Corneal Stroma Thickness Changes after Myopic Laser Corneal Refractive Surgery

Three-year results of central corneal stromal thickness reduction in small-incision lenticule extraction for high myopia correction

BACKGROUND

To investigate the long-term corneal stromal remodeling and central stromal thickness (CST) reduction accuracy after small-incision lenticule extraction (SMILE) for high myopia correction.

METHODS

This prospective study included 30 patients (50 eyes) who had undergone SMILE. Measurements of CST reduction using optical coherence tomography were performed at 1 month, 6 months, 1 year, and 3 years after surgery. Correlations were performed between planned and achieved CST reductions.

RESULTS

The study enrolled 50 eyes of 30 patients. The mean spherical equivalent was -9.25±1.52 D(diopters). The postoperative CST increased in the first month after surgery and remained stable for a year. Thereafter, it remained stable during follow-up from 1 to 3 years postoperatively. The predicted CST reduction was 146.4 ± 10.3 μm. The achieved CST reductions at 1 month, 6 months, 1 year, and 3 years after surgery were 135.3 ± 12.1 μm, 130.8 ± 10.6 μm, 125.9 ± 9.4 μm, and 122.2 ± 10.6 μm, respectively. An overestimation of CST reduction was observed three years after surgery. Correlation analysis revealed a strong correlation between planned and achieved CST reductions; however, no correlation was found between CST reductions predicted error and the planned CST reductions.

CONCLUSION

During long-term follow-up, our findings revealed a significant stromal remodeling following SMILE in patients with high myopia. Therefore, clinicians should consider it when screening patients with high myopia for SMILE.

Myopic Regression after FS-LASIK and SMILE

PURPOSE

To compare the degree of myopic regression after myopia correction with either femtosecond laser-assisted in situ keratomileusis (FS-LASIK) or small-incision lenticule extraction (SMILE) over 18 months.

METHODS

Patients undergoing FS-LASIK or SMILE surgery for myopia correction were retrospectively recruited. The propensity scores were used to match patients by age and preoperative manifest spherical equivalent (SEQ) from these 2 groups. Myopic regression was analyzed using the Cox proportional hazard model.

RESULTS

A total of 416 eyes of 416 patients undergoing FS-LASIK and 416 eyes of 416 patients undergoing SMILE were matched. Using 1-month SEQ as baseline, the SEQ regression values after FS-LASIK were 0D, -0.17 ± 0.69D, -0.24 ± 0.65D, -0.31 ± 0.65D, -0.32 ± 0.63D, and -0.33 ± 0.62D and the SEQ regression values after SMILE were 0D, -0.07 ± 0.75D, -0.18 ± 0.77D, -0.23 ± 0.82 D, -0.21 ± 0.77D, and -0.24 ± 0.68D at 1, 3, 6, 9, 12, and 18 months, respectively. The Cox proportional hazard model showed that preoperative manifest SEQ (P = 0.021) and designed optical zone (P = 0.048) are significant predictors. The selected surgical procedure had no significant effect on predicting myopic regression (P = 0.470). The cumulative survival rates of myopic regression were 54.74% and 42.10% in the FS-LASIK group and 58.66% and 43.83% in the SMILE group, at 12 and 18 months, respectively (log-rank test, P = 0.11).

CONCLUSIONS

After matching based on age and preoperative manifest SEQ, we found that higher myopia and a smaller optical zone contribute significantly to the development of myopic regression after undergoing FS-LASIK or SMILE surgery at 18 months. The selected surgical procedure, however, does not affect the likelihood of myopic regression.

Epithelial thickness remodeling after small incision lenticule intrastromal keratoplasty in correcting hyperopia measured by RTVue OCT

PURPOSE

To characterize the in vivo corneal epithelial thickness (CET) remodeling profile in a population of eyes after small incision lenticule intrastromal keratoplasty (SMI-LIKE) for hyperopia.

METHODS

The CET profile was measured by RTVue-100 Fourier-domain OCT system across the central 6-mm diameter of the cornea of 17 eyes from 12 subjects (five males and seven females) who accepted corneal stromal lens implantation surgery for correcting hyperopia. The CET were measured at positions with a radius of 0-1.0 mm, 1.0-2.5 mm (divided into eight quadrants) and 2.5-3.0 mm (divided into eight quadrants) from the corneal center. Corneal maximum simulated keratometry (Km) was measured by Pentacam HR anterior segment analyzer to analyze CET changes. The examination data of subjects were collected in four time periods, which were preoperative, short-term postoperative (one week after surgery), mid-term postoperative (the last review within 3-6 months after surgery), and long-term postoperative (the last review over 1-2.5 years after surgery). The changes of CET were compared and analyzed in the four time periods.

RESULTS

Mean CET in 0-1.0 mm, 1.0-2.5 mm and 2.5-3.0 mm of the cornea decreased in one week after surgery, respectively, as compared to CET in the preoperative period, which turned from 55.06 ± 0.82 μm、54.42 ± 0.75 μm、53.46 ± 0.60 μm to 51.18 ± 1.05 μm (P = 0.005), 49.38 ± 0.70 μm (P = 0.000), 51.29 ± 0.59 μm (P = 0.025). In the mid-term postoperative period, mean CET in 0-1.0 mm and 1.0-2.5 mm areas kept thinner than mean CET in the preoperative period, CET in 0-1.0 mm is 50.59 ± 0.76 μm (P = 0.000),CET in 1.0-2.5 mm is 50.23 ± 0.57 μm (P = 0.000), while mean CET in 2.5-3.0 mm area recovered to the same thickness as the preoperative level, which is 54.36 ± 0.66 μm (P = 1.000), until the long-term period, CET stabilized in the above doughnut pattern.

CONCLUSIONS

After stromal lenticule implantation for hyperopia, CET showed a remodeled form of thinning in the 0-2.5 mm area and thickening in the 2.5-3.0 mm area, and remained stable within one year after surgery.

Predictability of the Achieved Lenticule Thickness in Keratorefractive Lenticule Extraction for Myopia Correction

PURPOSE

To assess the predictability between the SCHWIND ATOS femtosecond laser (SCHWIND eye-tech-solutions) read-out and achieved lenticule thickness measured using MS-39 anterior segment optical coherence tomography (ASOCT; CSO) at the corneal vertex at postoperative 1 day and 1 and 3 months of follow-up.

METHODS

This retrospective case series included 130 eyes of 65 consecutive patients who were treated with SmartSight (SCHWIND eye-tech-solutions) lenticule extraction. Sixty-four percent of patients were women with a mean spherical refraction of -4.98 ± 1.19 diopters (D) and mean astigmatism of 0.53 ± 0.64 D. The measurements were performed using AS-OCT with the Phoenix Software v 4.1.1.5. Lenticule thickness was obtained by the subtraction method between preoperative and postoperative total corneal thickness.

RESULTS

A lower reduction in central corneal thickness (CCT) compared to the laser read-out for all three follow-up visits can be observed. Essentially, the reduction in CCT was identical at 3 months versus 1 month. The reduction in CCT was lowest at postoperative 1 day. Examining the 1- and 3-month data (essentially equivalent), one can see a best fit of y = 0.94 x -7 µm.

CONCLUSIONS

The reduction in CCT was stable from 1 month of follow-up. The stable reduction in CCT was -6% (-1.5% after accounting for design decisions) and -7 µm lower than the respective laser read-out. The findings are predictable, showing a certain level of the lenticules becoming slightly thinner than their respective laser read-outs. [J Refract Surg. 2023;39(11):728-735.].

Comparison of central corneal thickness treated with small incision lenticule extraction, femtosecond laser-assisted in situ keratomileusis, or laser-assisted subepithelial keratomileusis for myopia

To compare the central corneal thickness (CCT) treated with small incision lenticule extraction (SMILE), femtosecond laser-assisted in situ keratomileusis (FS-LASIK), or laser-assisted subepithelial keratomileusis (LASEK) for myopia correction. This was a retrospective case series study. Patients who had completed 1-year follow-up after receiving SMILE, FS-LASIK, or LASEK at our hospital from January 2019 to July 2021 were included. Pentacam pachymetry was performed to measure the CCT. The predicted CCT reduction was obtained through laser platform. The measured CCT reduction was defined as the difference between the preoperative and postoperative CCT using Pentacam pachymetry. There were 100 eyes treated by SMILE, 100 eyes by FS-LASIK, and 100 eyes by LASEK. There was a significant difference in predicted CCT reduction among the three surgeries (P < 0.001), but no significant difference in measured CCT reduction postoperatively (PGroup = 0.373). At 1 year postoperatively, the CCT reduction was overestimated by 17.85 ± 5.36 µm in the SMILE group, underestimated by 4.31 ± 7.08 µm in the FS-LASIK group, and underestimated by 7.60 ± 8.28 µm in the LASEK group (PGroup < 0.001, PTime < 0.001). In the FS-LASIK group, the difference between predicted and measured CCT reduction was not related to the predicted CCT reduction (P = 0.095). The laser platform may overestimate the CCT reduction for SMILE and underestimate it for FS-LASIK and LASEK. FS-LASIK has a much higher level of accuracy in CCT reduction, which is not influenced by refractive correction.

Comparison of planned versus achieved central corneal stromal thickness reduction in SMILE versus FS-LASIK: a retrospective study

Accuracy of planned corneal stromal thickness (CST) reduction is essential to the safety of laser vision correction. This study was to compare the accuracy of the planned central CST reduction in small incision lenticule extraction (SMILE) and femtosecond laser-assisted in situ keratomileusis (FS-LASIK). A total of 77 patients (43 for SMILE, 34 for FS-LASIK using Custom-Q algorithm) were included in this retrospective study. At postoperative 6-18 months, the central CST reduction was overestimated by 18.49 ± 6.42 μm in the SMILE group (P < 0.001) and underestimated by 2.56 ± 7.79 μm in the FS-LASIK group (P = 0.064). The planned-achieved difference (PAD) of central CST reduction was positively correlated with preoperative manifest refraction spherical equivalent (MRSE) and with planned central CST reduction in both groups. When calculated by manifest refraction (MR) without nomogram adjustment, the central CST reduction was overestimated by 11.14 ± 6.53 μm in the SMILE group and underestimated by 2.83 ± 7.39 μm in the FS-LASIK group. The PAD of central CST reduction without nomogram was significantly narrowed in SMILE and maintained in FS-LASIK, suggesting estimation using MR without nomogram adjustment may be feasible for SMILE and FS-LASIK in clinical practice.

Comparison of Predictability in Central Corneal Thickness Reduction After SMILE and FS-LASIK for High Myopia Correction

INTRODUCTION

To compare central corneal thickness (CCT) reduction after small incision lenticule extraction (SMILE) and femtosecond laser-assisted in situ keratomileusis (FS-LASIK) in eyes with high myopia.

METHODS

In this prospective, consecutive study, 70 eyes with high myopia undergoing SMILE (n = 35) or FS-LASIK (n = 35) were recruited. Corneal topography images were acquired using the Pentacam HR imaging system preoperatively and at 1 day, 1 month, and 6 months postoperatively. Predicted CCT reduction was extracted directly from the VisuMax femtosecond laser system or MEL 80 excimer laser platform. The achieved CCT reduction was determined using corneal thickness difference maps from the Pentacam. Comparative statistics and linear regression analyses were performed to evaluate the predictability in stromal thickness reduction.

RESULTS

The mean predicted CCT reductions were 152.9 ± 6.7 μm and 150.9 ± 7.3 μm in the FS-LASIK and SMILE groups, respectively, with no statistical difference. For each follow-up time, no significant difference was noted in the two groups in the achieved CCT reduction. At 6-month follow-up, the CCT reductions were overestimated to be 23.06 ± 6.97 µm and 28.29 ± 13.92 µm in the SMILE and FS-LASIK groups, respectively (P = 0.003), showing statistical difference. Regression analysis revealed that the positive correlation between achieved and predicted CCT reductions was stronger in SMILE (R² = 0.5065, P < 0.001) than in FS-LASIK (R² = 0.2237, P = 0.004). Overestimated CCT reduction was not correlated with predicted CCT reduction in either group.

CONCLUSIONS

Systematically overestimated CCT reduction is found after SMILE and FS-LASIK in high myopia correction. Deviations between planned and achieved CCT reductions are more pronounced in FS-LASIK than in SMILE.

Twelve-month outcomes of a new refractive lenticular extraction procedure

BACKGROUND

To evaluate the 12-month refractive and visual outcomes of Small Incision Guided Human-cornea Treatment (SmartSight®, SCHWIND eye-tech-solutions, Kleinostheim, Germany) in the treatment of myopia corrections with low to moderate astigmatism with the use of a new femtosecond laser system.

METHODS

221 eyes of 114 patients consecutively treated with SmartSight lenticule extraction were assessed. The mean age of the patients was 28±6 years at the time of treatment with a mean spherical equivalent refraction of -6.26±2.17D and mean astigmatism of 0.92±0.68D. Monocular corrected distance visual acuity (CDVA), uncorrected distance visual acuity (UDVA) were assessed pre- and post-operatively. Refractive changes have been determined in terms of changes in refraction, as well as changes in keratometric readings. The changes in central epithelial thickness have been determined.

RESULTS

At twelve months post-operatively, mean UDVA was 20/21±2. Spherical equivalent showed a residual refraction of +0.48±0.31D with refractive astigmatism of 0.13±0.18D postoperatively. There was a slight decrease of -0.1 Snellen lines at 12-months follow-up. The same correction was determined using changes in refraction, as well as changes in keratometric readings. The central epithelial thickness increased by +3±2µm. Spherical equivalent correction within ±0.50D was achieved in 199 eyes (90%), and cylindrical correction in 221 (100%). Preoperative corrected distance visual acuity (CDVA) was 20/20 or better in 213 eyes (96%), and postoperative uncorrected (UDVA) was 20/20 or better in 205 eyes (93%). No eye had lost two or more Snellen lines of CDVA.

CONCLUSIONS

Myopic astigmatism correction with SmartSight provided good results for efficacy, safety, predictability, and visual outcomes at the twelve months of follow up. The central epithelial thickness barely increased by 3±2µm.

Comparison of Changes in Corneal Thickness and Curvature After Myopia Correction Between SMILE and FS-LASIK

PURPOSE

To comparatively investigate the changes in corneal thickness and curvature between small incision lenticule extraction (SMILE) and femtosecond laser-assisted laser in situ keratomileusis (FS-LASIK).

METHODS

Data were collected preoperatively and postoperatively at 1 week, 1 month, and 3 months, including central corneal thickness (CCT), midperipheral corneal thickness (MPCT), peripheral corneal thickness (PCT), central keratometry (CK), midperipheral keratometry (MPK), and peripheral keratometry (PK), using a dual rotating Scheimpflug analyzer.

RESULTS

At 1 week postoperatively, changes in CCT, MPCT, PCT, and PK were significantly greater in the SMILE group than in the FS-LASIK group (P = .003 for CCT and < .001 for MPCT, PCT, and PK), but no significant inter-group differences were found between changes in CK and MPK (P = .883 and .513, respectively). CCT, MPCT, and PK values showed a similar increase in both groups, but significantly more increase in PCT and significantly less increase in CK and MPK was observed in the SMILE group from 1 week to 3 months after surgery. At 3 months postoperatively, CCT and MPCT were significantly thinner in the SMILE group than in the FSLASIK group (P = .018 and .022, respectively), and there was no significant difference in PCT, CK, MPK, and PK between the two groups.

CONCLUSIONS

SMILE caused significantly more corneal thinning at the center, midperiphery, and periphery, and more corneal flattening at the periphery, compared to FS-LASIK. A different pattern of corneal remodeling was observed between the two surgeries during the 3-month follow-up period. [J Refract Surg. 2023;39(1):15-22.].

Corneal Epithelium Thickness and Refractive Changes After Myopic Laser Corneal Refractive Surgery

PURPOSE

To evaluate the postoperative changes in corneal epithelium thickness and refractive power after femtosecond laser-assisted laser in situ keratomileusis (LASIK) and small incision lenticule extraction (SMILE) for myopia correction using anterior segment optical coherence tomography (OCT) with an integrated Placido disc topographer.

METHODS

The VisuMax 500-kHz femtosecond laser (Carl Zeiss Meditec AG) and Amaris 750 excimer laser (SCHWIND eye-tech-solutions) were used. Central, paracentral, and 6-mm epithelial thickness values were obtained, and the change in the value of epithelial thickness was calculated. Changes in the refractive power of the epithelium were also evaluated. The repeatability of this new measurement was also analyzed using the intraclass correlation (ICC). The total follow-up period was 6 months.

RESULTS

A total of 77 LASIK eyes were matched with 77 SMILE eyes. Mean spherical equivalent was -3.92 ± 1.67 diopters (D) for LASIK versus -4.02 ± 1.63 D for SMILE (P = .356). Epithelial thickness parameters significantly and equally thickened in both types of surgery. The change in the value of epithelial thickness was positively correlated with spherical aberration. Analysis of the refractive power of the corneal epithelial layer (ICC > 0.70) showed a tendency for the postoperative myopization of the refractive component of this layer (-0.11 D for SMILE and -0.53 D for LASIK at 3 mm) and an increase in its cylinder and aberrometry. Increasing postoperative spherical aberration and epithelial thickness increased myopization of the epithelial refractive sphere (P < .05).

CONCLUSIONS

Corneal epithelium thickens similarly after LASIK and SMILE, being slightly higher after SMILE. This correlates with the induced spherical aberration. Corneal epithelium thickening induces myopization of its refractive power, which accounts for a slight regression of the net refractive power change on the treated cornea. [J Refract Surg. 2022;38(9):602-608.].

Accuracy of the Preoperative Predicted Percentage of Tissue Altered Calculation in Refractive Surgery Planning for Myopic LASIK

PURPOSE

To determine the reliability of the percentage of tissue altered (PTA) calculation as part of the planning strategy for myopic laser in situ keratomileusis (LASIK) by comparing the estimated PTA provided by preoperative calculation to the postoperative PTA actually achieved in microkeratome-assisted myopic LASIK.

METHODS

This retrospective study included 3,624 eyes of 3,624 patients who underwent mechanical microkeratome-assisted LASIK surgery for myopic correction. The calculated preoperative PTA values based on the planned flap thickness and ablation depth were compared with the actual achieved postoperative PTA using the difference of corneal central thickness postoperatively for assessing the achieved ablation depth and the intraoperative ultrasound-assisted flap thickness measurement. Regression analysis was performed to reveal preoperative parameters that might influence PTA calculation accuracy.

RESULTS

The mean difference between the estimated and achieved PTA was 0.451 ± 3.45% (P < .001) (95% CI: 0.3708 to 0.5322) with a preoperative and postoperative mean PTA of 31.07 ± 4.07% and 31.52 ± 5.78%, respectively. The differences between the achieved and planned maximum ablation depth and flap thickness were 4.32 ± 13.70 µm (P < .001) and -1.61 ± 13.66 µm (P < .001), respectively.

CONCLUSIONS

Although a statistically significant difference was found between the preoperative calculated PTA and actually achieved PTA, the difference in PTA value (less than 1%) was clinically non-significant and indicated a highly reliable metric for preoperative refractive surgery planning. [J Refract Surg. 2022;38(7):422-427.].

Corneal Lenticule Creation Using a New Solid-State Femtosecond Laser Measured by Spectral Domain OCT in a Porcine Eye Model

Purpose

To determine the accuracy and precision of corneal lenticule creation with a new solid-state femtosecond laser in a porcine eye model.

Methods

Corneal lenticule creation was performed using a new solid-state femtosecond laser on 60 porcine eyes with 10 subgroups. Optical coherence tomography images were acquired immediately after laser treatment. Cap thickness (CT), cap diameter (CD), and lenticule thickness (LT) were measured manually by three independent readers. Additionally, CT and LT were measured by an automated algorithm (aLT, aCT).

Results

Measured LT was significantly greater than the intended LT (average difference 14.3 ± 5.6 µm, P < 0.001). aLT was closer but still significantly different from the intended LT (-2.9 ± 5.8 µm, P < 0.001). Measured CT showed no significant difference from the intended CT (2.6 ± 13.3, P = 0.145). aCT was significantly smaller compared to the intended CT (-9.6 ± 13.6, P < 0.001). Measured CD was significantly smaller compared to the intended CD (-0.21 ± 0.20 mm, P < 0.001). All lenticules were cut as planned with no laser-related complications.

Conclusions

This new solid-state femtosecond laser used in our trial provides corneal lenticule creation in a porcine eye model comparable to other established systems. However, measuring those lenticules in the provided setting seems too challenging even when using semiautomated algorithms, which seems to be due to the experimental setting of the trial.

Translational Relevance

This trial shows the precision and repeatability of corneal cuts performed by a new femtosecond laser that could translate to refractive corneal lenticule surgery.