Abstract
Purpose:
To investigate 7-year changes in corneal densitometry (CD) and its influencing factors following small incision lenticule extraction (SMILE) for moderate to high myopia.
Methods:
A total of 32 patients (52 eyes) with a mean spherical equivalent (SE) refraction of -6.30 ± 1.30 D who underwent SMILE were recruited for this prospective study. Pre- and postoperative CD and corneal aberrations were measured by an oculus Pentacam system, and patients were followed up to 7 years. The CD evolution over time and its influencing factors were explored.
Results:
No complications were observed after SMILE. There was a significant time effect on postoperative CD (P < 0.05). The CD values at the 0-to-2 and 2-to-6 mm of the anterior layer increased at the first day (P < 0.05) and declined to baselines at 1 year postoperatively; after that, CD values at any zone and any corneal layer presented a decreasing trend over time (all P < 0.05). Multivariate analysis showed that postoperative CD was positively correlated with age and optical zone (OZ), whereas negatively correlated with SE, corrected distance visual acuity (CDVA), and corneal aberrations (higher-order aberrations, Coma0 and Coma90) (all P < 0.05). Generalized estimating equation revealed that age, SE, CDVA, OZ, and Coma0 were the main factors influencing postoperative CD values (all P < 0.05).
Conclusion:
CD manifested a long-term decreasing trend in moderate to high myopia treated with SMILE. Its main influencing factors are age, SE, CDVA, OZ, and Coma0.
Keywords: Corneal densitometry, myopia, small incision lenticule extraction
Corneal transparency is a key element of the refractive index of a healthy cornea.[1,2] The corneal transparency following refractive surgery is not only related to the outcome, but also one of the indicators of surgical safety. Corneal densitometry (CD) can be objectively measured by Pentacam Scheimpflug imaging technology, and it quantitatively assesses the transparency of the cornea,[3,4] which can be used to monitor corneal pathological changes, such as interface reactions and keratinocyte activation, or turbidity, after corneal refractive surgery.[5]
The safety and efficacy of small incision lenticule extraction (SMILE), which is a minimally invasive corneal refractive procedure for myopia and myopic astigmatism, are supported by a large amount of clinical data.[6,7,8,9,10,11,12] Compared to other corneal refractive surgeries, SMILE is flapless and minimally invasive, which is conducive to maintaining the stability of the corneal structure.[8] During SMILE procedure, a femtosecond laser scans the stromal layer twice, and then the lenticule is manually removed; this could affect the transparency of the cornea.[9,10,11,12]
Understanding the CD changes following SMILE is beneficial for optimizing surgery and evaluating the prognosis.[9,10,13] Whether the CD values following SMILE are associated with patient age, preoperative refraction, optical zone (OZ), residual stromal bed (RSB), corneal aberrations, etc., has not been thoroughly investigated. Our study aimed to objectively evaluate the continuous CD changes following SMILE, and its influencing factors. To our knowledge, this is the longest and most frequent monitoring study on the changes in CD values following SMILE-treated patients with moderate to high myopia.
Methods
Prospective study
This prospective study began following the approval by the Ethical Committee of Hospital Review Board (No. 2013015-1). A total of 32 patients (14 men, 18 women; 52 eyes) undergoing SMILE with a mean spherical equivalent (SE) refraction of -6.20 ± 1.30D (range: −4.00 to −8.75D) were recruited. All patients signed informed consent forms before their inclusion in the study, and the study observed the tenets of the Declaration of Helsinki. The inclusion criteria for patients were: -10.00 D≤SE ≤-3.00 D, astigmatism <1.25 D, and residual stromal bed (RSB) ≥280 µm. Patients with systemic or ocular disorders were excluded.
Observation index
All participants underwent a preoperative ophthalmic evaluation that included uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), and manifest refraction; CD values and corneal aberrations were measured by Pentacam (Oculus Optikgeräte GmbH, Wetzlar, Germany).
CD measurement
Corneal densitometry was determined using the Pentacam HR (Oculus, Germany). The methodology used was comparable to the one described by Wei et al., with some wording reproduced in the methods section.[10] We evaluated corneal densitometry expressed in gray scale units (GSU) ranging from 0 (100% transparent) to 100 (completely opaque, 0% transparent). The CD values on each of the concentric annuli of diameters (0–2 mm, 2–6 mm, and 6–10 mm), and each layer (anterior [the first 120 µm of the corneal thickness], central [between the anterior and posterior layer], posterior layer [the last 60 µm of the cornea], and total layer [the complete corneal thickness]) were recorded.
Surgical procedures
The SMILE procedures were performed by the same surgeon (XZ) using the VisuMax 500-kHz femtosecond laser system (Carl Zeiss Meditec, Jena, Germany). The lenticule diameter, cap diameter, and corneal cap thickness were set at 6.5 mm, 7.5 mm at a depth of 120 µm (90° single-side cut, 2 mm length), and 100–120 µm, respectively. This procedure has been described thoroughly in previous reports.[7,14] After the surgery, each patient received standard topical treatment with antibiotics, steroids, and artificial tear eye drops.
Follow-up
The patients were monitored for a duration of 7 years. UDVA, CDVA, manifest refraction, CD values, and corneal aberrations were assessed prior to surgery and at postoperative intervals of 1 day, 1 year, 3 years, 5 years, and 7 years. Participants (3 eyes of 2 subjects at 1 year, 4 eyes of 3 subjects at 3 years, 7 eyes of 5 subjects at 5 years, 2 eyes of 1 subject at 7 years) were excluded, respectively, from the time point as they failed to show up for follow-up.
Statistical analysis
Data were analyzed using SPSS (Version 26.0, IBM, Chicago, USA). Mixed linear models for repeated measurements were constructed to analyze changes in CD values over time. The relationship between postoperative CD and surgery-related parameters, such as age, SE, CDVA, OZ, RSB, and corneal aberrations were evaluated by a generalized estimating equation (GEE) to explore the influencing factors of total CD long-term after SMILE. Statistical significance was defined as P < 0.05.
Results
All the eyes (14 men, 18 women; 52 eyes) underwent uneventful SMILE without complications. The preoperative baseline characteristics are shown in Table 1. The mean preoperative SE was -6.30 ± 1.30 D. The safety and efficacy indices were 1.10 ± 0.16 and 1.03 ± 0.16, respectively; 94.2% of the eyes (49 eyes) achieved a postoperative UDVA ≥20/20 and 100% of the eyes achieved a postoperative CDVA ≥ 20/20. Furthermore, 96.2% of the eyes (50 eyes) were within ± 1.00 D, and 86.5% (45 eyes) were within ± 0.50 D [Supplementary Fig. 1 (3.5MB, tif) ].
Table 1.
Patients’ baseline demographics before SMILE (Mean±SD)
| Characteristic | Mean±standard deviation | |
|---|---|---|
| Sex (M/F) | 14/18 | |
| Age (years) | 30.54±8.58 | |
| Spherical equivalent (D) | -6.30±1.30 | |
| Central corneal thickness (μm) | 549.15±34.12 | |
| Optical zone (mm) | 6.60±0.08 | |
| Lenticule thickness (μm) | 125.76±21.60 | |
| Residual bed thickness (μm) | 319.79±36.30 |
Corneal densitometry evolution
All CD values, represented in grayscale units (GSU), are summarized in Table 2. The time effect of the postoperative CD values was statistically significant (P < 0.05). Regarding the early postoperative changes, no significant differences were observed for any layer of the cornea. However, for specific ring zone, CD values corresponding to the 0-to-2 and 2-to-6 mm annuli of the anterior layer, and 0-to-2 mm annulus of the total layer, increased significantly at the first day, and then returned to baselines at 1-year after surgery when compared with preoperative values (P < 0.05). And CD values corresponding to the 0-to-2 mm annulus of the posterior layer at 1 year showed a slight but statistically significant decrease when compared to baseline and postoperative one day values (P < 0.05).
Table 2.
The corneal densitometry before and after SMILE
| Variables | Pre-op | Post-op |
||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1-day | 1-year | 3-year | 5-year | 7-year | ||||||||
| AL0-2mm | 25.70±3.02 | 30.24±6.31a | 23.91±3.75b | 19.77±2.01abc | 18.96±2.85abc | 16.49±3.88abcd | ||||||
| AL 2-6mm | 23.50±2.77 | 27.52±5.37a | 22.41±3.28b | 18.40±1.85abc | 17.69±2.49abc | 15.53±3.47abcd | ||||||
| AL6-10mm | 24.64±4.34 | 25.34±5.52 | 22.22±3.84 | 18.77±2.78ab | 18.94±3.41ab | 17.58±3.54abc | ||||||
| TAL | 25.90±3.86 | 27.40±5.48 | 24.41±2.64 | 20.37±1.54abc | 20.53±2.73abc | 18.10±2.77abc | ||||||
| CL0-2mm | 17.19±1.84 | 18.18±2.56 | 16.07±2.01 | 13.44±1.49abc | 12.61±2.13abc | 10.49±2.06abcde | ||||||
| CL2-6mm | 15.72±1.71 | 16.32±2.05 | 15.13±1.91 | 12.54±1.24abc | 11.72±1.73abc | 9.89±1.84abcde | ||||||
| CL6-10mm | 17.30±2.98 | 17.07±3.09 | 16.29±2.56 | 13.61±1.78abc | 13.53±2.19abc | 12.03±2.30abc | ||||||
| TCL | 18.02±2.59 | 17.80±2.90 | 16.89±2.02 | 14.50±1.53abc | 13.84±2.03abc | 12.02±2.14abcd | ||||||
| PL0-2mm | 14.74±2.24 | 14.93±2.20 | 13.41±1.66ab | 11.35±1.28abc | 9.58±2.08abcd | 9.29±1.43abcd | ||||||
| PL2-6mm | 13.95±2.06 | 14.22±1.86 | 13.18±1.42 | 11.11±1.13abc | 9.55±1.79abcd | 9.04±1.35abcd | ||||||
| PL6-10mm | 16.00±2.93 | 16.21±2.82 | 15.39±2.14 | 12.96±1.75abc | 12.11±1.82abc | 11.45±2.34abc | ||||||
| TPL | 15.93±2.77 | 15.92±2.57 | 15.09±1.90 | 12.84±1.62abc | 11.59±2.05abc | 10.97±2.12abcd | ||||||
| TL0-2mm | 19.18±2.29 | 21.12±3.55a | 17.79±2.03b | 14.86±1.49abc | 13.72±2.00abc | 12.10±2.37abcd | ||||||
| TL2-6mm | 17.81±2.19 | 19.34±2.99 | 16.87±2.05b | 13.97±1.27abc | 12.97±1.70abc | 11.48±2.17abcd | ||||||
| TL6-10mm | 19.49±3.64 | 19.54±3.72 | 18.15±3.10 | 14.95±2.03abc | 14.86±2.28abc | 13.68±2.72abc | ||||||
| TTL | 20.08±2.91 | 20.52±3.49 | 18.67±2.02 | 15.97±1.50abc | 15.23±1.94abc | 13.71±2.33abcd | ||||||
Note: a, vs Pre-op P<0.05; b, vs 1-day Post-op, P<0.05; c, vs 1-year Post-op; d, vs 3- year Post-op; e, vs 5- year Post-op. Pre-op, preoperative; Postop, postoperative; AL, anterior layer; TAL, anterior layer in all zones; CL, central layer; TCL, central layer of all zones; PL, posterior layer of all zones; TPL, posterior layer of all zones; TL, total layer; TTL, total layer of all zones
On the contrary, the long-term changes of CD values for each layer were detected at 3, 5, and 7-year follow-up points when compared to that at first year postoperatively. Specifically, CD values for any ring zones of each layer were decreased significantly at 3, 5, and 7-year points (all P < 0.05). What more, CD values corresponding to the 0-to-2 and 2-to-6 mm annuli of the three layers at 5 and 7-years, were further decreased when compared to the values at 3-years (all P < 0.05). And the values corresponding to the 0-to-2 and 2-to-6 mm annuli of the central layer at 7-years were also decreased when compared to the values at 5-years (all P < 0.05).
Corneal aberrations variations over time
Pre- and postoperative corneal aberrations were also evaluated using the Pentacam system, which were summarized in Table 3. Except for Coma0, the time effect of the higher-order aberrations (F = 4.486, P = 0.001), spherical aberration (F = 4.327, P = 0.001), and Coma90 (F = 5.407, P = 0.000) was statistically significant (P < 0.05). That is, compared to baselines, the postoperative higher-order aberrations (HOAs), spherical aberration, and Coma90 were significantly increased. However, significant differences in these aberration components were not detected among the follow-up time points.
Table 3.
The corneal aberrations before and after SMILE
| Variables | Pre-op | Post-op |
||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1-day | 1-year | 3-year | 5-year | 7-year | ||||||||
| HOAs | 0.29±0.18 | 0.56±0.43a | 0.47±0.16 | 0.45±0.11 | 0.47±0.15 | 0.48±0.20a | ||||||
| Coma0 | 0.42±1.43 | 1.21±2.77 | 1.26±2.86 | 1.50±2.52 | 0.98±2.12 | 1.03±3.23 | ||||||
| Coma90 | - 0.31±1.79 | - 1.24±2.56 | -2.41±1.86a | - 2.14±1.51a | -1.84±2.00 | - 2.40±2.13a | ||||||
| SA | 0.05±0.53 | 0.80±1.02a | 0.58±0.76 | 0.70±0.69a | 0.70±0.78a | 0.71±0.67a | ||||||
Note: a, vs Pre-op P<0.05; b, vs 1-day Post-op, P<0.05; c, vs 1-year Post-op; d, vs 3- year Post-op; e, vs 5- year Post-op. Pre-op, preoperative; Post-op, postoperative; HOAs, higher-order aberrations; SA, spherical aberration
Influencing factors of corneal densitometry after SMILE
Multivariate analysis was conducted to explore the correlations between CD values at different layers and clinical parameters [Table 4]. In this long-term observation, regarding corneal higher-order aberration, HOAs and Coma0 were the main factors influencing postoperative CD. Specifically, HOAs were negatively correlated with CD of central surrounding annulus (2-to-6 mm) at the anterior 120 µm layer (B = -14.704, P = 0.038), and Coma0 was inversely correlated with CD of any regions at any layers (all P < 0.05). In terms of surgical-related parameters, age was one of the influencing factors for CD corresponding to paracentral annulus (6-to-10 mm) at each corneal layer (all P < 0.05). SE was the influencing factor for CD corresponding to central zone (0-to-2 mm) at central layer, central surrounding annulus at anterior and central layers, paracentral annulus at all three corneal layers. That is, SE was negatively correlated with postoperative CD (all P < 0.05). Similarly, OZ was the influencing factor for CD corresponding to central surrounding annulus at anterior and central layers, paracentral annulus at all three corneal layers. That is, OZ was positively correlated with postoperative CD (all P < 0.05). And CDVA was also the influencing factor for CD corresponding to central zone at central layer, central surrounding annulus at all layers, and paracentral annulus at posterior layer. That is, CDVA was negatively correlated with postoperative CD (all P < 0.05), while RSB was positively correlated to CD corresponding to central surrounding annulus at anterior and posterior layers (both P < 0.05).
Table 4.
Multivariate analysis between postoperative corneal densitometry at different regions and layers and clinical parameters
| Variables | B Coefficient P Value |
0-2mm |
2-6mm |
6-10mm |
||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| AL | CL | PL | AL | CL | PL | AL | CL | PL | ||||||||||||
| Age | B | -0.111 | -0.030 | 0.087 | -0.039 | 0.032 | 0.108 | -0.393 | 0.319 | 0.327 | ||||||||||
| P | 0.393 | 0.742 | 0.372 | 0.743 | 0.719 | 0.245 | 0.032 | 0.009 | 0.006 | |||||||||||
| SE | B | -0.811 | -1.102 | -0.355 | -1.250 | -1.208 | -0.683 | -2.064 | -1.731 | -1.432 | ||||||||||
| P | 0.117 | 0.023 | 0.520 | 0.035 | 0.019 | 0.215 | 0.000 | 0.000 | 0.011 | |||||||||||
| OZ | B | 6.913 | 12.155 | 1.164 | 25.944 | 18.519 | 15.005 | 21.065 | 11.758 | 26.940 | ||||||||||
| P | 0.422 | 0.067 | 0.891 | 0.000 | 0.004 | 0.060 | 0.041 | 0.045 | 0.005 | |||||||||||
| CDVA | B | -1.434 | -6.598 | -3.682 | -13.568 | -10.839 | -12.211 | -6.931 | -1.255 | -16.491 | ||||||||||
| P | 0.731 | 0.015 | 0.300 | 0.000 | 0.000 | 0.000 | 0.213 | 0.686 | 0.000 | |||||||||||
| RSB | B | 0.010 | 0.009 | 0.009 | 0.015 | 0.011 | 0.011 | 0.003 | 0.004 | 0.005 | ||||||||||
| P | 0.290 | 0.176 | 0.103 | 0.031 | 0.059 | 0.027 | 0.742 | 0.451 | 0.322 | |||||||||||
| HOAs | B | -11.076 | -5.888 | -4.272 | -14.704 | -8.475 | -6.174 | -2.558 | -0.965 | 0.077 | ||||||||||
| P | 0.355 | 0.369 | 0.461 | 0.038 | 0.070 | 0.213 | 0.799 | 0.845 | 0.991 | |||||||||||
| Coma0 | B | -0.973 | -0.805 | -0.993 | -0.775 | -0.770 | -0.912 | -1.264 | -0.782 | -0.899 | ||||||||||
| P | 0.020 | 0.010 | 0.002 | 0.009 | 0.003 | 0.002 | 0.003 | 0.000 | 0.004 | |||||||||||
| Coma90 | B | -0.311 | -0.301 | -1.751 | -0.319 | -0.457 | -1.492 | -0.613 | 0.042 | -0.904 | ||||||||||
| P | 0.846 | 0.795 | 0.157 | 0.785 | 0.650 | 0.190 | 0.711 | 0.963 | 0.485 | |||||||||||
| SA | B | -0.205 | 0.966 | -0.977 | 1.925 | 2.080 | 0.602 | 1.183 | 1.410 | 0.432 | ||||||||||
| P | 0.913 | 0.509 | 0.437 | 0.213 | 0.127 | 0.612 | 0.584 | 0.286 | 0.754 | |||||||||||
AL, anterior layer; CL, central layer; PL, posterior layer; SE, spherical equivalent; CDVA, corrected distance visual acuity; OZ, optical zone; RSB, residual stromal bed; HOAs, higher-order aberrations; SA, spherical aberration
When analyzing the full depth of corneal layer, multivariate analysis showed that age was positively correlated with densitometry of the total posterior layer (B = 0.267, P < 0.05); HOAs and Coma90 were negatively correlated with densitometry at the total anterior layer (both P < 0.05). SE was negatively correlated with densitometry at any depth except for total posterior layer (all P < 0.01). CDVA and Coma0 were negatively correlated with densitometry at any depth (all P < 0.05). And OZ was positively correlated with densitometry at any depth (all P < 0.01) [Table 5].
Table 5.
Multivariate analysis between corneal densitometry at full depth and clinical parameters
| Variables | B Coefficient P Value |
TL 0-2mm | TL2-6mm | TL6-10mm | TAL | TCL | TPL | TTL | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Age | B | 0.000 | -0.008 | 0.266 | 0.274 | 0.194 | 0.267 | 0.233 | ||||||||
| P | 0.999 | 0.941 | 0.072 | 0.248 | 0.128 | 0.048 | 0.126 | |||||||||
| SE | B | -0.879 | -0.801 | -1.164 | -1.888 | -1.468 | -0.906 | -1.457 | ||||||||
| P | 0.068 | 0.236 | 0.188 | 0.001 | 0.003 | 0.153 | 0.007 | |||||||||
| CDVA | B | -4.655 | -9.011 | -1.818 | -13.513 | -12.039 | -24.758 | -16.049 | ||||||||
| P | 0.087 | 0.051 | 0.843 | 0.049 | 0.000 | 0.000 | 0.000 | |||||||||
| OZ | B | 9.234 | 13.286 | 6.287 | 27.183 | 18.620 | 30.225 | 24.880 | ||||||||
| P | 0.180 | 0.203 | 0.724 | 0.002 | 0.005 | 0.001 | 0.001 | |||||||||
| RSB | B | 0.011 | 0.009 | -0.001 | 0.025 | 0.012 | 0.010 | 0.012 | ||||||||
| P | 0.118 | 0.132 | 0.895 | 0.163 | 0.129 | 0.095 | 0.161 | |||||||||
| HOAs | B | -8.823 | -6.092 | 6.303 | -23.964 | -12.570 | -9.114 | -14.011 | ||||||||
| P | 0.218 | 0.380 | 0.596 | 0.034 | 0.068 | 0.134 | 0.061 | |||||||||
| Coma90 | B | -1.125 | -0.068 | 0.860 | -3.331 | -1.386 | -2.263 | -2.218 | ||||||||
| P | 0.364 | 0.960 | 0.683 | 0.025 | 0.195 | 0.074 | 0.072 | |||||||||
| Coma0 | B | -0.990 | -0.720 | -0.791 | -1.314 | -1.049 | -1.112 | -1.124 | ||||||||
| P | 0.004 | 0.018 | 0.041 | 0.000 | 0.000 | 0.001 | 0.000 | |||||||||
| SA | B | -0.649 | 2.893 | 3.723 | -0.828 | 0.560 | -0.827 | -0.093 | ||||||||
| P | 0.650 | 0.135 | 0.255 | 0.777 | 0.729 | 0.528 | 0.956 |
TL, total layer of a certain zone; TAL, anterior layer of all zones; TCL, central layer of all zones; TPL, posterior layer of all zones; TTL, total layer of all zones; SE, spherical equivalent; CDVA, corrected distance visual acuity; OZ, optical zone; RSB, residual stromal bed; HOAs, higher-order aberrations; SA, spherical aberration
Discussion
Corneal densitometry as an indicator of corneal transparency is critical for optimizing surgical outcomes after SMILE. A long-term CD observation of this procedure would be of great value.[9,10,13] This 7-year prospective study, which assessed the continuous changes and the influencing factors of CD values following SMILE, aimed to help refractive surgeons objectively evaluate postoperative corneal response, thus allowing these to be monitored long-term.
We found a significant time effect on postoperative CD values. At the early stage after SMILE, the increase of CD values was only detected at first day at the central and surrounding central zone of the anterior layer, which are the ones most likely to be manually affected by SMILE procedure, then declined back to baselines at 1 year. After that, a decreasing trend for CD values at any regions and any corneal layer was observed in this long-term study. At the 3-, 5-, and 7-year follow-up visits, the CD values exhibited significant reductions in comparison to the baseline measurements. Furthermore, after 7 years following SMILE surgery, the CD values in both the central and surrounding central zones showed a subsequent decrease in comparison to the values observed at 3 and 5 years at full corneal depth. The CD values rose on postoperative day one, possibly due to corneal edema, small lacuna, and cellular debris at the interface. It has been reported that cell debris found in the affected area could heighten light scattering on day one following SMILE.[14,15] Another potentially contributing factor could be microdistortions, which were frequently observed in Bowman’s layer one day after SMILE.[16,17,18] The postoperative early-stage CD values confirm prior findings in SMILE and other refractive surgeries, such as FS-LASIK and PRK.[9,10,19] Although cohort studies conducted over 3 years[9] and 5 years[10] in SMILE and FS-LASIK support a trend of decreasing CD values, this must not be considered indicative of a tendency towards keratoconus or ectasia. It is important to note that a rise in CD values is frequently linked to subclinical keratoconus.[20] The alterations in CD values after 7 years post SMILE is distinct from the age-related variations observed in healthy corneas.[21,22] There are two potential reasons for this contrasting pattern. Firstly, it is important to note that our subjects, with an average age of 30.5 ± 8.6 years, were monitored longitudinally for 7 years, which contrasts with the prior research that concentrated on CD value shifts in 10-year intervals related to aging.[21,22] Furthermore, stromal thinning that occurs from SMILE may lead to a decrease in corneal cell and basal nerve density.[23] Consequently, a more precise assessment of posterior backscattering could be made, potentially resulting in reduced CD values.
In this study, the influencing factors of postoperative CD values are investigated. Multivariate analysis was carried out along two dimensions, one for CD in different zones at each layer and the other for CD in all layers. Age was found to be statistically correlated with CD values at peripheral annulus in the anterior, central, and posterior layers, but only positively correlated with the posterior layer of all zones. Similar results have been reported in healthy individuals.[24,25] Our data have demonstrated that postoperative CD values decreased as the degree of myopia increased, which is congruent with Dong et al.’s study[26] that observed decreased CD values in areas of the cornea with high myopia compared to normal corneal regions. Similarly, reduced CD values indicate improved postoperative CDVA in all three layers, namely the central zone of the central layer, the surrounding central annulus of all layers, and the posterior annulus of the peripheral layer. The thickness of the RSB was found to have a positive correlation with CD values in the surrounding central annulus at both anterior and posterior layers. This might imply that the thinner the RSB, the better the transparency of the anterior and posterior cornea. Our clinical experience suggests that a larger OZ typically results in a thicker lenticule and a thinner RSB, which is positively correlated with CD values as previously mentioned. Abundant reporting exists regarding the increase in HOAs induced by SMILE.[10,27] In the long term, we detected the induction of HOAs and observed that corneal HOAs, specifically horizontal coma (Coma0), exhibited a negative correlation with CD values in any zone of any corneal layer. Coma is a vital indicator of visual quality after refractive surgery. During our investigation, we found that the increase in Coma0 was insignificant, and postoperative Coma0 fluctuated over time, accompanying with the reduction of CD values. The current study presents evidence that modifications to Coma0 do not impact the transparency of the cornea when treating patients with SMILE.
The research has some limitations. Although the study involved a relatively small sample size, subjects were followed up at various time points to enhance statistical power and efficiency. Statistical analysis of the data was conducted using Mixed Linear Models and Generalized Estimation Equations. Additionally, we solely assessed surgery-specific objective parameters, which could have been more rigorous with the inclusion of anterior segment OCT and confocal microscopy.
Conclusion
CD manifested a long-term decrease trend in moderate to high myopia treated with SMILE. Its main influencing factors are age, SE, CDVA, OZ, and Coma0.
Ethical approval
Ethical Committee of Eye and ENT Hospital Review Board (No. 2013015-1).
Financial support and sponsorship
Supported by the National Natural Science Foundation of China (Grant No. 81770955), Project of Shanghai Science and Technology (Grant No. 20410710100) (Grant No. 21Y11909800), and Clinical Research Plan of SHDC (SHDC2020CR104).
Conflicts of interest
There are no conflicts of interest.
(A-F) Refractive outcomes during the follow-up period after small-incision lenticule extraction (SMILE). (A) Uncorrected distance visual acuity; (B) Change in corrected distance visual acuity; (C) Spherical equivalent attempted vs- achieved; (D) Spherical equivalent refractive accuracy; (E) Refractive astigmatism; (F) Stability of spherical equivalent refraction
Acknowledgments
We would like to thank Editage (www.editage.cn) for English language editing.
References
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Supplementary Materials
(A-F) Refractive outcomes during the follow-up period after small-incision lenticule extraction (SMILE). (A) Uncorrected distance visual acuity; (B) Change in corrected distance visual acuity; (C) Spherical equivalent attempted vs- achieved; (D) Spherical equivalent refractive accuracy; (E) Refractive astigmatism; (F) Stability of spherical equivalent refraction