Study of Corneal and Retinal Thicknesses at Five Years after FS-LASIK and SMILE for Myopia

doi:10.21203/rs.3.rs-4144846/v1

BACKGUOND: This study aimed to compare corneal and retinal thicknesses at 5 years after femtosecond laser-assisted in situ keratomileusis (FS-LASIK) and small incision lenticule extraction (SMILE) for myopia, investigate the effect of epithelial remodeling on refractive status.

METHODS: Patients who received FS-LASIK or SMILE 5 years before were enrolled in this cross-sectional study. After 1:1 propensity score matching, each surgical group obtained 177 patients (177 eyes). Visual acuity and refraction were measured in this visit at 5 years after surgery. Corneal and retinal thickness maps were generated by spectral domain optical coherence tomography (SD-OCT). The Quality of Vision (QoV) questionnaire was used to assess visual symptoms and overall satisfaction. Thicknesses between groups were compared, contributing factors were analyzed, and correlations between epithelial thickness (ET), postoperative refractive status and overall satisfaction were evaluated.

RESULTS: Compared to SMILE group, central and paracentral ET of FS-LASIK group were thicker, pericentral ET was thinner, and the discrepancy of ET between central and pericentral zones was larger (all P<0.05). This discrepancy was correlated with postoperative spherical equivalent (SE) and myopic regression (all P<0.05) in both groups, but not correlated with QoV scores and overall satisfaction (both P>0.05). There was no statistical difference in stromal thickness (ST) and total corneal thickness (all P>0.05). Most annuluses of ET and ST were linearly related to preoperative SE (all P<0.05). Statistical difference of ET between high and super-high myopia groups only was found in supratemporal section of pericentral zone in SMILE group (P=0.012). Macular thickness, ganglion cell complex thickness and retinal nerve fiber layer thickness of two groups had no statistical difference (all P>0.05).

CONCLUSION: The tendency that ET in central zone was thicker than peripheral zone was more obvious at 5 years after FS-LASIK compared to SMILE. This tendency was also more significant in patients with higher degree of myopia. Epithelial remodeling might be one of the reasons of myopic regression, but it had little effect on patients' subjective visual quality and satisfaction. Retinal thickness was not affected by these two surgical methods.

Corneal Thickness

Retinal Thickness

Myopia

SMILE

FS-LASIK

Femtosecond laser-assisted in situ keratomileusis (FS-LASIK) and small incision lenticule extraction (SMILE) have become the preferred surgical procedures for correcting myopia and myopic astigmatism.[1] FS-LASIK has been in clinical practice for years, and its efficacy and safety also have been confirmed.[2] SMILE, in which the intrastromal lenticule is removed through a small incision of 2 mm in length so the side effects associated with the flap creation could be avoided,[3] gains popularity progressively.[4] Corneal epithelium has a potential to compensate the change of surface or irregular of stroma by altering its thickness.[5] After FS–LASIK and SMILE such remodeling has been observed in short-term follow-up.[6–11]For the accuracy of myopic correction and the prevention of ectasia after surgery, precise prediction of stromal reduction is important. But previous studies have revealed the deviation between the amount of planed and achieved stroma reduction.[12–14] Thus, further studies with longer follow-up period are needed to describe the characteristics of epithelial and stromal distribution and analyze their influence. In addition, myopia with elongation of ocular axial length could bring a series of fundus changes, and the long-term safety of fundus after surgery is of concern.

This study aimed to observe the epithelial and stromal thickness distribution at 5 years after these two different surgical methods by spectral domain optical coherence tomography (SD-OCT), explore their contributing factors and effect on refractive status, and evaluate the surgical safety in terms of retinal thickness.

This is a cross-sectional study. A visit at 5 years after surgery was conducted for patients who received FS-LASIK and SMILE for myopia and myopic astigmatism in Beijing Tongren Hospital from January 2015 to November 2017. The inclusion criteria were as follows: over 18 years of age at surgery; preoperative spherical equivalent (SE) ranged from − 12.00D to -1.00D; diopter increase≥-1.00D within 2 years; preoperative correction distance visual acuity (BCDVA) was 20/20 or better; stopped wearing corneal contact lens for 2 weeks, rigid contact lenses for 1 month and orthokeratology lens for more than 3 months. The exclusion criteria were as follows: presence or history of ocular and systemic diseases, or history of ocular surgery. All eligible patients participating in this study were divided into two surgical groups, and their age at surgery, gender, preoperative SE, flattest keratometry (Kf) and steepest keratometry (Ks) measured by TMS-4 (Tomey Corporation, Nagoya, Japan), central corneal thickness (CCT) measured by Lenstar LS 900 (Haag-Streit AG, Koeniz, Switzerland), and fundus examination using binocular indirect ophthalmoscopy were also consulted and recorded.

This study was approved by the Institutional Review Board of Beijing Tongren Hospital and adhered to the tenets of the Declaration of Helsinki. Written informed consent was obtained from all participants.

SURGICAL TECHNIQUE

Surgeries were performed by three experienced surgeons (FZ, CZ, YQ). In FS-LASIK VisuMax femtosecond laser system (Carl Zeiss Meditec AG, Jena, Germany) was used to create the corneal flap. The pulse energy was 130nJ. The intended flap thickness was 95–110µm. The position of the hinge was set at the 12 o’clock. Lifting the flap, the VISX STAR S4 (VISX Inc, Santa Clara, California, America) excimer laser was used to perform stromal ablation, and the diameter of the optical zone was 6.0-6.5mm. Then the flap was carefully repositioned over the stroma.

SMILE was performed using the VisuMax 500-kHz femtosecond laser system (Carl Zeiss Meditec AG, Jena, Germany). The pulse energy was 130nJ. The intended cap thickness was 110–130µm and the diameter of the optical zone was 6.0-6.5mm. A 2-mm incision was made at the position of 11 or 12 o'clock. When the upper and lower layers had been separated, the lenticule was removed through the incision.

MEASUREMENTS

All patients received a complete clinical examination in this visit at 5 years after surgery, including uncorrected visual acuity (UCDVA) and BCDVA, noncontact tonometry, slit-lamp evaluation, manifest refraction, fundus examination. Refractive data of early postoperative period were lacked in our cross-sectional study, so myopic regression was defined as UCDVA of < 20/20, and a myopic shift of > 0.50D from target refraction of 0.00D.

The Quality of Vision (QoV) questionnaire was used to assess 10 visual symptoms including glare, halos, hazy vision, starbursts, blurred vision, double or multiple images, distortion, fluctuation in vision, focusing difficulties, and difficulty judging distance or depth perception. Each symptom has 3 rating scales: frequency (0 = never, 1 = occasionally, 2 = quite often, 3 = very often), severity (0 = not at, 1 = mild, 2 = moderate, 3 = severe), and bothersome (0 = not at all, 1 = a little, 2 = quite, 3 = very).[15] Patients were also asked about their overall satisfaction with the procedure (0 = very dissatisfied, 1 = dissatisfied, 2 = general, 3 = less satisfied, 4 = satisfied, 5 = very satisfied).

RTVue-100 (Optovue, Inc., Fremont, USA) was used to measure corneal and retinal thicknesses and performed by a single trained investigator (JL) at all visits. “Pachymetry” scan mode was selected to measure epithelial thickness (ET), stromal thickness (ST) and total corneal thickness (CT). The scan was triggered manually when both horizontal and vertical reflection stripes were simultaneously observed. Maps were divided into 17 sections: a central 2 mm diameter zone, 8 paracentral sections within an annulus between the 2- and 5- mm diameter rings, and 8 pericentral sections within an annulus between the 5- and 6- mm diameter rings. △ET-1, △ST-1 and △CT-1 were calculated as mean ET, ST and CT in central zone minus that in paracentral zone respectively. And △ET-2, △ST-2 and △CT-2 represented the differences between central and pericentral zones. “Retina” scan mode was selected to measure the macular thickness (MT). The scan was manually triggered after aligning the fovea. MT maps were divided into 9 sections: fovea represented the area within 1mm diameter, 4 sections of parafovea within an annulus between the 1- and 3- mm diameter rings, 4 sections of perifovea within an annulus between the 3- and 5- mm diameter rings. “GCC” scan mode was selected to measure the ganglion cell complex (GCC) thickness which contained internal limiting membrane to the inner plexiform layer, centered 1mm temporal to fovea and across 6-mm dimeter area. Thicknesses in the superior and inferior zones were recorded. “ONH” scan mode was selected to measure the retinal nerve fiber layer (RNFL) thickness. Aligning the optic disc and the scan was triggered manually. The diameter of the scan was 3.4mm. And the RNFL maps included temporal, superior, nasal and inferior zones. Data of patients’ right eyes were selected for statistics.

STATISTICAL ANALYSIS

SPSS (version 26; SPSS Inc., Chicago, IL, USA) was used for statistical analysis. A descriptive statistic was used to calculate the mean and standard deviations. The normality of all data samples was checked using the Kolmogorov-Smirnov test. 1:1 propensity score matching (PSM) was used to control age at surgery, gender and preoperative SE. Match tolerance was set to 0.02. Independent-samples T test was used for normally distributed data when comparing, and the Mann-Whitney U test for non-normally distributed data. Paired t-test was used to detect the asymmetry of cornea. Linear regression analysis was used to identify the contributing factors. Spearman correlation analysis was used to assess the correlation between ET, postoperative SE, myopic regression and overall satisfaction. A P value of less than 0.05 was considered significant.

A total of 575 patients (575 eyes) participated in the study. In FS-LASIK group 339 patients (339 eyes) completed postoperative OCT examination, and 183 patients (183 eyes) in SMILE group. After PSM each group included 177 patients (177 eyes) ultimately, and the baseline demographic and clinical characteristics were comparable between groups (Table 1).

CORNEAL AND RETINAL THICKNESS PROFILES

The mean values of ET, ST and CT at 5 years after surgery were shown in Figure 1. In both groups ET of the temporal and inferior sections were significantly thicker than nasal and superior sections (all P<0.001), which was contrary to the ST and CT (all P<0.001).

The average MT of two groups were shown in Figure 2. 119 patients of FS-LASIK group and 175 patients of SMILE group completed GCC and RNFL thickness measurements. The GCC and RNFL thickness profiles were also shown in Figure 2.

COMPARISON OF FS-LASIK AND SMILE

Except supratemporal and superior sections of paracentral zone, all other sections demonstrated significant variation of ET (P of central zone, temporal, supratemporal, superior, supranasal and nasal sections of paracentral zone were 0.013, 0.013, 0.064, 0.128, 0.032, 0.003, other P<0.001, Figure 1). In FS-LASIK group ET was thicker in the central and paracentral zones (P=0.013, 0.001) and thinner in pericentral zones (P<0.001) than that in SMILE group. And △ET-2 was statistically different between groups (P<0.001) (Table 2 and Figure 3). There was no statistical difference of ST and CT in all sections (all P>0.05, Figure 1), same as the mean ST and CT of 3 annuluses, △ST-1, △ST-2, △CT-1 and △CT-2 (all P>0.05, Table 2 and Figure 3).

There was no significant difference in MT between groups, so as the GCC and RNFL thicknesses (all P>0,05).

CORRELATION ANALYSIS

Multiple linear regression analysis reveals that ET would be thicker in central and paracentral zones with higher degree of myopia, so as △ET-1 and △ET-2 (all P<0.05). The mean ET of annuluses were correlated with gender (P<0.05). Age at surgery and corneal curvature only correlated with a few parameters in two groups (P<0.05) (Additional file 1). It was worth noting that after 1:1 PSM controlling age at surgery and gender, we also compared ET of patients with super-high myopia (preoperative SE <-8.00D) and those with high myopia (preoperative SE ranging from -8.00D to -6.00D) in two surgical groups. 20 patients in all were enrolled in FS-LASIK group and 16 patients in SMILE group. Ks, Kf and CCT preoperative were comparable. Mann-Whitney U test showed that statistical difference only was found in supratemporal section of pericentral zone in SMILE group (P=0.012, Table 3).

ST would be thinner with higher degree of myopia (P<0.05). In FS-LASIK group the mean ST in central and paracentral zones, and △ST-2 were correlated with gender (P<0.05), so as △ST-1 and △ST-2 in SMILE group. All parameters of FS-LASIK group were affected by corneal curvature (P<0.05). And all parameters in both groups were not associated with age at surgery (all P>0.05) (Additional file 2).

The median and interquartile range of postoperative UCDVA and postoperative SE were -0.10 (-0.10, 0.00) logMAR and 0.000D (0.000, 0.000) D in FS-LASIK group, -0.10 (-0.10, 0.00) logMAR and 0.000D (-0.468, 0.000) D in SMILE group. ET in the central zone of FS-LASIK group and △ET-1 and △ET-2 of two groups were negatively correlated with SE postoperative (P=0.023, 0.001, <0.001, 0.008 and 0.036). Myopic regression occurred in 15.25% of patients in FS- LASIK group and 12.99% in SMILE group. ET in the central zone and △ET-1 of the FS-LASIK group and △ET-2 of two groups were correlated with myopic regression (P=0.035, 0.001, <0.001and 0.016). 151 patients of FS-LASIK group and 142 patients of SMILE group completed the questionnaire which revealed that there was no relevance between subjective visual quality (QoV scores and overall satisfaction) and ET (ET of annuluses, △ET-1 and △ET-2) in two groups (all P>0.05, Table 4 and Table 5).

In all enrolled patients, MT in perifovea (P<0.001), GCC (P=0.008, 0.010), and superior (P=0.004), nasal, inferior RNFL (both P<0.001) were linearly positively correlated with SE preoperative.

Using SD-OCT, our cross-sectional study revealed the distribution of epithelium at 5 years postoperatively. This is the longest visit from surgery as we know describing the differences of distribution patterns, and our result supported and complemented to the previous observation within 3 months postoperative.[9] ET increases greater after FS-LASIK than SMILE in the central 5-mm diameter zone, and less in 5–6 mm diameter zone. Reasons might account for this. These surgical methods have different effects on the stroma, and epithelial remodeling as the compensation is also affected. In FS-LASIK, the actual ablation at the paracentral zone might be deeper for compensation of the laser-induced spherical aberration, and the greatest epithelial thickening was observed in 3–4 mm diameter zones,[16] which was included in the paracentral zone of our study. At the same time, considering the correlation between ET and spherical aberration which is greater after FS-LASIK, higher epithelial thickening was also expected.[17] Differently, optical zone transits smoothly from the center to the periphery in SMILE. Centrifugal increase after SMILE was presumed to be related to biomechanical change and central stroma remodeling.[9] Furthermore, ET increased until 3 months in the central zone after FS-LASIK, but until 1 month after SMILE, and different distribution patterns between these two surgeries appeared at 1 month postoperatively.[6, 9, 10] Then ET tends to be stable up to 2 years.[7] Therefore we speculated that different distribution patterns mainly developed shortly after surgery and persisted up to 5 years, although this study only included one visit at 5 years after surgery.

To verify the different distribution patterns of ET and their impact after two surgical methods, we analyzed that △ET-1 and △ET-2 was larger in FS-LASIK group, and the difference of △ET-2 was statistically significant. Central epithelium thickening greater than periphery can be regarded as a convex lens adding on the corneal surface,[18] and increasing ET might induce myopization of its refractive power.[17] △ET-1 and △ET-2 of two groups were negatively correlated with SE postoperative in our study, which was similar to the results after LASIK and SMILE.[10, 11] △ET-1 of FS-LASIK group and △ET-2 of two groups were also correlated with myopic regression. Hence, we considered that epithelial remodeling may be one of the factors affecting refractive status after surgery, especially after FS-LASIK. On the other hand, the mean ET of annuluses and their discrepancies were not relevant to the subjective visual quality of patients, which included complaints of visual symptoms and overall satisfaction. We noticed that the myopic shift in refraction was quite subtle (the average postoperative SE was − 0.213D in FS-LASIK group and − 0.298D in SMILE group), which might explain such irrelevance. Combined with the conclusion about time course of epithelial remodeling mentioned above, the monitoring of ET and the use of drugs such as steroid and anti-glaucoma in the short term postoperative are more important than the long term.[18–20]

The remodeling induced by surgery did not alter the vertical distribution characteristic of epithelium 5 years postoperatively.[21] The asymmetry might be related to the gravity-dependent flow of tear film and mechanical force exerted by eyelid when blinking.[22] And especially after FS-LASIK, the reducing of corneal sensitivity and lower blink rate increase the influence of incomplete blinks and result in epitheliopathy.[23] Although this change mostly occurred within 6 months after surgery, punctate stains were found in inferior area among a small number of patients complaining of dry eye in our study. The horizontal asymmetry was opposite to that in primary eyes, but consistent with other studies after surgery.[6, 10] Large curvature gradient change caused by astigmatism correction might be the reason.

Various studies have shown that epithelial remodeling is related to the attempted myopic correction.[6, 8, 10, 16] Moreover, △ET-1 and △ET-2 increased with higher degree of myopia in this long-term research. Based on the correlation of ET and SE postoperative described above, higher degree of unevenness of ET in annuluses might affect the stability of refractive state in patients with higher myopia. But to our surprise, in most of sections there was no statistic variation of ET between patients with high and super-high myopia 5 years after both surgeries, which seemed to be different from short-term postoperative outcome.[24] This result might be limited by the small number of cases after matching, and the difference of diopters between groups did not broadened enough. The relationship between epithelial remodeling and visual quality in patients with high or super-high myopia was still unconclusive. And the upper limit of the degree of epithelial remodeling with the amount of correction remains unknown.

Our correlation analyzation suggested the stability of ET for young patients at 5 years after surgery. However, Luft et al. found that age diminished the effect of surgical refractive correction on epithelial remodeling.[8] The long-term consequences of this interaction require further studies with a wider range of ages. In both groups male patients had thicker ET. According to the previous research, gender would not increase the prediction of epithelium thickening,[8] difference between gender was consistent with primary eyes,[21] and might result from the influence of gonadal hormones on ocular development.[25] Ks and Kf impacted on the mean ET in pericentral zone in the FS-LASIK group, and they were also negatively associated with △ET-1 and △ET-2 in the SMILE group. It was reported that patients with flatter corneas whose postoperative curvatures changed greater, which was ascribed to the less eyelid pressure.[26]

In terms of ST, Luft et al. observed the increase of central stroma until the end of their 1 year of observation.[14] Our study shows the long-term results without interference of measurement, including wound healing mediated by keratocyte in interface and short-term postoperative corneal edema. There were vertical and horizontal asymmetries of ST in both surgical groups. Surgeries preserved the distribution characteristics of primary eyes.[27] In current studies, when higher refractive error was corrected, the discrepancy between the planned and achieved lenticule thickness, or stromal reduction would be larger.[12, 14] Despite this mismatch, our study confirmed that residual ST was still linearly correlated to SE preoperative after these two surgical methods.

In this study there was no statistical difference of ST between groups. SMILE is considered to conduct more stromal ablation clinically. However, underestimation and overestimation of actual central stromal reduction after FS-LASIK and SMILE were reported in the follow-up of 6 months respectively.[13] For FS-LASIK, laser ablation increasing progressively with depth would cause over ablation. In SMILE, the anterior-most stomal lamellae with high cohesive tensile strength can be preserved better and the posterior stroma thickening might be the major contributor to the discrepancy.[28–30] Clinically, FS-LASIK is commonly recommended for patients with thin cornea or high myopia. We cannot completely attribute the result to over ablation and stromal remodeling. But the preoperative CCT was comparable between groups, so our 5-year ST results were still valuable for reference. Alio et al. demonstrated that as the level of ametropias was greater, the misestimation would be higher, and real ST changes became equivalent, which proved the reliability of our study.[12]

MT, GCC and RNFL thicknesses at 5 years postoperatively were not significantly different between two groups, which was consistent with studies conducted in the short period after surgery.[31, 32] And no maculopathy was found in all patients participating. MT in perifovea, GCC, and superior, nasal, inferior RNFL were positively correlated with SE preoperative, matching the studies on retinal alterations in myopia.[33, 34] It should be noted that postoperative aberration and change of refractive properties will affect the measurement of RNFL,[35] and tilted optic disc might cause the displacement of OCT scan circle and overestimation of temporal RNFL.[36]

There are limitations to our study. OCT was not a routine preoperative examination 5 years before in our hospital, so the baseline data of all patients were hard to be collected. Though the inherent defects of cross-sectional study could not be avoided, we focused on the characteristics of corneal and retinal thickness at 5 years after two surgeries. Considering the relatively large number of patients included in this study, we did not specify the size of the optical zone and the thickness of the corneal flap or cap. However, previous studies have shown that epithelium thickens greater with smaller the optical zone.[10, 19] And the thickness of corneal flap or cap might be associated with the anterior surface curvature and biomechanics.[37–39] The improved SD-OCT covering a 9-mm diameter range had already been applied in clinical research.[6, 10] However, the clinical significance of longer follow-up period beyond the 6-mm diameter range needs to be investigated further.

In summary, the tendency that ET in central zone was thicker than peripheral zone was more obvious at 5 years after FS-LASIK compared to SMILE. This characteristic of thickness distribution was also more significant in patients with higher degree of myopia, though no statistical difference of ET was found between patients with high and super-high myopia. Epithelial remodeling might be one of the reasons of myopic regression, but it had little effect on patients' subjective visual quality. Moreover, ST was affected by over ablation and stromal remodeling respectively. And there were asymmetries of ET, ST and CT after two surgeries. Attention should be paid to the corneal remodeling to ensure the accuracy and stability of surgery. Retinal thicknesses were not affected by these two surgeries.

ETHICS APPROVAL AND CONSENT TO PARTICIPATE

This study was conducted in compliance with the Declaration of Helsinki and was approved by the ethics committee of Beijing Tongren Hospital affiliated to Capital Medical University. Informed consent was obtained from all patients.

CONSENT FOR PUBLICATION

Not applicable.

AVAILABILITY OF DATA AND MATETIALS

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

CONSENT FOR PUBLICATION

The authors declare that they have no competing interests.

FUNDING

This study was supported by Research and Transformation Application of Capital Clinical Diagnosis and Treatment Technology by Beijing Municipal Commission of Science and Technology (number Z201100005520043).

AUTHORS’ CONTRIBUTIONS

FZ provided supervision and final approval, made critical revision of the manuscript and fund application, designed the research and conducted the surgeries. JL conducted the research, collected data, performed statistical analysis and wrote the manuscript. XL and YX interpreted data and made revision of the manuscript. MS made revision of the manuscript. All authors read and approved the final manuscript.

ACKNOWLEDGEMENTS

The authors thank Professor Zhai and the colleagues from the Refractive Surgery Center, Beijing Tongren Eye Center of Beijing Tongren Hospital for supporting and assistance with collecting preoperative data and completing postoperative measurements.

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TABLE 1 Demographic and Clinical Characteristics of FS-LASIK Group and SMILE Group^a
Parameter	FS-LASIK (n=177)	SMILE (n=177)	P^b
Male/Female	55/122	57/120
Age (y)	26.46±5.77 (18～46)	27.02±5.11 (18～43)
SE (D)	-5.177±1.632 (-9.500～-1.000)	-5.248±1.596 (-9.250～-2.375)
Ks (D)	43.66±1.48 (40.58～47.40)	43.61±1.41 (39.80～47.16)	0.777
Kf (D)	42.56±1.42 (38.95～46.87)	42.63±1.36 (38.81～45.91)	0.646
CCT (μm)	537.10±30.06 (473～617)	541.19±24.81 (481～640)	0.163
FS-LASIK=femtosecond laser-assisted in situ keratomileusis; SMILE=small incision lenticule extraction; Age=age at surgery; SE=preoperative spherical equivalent; Ks=preoperative steepest keratometry; Kf=preoperative flattest keratometry; CCT=preoperative central corneal thickness; D=dioptors; ^aAll values are presented as mean ± standard deviation (range); ^bIndependent-samples T test.

TABLE 2 Comparison of Corneal Thicknesses Between FS-LASIK Group and SMILE Group^a
Parameter	FS-LASIK (n=177)	SMILE (n=177)	P^b
Epithelial thickness (μm)
Central	58.92±3.90 (48.17~73.96)	57.86±4.14 (48.19~70.25)	0.013
Paracentral	58.66±3.20 (48.03~69.45)	57.52±3.37 (49.12~66.17)	0.001
Pericentral	53.87±3.11 (43.37~60.99)	55.78±2.71 (48.98~63.11)	<0.001
△ET-1	0.26±2.10 (-5.29~8.04)	0.33±1.62 (-5.13~5.75)	0.713
△ET-2	5.05±3.94 (-2.75~17.74)	2.07±3.17 (-4.87~13.04)	<0.001
Stromal thickness (μm)
Central	403.68±31.90 (336.93~500.50)	406.68±22.34 (348.02~482.05)	0.306
Paracentral	447.38±30.16 (380.61~527.89)	449.81±21.79 (395.41~529.74)	0.385
Pericentral	501.73±29.81 (440.97~583.88)	501.92±22.87 (447.62~560.47)	0.945
△ST-1	-43.70±7.27 (-63.86~-25.68)	-43.13±6.06 (-61.99~-28.45)	0.427
△ST-2	-98.05±15.91 (-142.00~-56.22)	-95.24±13.21 (-138.58~-58.04)	0.072
Total corneal thickness (μm)
Central	462.60±31.79 (394.70~554.85)	464.54±22.24 (404.47~540.32)	0.507
Paracentral	506.04±30.11 (438.90~585.18)	507.34±21.85 (450.83~589.21)	0.643
Pericentral	555.60±30.06 (493.31~639.83)	557.71±23.00 (505.81~620.09)	0.458
△CT-1	-43.44±7.24 (-62.16~-24.89)	-42.80±6.08 (-61.39~-29.14)	0.370
△CT-2	-92.99±15.06 (-132.39~-51.51)	-93.17±12.52 (-135.07~-58.72)	0.904
FS-LASIK=femtosecond laser-assisted in situ keratomileusis; SMILE=small incision lenticule extraction; △ET-1=the difference of mean epithelial thickness between central and paracentral zones; △ET-2=the difference of mean epithelial thickness between central and pericentral zones; △ST-1=the difference of mean stromal thickness between central and paracentral zones; △ST-2=the difference of mean stromal thickness between central and pericentral zones; △CT-1=the difference of mean total corneal thickness between central and paracentral zones; △CT-2=the difference of mean total corneal thickness between central and pericentral zones; ^aAll values are presented as mean ± standard deviation; ^bIndependent-samples T test.

TABLE 3 Comparison of Epithelial Thickness (μm) Between Patients with High and Super-High Myopia^a
Group	n	Central	Paracentral	Pericentral	△ET-1	△ET-2
FS-LASIK
Super-High	10	62.10 (59.89, 63.66)	58.63 (57.70, 59.86)	50.60 (48.25, 52.86)	2.58 (1.32, 5.08)	11.64 (9.05, 13.70)
High	10	60.62 (57.73, 62.87)	58.69 (57.51, 61.04)	49.38 (48.12, 54.46)	1.73 (-0.96, 3.52)	9.94 (8.47, 11.93)
P^b		0.353	0.853	0.912	0.247	0.436
SMILE
Super-High	8	58.94 (55.87, 65.93)	58.03 (55.64, 63.42)	53.45 (53.00, 56.71)	1.18 (-0.10, 3.52)	4.84 (2.48, 10.77)
High	8	57.61 (56.02, 61.15)	58.13 (56.77, 59.68)	55.92 (54.63, 56.83)	-0.35 (-1.00, 1.00)	2.48 (0.71, 5.84)
P^b		0.959	0.878	0.279	0.065	0.105
FS-LASIK=femtosecond laser-assisted in situ keratomileusis; SMILE=small incision lenticule extraction; △ET-1=the difference of mean epithelial thickness between central and paracentral zones; △ET-2=the difference of mean epithelial thickness between central and pericentral zones; ^aThickness values are presented as median (Q₂₅,Q₇₅); ^b Mann-Whitney U test.

TABLE 4 Correlations Between Epithelial Thickness, Postoperative Spherical Equivalent and Myopic Regression^a
Group	Postoperative SE		Myopic Regression
	r	P	r	P
FS-LASIK (n=177)
Central	-0.171	0.023	0.159	0.035
Paracentral	-0.09	0.233	0.093	0.22
Pericentral	0.138	0.066	-0.102	0.176
△ET-1	-0.241	0.001	0.256	0.001
△ET-2	-0.311	<0.001	0.306	<0.001
SMILE (n=177)
Central	-0.092	0.225	0.134	0.075
Paracentral	-0.029	0.699	0.107	0.157
Pericentral	0.079	0.294	-0.016	0.833
△ET-1	-0.199	0.008	0.147	0.051
△ET-2	-0.158	0.036	0.181	0.016
FS-LASIK=femtosecond laser-assisted in situ keratomileusis; SMILE=small incision lenticule extraction; SE=preoperative spherical equivalent; △ET-1=the difference of mean epithelial thickness between central and paracentral zones; △ET-2=the difference of mean epithelial thickness between central and pericentral zones; ^aSpearman correlation analysis.

TABLE 5 Correlations Between Epithelial Thickness, QoV Scores and Overall Satisfacion^a
Group	QoV Scores		Overall Satisfacion
	r	P	r	P
FS-LASIK (n=151)
Central	0.046	0.537	-0.088	0.283
Paracentral	0.090	0.271	-0.073	0.37
Pericentral	0.086	0.297	0.078	0.34
△ET-1	-0.039	0.637	-0.079	0.334
△ET-2	-0.017	0.837	-0.188	0.149
SMILE (n=142)
Central	0.091	0.280	-0.019	0.819
Paracentral	0.099	0.239	-0.011	0.892
Pericentral	0.054	0.522	-0.031	0.711
△ET-1	-0.013	0.876	-0.002	0.984
△ET-2	0.027	0.747	0.031	0.715
QoV=Quality of Vision; FS-LASIK=femtosecond laser-assisted in situ keratomileusis; SMILE=small incision lenticule extraction; △ET-1=the difference of mean epithelial thickness between central and paracentral zones; △ET-2=the difference of mean epithelial thickness between central and pericentral zones; ^aSpearman correlation analysis.

No competing interests reported.

Study of Corneal and Retinal Thicknesses at Five Years after FS-LASIK and SMILE for Myopia

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Abstract

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BACKGROUND

METHODS

SURGICAL TECHNIQUE

MEASUREMENTS

STATISTICAL ANALYSIS

RESULTS

DISCUSSION

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References

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Supplementary Files

Status:

Journal Publication

Version 1