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. 2025 Apr 16;5(3):196–204. doi: 10.1016/j.aopr.2025.04.003

Early postoperative changes in corneal densitometry after SMILE with 120-μm and 130-μm cap thickness: a comparative study

Shirou Wu a,b, Hongying Jin a,b, Ting Wan a,b,⁎
PMCID: PMC12274913  PMID: 40688735

Abstract

Objective

To compare early postoperative changes in corneal densitometry after small incision lenticule extraction using 120-μm and 130-μm cap thicknesses.

Methods

69 eyes of 39 patients who underwent small incision lenticule extraction (SMILE) with cap thicknesses of 120-μm (n ​= ​34) and 130-μm (n ​= ​35) were included in this study. The corneal densitometry (CD) of three zones (0–2 ​mm, 2–6 ​mm, and 6–10 ​mm) of the anterior, central, and posterior corneal layers was evaluated before, one week, and one month after surgery. Uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), spherical degree, cylinder degree, and spherical equivalent (SE) were also analyzed and compared between the two groups before and after surgery.

Results

Little difference was detected between the two groups in postoperative UCVA, BCVA, SE, and cylinder degree at one month. No statistically significant differences were found between the preoperative and postoperative CD values for the overall (0–12 ​mm) cornea and the 6–10 ​mm zone in either group. A significant increase in CD was observed in the 0–2 ​mm and 2–6 ​mm zones of the anterior layer in the 120-μm group, and the 0–2 ​mm zone of the central layer in the 130-μm group, one week postoperatively. These changes persisted for one month after surgery (P ​< ​0.05). The CD in the central layer (0–2 ​mm and 2–6 ​mm) in the 120-μm group, as well as the total layer (0–2 ​mm and 2–6 ​mm) in both groups, significantly increased at the first postoperative week but returned to preoperative levels by one month after surgery. Additionally, the CD of the anterior layer (0–2 ​mm and 2–6 ​mm) increased significantly in the first week postoperatively, decreased significantly one month postoperatively, but remained significantly higher than baseline in the 130-μm group. In both groups, the CD of the posterior 0–2 ​mm zone decreased. Moreover, the increase in CD in the anterior 2–6 ​mm zone one week postoperatively was significantly higher in the 120-μm group compared to the 130-μm group (ΔCD 2.4 ​± ​1.55 vs. 1.64 ​± ​0.87, P ​= ​0.014). Postoperative corneal wavefront aberrations were significantly higher in the 120-μm group than in the 130-μm group.

Conclusions

CD increased mainly in the 0–6 ​mm zone of the anterior layer in the early phase after the SMILE procedure with both the 120-μm and 130-μm groups. The increase in CD in the anterior 2–6 ​mm zone one week postoperatively was higher in the 120-μm group than in the 130-μm group. SMILE with 120-μm and 130-μm cap thickness were both efficient and safe, but eyes with a 120-μm cap thickness showed higher postoperative corneal wavefront aberrations.

Keywords: SMILE, Corneal densitometry, Cap thickness, Wavefront aberration

1. Introduction

Small Incision Lenticule Extraction (SMILE) is a minimally invasive corneal refractive surgery. It creates a lenticule in the anterior corneal stroma using only a femtosecond laser, which is then dissected and removed through a 2 ​mm arcuate side cut.1,2 SMILE has become widely accepted for correcting myopia as well as myopic astigmatism because numerous clinical reports have confirmed its efficacy, safety, predictability, stability, and the unnecessary of flap creation.2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 Despite the minimal wound, corneal clarity has been shown to decrease after SMILE surgery, and it is an important aspect of the failure to obtain clear and bright vision in the early phase after surgery.6,17,18 As previously reported, corneal densitometry (CD) values would significantly increase in the early phase after SMILE surgery, and then gradually return to the baseline level.19,20 That might lead to a haze-like phenomenon under a slit-lamp examination, which could be the reason why some patients complained of a transient foggy feeling vision in the early phase after the SMILE procedure. Aside from the precision, long-term stability, and safety of the SMILE surgery, rapid vision recovery is also an important issue that patients are highly concerned about. Thus, efforts should be made to alleviate the postoperative CD increase in the early stage.

Generally speaking, a thinner flap is preferred to retain the residual corneal stromal bed thickness as much as possible. It is because a residual corneal stromal bed with sufficient thickness is a key factor for preventing corneal ectasia. Currently, the ideal cap thickness of SMILE surgery has not been officially standardized. The 120-μm cap thickness is commonly used as a standard parameter.21,22 However, some other ophthalmologists prefer to use the 130-μm cap thickness, especially in patients with corneal thickness more than 550 ​μm. It is because the thinner cap had more focal opaque bubble layer (OBL) than the thicker cap,23 which would cause trouble in the following lenticule dissection and lead to more obvious postoperative wound healing reaction. Although previous studies have demonstrated that little difference was found for the efficacy and safety of SMILE with different cap thickness, few studies have directly analyzed the difference in corneal density change after SMILE with different cap thickness.

Previously, corneal transparency was subjectively evaluated by the surgeon via slit lamp microscopy, which is rough and difficult for analyzing. With the development of scheimpflug photography technique, corneal clarity could be objectively quantified and described as a CD value with high accuracy, reproducibility, and repeatability.24 Objective visual quality assessments provide a more accurate reflection of subjective visual dissatisfaction compared to traditional visual acuity measurements.25 This technique has already been widely used to observe and monitor the corneal stromal reactions, such as keratocyte activation and the haze-like reaction after surgery.24,26 Previous studies mainly focused on comparing the postoperative CD change between SMILE and other corneal refractive surgeries. 17,27 To the best of our knowledge, the difference in corneal clarity after SMILE with different cap thicknesses remains unclear. In the present study, we acquired and compared the CD change after SMILE surgery with different cap thicknesses (120-μm and 130-μm) at baseline, 1 week, and 1 month postoperatively using the Pentacam Scheimpflug system. Meanwhile, the refractive and visual results were also evaluated and compared within and between the 120-μm and 130-μm caps. This study would be beneficial for providing information for choosing optimal cap thickness in SMILE surgery.

2. Methods

2.1. Subjects

This prospective study included 69 eyes of 39 patients who underwent the SMILE procedure with cap thickness of 120-μm (n ​= ​34) and 130-μm (n ​= ​35) at the Eye Center, Second Affiliated Hospital, Medical School of Zhejiang University from Sep 2023 to Dec 2023. All surgeries were completed by the same experienced surgeon (WT). This study was approved by the Ethical Committee of the Hospital Review Board (No.2017-017), and all procedures adhered to the principles of the Declaration of Helsinki. All patients recruited in the study signed an informed consent form. The study has been registered at http://www.chictr.org.cn as No. ChiCTR-ORC-17011040. Patients were enrolled according to the following inclusion criteria: corneal thickness >550 ​μm, age ≥18 years old with stable refraction for at least 2 years, -9 D ​< ​spherical equivalent (SE) ​< ​−0.5 D, stop wearing soft contact and orthokeratology lens for at least one week and three months before the examination respectively. The exclusion criteria included: risk of keratectasia, severe dry eye, and a history of systemic or ocular disease. The cap thickness was chosen based on corneal thickness, lenticule thickness, and the residual stromal thickness (RST). If the expected RST was more than 300 ​μm, the 130-μm cap thickness was selected; if the expected RST was less than 300 ​μm, the 120-μm cap thickness was selected.

2.2. Small-incision lenticule extraction procedure

Refractive corrections were performed by using a VisuMax femtosecond laser system (Carl Zeiss Meditec AG, Jena, Germany) for all patients. The surgical settings included the intended cap thickness as 120-μm or 130-μm, the lenticule diameter as 6.5 ​mm, the cap diameter as 7.5 ​mm, and the minimum lenticule side cut thickness as 10 ​μm. The optical zone diameter was equal to the lenticule diameter in patients with purely spherical refractive error, and a transition zone was added to convert the oval lenticule into a circle in patients with astigmatism. The exact details of the surgical procedure have been thoroughly described previously. 28,29 Firstly, the posterior surface of the refractive lenticule spiral was created, and then the anterior surface of the refractive lenticule spiral was formed. After that, a 2 ​mm side cut was created at 120° to provide access to the lenticule. Finally, the refractive lenticule was dissected by a spatula through the side cut and removed by forceps. All patients received a topical antibiotic for seven days, a topical steroid for two weeks, and artificial tears for more than four weeks postoperatively.

2.3. Preoperative and postoperative evaluation

Routine examinations were conducted preoperatively and one week, one month postoperatively. The following parameters were measured at each visit: logarithm of the minimal angle of resolution (LogMAR) of uncorrected distance visual acuity (UDVA), LogMAR of corrected distance visual acuity (CDVA), SE, CD values, IOP, and a standard slit-lamp biomicroscopic examination. The IOP was measured using a noncontact tonometer (KT-500, Kowa, Japan). All intra- and post-operative complications were recorded.

2.4. Corneal densitometry analysis

The CD values were examined using the Pentacam system (Oculus, Germany), with results expressed in grayscale units (GSUs), ranging from 0 (completely transparent) to 100 (opaque). The CD values on each of the concentric annuli of cornea (0–2 ​mm, 2–6 ​mm, 6–10 ​mm, 0–12 ​mm), and each layer (the anterior 120 ​μm of the corneal thickness, central [between the anterior and posterior layer], the posterior 60 ​μm of the cornea thickness, and total layer [the complete corneal thickness]) were recorded.30,31 However, the zone of peripheral 10–12 ​mm was not included because the repeatability and reproducibility of this zone are low. 24

2.5. Corneal wavefront aberration analysis

The corneal wavefront aberrations, including the anterior, posterior, and total of the 6 ​mm diameter around the corneal vertex, were examined using the Scheimpflug system.28 Specifically, the root mean square (RMS) values of the total higher-order aberrations (HOAs), spherical aberration (SA), horizontal coma (coma 0°), vertical coma (coma 90°), trefoil 0°, and trefoil 30° were examined.

2.6. Statistical analysis

SPSS software (version 22; SPSS, USA) was used to conduct the statistical analyses. Continuous variables were presented as mean ​± ​standard deviation (SD). The Kolmogorov–Smirnov test was used to investigate the normality of all data samples. A paired sample t-test and an analysis of variance (ANOVA) were used to compare measurements before and after surgery. An independent sample t-test was used for intergroup comparisons. To evaluate the relationship between cap thickness and postoperative HOA while adjusting for potential confounding factors, multiple linear regression models were applied. P ​< ​0.05 was considered statistically significant.

3. Results

3.1. Study population

Sixty-nine eyes of thirty-nine patients were included in this study. The preoperative demographic characteristics of the eyes, including SE, minimum corneal thickness (MCT), and lenticule thickness (LT), were presented. The eyes with 120-μm cap thickness had higher SE and LT than those with 130-μm cap thickness (P ​< ​0.05). All the patients finished the one-month observation. All surgical procedures were uneventful, and no obvious complications were observed during the follow-up time (see Table 1).

Table 1.

Preoperative demographic information of eyes undergoing SMILE with 120- and 130-μm cap thickness (mean ​± ​SD and range).

Parameter 120-μm 130-μm P
Eye(n) 34 35
Sex (M/F) 15/14 17/18 0.716
Age (y) 28. 15 ​± ​7.72 28.40 ​± ​6.34 0.882
IOP(mmHg) 15.95 ​± ​2.82 14.40 ​± ​2.11 0.011∗
SE(D) −5.51 ​± ​1.84 −3.71 ​± ​1.39 < 0.001∗
Sphere (D) −5. 12 ​± ​1.87 −3.34 ​± ​1.29 < 0.001∗
Cylinder (D) −0.79 ​± ​0.62 −0.74 ​± ​0.64 0.179
MCT (-μm) 565.00 ​± ​10.46 575.51 ​± ​14.24 0.001∗
LT (-μm) 115.35 ​± ​25.88 90.74 ​± ​20.94 < 0.001∗
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Group 1: cap thickness ​= ​120-μm; Group 2: cap thickness ​= ​130-μm; SD: standard deviation; D: diopters; IOP: intraocular pressure; SE: spherical equivalent; MCT: minimum corneal thickness; LT: lenticule thickness; ∗P ​< ​0.05 between 120-μm and 130-μm cap thickness.

3.2. Visual and refractive outcomes

3.2.1. Efficacy and safety

Table 2 shows the visual outcomes before and after surgery. At one week after surgery, the LogMAR CDVA and the LogMAR UDVA improved in the 120-μm group compared to preoperative values. In the 130-μm group, LogMAR UDVA also improved, but LogMAR CDVA did not show a significant difference. At one month postoperatively, both LogMAR UDVA and LogMAR CDVA improved in both groups (P ​< ​0.05). At one week postoperatively, the LogMAR UDVA and LogMAR CDVA were significantly higher in eyes with 120-μm cap thickness compared with eyes with 130-μm cap thickness (P ​= ​0.032, 0.002). However, the LogMAR UDVA and LogMAR CDVA were similar between groups one month postoperatively (all P ​> 0.05). Furthermore, the preoperative CDVA and one month postoperative UDVA of each group are compared and presented in Fig. 1A, in which the efficacies of the two groups were shown by calculating the cumulative percentage of preoperative CDVA and postoperative UDVA. The efficacy index at one month was 1.00 ​± ​0.04 in eyes with 120-μm cap thickness, and 1.02 ​± ​0.10 in eyes with 130-μm cap thickness, respectively, without a significant intergroup difference (P ​> ​0.05). 97% (33 eyes) in the 120-μm group and 94% (33 eyes) in the 130-μm group had UDVA equal to or better than the preoperative CDVA (Fig. 1B). Meanwhile, the change in CDVA in both groups one month postoperatively was shown in Fig. 1C. The safety index of the procedure was 1.02 ​± ​0.06 in the120-μm group and 1.06 ​± ​0.11 in the 130-μm group, with no significance between groups (P ​> ​0.05).

Table 2.

Visual and refractive outcomes preoperatively and postoperatively.

Group Pre-op (mean ​± ​SD)
 1w Post-op (mean ​± ​SD)
 1m Post-op (mean ​± ​SD)
UDVA (LogMAR) CDVA (LogMAR) SE (D) Cylinder (D) UDVA (LogMAR) CDVA (LogMAR) SE(D) Cylinder (D)) UDVA (LogMAR) CDVA (LogMAR) SE(D) Cylinder (D)
120-μm 0.72 ​± ​0.21 0.00 ​± ​0.01 −5.51 ​± ​1.84 −0.79 ​± ​0.62 0.00 ​± ​0.04# −0.01 ​± ​0.02# −0.03 ​± ​0.18 −0. 14 ​± ​0.24 0.00 ​± ​0.03# −0.01 ​± ​0.02# −0.03 ​± ​0.17 −0. 13 ​± ​0.26
130-μm 0.52 ​± ​0.20 0.01 ​± ​0.03 −3.71 ​± ​1.39 −0.74 ​± ​0.64 0.02 ​± ​0.04# 0.02 ​± ​0.04 0.00 ​± ​0.16 −0.06 ​± ​0.22 0.00 ​± ​0.03# −0.01 ​± ​0.03# 0. 10 ​± ​0.39 −0. 15 ​± ​0.25
P < 0.001∗ 0.252 < 0.001∗ 0.179 0.032∗ 0.002∗ 0.483 0.179 0.939 0.312 0.089 0.683
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UDVA: Uncorrected distance visual acuity; CDVA: Corrected distance visual acuity; SE: Spherical Equivalent; LogMAR: Logarithm of the minimal angle of resolution.

∗P ​< ​0.05 between the 120-μm and 130-μm cap groups.

#P ​< ​0.05 between postoperative and preoperative values.

Fig. 1.

Fig. 1

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Refractive and visual outcome after SMILE with different cap thickness. (A). Cumulative percentage of preoperative CDVA and postoperative UDVA at the one-month visit after SMILE with different cap thickness. (B). Difference between postoperative CDVA and postoperative UDVA at the one-month visit after SMILE with different cap thickness. (C). Change in CDVA in SMILE with different cap thicknesses at one month postoperatively. (D). A scatter plot of the attempted versus the achieved spherical equivalent correction one month postoperatively after SMILE with different cap thickness. (E). Accuracy of the spherical equivalent refraction in SMILE with different cap thickness at the one-month visit after surgery. (F). Accuracy of the cylinder refraction at one month postoperatively. (G). Stability of the spherical equivalent at one week and one month postoperatively after SMILE with different cap thickness. CDVA: corrected distance visual acuity; UDVA: uncorrected distance visual acuity; D: diopters; preop: preoperative; postop: postoperative; w: week; m: month.

3.2.2. Predictability

Scatter plots with a linear regression analysis of the attempted versus the achieved SE at one month postoperatively are shown in Fig. 1D. The correlation between attempted and achieved SE refraction was high. The coefficient of determination (r2) was 0.99 in the 120-μm group and 0.90 in the 130-μm group. At one month postoperatively, 100% of eyes in the 120-μm group and 85% in the 130-μm group had an SE within ±0.5 D (P ​< ​0.05 between groups). Furthermore, 100% of eyes in the120-μm group and 97 ​% in the 130-μm group had an SE within ±1.0 D of the intended SE correction (Fig. 1E). For cylinder correction, 82% of eyes in the 120-μm group and 81% in the 130-μm group had a refractive error within ±0.5 D. Meanwhile, 82% in the 120-μm group and 84% in the 130-μm group had cylinder values within ±1.0 D (Fig. 1F, P ​> ​0.05 between groups). The mean SE remained stable in the early postoperative period, measuring −0.02 ​± ​0.38 D in the 120-μm group and 0.11 ​± ​0.34 D in the 130-μm group at one month (Fig. 1G, P ​> ​0.05 between groups).

3.3. Corneal densitometry

Table 3 compares preoperative and postoperative CD. No significant differences were found in the overall corneal (0–12 ​mm) CD or the 6–10 ​mm zone across all layers in either group. However, the CD of the anterior and central layers increased in some zones, while the CD of the posterior layer decreased in the early postoperative period in both groups. Specifically, in the 120-μm group, the CD in the 0–2 ​mm and 2–6 ​mm zones of the anterior layer increased significantly at one week and remained elevated at one month. The CD in the 0–2 ​mm and 2–6 ​mm zones of both the central and total layers also increased significantly in the first week but returned to baseline levels by one month. In the 130-μm group, the CD in the 0–2 ​mm zone of the central layer increased significantly at one week and remained elevated at one month. The CD in the 0–2 ​mm and 2–6 ​mm zones of the total layer also increased significantly in the first week but returned to baseline levels by one month. However, in the 0–2 ​mm and 2–6 ​mm zones of the anterior layer, the CD increased at one week, and then decreased significantly at one month, which were still significantly higher than the baseline level in the 130-μm group. Furthermore, we compared the CD changes after surgery between the two groups (Table 4). The increase in CD in the 2–6 ​mm zone of the anterior layer at one week was significantly greater in the 120-μm group than in the 130-μm group (2.4 ​± ​1.55 vs 1.64 ​± ​0.87, P ​= ​0.014, Fig. 2). However, no significant difference was observed in overall corneal CD or other regions.

Table 3.

Comparison of corneal densitometry (CD) between SMILE with 120-μm cap thickness and 130-μm cap thickness.

CD(GSU) Preoperative
 Postoperative one week
 Postoperative one month
120-μm 130 ​μm P-value 120-μm 130 ​μm 120-μm 130 ​μm
Anterior layer 0–2 ​mm 19.87 ​± ​20.73 20.73 ​± ​1.75 0.094 22.23 ​± ​2.73∧ 22.60 ​± ​1.64#$ 21.23 ​± ​3. 15∧ 21.69 ​± ​1.46#$
2–6 ​mm 17.95 ​± ​2.23 18.85 ​± ​1.53 0.053 20.35 ​± ​2.31∧ 20.49 ​± ​1.47#$ 19.43 ​± ​2.52∧ 19.82 ​± ​1.36#$
6–10 ​mm 19. 16 ​± ​3.54 20.93 ​± ​2.86 0.026∗ 19.6 ​± ​3.31 20.43 ​± ​2.91 19.27 ​± ​4.26 20.82 ​± ​3.82
0–12 ​mm 21.56 ​± ​2.91 22.35 ​± ​1.74 0.172 22.41 ​± ​2.33 22.66 ​± ​1.79 21.56 ​± ​2.58 22.49 ​± ​1.89
Central layer 0–2 ​mm 12.56 ​± ​0.82 12.86 ​± ​0.74 0.120 12.89 ​± ​1.08∧ 13. 12 ​± ​0.85# 12.87 ​± ​1.01 13.36 ​± ​0.8#
2–6 ​mm 11.46 ​± ​0.72 11.73 ​± ​0.63 0.102 11.65 ​± ​0.9∧ 11.81 ​± ​0.71 11.62 ​± ​0.85 12.05 ​± ​0.7#
6–10 ​mm 12.45 ​± ​2.02 13.04 ​± ​1.92 0.220 12.52 ​± ​2.05 12.71 ​± ​2.09 12.54 ​± ​2.29 13. 19 ​± ​2.39
0–12 ​mm 13.68 ​± ​1.65 13.64 ​± ​1.08 0.906 13.51 ​± ​1.56 13.43 ​± ​1.22 13.45 ​± ​1.55 13.73 ​± ​1.23
Posterior layer 0–2 ​mm 9.33 ​± ​2.49 10.74 ​± ​2.10 0.013∗ 8.92 ​± ​2.71 10.07 ​± ​2.12# 8.28 ​± ​2.39∧ 10.07 ​± ​2.34
2–6 ​mm 8.72 ​± ​2.18 9.89 ​± ​1.81 0.018∗ 8.63 ​± ​2.34 9.5 ​± ​1.82 8.09 ​± ​2.1 9.51 ​± ​2.03
6–10 ​mm 10. 16 ​± ​2.23 11.37 ​± ​2.1 0.023∗ 10.23 ​± ​2.31 11.04 ​± ​2.27 9.81 ​± ​2.43 11.23 ​± ​2.53
0–12 ​mm 10.36 ​± ​2.37 11.29 ​± ​1.89 0.076 10. 18 ​± ​2.43 10.91 ​± ​1.9 9.7 ​± ​2.29 10.97 ​± ​2.16
Total thickness 0–2 ​mm 13.92 ​± ​1.83 14.77 ​± ​1.43 0.035∗ 14.66 ​± ​2.08∧ 15.27 ​± ​1.44# 14. 11 ​± ​1.9 15.05 ​± ​1.38
2–6 ​mm 12.71 ​± ​1.64 13.49 ​± ​1.26 0.030∗ 13.54 ​± ​1.75∧ 13.93 ​± ​1.26# 13.05 ​± ​1.6 13.79 ​± ​1.24
6–10 ​mm 13.93 ​± ​2.39 15. 1 ​± ​2.13 0.034∗ 14. 12 ​± ​2.34 14.73 ​± ​2.3 13.87 ​± ​2.83 15.07 ​± ​2.73
0–12 ​mm 15.2 ​± ​2.17 15.76 ​± ​1.39 0.204 15.36 ​± ​1.96 15.67 ​± ​1.52 14.9 ​± ​1.93 15.74 ​± ​1.59
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∗P ​< ​0.05, significantly different in preoperative values between 120-μm cap thickness and 130-μm cap thickness.

∧P ​< ​0.05, significantly different from the preoperative values in eyes with 120-μm cap thickness.

#P ​< ​0.05, significantly different from the preoperative values in eyes with 130-μm cap thickness.

$P ​< ​0.05, significantly different in CD values between one week and one month postoperatively in each group.

Table 4.

Comparison of the corneal densitometry (CD) change between SMILE with 120-μm cap thickness and 130-μm cap thickness.

CD(GSU) Post one week vs Pre-operative
 Post one month vs Pre-operative.
120-μm 130-μm P-value 120-μm 130-μm P-value
Anterior
layer		 0–2 ​mm 2.36 ​± ​1.64 1.87 ​± ​0.9 0.127 1.35 ​± ​2.69 0.95 ​± ​1.8 0.470
2–6 ​mm 2.4 ​± ​1.55 1.64 ​± ​0.87 0.014 1.48 ​± ​2.2 0.97 ​± ​1.53 0.266
6–10 ​mm 0.44 ​± ​2.47 −0.5 ​± ​2.15 0.097 0. 11 ​± ​2.67 −0.11 ​± ​2.29 0.725
0–12 ​mm 0.86 ​± ​1.98 0.31 ​± ​1.36 0.186 0 ​± ​2.46 0. 14 ​± ​1.66 0.777
Central
layer		 0–2 ​mm 0.32 ​± ​0.55 0.27 ​± ​0.61 0.681 0.31 ​± ​0.84 0.5 ​± ​0.79 0.341
2–6 ​mm 0. 19 ​± ​0.42 0.09 ​± ​0.54 0.368 0. 16 ​± ​0.73 0.33 ​± ​0.68 0.339
6–10 ​mm 0.07 ​± ​0.92 −0.33 ​± ​1.17 0.124 0.09 ​± ​1.08 0. 15 ​± ​1.04 0.823
0–12 ​mm −0.18 ​± ​0.79 −0.21 ​± ​0.75 0.839 −0.23 ​± ​1.15 0.09 ​± ​0.77 0.176
Posterior
layer		 0–2 ​mm −0.4 ​± ​1.51 −0.67 ​± ​1.31 0.426 −1.05 ​± ​2.18 −0.67 ​± ​1.81 0.438
2–6 ​mm −0.1 ​± ​1.29 −0.39 ​± ​1.14 0.313 −0.64 ​± ​1.92 −0.38 ​± ​1.58 0.548
6–10 ​mm 0.07 ​± ​1.35 −0.33 ​± ​1.37 0.221 −0.35 ​± ​1.84 −0.15 ​± ​1.5 0.619
0–12 ​mm −0.18 ​± ​1.39 −0.38 ​± ​1.18 0.527 −0.66 ​± ​2.02 −0.32 ​± ​1.61 0.443
Total
thickness		 0–2 ​mm 0.74 ​± ​1.11 0.5 ​± ​0.82 0.308 0. 19 ​± ​1.57 0.27 ​± ​1.29 0.798
2–6 ​mm 0.83 ​± ​1 0.45 ​± ​0.78 0.079 0.34 ​± ​1.39 0.3 ​± ​1.1 0.892
6–10 ​mm 0. 19 ​± ​1.47 −0.37 ​± ​1.49 0.119 −0.06 ​± ​1.66 −0.03 ​± ​1.42 0.935
0–12 ​mm 0. 16 ​± ​1.33 −0.09 ​± ​0.93 0.367 −0.3 ​± ​1.74 −0.02 ​± ​1.16 0.432
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Fig. 2.

Fig. 2

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Corneal densitometry change of different corneal zones one week and one month after SMILE with 120um and 130um cap thickness.

Complete data are provided in Table 4

∗P ​< ​0.05 between 120-μm and 130-μm cap thickness.

3.4. Wavefront aberrations

No statistically significant intergroup differences in corneal wavefront aberrations were found before surgery (Table 5). HOAs and the coma 90° of both the front cornea and the total cornea increased significantly at one week postoperatively in the 120-μm group. At one month after surgery, HOAs, SA, and the coma 90° of the front and total cornea in the 120-μm group, and HOAs and the coma 90° of both the front and total cornea in the 130-μm group significantly increased. At one week after surgery, HOAs of the front cornea and the SA of both the front and total cornea were significantly higher in the 120-μm group compared with the values in the 130-μm group. Similarly, at one month, HOAs and the SA of both the front and total cornea remained significantly higher in the 120-μm group compared with the respective values in the 130-μm group.

Table 5.

The corneal aberrations before and after SMILE with 120-μm cap thickness and 130-μm cap thickness.

Wavefront Aberrations Preoperative
 Postoperative one week
 Postoperative one month
120-μm 130-μm P 120-μm 130-μm P 120-μm 130-μm P
Front cornea
HOAs 0.39 ​± ​0.01 0.37 ​± ​0.01 0.255 0.57 ​± ​0.05# 0.45 ​± ​0.02 0.026∗ 0.58 ​± ​0.03# 0.49 ​± ​0.02& 0.014∗
Spherical aberration 0.28 ​± ​0.02 0.25 ​± ​0.01 0.068 0.34 ​± ​0.03 0.24 ​± ​0.02 0.002∗ 0.35 ​± ​0.02# 0.27 ​± ​0.02 0.007∗
Trefoil 30° −0.02 ​± ​0.02 −0.02 ​± ​0.02 0.911 −0.03 ​± ​0.02 0.02 ​± ​0.02 0.095 −0.01 ​± ​0.02 0.01 ​± ​0.02 0.387
coma 90° −0.03 ​± ​0.02 −0.03 ​± ​0.03 0.880 −0. 17 ​± ​0.06# −0. 14 ​± ​0.04 0.641 −0.21 ​± ​0.04# −0.21 ​± ​0.03& 0.939
coma 0° 0.02 ​± ​0.02 0.01 ​± ​0.02 0.690 0.07 ​± ​0.04 0.04 ​± ​0.03 0.438 0.05 ​± ​0.04 0.03 ​± ​0.03 0.592
Trefoil 0° 0.00 ​± ​0.01 0.01 ​± ​0.01 0.660 0.00 ​± ​0.02 −0.01 ​± ​0.02 0.635 0.01 ​± ​0.02 0.02 ​± ​0.02 0.782
Back cornea
HOAs 0. 18 ​± ​0.01 0. 19 ​± ​0.01 0.842 0. 19 ​± ​0.01 0. 19 ​± ​0.01 0.863 0. 19 ​± ​0.01 0. 19 ​± ​0.01 0.299
Spherical aberration −0. 16 ​± ​0.01 −0. 15 ​± ​0.01 0.705 −0. 15 ​± ​0.01 −0. 16 ​± ​0.01 0.483 −0. 16 ​± ​0.01 −0. 15 ​± ​0.02 0.254
Trefoil 30° −0.03 ​± ​0.01 −0.02 ​± ​0.01 0.523 −0.04 ​± ​0.01 −0.04 ​± ​0.01 0.834 −0.03 ​± ​0.01 −0.03 ​± ​0.01 0.430
coma 90° −0.01 ​± ​0.01 −0.03 ​± ​0.01 0.091 −0.01 ​± ​0.01 −0.02 ​± ​0.01 0.551 −0.01 ​± ​0.01 −0.02 ​± ​0.01 0.129
coma 0° −0.01 ​± ​0.01 −0.01 ​± ​0.01 0.468 0.00 ​± ​0.01 −0.01 ​± ​0.01 0.335 −0.01 ​± ​0.01 0.00 ​± ​0.01 0.471
Trefoil 0° 0.00 ​± ​0.01 0.00 ​± ​0.01 0.541 0.00 ​± ​0.01 0.00 ​± ​0.01 0.964 0.00 ​± ​0.01 0.01 ​± ​0.01 0.592
Total cornea
HOAs 0.38 ​± ​0.01 0.37 ​± ​0.01 0.431 0.58 ​± ​0.05# 0.47 ​± ​0.02& 0.052 0.60 ​± ​0.03# 0.51 ​± ​0.02& 0.026∗
Spherical aberration 0.24 ​± ​0.02 0.20 ​± ​0.01 0.053 0.30 ​± ​0.03 0. 19 ​± ​0.02 0.002∗ 0.30 ​± ​0.02# 0.22 ​± ​0.02 0.013∗
Trefoil 30° −0.04 ​± ​0.02 −0.04 ​± ​0.02 0.823 −0.06 ​± ​0.02 −0.01 ​± ​0.02 0.104 −0.04 ​± ​0.02 −0.01 ​± ​0.02 0.402
coma 90° −0.04 ​± ​0.02 −0.06 ​± ​0.03 0.636 −0. 19 ​± ​0.06# −0. 17 ​± ​0.04 0.755 −0.24 ​± ​0.04# −0.25 ​± ​0.03& 0.941
coma 0° 0.02 ​± ​0.02 0.01 ​± ​0.02 0.554 0.07 ​± ​0.04 0.03 ​± ​0.03 0.381 0.05 ​± ​0.04 0.03 ​± ​0.03 0.635
Trefoil 0° 0.01 ​± ​0.01 0.01 ​± ​0.02 0.882 0.00 ​± ​0.02 −0.02 ​± ​0.02 0.573 0.02 ​± ​0.02 0.03 ​± ​0.02 0.708
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#P ​< ​0.05, significantly different from the preoperative values in eyes with 120-μm cap thickness. &P ​< ​0.05, significantly different from the preoperative values in eyes with 130-μm cap thickness. ∗P ​< ​0.05, significantly different between the 120-μm cap group and the 130-μm cap group.

Furthermore, we also did multivariate analysis between postoperative corneal densitometry at different regions and layers and postoperative high-order aberration (Table 6) at one month postoperatively. In the 120-μm group, HOAs were positively correlated with the corneal densitometry of each layer of the 0–2 ​mm and 2–6 ​mm zones, as well as the central and posterior layers of the overall (0–12 ​mm) cornea. Trefoil 30° was negatively correlated with the corneal densitometry, except for the anterior layer of the 0–2 ​mm zone, the anterior and central layers of the 6–10 ​mm zone, and the central layer of the overall cornea. Coma 90° was positively correlated with the corneal densitometry, except for the anterior and posterior layers of the 0–2 ​mm and 2–6 ​mm zones, as well as the anterior and central layer of the 6–10 ​mm zone. SA was negatively correlated with most of the layers of the 0–2 ​mm and 2–6 ​mm zones. No significant correlations were found between either coma 0° or Trefoil 0° and the corneal densitometry of any layer. In the 130-μm group, no significant correlations were found between HOAs, SA, Trefoil 30°, or coma 90° and the corneal densitometry. However, Trefoil 0° was positively correlated with the central layer of the 0–2 ​mm and 2–6 ​mm zones, and coma 0° was positively correlated with the anterior layer of the 6–10 ​mm zone.

Table 6.

Multivariate analysis between postoperative corneal densitometry at different regions and layers and postoperative high-order aberration.

Variables 0–2 ​mm
 2–6 ​mm
 6–10 ​mm
 0–12 ​mm
AL
 CL
 PL
 TL
 AL
 CL
 PL
 TL
 AL
 CL
 PL
 TL
 AL
 CL
 PL
 TL
B B B B B B B B B B B B B B B B
120 ​μm HOA 6.52∗ 2.60∗ 6.22∗ 5.07∗ 5.68∗ 1.89∗ 5.41∗ 4.35∗ 0.19 −0.001 4.17∗ 1.48 3.19 2.38∗ 6.02∗ 3.85∗
SA −5.75 −1.69 −6.14∗ −4.49∗ −4.94∗ −1.31 −5.49∗ −3.94∗ 2.73 1.75 −2.67 0.57 −2.54 −1.16 −5.29∗ −2.98
Trefoil 30° −5.79 −2.40∗ −7.39∗ −5.25∗ −5.39∗ −2.07∗ −6.40∗ −4.57∗ −9.35 −3.26 −5.86∗ −6.19∗ −7.09∗ −2.74 −6.35∗ −5.35∗
coma 90° 1.78 1.31∗ 2.01 1.69∗ 1.55 0.90∗ 1.58 1.33∗ 3.10 1.87 2.23∗ 2.40∗ 2.35∗ 2.37∗ 2.63∗ 2.45∗
coma 0° 1.82 0.82 3.26 1.98 0.54 0.07 2.96 1.29 −2.31 −2.96 −0.34 −1.84 −0.06 −1.59 1.14 −0.21
Trefoil 0° 0.97 0.33 0.08 0.47 0.99 0.28 0.12 0.48 1.32 0.77 1.15 1.06 −0.19 −0.31 0.02 −0.16
130 ​μm HOA −2.83 −0.58 −2.95 −2.12 −2.15 −0.87 −2.73 −1.91 −4.07 −3.35 −4.31 −3.88 −2.04 −1.96 −3.52 −2.50
SA 1.00 1.00 1.22 1.05 0.61 0.91 1.25 0.96 0.31 0.35 1.19 0.64 1.17 0.42 0.77 0.77
Trefoil 30° 0.59 1.34 0.92 0.95 0.28 0.98 0.66 0.68 3.68 2.64 1.11 2.49 1.25 1.64 0.86 1.29
coma 90° 0.16 0.22 −1.99 −0.51 −0.01 0.09 −1.43 −0.45 −0.11 0.34 −0.87 −0.17 −1.08 0.14 −1.20 −0.74
coma 0° −1.24 −0.11 0.05 −0.43 −0.25 0.29 0.21 0.11 7.39∗ 3.95 2.49 4.59 2.09 1.33 0.83 1.35
Trefoil 0° 1.64 1.37∗ 1.96 1.68 1.31 1.10∗ 1.70 1.38 1.79 2.05 2.61 2.15 2.18 1.70 2.10 1.95
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B = Coefficient, ∗P ​< ​0.05, Postoperative corneal densitometry was correlated with postoperative higher-order aberration.

4. Discussion

The cornea is a critical part of the refractive medium of the visual system, and the corneal transparency is essential for keeping clear vision. Therefore, good corneal transparency after corneal refractive surgery is critical for achieving good visual quality, and is considered as a crucial element for the surgical efficacy and safety. SMILE is a minimal invasive surgery, and the inflammatory response after SMILE has been demonstrated to be much less than that after femtosecond laser-assisted laser in situ keratomileusis.32,33 Nevertheless, previous reports showed that SMILE could reduce corneal transparency in the early postoperative phase as a result of edema or inflammation.17,34,35 This may be one of the reasons why there is a transient haze-like reaction in the cornea, and patients experience slower visual recovery in the early phase after SMILE.

In the present study, we investigated the CD changes at one week and one month after SMILE with two different cap thicknesses (120-μm and 130-μm). We found that the difference between preoperative and postoperative CD of the overall (0–12 ​mm) cornea and the 6–10 ​mm cornea of all layers was not statistically significant in either the 120-μm group or the 130-μm group. However, the CD in the 0–2 ​mm and 2–6 ​mm zones of the anterior layer of the 120-μm group and the 0–2 ​mm zone of the central layer of the 130-μm group increased significantly at one week and remained elevated at one month. The CD of the 0–2 ​mm and 2–6 ​mm zones of the central layer in the 120-μm group, as well as the 0–2 ​mm and 2–6 ​mm zones of the total layer in both groups, increased significantly in the first week and returned to baseline levels at one month. Furthermore, the CD of the 0–2 ​mm and 2–6 ​mm zones of the anterior layer increased significantly in the first week after surgery, and then decreased significantly one month postoperatively, but were still significantly higher than the baseline level in the 130-μm group. We supposed that the CD value increased in the early phase after SMILE may be the result of corneal edema, cellular debris at the interface, small lacuna, and also microdistortions in Bowman's layer. 35, 36, 37 Moreover, the increased light scatter after SMILE surgery may be due to the keratocyte activation and the increased reflectivity from the extracellular matrix in the anterior layer of the corneal stroma. Consistent with our results, Zhang38 showed that the CD value increased mainly in the 0–2 ​mm and 2–6 ​mm zones of the anterior layer at the first week follow-up postoperatively, and then returned to the baseline levels. Yang39 revealed that the CD values at the anterior 0–2 ​mm and 2–6 ​mm zones significantly increased at the first postoperative day and returned to the baseline level at one year after SMILE. Meanwhile, Lazaridis19 showed that the CD in the anterior 0–6 ​mm zones of the cornea after SMILE surgery is higher than the preoperative level three months postoperatively. Furthermore, Agca40 reported increased corneal density in the anterior layer in the first three months after SMILE surgery, as observed through confocal microscopy. Nevertheless, there are also some reports with controversial results. Wei 6 and Han41 showed a significant CD decrease several years after SMILE surgery. Cai42 demonstrated that no significant differences were found for the CD of all zones of the three layers between pre- and three months postoperative levels in SMILE surgery. We suppose the discrepancy between our results and those reports may be because our study focused on the early postoperative phase, with a follow-up of only one month, which is much shorter than in those other studies.

It is an interesting aspect that the CD of the posterior 0–2 ​mm zone decreased during the early phase after SMILE surgery in both the 120-μm and 130-μm groups. Similarly, Lazaridis19 and Xu43 showed that the postoperative CD of the posterior layer reduced significantly after SMILE surgery compared to the preoperative level. Besides, Li 44 demonstrated that corneal cell density and basal nerve density decreased after SMILE surgery via in vivo confocal microscopy. Thus, it is possible that the thinning of corneal stroma leads to fewer backscatter particles from the keratocytes and collagen fibrils, which would promote the image quality and result in increased posterior stroma transparency.

It is common sense that sufficient residual corneal stromal bed thickness is of great importance for preventing iatrogenic keratectasia.45,46 As a result, a thinner cap and a thicker residual cornea bed are more preferable. However, the incidence and severity of OBL were usually higher in the SMILE surgeries with a thinner cap. 23,47 The OBL is a diaphanous bubble layer produced in the corneal stroma during femtosecond laser scanning. It is associated with the corneal fiber compactness, which makes the gas difficult to overflow. Usually, slight OBL has little impact on the surgical process. However, moderate and severe OBL may increase the difficulty of lenticule dissection, which would lead to more surgical manipulation, greater corneal stromal injury, and postoperative inflammatory reaction. 48 It has been reported that the keratocyte response after SMILE was much higher than compared with femtosecond intrastromal lenticule creation. It suggests that surgical manipulation, rather than the femtosecond laser, may be the cause of the postoperative keratocyte response in the cornea after SMILE surgery.33 In the present study, it is shown that the CD increase of the anterior 2–6 ​mm zone at one week after surgery was significantly higher in the 120-μm group as compared with the 130-μm group. Similarly, several reports have demonstrated that SMILE with a thicker corneal cap has much less corneal wound healing response49,50. Liu50 also showed that the anterior surfaces of lenticules with a 140-μm cap were smoother as compared with those with a 120-μm cap. Previous studies have shown that the anterior corneal stroma exhibits higher tensile strength,51 which may contribute to increased resistance during lenticule dissection in SMILE surgeries with a 120-μm cap thickness. Additionally, the more compact arrangement of collagen fibers52 at the anterior stroma could hinder the dispersion of gas bubbles generated by the femtosecond laser, leading to a denser OBL formation. This, in turn, may increase the difficulty of lenticule separation, requiring more forceful dissection and potentially resulting in greater surgical trauma and a more pronounced inflammatory response. The observed increase in cellular response in the 120-μm cap group is consistent with these findings.

With the advancement of refractive surgery, more attention has been paid to the postoperative visual quality. HOAs induced by corneal refractive surgeries can cause symptoms such as glare, halos, and starbursts,53 which decrease the patients’ satisfaction.

In the present study, we detected a significant increase in corneal wavefront aberration after SMILE, which is consistent with previous reports.39,54 Specifically, we found that HOAs and vertical coma of both the front cornea and total cornea significantly increased at one week after operation in the 120-μm group. Meanwhile, SA of the front and total cornea in the 120-μm group, while HOAs and vertical coma of the front cornea and total cornea in both the 120-μm and 130-μm groups significantly increased one month after operation. Moreover, the HOAs of the front cornea and the SA of both the front and total cornea were significantly higher in the 120-μm group compared with those values in the 130-μm group at one week and one month postoperatively. Similar to our results, Liu55 found that the corneal SA was significantly higher in the 110-μm group as compared with the 150-μm cap group at three months postoperatively. However, El-Massryl56 reported no statistically significant differences in the HOAs between the cap depths of 100-μm and 160-μm. There is also another report showing no significant differences between SMILE with 120-μm and 140-μm caps in terms of coma, trefoil, SA, residual fourth-order aberrations, and total HOAs three months postoperatively.50 The differences in results may be attributed to the difference in follow-up time points. Our results showed that the 120-μm group had higher HOA values than the 130-μm group during the early postoperative period. We hypothesize that this may be related to the cap location being closer to Bowman's layer and the superficial corneal nerve plexus, which could induce a more pronounced early biological response or anterior surface microstructural changes. These early postoperative changes, including epithelial remodeling and corneal biomechanical stress distribution, may contribute to higher HOA values in the early period. However, such differences may diminish over time as corneal healing and remodeling progress.

Previous studies have reported that there was no relationship between the changes in CD values with the visual function, such as corneal wavefront aberrations.6 Han et al.17 also demonstrated no correlation between CD values and alterations in corneal aberration. However, Xu43 found that there was a negative correlation between the CD values (0–6 ​mm) and corneal oblique trefoil. In the present study, we found that HOAs and coma 90° were positively related to corneal densitometry, while SA and Trefoil 30° were negatively related to corneal densitometry in the 120-μm group, but not in the 130-μm group. We suppose that the higher CD change in the 120-μm group may be one of the reasons why eyes with 120-μm cap thickness have more severe postoperative cornea wavefront aberration, which needs further studies to explore.

In the present study, we also compared the refractive and visual outcomes after SMLE with different cap thicknesses. We confirmed that both 120-μm and 130-μm cap thicknesses were effective and safe for correcting myopia and myopic astigmatism. No significant differences were found in postoperative UCVA, DCVA, SE, and cylinder between the 120-μm and 130-μm cap groups in 1 month. Those results were consistent with previous studies.49,57, 58, 59

Furthermore, our study demonstrated that the 120-μm cap thickness was associated with greater early postoperative increases in CD. This suggests a more pronounced wound-healing response and potential anterior surface microstructural changes. Additionally, the 120-μm group exhibited significantly higher values of HOAs and SA at both one week and one month postoperatively, indicating a temporary compromise in visual quality. However, the thinner cap may offer biomechanical advantages by preserving a thicker residual stromal bed, which is important for preventing iatrogenic ectasia, especially in patients with relatively thinner corneas. In contrast, the 130-μm cap thickness group showed relatively lower early postoperative CD changes and wavefront aberrations, reflecting a milder wound-healing response and potentially smoother refractive interfaces. The thicker cap also resulted in fewer complications related to intraoperative manipulation, such as OBL in the early phase. Nevertheless, it leaves a thinner residual stromal bed, which may limit its applicability in patients with thin corneas. Taken together, the 120-μm cap may be more suitable for patients requiring greater residual stromal preservation, while the 130-μm cap provides better early optical quality and induces less inflammation. Selection of the appropriate cap thickness should therefore be individualized based on corneal thickness, visual quality expectations, and risk of biomechanical instability.

This study has several limitations. Firstly, the sample size in each group was relatively small, which might increase the risk of a Type Ⅱ statistical error when analyzing the difference between the 120-μm and 130-μm cap. Secondly, the follow-up time is relatively short at 1 month after SMILE surgery. It is because we focused on the CD change in the early postoperative period. Thus, further studies with a larger sample size and longer follow-up time are needed to figure out the difference in longer-term CD change and visual outcomes between the 120-μm and 130-μm caps.

5. Conclusions

SMILE with 120-μm and 130-μm cap thicknesses were both efficacious and safe. The CD increased mainly in the 0–6 ​mm zone of the anterior layer in the early phase after the SMILE procedure with both 120-μm cap and 130-μm cap. The increase in CD of the anterior 2–6 ​mm zone 1 week postoperatively was higher in the 120-μm cap group as compared to the 130-μm group. Moreover, the postoperative corneal wavefront aberrations with a 120-μm cap were significantly higher than those with a 130-μm cap. Thus, higher cap thickness may be better for postoperative corneal clarity and wound healing.

Study approval

The authors confirm that any aspect of the work covered in this manuscript that involved human patients or animals was conducted with the ethical approval of all relevant bodies and the study was performed in accordance with the Declaration of Helsinki, and the protocol was approved by the Ethical Committee of the Second Affiliated Hospital of Zhejiang University School of Medicine (No.2017-017).

Author contributions

SW: conceptualization, design, writing original draft and revision; HJ: conceptualization, design, writing original draft; TW: conceptualization, supervision, manuscript review, and editing. All authors reviewed the results and approved the final version of the manuscript.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Thanks to all the peer reviewers for their opinions and suggestions.

Abbreviations

CD

corneal densitometry

CDVA

corrected distance visual acuity

GSU

grayscale units

HOA

higher-order aberrations

LogMAR

Logarithm of the minimal angle of resolution

LT

lenticule thickness

MCT

minimal corneal thickness

OBL

opaque bubble layer

RMS

root mean square

RST

residual stromal thickness

SD

standard deviation

SE

Spherical Equivalent

SMILE

Small Incision Lenticule Extraction

UDVA

Uncorrected distance visual acuity

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