (D) Accuracy of SE refraction. (E) Refractive astigmatism before and 6 months after SMILE. (F) Stability of SE refraction from before to 6 months after SMILE. D = diopters; SD = standard deviation.

Figure A.

Standard graphs for reporting refractive surgery, showing the visual and refractive outcomes for 60 myopic eyes after small incision lenticule extraction (SMILE). (A) Uncorrected distance visual acuity (UDVA) before and 6 months after SMILE. (B) Changes in the number of Snellen lines of corrected distance visual acuity (CDVA). (C) Target and achieved spherical equivalent (SE). (D) Accuracy of SE refraction. (E) Refractive astigmatism before and 6 months after SMILE. (F) Stability of SE refraction from before to 6 months after SMILE. D = diopters; SD = standard deviation.

A positive correlation was found between target and achieved spherical equivalent (SE) refraction (r2 = 0.95, P < .001) (Figure AC). The mean SE refraction in all 60 eyes was −0.09 ± 0.37 D at 6 months. For all eyes, 54 patients (90%) were within ±0+.50 D of the indented SE refraction and 60 patients (100%) were within ±1.00 D.

Mean residual astigmatism was −0.47 ± 0.35 D. Residual refractive astigmatism was 0.50 D or less in 45 of the eyes (75%) and 1.00 D or less in all 60 eyes (100%) (Figure AE). The mean SE refraction was stable from 1 to 6 months (Figure AF).

Changes in Epithelial Thickness

Table A (available in the online version of this article) shows epithelial thickness values for all zones at 1 week and 1, 3, and 6 months postoperatively, and a comparison with preoperative values. Figure 1 shows the mean change in epithelial thickness after surgery. Central epithelial thickness stability was reached at 3 months postoperatively. Significant postoperative epithelial thickening was observed in all regions of interest examined except the infranasal, infratemporal, and inferior zones, taking a lenticular appearance. Mean central epithelial thickness was 53.7 ± 4.0 µm before SMILE and 57.1 ± 4.1 µm 6 months after SMILE (P < .001). Figure B (available in the online version of this article) shows central epithelial thickening on SDOCT for a 32-year-old man with a refractive error of −6.50 −0.50 @ 150° for the right eye before surgery and 6 months after myopic SMILE.

Mean Epithelial Thickness (µm) in All Zones and Comparison of the Values Before and After SMILEa

Table A:

Mean Epithelial Thickness (µm) in All Zones and Comparison of the Values Before and After SMILE

Spectral-domain optical coherence tomography (RTVue; Optovue, Inc), showing mean epithelial thickness change profiles over the 6-mm diameter zone 6 months after small incision lenticule extraction (SMILE). Epithelial thickness was measured in micrometers and mean values ± standard deviation are presented. Orange areas display significant thickening. Green areas display no significant thickening. Significant thickening was observed in all zones except the intranasal, infratemporal, and inferior zones (S = superior; I = inferior; N = nasal; T = temporal).

Figure 1.

Spectral-domain optical coherence tomography (RTVue; Optovue, Inc), showing mean epithelial thickness change profiles over the 6-mm diameter zone 6 months after small incision lenticule extraction (SMILE). Epithelial thickness was measured in micrometers and mean values ± standard deviation are presented. Orange areas display significant thickening. Green areas display no significant thickening. Significant thickening was observed in all zones except the intranasal, infratemporal, and inferior zones (S = superior; I = inferior; N = nasal; T = temporal).

Spectral-domain optical coherence tomography (RTVue; Optovue, Inc) for a 32-year-old man with a refractive error of −6.50 −0.50 @ 150° in the right eye (A and B) before surgery and (C and D) 6 months after myopic small incision lenticule extraction (SMILE). Pachymetry map showing central corneal thinning corresponding to the ablation zone. The epithelium map shows epithelial thickening, which is greatest in the central zone.

Figure B.

Spectral-domain optical coherence tomography (RTVue; Optovue, Inc) for a 32-year-old man with a refractive error of −6.50 −0.50 @ 150° in the right eye (A and B) before surgery and (C and D) 6 months after myopic small incision lenticule extraction (SMILE). Pachymetry map showing central corneal thinning corresponding to the ablation zone. The epithelium map shows epithelial thickening, which is greatest in the central zone.

A positive significant correlation between the degree of myopia corrected and postoperative epithelial thickening was observed in the central zone (R2 = 0.60, P < .001) (Figure 2).

Scatter plot showing a positive correlation between central epithelial thickening (µm) and target spherical equivalent (diopters [D]). Spearman rank correlation: R2 = 0.60, P < .001.

Figure 2.

Scatter plot showing a positive correlation between central epithelial thickening (µm) and target spherical equivalent (diopters [D]). Spearman rank correlation: R2 = 0.60, P < .001.

Lenticule Thickness Accuracy

The mean preoperative programmed central thickness of the extracted refractive lenticule (including the 10% overcorrection nomogram) indicated in the femtosecond laser readout was 88.4 ± 21.3 µm (range: 48 to 133 µm). The mean central lenticule thickness measured by SD-OCT was 72.7 ± 17.6 µm (range: 40 to 108 µm) 6 months after SMILE. The central lenticule thickness actually achieved was below the target in all 60 eyes, by a mean of 16 ± 6.1 µm (range: +2 to +33 µm) (P < .001).

There was a positive significant correlation between the programmed and achieved lenticule thicknesses (R2 = 0.92, P < .001) (Figure 3), and between lenticule thickness error and attempted SE refraction (R2 = 0.47, P < .001) (Figure 4).

Scatter plot showing a positive correlation between target and achieved lenticule thickness (µm). The mismatch between target and achieved lenticule thickness increased with lenticule thickness. Spearman's rank correlation: R2 = 0.92, P < .001. The red line indicates equality between the achieved and target lenticule thickness.

Figure 3.

Scatter plot showing a positive correlation between target and achieved lenticule thickness (µm). The mismatch between target and achieved lenticule thickness increased with lenticule thickness. Spearman's rank correlation: R2 = 0.92, P < .001. The red line indicates equality between the achieved and target lenticule thickness.

Scatter plot showing a positive correlation between lenticule thickness error (µm), and target spherical equivalent (diopters [D]). The achieved lenticule thickness was less than the target thickness, and this mismatch increased with the degree of myopia. Spearman's rank correlation: R2 = 0.47, P < .001.

Figure 4.

Scatter plot showing a positive correlation between lenticule thickness error (µm), and target spherical equivalent (diopters [D]). The achieved lenticule thickness was less than the target thickness, and this mismatch increased with the degree of myopia. Spearman's rank correlation: R2 = 0.47, P < .001.

Confocal Microscopy

Table B (available in the online version of this article) summarizes the various confocal microscopy results obtained. Cell density remained stable in each layer of the epithelium. Figure 5 shows the different layers of the epithelium during follow-up.

Confocal Microscopy Results

Table B:

Confocal Microscopy Results

Corneal confocal microscopy images of the epithelium (A, B, C, D represent superficial cells; E, F, G, H represent wing cells; I, J, K, L represent basal cells) in one eye before surgery and 1, 3, and 6 months after small incision lenticule extraction (SMILE). The white bar represents 100 µm. The Heidelberg retina tomography images correspond to an area of 400 × 400 µm.

Figure 5.

Corneal confocal microscopy images of the epithelium (A, B, C, D represent superficial cells; E, F, G, H represent wing cells; I, J, K, L represent basal cells) in one eye before surgery and 1, 3, and 6 months after small incision lenticule extraction (SMILE). The white bar represents 100 µm. The Heidelberg retina tomography images correspond to an area of 400 × 400 µm.

Subbasal nerve density was 22.11 ± 3.43 mm/mm2 preoperatively, 11.16 ± 4.73 mm/mm2 at 1 month, 14.69 ± 3.97 mm/mm2 at 3 months, and 16.47 ± 3.85 mm/mm2 at 6 months. It was significantly lower than the preoperative value at all postoperative visits (P < .001 for all comparison). Subbasal nerve density at 6 months was 74.5% of the initial density. Epithelial thickening was not correlated with the decrease in subbasal nerve density (r2 = 0.083, P = .56). Figure C (available in the online version of this article) shows the subbasal nerves of one patient during follow-up.

Corneal confocal microscopy images of the subbasal nerve plexus in one patient (A) before surgery and (B) 1, (C) 3, and (D) 6 months after small incision lenticule extraction (SMILE). Subbasal nerve density was low at all visits, and regeneration was incomplete at 6 months. The white bar represents 100 µm. The Heidelberg retina tomography images correspond to an area of 400 × 400 µm.

Figure C.

Corneal confocal microscopy images of the subbasal nerve plexus in one patient (A) before surgery and (B) 1, (C) 3, and (D) 6 months after small incision lenticule extraction (SMILE). Subbasal nerve density was low at all visits, and regeneration was incomplete at 6 months. The white bar represents 100 µm. The Heidelberg retina tomography images correspond to an area of 400 × 400 µm.

The density of dendritic cells was significantly higher after surgery than before surgery (P < .001 for all comparison). Epithelial thickening was not correlated with the increase in dendritic cell density at 6 months (r2 = 0.07, P = .41).

The number of white pixels at the cutting interface at 6 months was not correlated with epithelial thickening (r2 = 0.06, P = .29) or lenticule thickness error (r2 = 0.07, P = .22).

Anterior IF and Posterior IF keratocyte densities were significantly higher before than after surgery (P < .001 for all comparisons). Keratocyte density at the interface was not significantly different at any of the postoperative follow-up visits. Figure D (available in the online version of this article) shows the Anterior IF, interface, and Posterior IF keratocytes of two patients, 6 months after SMILE.

Corneal confocal microscopy images of the stroma of two patients 6 months after small incision lenticule extraction (SMILE). Keratocytes are present (A and B) above and (E and F) below the cutting interface, corresponding to stromal wound healing. (C and D) At the cutting interface, we can see rare keratocytes with a hyperreflective extracellular matrix and hyperreflective particles. The white bar represents 100 µm. The Heidelberg retina tomography images correspond to an area of 400 × 400 µm. Pre-IF = 10 µm anterior to the interface; Post-IF = 10 µm posterior to the interface.

Figure D.

Corneal confocal microscopy images of the stroma of two patients 6 months after small incision lenticule extraction (SMILE). Keratocytes are present (A and B) above and (E and F) below the cutting interface, corresponding to stromal wound healing. (C and D) At the cutting interface, we can see rare keratocytes with a hyperreflective extracellular matrix and hyperreflective particles. The white bar represents 100 µm. The Heidelberg retina tomography images correspond to an area of 400 × 400 µm. Pre-IF = 10 µm anterior to the interface; Post-IF = 10 µm posterior to the interface.

Discussion

In this study, we found that central epithelial thickness increased by a mean of 3.4 ± 4.1 µm after surgery. An 18% mismatch (16 ± 6.1 µm) between the target and achieved lenticule thicknesses was also observed 6 months after myopic SMILE. Both epithelial thickening and lenticule thickness error were correlated with the degree of myopia correction, and resulted in slight undercorrection with a 10% overcorrection nomogram for low to moderate myopia. IVCM revealed that epithelial thickening and lenticule thickness error were not correlated with fibrosis at the cutting interface.

Efficacy and safety were good and compared well with recent reports for myopic SMILE.15 Refraction remained stable after surgery in our patients, comparing well with recent reports for SMILE procedures,16 and no myopic regression was observed at 6 months postoperatively. However, a mean undercorrection of −0.09 + 0.37 D was observed. This trend toward undercorrection after SMILE procedures has also been reported in other studies. Kim et al16 reported a mean undercorrection of −0.13 and −0.24 D 12 months after SMILE in patients with a preoperative SE of −5.05 and −7.67 D, respectively. Ganesh et al17 also reported undercorrection by a mean of −0.28 D, with a preoperative SE of −4.42 D. Reinstein et al9 and Luft et al10 reported lower levels of undercorrection by a mean of −0.04 and −0.11 D, respectively. Assuming that a 10% overcorrection nomogram was used, this would appear to be insufficient for low to moderate myopia. Based on our figures and graphs, a 7% overcorrection nomogram would seem suitable for −1.00 to −3.00 D laser correction. For higher correction of −7.00 D, a 12% overcorrection would probably be better to improve refractive outcomes. However, such stratification based on our sample size makes the analysis less reliable, because we did not have the same number of eyes for each diopter of myopia from −1.00 to −8.00 D.

The significant increase in central epithelial thickness observed here indicates that corneal remodeling occurs after SMILE. Corneal epithelial thickness measurements by SD-OCT after SMILE have already been reported.5–8 Our findings are consistent with those of Kanellopoulos,7 who observed a statistically significant thickening of the corneal epithelium, by a mean of 4.63 µm at 24 months after SMILE, using AS-OCT (RTVue 100). Reinstein et al18 detected an increase in central epithelium thickening of 15 ± 5 µm 3 months after surgery, but they used very high-frequency ultrasound. Luft et al8 found a positive correlation between the central epithelial thickening observed on SD-OCT and spherical equivalent. Our findings confirm that the extent of epithelial hyperplasia following SMILE is strongly correlated with greater surgical refractive correction.

Knowledge about the underlying mechanism of epithelial remodeling after corneal refractive surgery is limited. The initial epithelial thinning may reflect a temporary change in response to wound creation and ocular dryness, but the subsequent epithelial thickening may reflect a compensatory mechanism in response to the change in curvature after stroma subtraction. Support for this hypothesis has been provided by several studies, which have highlighted the role of the epithelium in the regression effect.19,20 Our results are consistent with the hypothesis that the rate of change in curvature can explain epithelial growth. We show here that epithelial remodeling is not linked to corneal nerve density, dendritic cell density, or fibrosis at the cutting interface.

Unmyelinated sensory nerve endings are derived from the ophthalmic branch of the trigeminal nerve, from a dense whorl-like plexus below the basal layer of epithelial cells. Corneal healing is modulated by severance and regrowth of the corneal nerve21 and epithelial healing follows reinnervation.22 In the current study, a 25% reduction in subbasal nerve density was found at 6 months postoperatively. Other studies reported a mean reduction of 20% to 50% in subbasal nerve density at 6 months after SMILE and 50% to 62% after LASIK23,24 and showed that SMILE causes less damage to corneal innervation in the 3 to 6 months postoperative period compared with LASIK.23,24 In accordance with previous studies,8 we also found that epithelial thickening had stabilized by 3 months, a time point that does not coincide with nerve regrowth, thereby confirming that the rate of change in curvature accounts for epithelial growth.

IVCM showed that epithelial thickening could be explained by an increase in the number of cell layers, and not by cell hypertrophy. Such epithelial remodeling was already reported in a previous study after LASIK and photorefractive keratectomy.25 The epithelium appears to play a significant role in the early postoperative period after myopic SMILE, resulting in a need for initial overcorrection to ensure that refraction regresses back to the target value.

Another purpose of this study was to determine lenticule thickness accuracy.10,19 Only a few studies have assessed lenticule thickness accuracy by SDOCT. These studies reported a systematic discrepancy between the VisuMax lenticule thickness readout and the central stromal thickness reduction actually achieved. Reinstein et al9 speculated that the difference between the estimated and measured lenticule thicknesses was similar for both low and high myopia. Luft et al10 and Wang et al19 suggested that the discrepancy between estimated and measured lenticule thickness increases with refractive correction. Greater differences were observed in the high myopia group (12.3 ± 8.8 µm) and the very high myopia group (17.9 ± 6.9 µm).19 Biomechanical changes occurring after SMILE might account for this systematic difference. The central stroma might expand after SMILE due to tension release following the disruption of the stromal collagen lamellae during lenticule formation,9 in a process similar to the peripheral stromal expansion observed after LASIK.26

This hypothesis is consistent with the significant correlation between the discrepancy between target and achieved lenticule discrepancy and refractive correction. However, Luft et al10 suggested that other factors might also be involved. Keratocyte-mediated wound healing at the laser-cut interface may cause postoperative stromal thickening. Liu et al27 reported the presence of intense keratocyte activity and reflective particles in the interface layer. These results are based on a subjective interpretation of images at the interface level, without quantitative evaluation. We assessed keratocyte density and estimated fibrosis at the interface. The keratocytes observed were activated, but their density at the interface was very low. Anterior IF and Posterior IF keratocyte densities were also significantly lower than preoperative keratocyte density. Furthermore, evaluations of interface fibrosis based on counts of the white pixels per image revealed no correlation with the mismatch between target and achieved lenticule thickness. The hypothesis that stromal remodeling explains the mismatch between target and achieved lenticule thickness is, therefore, consistent with our IVCM results.

Anterior IF and Posterior IF keratocyte density decreased after surgery and showed no signs of recovery 6 months after SMILE, consistent with the findings of previous studies.28,29 The most likely explanation for this decrease in keratocyte density is keratocyte apoptosis mediated by epithelial and tear film cytokines.13 Erie et al29 reported a persistent decrease in keratocyte density after photorefractive keratectomy and LASIK. Keratocytes in the middle and posterior stromal layers appear to be less affected by photorefractive keratectomy than anterior keratocytes. After LASIK, the rate of keratocyte loss appears to be greatest in the stroma immediately anterior and posterior to the ablation interface. Although the role of stromal keratocytes in the maintenance of corneal health and the clinical significance of the loss of keratocytes remain to be determined, it could be concerning in terms of risk of ectasia after correction of high myopia with SMILE.

We evaluated stromal fibrosis by counting the number of white pixels, but fibrosis can also be assessed by measuring corneal backscatter. Indeed, a technique known as confocal microscopy through-focusing has been developed to collect and quantify three-dimensional information from the cornea. This technique can be used to measure relative levels of stromal cell and extracellular matrix backscatter.30 Unfortunately, we were unable to obtain such measurements because we do not have the hardware or software required.

Finally, we confirm that epithelial thickening and lenticule thickness error increase with the degree of myopia 6 months after SMILE. Our sample size consisted of 60 eyes from 30 patients, but quantitative analysis allowed by IVCM provides robust data on epithelial thickening and lenticule thickness error after myopic SMILE. Stromal remodeling may be the root cause of the observed discrepancy between the estimated and measured lenticule thicknesses. The 10% overcorrection nomogram appears to be sufficient for low to moderate myopia. Adapting the programmed VisuMax readout for lenticule thickness, especially for eyes with high and very high myopia, may therefore decrease the need for reenhancement procedures and increase predictability after SMILE.

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Patient Demographics and Ocular Characteristics at Inclusion (N = 60)

ParameterValue
Female/male (n)17/13
Eyes (n)60
Age (y)
  Mean ± SD33.24 ± 7.13
  Range21 to 50
CDVA (logMAR)
  Mean ± SD−0.08 ± 0.77 (20/16)
  Range0.0 (20/20) to −0.20 (20/12.5)
Target SE refraction correction in primary treatment (D)
  Mean ± SD−3.99 ± 1.50
  Range−1.75 to −8.00
Preoperative corneal thickness (µm)
  Mean ± SD546.14 ± 27.77
  Range489 to 614
Preoperative mean keratometry (D)
  Mean ± SD44.02 ± 1.33
  Range40.95 to 47.15

Mean Epithelial Thickness (µm) in All Zones and Comparison of the Values Before and After SMILEa

Zone of EpitheliumPreoperative1 WeekPS1b1 MonthPM1b3 MonthsPM3b6 MonthsPM6b
Central53.7 ± 4.054.9 ± 4.3.13555.5 ± 3.9.00756.4 ± 4.1< .00157.1 ± 4.1< .001
Mid-peripheral
  Superior53.4 ± 3.854.5 ± 4.5.04855.0 ± 4.0.00456.2 ± 4.0< .00155.7 ± 4.2< .001
  Supranasal53.6 ± 3.654.6 ± 4.1.02755.1 ± 3.9.00255.1 ± 7.6.16155.6 ± 4.0< .001
  Nasal54.3 ± 3.855.0 ± 3.9.04055.6 ± 4.1.00456.2 ± 3.7< .00156.1 ± 3.9< .001
  Infranasal55.3 ± 4.055.8 ± 4.0.12556.4 ± 4.4.00556.8 ± 3.9< .00156.9 ± 4.0< .001
  Inferior55.6 ± 3.956.2 ± 4.3.10757.0 ± 4.4.00157.0 ± 3.5< .00157.1 ± 3.8< .001
  Infratemporal54.8 ± 4.055.9 ± 3.9.00156.6 ± 3.9< .00157.2 ± 3.5< .00157.1 ± 3.8< .001
  Temporal53.6 ± 4.155.8 ± 3.9< .00156.3 ± 3.7< .00157.2 ± 3.6< .00156.9 ± 3.7< .001
  Supratemporal53.1 ± 3.955.2 ± 4.2< .00155.6 ± 3.7< .00156.9 ± 4.2< .00156.6 ± 4.2< .001
Peripheral
  Superior52.9 ± 3.554.7 ± 4.9.00854.8 ± 4.3.00155.9 ± 4.0< .00155.6 ± 3.8< .001
  Supranasal53.4 ± 3.554.9 ± 4.4.00155.3 ± 4.2< .00156.0 ± 3.9< .00155.5 ± 3.8< .001
  Nasal54.0 ± 3.655.4 ± 4.7.00355.7 ± 5.2.00156.6 ± 4.3< .00156.1 ± 4.8< .001
  Infranasal55.4 ± 3.955.0 ± 4.8.42656.0 ± 4.9.20855.7 ± 5.0.49755.2 ± 5.5.807
  Inferior55.7 ± 3.954.4 ± 4.9.02356.1 ± 5.0.51855.3 ± 4.9.50454.7 ± 5.3.110
  Infratemporal55.0 ± 3.853.9 ± 4.3.04855.2 ± 4.9.73555.4 ± 4.9.45954.9 ± 5.4.807
  Temporal53.2 ± 3.455.0 ± 4.0< .00155.4 ± 4.0< .00156.1 ± 4.0< .00155.8 ± 4.5< .001
  Supratemporal52.6 ± 3.755.1 ± 4.2< .00155.1 ± 3.3< .00156.8 ± 3.9< .00156.5 ± 4.0< .001

Confocal Microscopy Results

ParameterPreoperative Mean ± SD1 Month3 Months6 Months



Mean ± SDPaMean ± SDPaMean ± SDPa
Epithelium (μm)
  Superficial cells904 ± 158997 ± 181.22b879 ± 174.19b975 ± 187.18b
  Wing cells4,307 ± 8794,432 ± 914.13b4,476 ± 925.15b4,219 ± 963.12b
  Basal cells7,461 ± 9017,419 ± 986.14b7,584 ± 1,070.16b7,641 ± 1,157.17b
Subbasal nerve density (mm/mm2)22.11 ± 3.4311.16 ± 4.74< .001b14.69 ± 3.97< .001b16.47 ± 3.85< .001b
Dendritic cell density (cells/mm2)32.1 ± 1967.3 ± 4< .001b55.4 ± 19< .001b48.6 ± 17< .001b
Keratocyte density (cells/mm2)
  10 μm anterior to the interface280.3 ± 66.2192.7 ± 57.4< .001b189.8 ± 64.3< .001b191.7 ± 59.8< .001b
  Interface30.2 ± 15.728.4 ± 17.1.11c29.4 ± 16.8.19c
  10 μm posterior to the interface253.9 ± 63.8151 ± 68.3< .001b154.8 ± 61.3< .001b148.7 ± 58.5< .001b
White pixel number per image27,651 ± 3,45622,081 ± 2,987.006c21,541 ± 2,412< .001c
Authors

From Centre Hospitalier National d'Ophtalmologie des Quinze-Vingts, Paris, France (NR, LT, IG, VB, LL, NB); and Sorbonne Université, INSERM, CNRS, Institut de la Vision, Paris, France (VB, LL, NB).

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

AUTHOR CONTRIBUTIONS

Study concept and design (NR, LL, NB); data collection (NR, IG); analysis and interpretation of data (NR, LT, IG, VB, LL, NB); writing the manuscript (NR, LT, LL, NB); critical revision of the manuscript (NR, LT, IG, VB, LL, NB); statistical expertise (NR, LT, VB, LL, NB); administrative, technical, or material support (NB); supervision (NB)

Correspondence: Nacim Bouheraoua, MD, PhD, Centre Hospitalier National d'Ophtalmologie des Quinze-Vingts, 28, rue de Charenton, Paris 75012, France. Email: nacim.bouheraoua@gmail.com

Received: March 05, 2020
Accepted: June 12, 2020

10.3928/1081597X-20200713-01

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