Introduction
Scanning Electron Microscopy (SEM) has been available since the mid-20th century, with its development beginning in the late 1930s and early 1940s. The first practical SEM was built in the 1960s and was introduced to ophthalmology in the 1970s. Its use was revolutionary, enabling scientists to visualize the fine structures of eye tissues with far greater resolution than light microscopy, significantly advancing the understanding and treatment of eye diseases. Having a SEM at hand offers the possibility to investigate lenticules created with ATOS SmartSight (SCHWIND eye-tech-solutions).
Methods
An investigation was conducted on 110 eyes that underwent Keratorefractive Lenticule Extraction (KLEx) surgery using the ATOS Femtosecond laser (SCHWIND eye-tech-solutions) at Cheongju First Eye Clinic in Cheongju, South Korea between December 10, 2023, and January 20, 2024. The patients were randomly assigned to either a conventional energy group (C-group) or a low-energy group (L-group). The C-group included 58 eyes, and the L-group included 52 eyes. Each group’s 10 corneal lenticules were analyzed in detail using SEM. The C-group used a pulse energy of 90 nJ with a spot distance of 3.0 μm, a track distance of 3.0 μm, and a total dose of 1000 mJ/cm². The L-group used a pulse energy of 80 nJ with a spot distance of 4.5 μm, a track distance of 3.0 μm, and a total dose of 593 mJ/cm².
Preoperative clinical parameters—such as age, gender, spherical equivalent (SEQ), best corrected distance visual acuity (CDVA), central corneal thickness (CCT), and residual stromal thickness (RST)—showed no statistically significant differences between the two groups.
Immediately after surgery, extracted corneal lenticules were fixed in 2.5% glutaraldehyde and stored at 4°C overnight. The specimens were subsequently sent to a specialized lab for dehydration and further fixation. SEM images were captured using the AxiaChemiSEM (Thermo Fisher Scientific, Massachusetts, USA) under vacuum conditions. Each lenticule’s upper central area was photographed at magnifications of 50x, 400x, and 800x. Figure 1 displays the SEM images of both groups at these magnifications.
Figure 1. SEM photographs at different magnifications in the C-group (top row) and L-group (bottom row).
Investigation
A comprehensive analysis, both qualitative and quantitative, was performed on the SEM images. For the qualitative analysis, the surface irregularity was assessed using a surface irregularity score.1 Three researchers independently evaluated the SEM photographs according to the surface regularity index described in Table 1. The individual scores were then averaged to obtain a final score.
For the quantitative analysis of the surface, the area and number of tissue bridges in the SEM images were determined with help of the image J 1.46 software. The process involved converting all images to grayscale and applying thresholding to enhance tissue bridges (Figure 2). The image J 1.46 software was then used to count the tissue bridges and calculate the area they occupied.
Figure 2. SEM images after grey scaling (A) and thresholding (B) using the image J software. Final image for count analysis is also shown (C).
Results
The L-group demonstrated better UCVA on the first day and at 1 week postoperatively. However, no significant differences were observed between the two groups in terms of BCVA or mean spherical equivalent at 1 week, 1 month, and 3 months after surgery.
Surface Regularity Score
The qualitative analysis, based on the Surface Regularity Score over three different magnifications and given by three independent researchers, revealed an average score of 7.27 ± 0.88 for the C-group and 13.63 ± 0.66 for the L-group, indicating a significantly smoother surface in the L-group (Figure 3A).
Figure 3. The qualitative analysis, based on the Surface Regularity Score over three different magnifications, indicated a significantly smoother surface in the L-group (A). Both the number (B) and the area (C) of tissue bridges were significantly lower in the L-group.
Quantitative analysis
The quantitative analysis showed that the total number of tissue bridges in the C-group was 4.91 ± 0.86, compared to 3.28 ± 0.99 (×10³ bridges/mm²) in the L-group (Figure 3B). The total area of tissue bridges was 92.90 ± 14.26 in the C-group and 67.51 ± 31.26 (×10³ µm²/mm²) in the L-group (Figure 3C). Both the number and the area of tissue bridges were significantly lower in the L-group, with P-values below .05, indicating statistical significance.
Conclusion
The result of KLEx surgery is influenced by various factors,2 and the degree of visual recovery varies depending on the shape of the cornea, the surgical area, and the preoperative spherical lens equivalent, among other factors.3 The concept of reducing the energy to enhance the early postoperative visual outcomes after lenticule extraction in the meantime is well-established.4,5
This study explored the characteristics of conventional and low energy KLEx using Schwind ATOS. The low energy SmartSight group showed better uncorrected visual acuity (UDVA) at the first day and the first week after surgery. Based on the findings with help of a SEM, the low energy group demonstrated a smoother surface considering qualitative and quantitative aspects. In conclusion, low energy SmartSight surgery resulted in faster initial vision recovery and a smoother lenticule surface than the conventional group. This can be attributed to the fact that as pulse energy decreases, the size of the photodisruption becomes smaller, and the asymmetrical setting where “spot distance is greater than track distance” is presumed to result in a smoother lenticule cutting surface.6 However, further studies with larger patient groups and longer follow-up periods are needed to confirm these findings.
1. Kunert KS, Blum M, Duncker GI, et al. Surface quality of human corneal lenticules after femtosecond laser surgery for myopia comparing different laser parameters. Graefes Arch Clin Exp Ophthalmol. 2011;249:1417-1424.
2. Mosquera SA, Darzi S, Pradhan K. Visual acuity improvement (supernormal vision) after SmartSight lenticule extraction procedures: a machine learning assisted approach. Research Square. 2024. PREPRINT (Version 1).
3. Moshirfar M, McCaughey MV, Reinstein DZ, et al. Small-incision lenticule extraction. J Cataract Refract Surg. 2015;41(3):652-665.
4. Donate D, Thaëron R. SMILE with low energy levels: assessment of early visual and optical quality recovery. J Refract Surg. 2019;35(5):285-293.
5. Ji YW, Kim M, Kang DSY, et al. Lower laser energy levels lead to better visual recovery after small-incision lenticule extraction: prospective randomized clinical trial. Am J Ophthalmol. 2017;179:159-170.
6. Amann J, Arba Mosquera S. Optimization of the spot spacings for reducing roughness in laser-induced optical breakdown processes for corneal laser vision correction. Photonics. 2024. 11(2):114.
