Posterior corneal elevation changes after small incision lenticule extraction

doi:10.21203/rs.3.rs-2193907/v1

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Posterior corneal elevation changes after small incision lenticule extraction

https://doi.org/10.21203/rs.3.rs-2193907/v1

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Background: To investigate posterior corneal surface (PCS) elevation changes and influencing factors after SMILE.

Methods: A total of 285 patients (559 eyes) were analyzed. The PCS elevation before the operation, 1 day, 1 week and 1 month after the operation were recorded, including cornea apex and the 2 mm, 4 mm, and 6 mm circle. Analyzed the correlation between the changes with age, equivalent spherical refraction, corneal thickness, lenticule thickness, corneal cap thickness, lenticule diameter, residual corneal thickness, corneal cap thickness/corneal thickness, lenticule thickness/corneal thickness and PCS curvature.

Results: There were significant differences in the PCS elevation of the center and 2 mm, 4 mm, and 6 mm circles at 1 day, 1 week and 1 month postoperation (P<0.0001) comparing to preoperation. There were no significant changes after 1 day (P>0.05). At the center and 2 mm circle, the PCS moved backward after the operation, which were positively correlated with preoperative values of K1, K2 and Km on the PCS (r>0, P<0.05). At the 4 mm and 6 mm circles, the PCS moved forward after the operation, while it was negatively correlated with the preoperative K1, K2 and Km of the PCS (r<0, P<0.05). There was no other influencing factors.

Conclusions: The PCS elevation changed significantly on the first day after SMILE surgery and then stayed stable. Within the safe range, the possibility of PCS expansion after SMILE is rare. SMILE surgery should be carefully selected for patients with excessive or excessively small PCS curvature.

Health sciences/Medical research/Outcomes research

Health sciences/Risk factors

small incision lenticule extraction (SMILE)

posterior corneal surface

posterior corneal

curvature

With the development of laser technology, corneal refractive surgery has entered a new stage. A femtosecond laser uses the principle of light blasting to cut tissue, which has high accuracy and safety ^[1]. Femtosecond laser small incision lenticule extraction (SMILE) using this technology is invasive and flapless, with good safety, efficacy and predictability ^[2–3]. However, as a refractive surgery, changes in corneal morphology and biomechanics could lead to corneal ectasia ^[4], which can lead to keratoconus and severe visual impairment ^[5]. When keratoconus occurs, the change in the posterior surface of the cornea occurs earlier than that of the anterior surface of the cornea. The protrusion of the posterior surface of the cornea is the initial signal of the occurrence of keratoconus. Observing the change in the posterior surface of the cornea is an effective method to detect the stability of the posterior surface of the cornea with good repeatability and stability ^[6–8]. The Pentacam anterior segment analyzer can be used to measure the height of the PCS to further predict the possibility of corneal ectasia ^[9–10]. In this study, we measure the height of the PCS before and after SMILE and to analyze the changes.

Participants

This study is a retrospective study and strictly abides by the Declaration of Helsinki. All patients signed an informed consent form. This study was approved by the Ethics Committee of the First Affiliated Hospital of the University of Science and Technology of China (USTC) ( 2022 RE272). A total of 285 patients (559 eyes) who underwent SMILE in the First Affiliated Hospital of USTC from June 2020 to March 2021 were selected. There were 172 males (336 eyes) and 113 females (223 eyes), with an average age of 24.28 ± 5.19 (range 18–41) years. And the average preoperative spherical equivalent (SD) was − 5.08 ± 1.65 (range − 9.62~ -1.00) D. The average corneal cap thickness was 119.30 ± 5.61 (range 90.00-140.00) µm. The average lenticule diameter was 6.65 ± 0.17 (range 6.00-6.80) mm. The average corneal thickness was 538.60 ± 25.31 (range 479.00-617.00) µm. The average lenticule thickness was 114.40 ± 22.62 (range 56.00-180.00) µm. The average residual corneal thickness was 304.80 ± 24.53 (range 275.00-394.00) µm.

Examination

All patients underwent detailed preoperative examination, including uncorrected visual acuity, best corrected visual acuity, computer optometry, retinoscopy optometry, comprehensive optometry, noncontact intraocular pressure, slit lamp microscopy, fundus examination, master examination, etc.

Pentacam Scheimpflug imaging: Application of Pentacam anterior segment analyzer (Oculus GmbH, Wetzlar, Germany) measured the patient in a scotopic environment, saving the examination results with image quality "OK". The corneal topography and the height map of the anterior and PCSs were obtained. The height of the posterior surface of the cornea refers to the best fit sphere in the central 8 mm area of the cornea. The posterior surface height of the corneal vertex is recorded. 270° corneal posterior surface height, and calculate the average of 2 mm, 4 mm, and 6 mm annular corneal posterior surface height. All operations were performed by experienced physicians.

Surgical procedure

SMILE surgery was performed by experienced physicians using Visual The Max femtosecond laser system (Carl Zeiss Meditec AG, Germany). After topical anesthesia with 0.5% proparacaine hydrochloride eye drops (15 ml: 75 mg, Alcon), the eyeball was fixed by a negative pressure suction ring. Laser scanning was started after confirming accurate positioning. The pulse frequency was set to 500 kHz. The incision was located at 12 o’clock and 2 mm widely. All lenticules were all taken out completely.

Postoperative examination

The patients routinely applied levofloxacin eye drops (5 ml: 24.4 mg, Santen Pharmaceutical Co., Ltd.), fluorometholone eye drops (5 ml: 5 mg, Santen Pharmaceutical Co., Ltd.), and sodium hyaluronate eye drops (5 ml: 5 mg, Santen Pharmaceutical Co., Ltd.). Uncorrected visual acuity, slit lamp microscopy, computer optometry, noncontact intraocular pressure and Pentacam anterior segment analyzer were reviewed at one day, one week, one month, three months and six months after the operation.

Statistical analysis

Statistical analysis was performed using Prism 9.3. Data are presented as the mean ± standard deviation. Repeated measures analysis of variance was used to compare the differences in the height of the PCS at different time points. Pearson correlation analysis was used to analyze the correlation between the change in corneal posterior surface height and age, preoperative spherical equivalent power, corneal thickness, lenticule thickness, residual corneal thickness, corneal cap thickness/corneal thickness, lenticule thickness/corneal thickness and posterior surface curvature and the correlation with corneal cap thickness and lenticule diameter using Spearman correlation analysis. P < 0.05 indicates statistical significance.

General Results

All patients underwent successful surgery without intraoperative or postoperative complications. All patients achieved visual acuity of 0.8 and above at 1 week and more than 6 months after surgery. The diopters were all within ± 1 D, and 95.2% (532 eyes) were within ± 0.5 D. No patient had refractive regression > 0.5 D. No patient had corneal ectasia or secondary keratoconus during follow-up.

Height of the PCS

At the point of corneal vertex, the height of the PCS was 1.200 ± 2.629 µm before the operation, 0.224 ± 3.242 µm 1 day after the operation, 0.209 ± 2.773 µm 1 week after the operation, and 0.163 ± 2.681 µm 1 month after the operation. There were significant differences when comparing the height of the PCS at 1 day, 1 week, and 1 month after the operation with the height before the operation (F = 52.01, P < 0.0001). The apex of the posterior surface of the cornea gradually moved backward at 1 day, 1 week and 1 month after the operation. There was no significant difference when comparing the PCS height at 1 week with 1 day after surgery (P = 0.999), 1 month with 1 day after surgery (P = 0.948), and 1 month with 1 week after surgery (P = 0.928). (Fig. 1)

At the 2 mm circle, the height of the PCS was 1.623 ± 1.607 µm before the operation. 0.908 ± 2.033 µm 1 day after the operation, 1.086 ± 1.664 µm 1 week after the operation, and 1 month after the operation was 1.063 ± 1.625 µm. There were significant differences when comparing the height of the PCS at 1 day, 1 week, and 1 month after the operation with the height before the operation (F = 44.63, P < 0.0001). One day after the operation, the posterior surface of the 2 mm circle moved backward and then moved forward to approach the preoperative height. There was no significant difference when comparing the PCS height at 1 week with 1 day after surgery (P = 0.070), 1 month with 1 day after surgery (P = 0.168), and 1 month with 1 week after surgery (P = 0.967). (Fig. 1)

At the 4 mm circle, the height of the PCS was − 1.733 ± 1.389 µm before the operation. -1.177 ± 1.730 µm 1 day after operation, -1.278 ± 1.423 µm 1 week after operation, and 1 month after operation was − 1.321 ± 1.379 µm. There were significant differences when comparing the height of the PCS at 1 day, 1 week, and 1 month after the operation with the height before the operation (F = 32.15, P < 0.0001). One day after the operation, the posterior surface of the 4 mm circle moved forward compared with the preoperative value and then moved backward to approach the preoperative height. There was no significant difference when comparing the PCS height at 1 week with 1 day after surgery (P = 0.532), 1 month with 1 day after surgery (P = 0.151), and 1 month with 1 week after surgery (P = 0.830). (Fig. 1)

At the 6 mm circle, the height of the PCS was − 6.085 ± 2.714 µm before the operation, -4.885 ± 4.246 µm 1 day after the operation, -4.983 ± 2.751 µm 1 week after the operation, and − 5.093 ± 2.568 µm 1 month after the operation. There were significant differences when comparing the height of the PCS at 1 day, 1 week, and 1 month after the operation with the height before the operation (F = 30.27, P < 0.0001). One day after the operation, the posterior surface of the 4 mm circle moved forward compared with the preoperative value and then moved backward to approach the preoperative value. There was no significant difference when comparing the PCS height at 1 week with 1 day after surgery (P = 0.950), 1 month with 1 day after surgery (P = 0.607), and 1 month with 1 week after surgery (P = 0.681). (Fig. 1)

Analysis of the influencing factors of the height change of the PCS

The difference between the corneal posterior surface height of the corneal vertex at pre-operation and 1 day, 1 week, and 1 month after the operation was calculated. Analyze the correlation between the change and possible factors. (Table 1)

The change in the height of the posterior surface of the corneal apex was correlated with the preoperative K2 value (r = 0.124, P = 0.003) and the preoperative Km value (r = 0.090, P = 0.033) at 1 day after the operation.

The change in the height of the posterior surface of the corneal apex was correlated with the preoperative spherical equivalent power (r = 0.101, P = 0.017), lenticule thickness (r= -0.112, P = 0.008), corneal cap thickness/corneal thickness (r = 0.086, P = 0.043), lenticule thickness/corneal thickness (r= -0.106, P = 0.012), preoperative K1 value (r = 0.124, P = 0.003), preoperative K2 value (r = 0.136, P = 0.001) and preoperative Km value (r = 0.119, P = 0.005) at 1 week after the operation.

The change in the height of the posterior surface of the corneal apex was correlated with the preoperative K1 (r = 0.148, P = 0.000 4), K2 (r = 0.141, P = 0.001), and Km value (r = 0.142, P = 0.001) at 1 month after the operation.

The difference between the posterior surface height of the corneal 2 mm circle at pre-operation and 1 day, 1 week, and 1 month after the operation was calculated. Analyze the correlation between the change and possible factors. (Table 2)

The change in the posterior surface height in the 2 mm circle was correlated with the preoperative K1 (r = 0.087, P = 0.040), K2 (r = 0.137, P = 0.001), and Km values (r = 0.001) 0.103, P = 0.015) at 1 day after the operation.

The change in the posterior surface height of the corneal 2 mm circle was correlated with the preoperative K1 (r = 0.103, P = 0.015), K2 (r = 0.123, P = 0.004) and Km values (r = 0.123, P = 0.004) 0.104, P = 0.014) at 1 week after the operation.

The change in the posterior surface height of the corneal 2 mm circle was correlated with the preoperative K1 (r = 0.107, P = 0.011), K2 (r = 0.109, P = 0.010), and Km values (r = 0.109, P = 0.010) 0.109, P = 0.010) at 1 month after the operation.

The difference between the posterior surface height of the corneal 4 mm circle at pre-operation and 1 day, 1 week, and 1 month after the operation was calculated. Analyze the correlation between the change and possible factors. (Table 3)

The change in the posterior surface height of the corneal 4 mm circle was correlated with lenticule thickness (r = 0.088, P = 0.038) and corneal cap thickness/corneal thickness (r= -0.084, P = 0.047) at 1 day after the operation.

The change in the posterior surface height of the corneal 4 mm circle was correlated with the preoperative K2 value (r= -0.099, P = 0.020) at 1 week after the operation.

The difference between the posterior surface height of the corneal 6 mm circle at pre-operation and 1 day, 1 week, and 1 month after the operation was calculated. Analyze the correlation between the change and possible factors. (Table 4)

The change in the posterior surface height of the corneal 6 mm circle was correlated with the preoperative K2 value (r= -0.090, P = 0.033) at 1 day after the operation.

The change in the posterior surface height of the corneal 6 mm circle was correlated with the preoperative K1 (r= -0.124, P = 0.003), K2 (r= -0.187, P < 0.0001), and Km values (r = -0.153, P = 0.000 3) 1 week after the operation.

The change in the posterior surface height of the corneal 6 mm circle was correlated with the corneal thickness (r = 0.085, P = 0.044), preoperative K2 (r= -0.095, P = 0.025), and Km value (r= -0.090, P = 0.033) at 1 month after the operation.

The number of people with myopia is increasing worldwidely. It is estimated that the global prevalence of myopia will reach 50% in 2050 ^[11]. According to the results of the 2014 Ministry of Education's national census on the physical fitness and health of school students, the prevalence of myopia in primary school students was approximately 45.71%, that in junior high school students was approximately 74.36%, that in senior high school was approximately 83.28%, and that the prevalence of myopia among college students was approximately 95%. The relevant literature after 2013 shows that the prevalence of myopia in adolescents aged 16 to 18 is approximately 84.8%. ^[12] These myopic patients may choose refractive surgery in adulthood. Refractive surgery has experienced the development from mechanical flap to laser flap, from with-flap to no-flap. Excimer laser in situ keratomileusis (LASIK) appeared in 1990 ^[13]. From 2001, a femtosecond laser was used to make corneal lamellar flaps in refractive surgery, that is, a femtosecond-assisted laser in situ keratomileusis (FS -LASIK)^[14]. In 2011, the first SILME operation was officially reported ^[15]. In the SMILE operation, the lenticule was made in the corneal stroma and taken out directly from a small incision, which shortened the operation time and avoided complications related to the corneal flap ^[16].

The biomechanical strength and resistance of corneal at different layers are different. Some studies have pointed out that the lamellar interweaving and corneal cross-linking of collagen fibers in corneal ensure the biomechanical strength of the cornea, which is essential to withstand intraocular pressure and maintain corneal shape. The anterior 1/3 is the strongest part ^[17–18], which includes the corneal epithelial layer, the Bowman’ membrane and the anterior stroma. The study found that compared with FS-LASIK surgery and superficial excimer laser keratectomy (PRK), SMILE surgery had a deeper cutting depth, smaller incision, preserved Bowman’s membrane, and avoided the absolute stress interruption of the anterior stromal layer. ^[19–21] However, some studies have also pointed out that for patients with the same SD, SMILE consumes more corneal tissue than other corneal refractive surgeries, which may lead to the loss of corneal biomechanical strength and resistance ^[22–23]. Combining these two factors, how corneal biomechanics change after SMILE still needs more in-depth research.

In recent years, the changes in corneal tissue morphology in ophthalmic surgery have attracted increasing attention, such as research on the changes in the PCS after LASIK ^[24]. The safety of the operation was evaluated by observing the changes in the posterior surface of the cornea. In addition to refractive surgery, changes in corneal morphology during intraocular surgery have also received attention, such as in cataract patients ^[25]. Studies have shown that the morphology of the PCS can be used as a reliable indicator for evaluating postoperative corneal stability because it is not affected by incision size, flap thickness, or tear film quality ^[26].

This study shows that there is a certain degree of change in the height of the PCS after SMILE. The apex of the posterior surface of the cornea gradually moved backward at 1 day, 1 week and 1 month after the operation. The 2 mm circle on the posterior surface of the cornea moved backward at 1 day after the operation and then moved forward to approach the preoperative position. The 4 mm circle and 6 mm circle on the posterior surface of the cornea moved forward at 1 day after the operation and then moved back to approach the preoperative position. This is different from the research results of Zhang et al. ^[27] which indicate that the height of the posterior surface of the cornea at the 4 mm circle gradually moved slightly backward compared with that before the operation, and the posterior surface of the cornea did not move forward in each area. Zhang et al. considered the results to be related to postoperative peripheral wound edema. Edema may result in thickening of the cornea, which results in a retrograde shift in the height of the posterior surface of the cornea at the 4 mm circle. The shape of the lenticule is with a thick central and thin periphery. Therefore, in this study, the vertex and the 2 mm circle cornea were moved backward because more tissue was cut in the center. On the other hand, the 4 mm circle and 6 mm circle were highly advanced in this study because less tissue was cut in the periphery. In this study, the operations were performed by fixed and experienced surgeons. More attention was given to protecting the incision, avoiding distraction of the incision during the separation and removal of lenticules. In addition, during lenticule separation, unnecessary intracorneal manipulation was avoided to reduce corneal edema in the central surgical area. There was no obvious corneal or peri-incision edema in all 559 eyes.

Athough the height of the PCS changed to a certain extent, all patients had satisfactory visual acuity. The results of Pentacam showed that in addition to the significant changes in the anterior surface morphology, the heights of the apex and 2 mm, 4 mm and 6 mm circles of the PCS on postoperative day 1 had statistically significant changes compared with preoperative ones. However, this change should be related to lenticule removal. At 1 week and 1 month after the operation, the data did not show significant differences when compared to the 1-day postoperative data. This result is consistent with the report of Zeng ^[28] on the changes in PCS height in the early postoperative stage of moderate to high myopia. We infer that the postoperative corneal posterior surface height may change to a certain extent, then remain stable and approach the preoperative status. In the safe range of corneal stroma ablation, slight changes in the height of the PCS are less likely to cause prosthetic ectasia and keratoconus.

This study is the first to explore the correlation of the postoperative height change of the PCS and the preoperative curvature of the PCS. The results show that the changes in the vertex position 2 mm circle are positively correlated with the curvature of the PCS. The changes in the height of the 4 mm circle and 6 mm circle on the PCS are negatively correlated with the curvature of the PCS. Although the correlation is weak, it still suggests that more attention should paid to in patients with too large or too small of a curvature of the PCS. Patients with flat corneal curvature are more likely to have refractive regression and need a higher nanogram value. This result is consistent with Zhou et al. ^[29], who found that a flat cornea increases the risk of regression. However, research has also indicated that compared with flat corneas, steep corneas are more unstable, and will increase the probability of refractive regression ^[30]. These findings suggest that we should be careful when patients have an extremely flat or steep cornea, including total and posterior surface curvature.

We also analyzed the correlation between the changes in PCS height and age, preoperative SE, corneal thickness, lenticules thickness, corneal cap thickness, lenticules diameter, residual corneal thickness, corneal cap thickness/corneal thickness, and thickness/corneal thickness. Previous studies have suggested that the change in the PCS is related to the residual corneal thickness ^[26] and corneal ablation ratio ^[31]. However, in our study, there were no significant findings, which is not consistent with our expectation. It is also different from studies that suggest that the residual stromal thickness ^[26] and the corneal ablation ratio ^[31] are correlated with the height of the PCS. This may be related to the preservation of the more resistant and stronger anterior corneal integrity. It is still necessary to strictly control the surgical indications and contraindications.

In this study, the height of the PCS tended to be stable 1 week and 1 month after the operation. Combined with the results of previous literature ^{[24, 26–28]}, no longer postoperative data analysis was included. SMILE is a safe, effective and stable treatment for correcting myopia. The morphology of the PCS changed 1 day after the operation and then tended to be stable. The risk of operation-induced ectasia and keratoconus is relatively small. More attention should be taken when patients are with too steep or too flat PCS.

Acknowledgements

The authors would like to thank the Ophthalmology Department of the First Affiliated Hospital of the USTC for all the support and the sponsors.

Conflict of Interest

All authors have no conflict of interest to declare.

Funding

Natural Science Foundation of Anhui (No. 2108085MH278) and Innovation and Entrepreneurship Support Program for Returnees in Anhui Province (No. 2021LCX006).

Contributions

SL contributed to the study design, data interpretation and revised the article.

LM contributed to the study design, collected and analyzed the data, and she drafted the article.

SW, CY, PH and LD contributed to the study design and implementation, and provided supportive contributions

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Tables 1 to 4 are available in the Supplementary Files section.

(Not answered)

Table1vertex.docx
Table 1 Analysis of the influencing factors on the change in the posterior surface height of the corneal apex after the operation. A positive r value indicates a positive correlation, while anegative r value indicates a negative correlation. P < 0.05 indicates a statistically significant correlation.
Table22mm.docx
Table 2 Analysis of influencing factors on the change in posterior surface height of corneal 2 mm circle after operation. A positive r value indicates a positive correlation, while a negative r value indicates a negative correlation. P < 0.05 indicates a statistically significant correlation.
Table34mm.docx
Table 3 Analysis of influencing factors on the change in posterior surface height of corneal 4 mm circle after operation. A positive r value indicates a positive correlation, while a negative r value indicates a negative correlation. P < 0.05 indicates astatistically significant correlation.
Table46mm.docx
Table 4 Analysis of influencing factors on the change in posterior surface height of corneal 6 mm circle after operation. A positive r value indicates a positive correlation, while a negative r value indicates a negative correlation. P < 0.05 indicates a statistically significant correlation.

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Posterior corneal elevation changes after small incision lenticule extraction

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