Introduction

Wavefront aberrations in the human eye can be classified as lower-order and higher-order aberrations (HOAs). Lower-order aberrations, comprising defocus and astigmatisms, correspond to the refractive sphere and cylinder errors in the human eye.^1–4 Studies^5,6 demonstrated that refractive sphere and cylinder errors dominate for nearly 90% of the wavefront aberrations in the population of normal eyes, with HOAs comprising the remaining 10%. Wavefront aberrations for small pupils are primarily influenced by refractive sphere and cylinder errors, making conventional correction such as spectacles, contact lenses, and laser vision correction effective for improving visual acuity.^5–9 However, it has been observed that HOAs have a significant impact on the retinal image quality, especially in night vision with large pupils in normal eyes and in postoperative eyes after laser vision correction.^7–9

Laser vision correction is a surgical procedure that uses a laser platform to reshape the cornea and correct the eye’s refractive sphere and cylinder errors, such as myopia, hyperopia, and astigmatism. Conventional correction through laser vision correction eliminates the need for spectacles and contact lenses thereby enhancing patients’ quality of life.^10,11 Laser-assisted in-situ keratomileusis (LASIK), initially pioneered by Pallikaris et al in 1990,¹² utilizes an excimer laser platform to reshape the corneal stroma after creating a flap through a microkeratome in earlier years^13,14 or a femtosecond laser when the iFS^® Advanced Femtosecond Laser (Johnson & Johnson Surgical Vision, Inc.) became available.^15,16 An alternative method for laser vision correction is laser-assisted lenticule extraction (LALEX) procedure, which uses a femtosecond laser to create a lenticule within the corneal stroma and achieves refractive correction by extracting the lenticule through a small incision. Small incision lenticule extraction (SMILE), first performed by Sekundo and Blum in 2008 with Zeiss VisuMax500 femtosecond laser platform,^17–22 has demonstrated comparable visual outcomes for conventional laser vision correction performed via a LASIK procedure.

Laser vision correction demands precise corneal tissue removal to achieve optimal visual outcomes. However, clinical observations indicate that HOAs, particularly coma and spherical aberration, tend to increase significantly after laser vision correction following a LASIK or SMILE procedure.^23–30 These HOAs can result in symptoms such as halos, glare, and reduced contrast sensitivity, especially for night vision when pupils become larger due to low-light conditions.^13,14

ELITA Femtosecond Platform is a new generation femtosecond (FS) laser platform developed for corneal refractive surgery by Johnson & Johnson Surgical Vision, Inc. (Milpitas CA, USA), which received CE Mark approval in March 2023 for SILK lenticule procedure and LASIK flap creation. The ELITA femtosecond laser system utilizes ultralow laser energy and a distinctive biconvex lenticule shape to perform a lenticule extraction SILK procedure for the correction of myopic refractive errors with and without astigmatism. In a clinical study involving 170 eyes, the visual outcomes at the 3-month refractive stability time point demonstrated that ELITA system is safe and effective for correcting myopic refractive errors with and without astigmatism.³¹ The purpose of this study is to investigate the changes of HOAs in surgical eyes following corneal lenticule extractions made by the SILK procedure using ELITA laser system.

Patients and Methods

Patients

This study was part of a prospective, single-arm, open-label clinical study³¹ conducted at one site in India (Narayana Nethralaya Eye Hospital, Bangalore) in accordance with the Declaration of Helsinki, ISO 14155:2011, and all other applicable laws and regulations. Human research ethics approval was obtained from the Narayana Nethralaya Ethics Committee (Bangalore, India) prior to beginning the study. The clinical study was registered on www.clinicaltrials.gov as NCT04200898. After written informed consent was obtained and confirmation that all eligibility criteria were met, the SILK procedure was performed by the same cornea-refractive surgeon who had previous experience with the SMILE procedure using VisuMax500 laser system.

A comprehensive preoperative evaluation was performed first by the surgeon for each patient to ensure that the patient was a suitable candidate for SILK procedure. Patients were required to be at least 18 years old and in good health to participate in the clinical study of the SILK procedure for laser vision correction. Inclusion criteria for eligible eyes included a myopic spherical equivalent (SE) refractive error of ≤−12.00 diopters (D) and astigmatism ≤−6.00 D, with at least 250 microns residual corneal stromal thickness after lenticule extraction. Additionally, patients were eligible to have uncorrected distance visual acuity (UCDVA) of 20/40 or worse, corrected distance visual acuity (CDVA) of 20/20 or better, and CDVA that is at least 2 lines better than UCDVA. Patients must also exhibit ≤0.75 D difference between cycloplegic and manifest refraction spheres, stable refractive error for at least 12 months, and have discontinued rigid contact lens wear for at least 4 weeks or soft contact lens wear for at least 2 weeks prior to surgery. Exclusion criteria for surgical eyes include corneal thickness less than 470 microns at the thinnest point, a history of prior intraocular or corneal surgery, corneal abnormalities, active ophthalmic diseases such as keratoconus, clinically significant dry eye syndrome, and evidence of glaucoma. Pupil size under mesopic lighting conditions was also assessed to evaluate the potential impact of postoperative complications such as halos and glare.

This study was part of the Phase II clinical study, which included refractive error range in the surgical eyes between −2.00 D and −6.00 D and astigmatism up to −1.00 D based on manifest refraction at optical infinity. Cyclotorsion correction after docking was available for astigmatism greater than −1. 00 D with SE limited to up to −6.00 D. It included 24 eyes that underwent a corneal refractive lenticule extraction SILK procedure for myopic correction at the Narayana Nethralaya Eye Institute, India. The SILK procedures were performed from April 6, 2021, to September 3, 2021, with preoperative and postoperative eye exams starting on March 27, 2021, and finishing on July 1, 2022.

Surgical Techniques

ELITA^TM Femtosecond Platform is a new generation of the commercial iFS^® Advanced Femtosecond Laser (Johnson & Johnson Surgical Vision, Inc). This innovative FS laser platform is designed to conduct both the flap treatment for a LASIK procedure and the lenticule treatment for a LALEX procedure. It utilizes an ultralow energy per laser pulse (laser energy of 40–90 nJ) along with 1.0 µm focused spot size and tight spot spacing (mean spot spacing of 1.0 µm) for corneal tissue cuts. These features allow for the precise creation of corneal incisions with minimal tissue-bridges. For the SILK procedure, this advanced FS laser platform empowers surgeons to perform lenticule extraction procedure for myopic correction, with or without astigmatism, up to a maximum of −12.00 D sphere, and astigmatism correction up to −6.00 D, with the sum of sphere and cylinder between −1.00 D and −12.00 D using minus cylinder convention based on the manifest refraction at optical infinity.

The SILK procedure with the ELITA system uses a unique biconvex lenticule shape for myopic correction with or without astigmatism. The diagram in Figure 1 illustrates the cross-section of an applanated biconvex lenticule created by the ELITA system within the corneal stroma. The lenticule cutting procedure starts with the creation of the posterior lenticule surface incision, followed by an angled ring cut, an anterior lenticule surface incision, and finishes with the entry incision. The refractive correction is equally applied to both the anterior surface and posterior surface of the lenticule. Each lenticule is meticulously designed with an optical zone for refractive power correction and a transition zone to mitigate the edge effect generated by the ring cut. To prevent tissue tearing at the lenticule’s edge during extraction, a non-refractive flat ranging from 15 to 30 µm thickness is added in the center of the lenticule. The non-refractive flat consists of a layer with a consistent thickness, serving to enhance the mechanical properties of the lenticule.

Figure 1 Biconvex lenticule incision for myopic correction with corneal cuts 1 for lenticule posterior surface, 2 for ring cut, 3 for anterior surface, and 4 for entry.

Surgical Procedure

The corneal lenticule extraction with the ELITA system is akin to a SMILE procedure performed with the Zeiss VisuMax500 FS laser, which includes docking, femtosecond laser application, lenticule dissection, and extraction.

Prior to treating the eye, the treatment parameters were programmed on the FS laser platform, based on the required correction for manifest refractive errors (sphere and cylinder error) as well as the keratometry measurements (K1 and K2). Additionally, this new FS laser supports corneal markings to ensure proper lenticule centration and compensation for cyclotorsion. Corneal marking was performed before placing the suction ring on the surgical eye for docking. If lenticule treatment includes an astigmatism correction, the surgeon first created two peripheral marks with surgical ink on the nasal and temporal cornea. These nasal and temporal marks were aligned with the horizonal axis while the patient was in an upright position. Subsequently, the surgeon placed a central mark with the surgical ink at the pupil’s center while the eye was viewing a coaxial fixation target under a surgical microscope when the patient was at supine position on the surgical bed. Once docking was achieved under the ELITA system, the surgeon could align the lenticule treatment center with the central mark and adjust the lenticule axis to compensate for any cyclorotation of the surgical eye.

Data Analysis

Each eye underwent a preoperative eye exam, as well as a postoperative eye exam at 1-day, 1-week, 1-month, 3-month and 9-month after the lenticule extraction by the SILK procedure. During the eye exam, manifest refraction for UCDVA and the best corrected distance visual acuity (BCDVA) were conducted using a phoropter. Wavefront aberrations up to the 6th order Zernike polynomials were measured using a commercial iDESIGN^® wavefront aberrometer (iDESIGN^® Refractive Studio, Johnson & Johnson Surgical Vision, Inc). iDESIGN^® Refractive Studio is a wavefront aberrometer, combined with a corneal topographer, which uses a Hartmann-Shark sensor to measure the whole eye’s wavefront aberrations at the pupil plane.

All wavefront measurements were conducted on each eye with a natural pupil while the eye was under mesopic light conditions (non-pharmacologically dilated). This study only included 24 eyes for which wavefront data were available with a pupil size equal to or larger than 6 mm at preoperative and postoperative iDesign wavefront measurements at 1-day, 1-week, 1-month, 3-month, and 9-month visits. Each wavefront measurement was then rescaled to a 6 mm pupil size for the analysis of HOAs. HOAs, such as comas, spherical aberration, and wavefront error of all higher-order aberrations (HOAs RMS) were analyzed based on a 6 mm pupil.

Statistical analyses were conducted using Microsoft Excel 2021 (Microsoft Corporation) to assess the changes of HOAs following SILK procedures in the 24 eyes. The data were analyzed using a paired study control method, where each eye’s preoperative HOAs were used as the control group to compare its postoperative HOAs at multiple time points. Student’s t-tests were performed to compare preoperative HOAs with postoperative HOAs at 1 day, 1 week, 1 month, 3 months, and 9 months. Differences were considered statistically significant when the p-value was less than 0.05.

Results

This prospective study included 24 eyes that underwent corneal lenticule extraction with the ELITA system and completed 9 months of follow-up. The mean age of patients was 22.6 ± 3.9 years, with 42% being women and 58% being men.

The ELITA system utilized ultralow laser energy per pulse, ranging from 58 nJ to 65 nJ, to create the lenticule’s posterior surface and anterior surface. Each lenticule treatment was configured with a 6 mm optical zone and 0.3 mm transition width. The anterior depth of the lenticule treatment was set as 110 μm, and a superior-entry incision width of 3 mm was set for extraction. A non-refraction flat, ranging from 15 μm to 30 μm thickness, was added to the center of each lenticule.

All lenticule cuts were successfully completed on the 24 surgical eyes using the ELITA system. There were no reported instances of suction loss throughout the entire study. In all cases, the lenticules were extracted successfully, with no or minimal additional dissection required.

Visual Outcomes at Postop 9-Month

Table 1 displays the mean manifest refractive errors of the 24 eyes measured both at the preoperative eye exams and at the 9-month postoperative eye exams. The mean manifest refractive SE of these 24 eyes changed from −3.82 to 1.26 D (range from −6.00 to −2.25 D) preoperatively to −0.20 ± 0.15 D (range from −0.50 to 0.00 D) at the 9-month postoperative assessments following the SILK procedures with the ELITA system. For all 24 eyes, the manifest refractions measured at postoperative 9-month visit were within ±0.50 D for manifest refraction sphere, manifest refraction cylinder, and manifest refraction SE.

Table 1 Preoperative and 9-Month Postoperative Refractive Errors of the 24 Eyes

Table 2 presents the visual outcomes for the UCDVA in the 24 eyes. It indicates that at the postoperative 9-month assessment, 63% of the surgical eyes achieved a UCDVA of 20/16 or better, while all 100% of the surgical eyes obtained a UCDVA of 20/20 or better.

Table 2 Postoperative Uncorrected Distance Visual Acuity (UCDVA) of the 24 Eyes

Figure 2 illustrates the comparison of BCDVA between the preoperative and the postoperative 9-month assessments, presented as a percentage of BCDVA for all eyes. It demonstrates that after lenticule extraction with the ELITA system, BCDVA increased by 37% for 20/16 and 4% for 20/20. More importantly, no patient experienced a loss of any visual acuity line at the 9-month postoperative evaluation. Specifically, at 9 months, no eye lost 1 line, 2 eyes gained 1 line, and 22 eyes (91.7%) had no change in BCDVA.

Figure 2 Comparison of BCDVA between the preoperative and postoperative 9-month assessments.

Figure 3 shows the linear regression analysis comparing the achieved manifest refractive SE correction at 9 months to the intended manifest refractive SE correction. With a regression line slope close to 1 (0.974) and a high R-squared value (0.9844), the scatter plot indicates that the achieved manifest refractive SE correction at the 9 months was on target in terms of magnitude. This demonstrates highly predictable visual outcomes following the SILK procedure using the ELITA system. All eyes (100%) achieved a visual outcome of manifest refractive SE within ±0.50 D.

Figure 3 Intended vs achieved manifest refractive SE at postoperative 9-month assessment of the 24 surgical eyes (Solid lines showing range of ±0.50 D).

Higher-Order Aberrations

Figure 4 displays the mean HOAs with a 6 mm pupil for the 24 eyes following lenticule extraction with the ELITA system. At 9 months, the average vertical coma Z(3, −1) was −0.054 ± 0.186 µm, horizontal coma Z(3, 1) was 0.016 ± 0.124 µm, the spherical aberration Z(4, 0) was 0.046 ± 0.163 µm, and the wavefront error of all higher-order aberrations HOAs RMS was 0.363 ± 0.115 µm with a 6 mm pupil. In comparison to the preoperative HOAs, the postoperative HOAs were significantly increased at the 1-day postoperative assessment (P = 0.015 for Z(3,-1); P = 0.001 for Z(4, 0); P = 0.000 for HOAs RMS) (as shown in Table 3). However, there was no significant difference in HOAs at 1-week, 1-month, 3-month and 9-month follow-up.

Table 3 Statistical Changes of Postoperative HOAs at Each Time Points Compared to Preoperative HOAs from the 24 Eyes

Figure 4 HOAs before and after corneal lenticule extractions.

Table 3 presents the statistical differences in the HOAs between the preoperative and postoperative assessments in the 24 surgical eyes. When comparing the preoperative HOAs, it is notable that there were no significant changes observed for horizontal coma Z(3, 1) and spherical aberration Z(4, 0), starting at the 1-week postoperative HOAs evaluation. However, the observed increase of vertical coma Z(3, −1) in the postoperative eyes could potentially be attributed to the superior position of the entry cut and centration errors during the lenticule treatments.

Discussion

The application of FS laser technology in LALEX has progressed significantly over the past 15 years. Compared to the FS-LASIK procedure for refractive surgery, the entire LALEX procedure only needs one laser for correcting myopia with or without astigmatism. The LALEX procedure requires less corneal tissue removal and involves a smaller incision, making it safer for patients with thinner corneas. Additionally, the LALEX procedure performs better in correcting high myopia, as it better maintains corneal structural integrity and biomechanical stability.

The potential benefits of the LALEX procedure over LASIK also include no flap-related complications both intraoperatively and postoperatively, reducing the risk of flap wrinkles, displacement, and dry eye disease.^25,32–34 Because the LALEX procedure involves a small incision of 2–4 mm, compared to the approximately 20 mm larger flap in LASIK, there is less damage to the corneal nerves, reducing the risk of postoperative dry eye. This makes the LALEX procedure suitable for patients who already have dry eye symptoms or concerns about developing dry eye.

This study was part of the phase II clinical study of the SILK procedure using the ELITA system, which included patients with mild to moderate myopia. It used a 6 mm large pupil to investigate the changes in HOAs in surgical eyes after the SILK procedure. Since all wavefront measurements were performed on each eye’s natural mesopic pupil, the sample size and wavefront data were limited by the availability of wavefront measurements obtained with a pupil size equal to or larger than 6 mm at all preoperative and postoperative visits. Because this study is a paired comparison of preoperative and postoperative aberrations in the same eyes, it allows for a smaller sample size due to the reduced variability of HOA measurements. The sample size of 24 eyes out of 170 used in this study is comparable to the 18 eyes out of 357 used for 6 mm pupil wavefront analysis in SMILE procedure submitted for FDA approval on October 4, 2018.³⁵

In this study, all surgical eyes achieved a good visual outcome SE within ±0.50 D of emmetropia and no eyes experienced any loss of visual acuity line at 9-month follow-up. Therefore, lenticule extraction with SILK procedure using the ELITA system is safe and predictable for myopic correction.

Wavefront aberrations measured in surgical eyes can offer comprehensive optical information both before and after laser vision correction, and a method used to evaluate the treatment outcome of refractive surgery. HOAs were evaluated in this study using a 6 mm pupil size, which is larger than the 5 mm pupil size usually used in other HOAs studies^28,29 to assess HOAs in surgical eyes after SMILE procedures. The results for a 6 mm pupil revealed low HOAs induced by the ELITA system in surgical eyes following the corneal lenticule extraction with SILK procedure. When compared to baseline preoperative HOAs, postoperative HOAs first increased at 1-day then subsequently stabilized starting at 1-week follow-up. This observation demonstrates a fast corneal wound healing response after the SILK procedure using the ELITA system for myopic correction.

The HOAs in surgical eyes after laser vision correction can vary with the preoperative eye exams, the laser technology and technique used in the refractive surgery, and the individual corneal healing response. Figure 5 compares the average HOAs with a 6 mm pupil at the preoperative eye exams and at the 3-month postoperative visit. It shows that small amounts of the 3rd order, 4th order and HOAs RMS, including coma, spherical aberration, trefoil, secondary astigmatism, and wavefront error all HOAs, are presented in a 6 mm pupil at the 3-month follow-up compared to baseline values. Compared to the postop 3-month HOAs after the SMILE procedure,³⁰ lenticule treatment for mild to moderate myopia made by the SILK procedure induced fewer HOAs in the surgical eyes at the 3-month postoperative assessment.

Figure 5 Postop 3-month HOAs in a 6 mm pupil after the SILK procedure using the ELITA system.

We believe the minimal induction of HOAs, and the rapid corneal recovery observed after the lenticule extraction made by SILK procedure can be attributable to several factors of the ELITA system, including the use of a high-precision digital encoder guided scanning system that achieves sub-micron accuracy in thickness profile scanning,³⁶ and the easy lenticule removal, which minimizes the unwanted deformation of corneal shape due to surgeon’s mechanical separation of the lenticule interfaces.³⁷

It is well known that the quality of the corneal treatment surface is correlated directly with the surgical eye’s HOAs, which are critical for achieving an optimal vision correction. Studies have reported less induction of HOAs with the femtosecond laser than microkeratomes during LASIK.^38,39 The quality of the corneal lenticular surface can be significantly impacted by some parameters of the FS laser, such as pulse repetition, pulse energy, focus spot size, and spot spacing. Research has demonstrated that the FS laser using lower energy pulse and tighter spot creates smoother corneal stromal beds, both in FS-LASIK flap procedures³⁹ and lenticule extraction procedures.⁴⁰ A recent study has also shown that the Clear procedure with Ziemer FEMTO LDV Z8, using lower FS energy and overlapping spot density, produced smoother lenticule surfaces for corneal lenticule extraction.⁴¹ Literature on SMILE procedures^42–44 has indicated that patients treated with the VisuMax500 using lower energy settings and smaller spot spacing (laser energy of 125–135 nJ; spot spacing of 4.5 μm) achieved significantly smoother separations, faster visual recovery, and improved visual outcomes.

The SILK procedure using the ELITA system employs a unique biconvex lenticule shape for myopic correction. It utilizes symmetric lenticule posterior and anterior lenticule shape to minimize surface mismatch between the two lenticular interfaces following lenticule removal.³¹ Mismatched lenticule anterior and posterior surfaces can potentially lead to corneal folds and wrinkles in the lenticule interface, resulting in corneal HOAs and light scattering after lenticule removal. Considering that the biomechanical strength of the cornea decreases as a function of the corneal depth moving from anterior to posterior, the biconvex lenticule shape also offers superior corneal biomechanics compared to a plano-convex lenticule shape, as it requires less tissue cutting in the anterior stroma.

Various studies have observed significant increases in spherical aberration following laser vision correction.^45–47 It is well known that there is an interaction between defocus and spherical aberration,⁴⁸ and the induced spherical aberration after laser vision correction not only degrades retinal image quality but also impacts the subjective refraction of defocus. Several studies^48–51 have proposed possible reasons for the induced spherical aberrations and have developed new treatment algorithms, including wavefront-optimized LASIK procedure and wavefront-guided LASIK procedure, aimed at reducing the spherical aberrations and HOAs in postoperative surgical eyes.

This study showed that spherical aberration was not induced after the lenticule extraction procedure with the ELITA system for myopic correction. This utilization of a flat patient interface for docking the surgical eye on the ELITA system eliminates the cosine effect and delivers uniform laser energy pulses across the entire treatment field of view. Additionally, the unique biconvex lenticule shape employed in lenticule extraction for myopic correction may lead to less corneal biomechanical response and epithelial remodeling, which contribute to the induced spherical aberration in surgical eyes after laser vision correction.⁵²

Inappropriate centration during the surgical procedure may result in the misplacement of the treatment optical zone on the surgical eye, which can subsequently lead to an increase of HOAs, such as the increased vertical coma and wavefront error of all HOAs^53–55 observed in this study. The current SILK procedure allows surgeons to adjust the treatment center after docking in two ways: either to the applanated pupil center or the applanated corneal central mark placed under a microscope before the lenticule treatment. However, using the applanated pupil center as the treatment center is not reliable for lenticule extraction due to either patient losing fixation during applanation from increased intraocular pressure or possible corneal displacement relative to the eye’s pupil caused by suction and docking process. In this study, the lenticule treatment center was manually adjusted by the surgeon to the central mark after docking. Nevertheless, the accuracy of manual centration depends on the experience and skill of the surgeons, which can result in centration errors caused by the misplacement of central mark under the microscope and the positioning of the treatment center on the ELITA system. The induced coma aberrations are expected to decrease when the auto-centration is available for the SILK procedure using the ELITA system.

Conclusions

This study has demonstrated that the SILK procedure using ELITA femtosecond laser system is efficient and predictable for the correction of myopic refractive errors with and without astigmatism. The changes in HOAs over a 9-month follow-up period following the SILK procedure were evaluated. One of the most distinctive feature settings in the SILK procedure compared to other lenticule extraction procedures is the utilization of ultralow laser energy pulses and an innovatively designed biconvex lenticule shape for myopic correction. In this study, the achieved visual outcomes were within ±0.50 D of emmetropia while inducing minimal HOAs in the surgical eyes after corneal lenticule extraction with SILK procedures using the ELITA system. Furthermore, rapid corneal recovery was demonstrated starting at the 1-week follow-up visit, and no spherical aberration was induced from the SILK procedures.