Characterization of Disrupted Tissue Interface Thickness for Keratorefractive Lenticule Extraction Procedure With ELITA Femtosecond Laser

Abstract

Purpose: This study identified and compared variables causing changes in corneal tissue structure following the smooth incision lenticular keratomileusis (SILK) procedure using the ELITA Femtosecond Laser System by characterizing the resulting disrupted tissue interface.

Methods: Seventy-one ex vivo porcine eyes and six human cadaver eyes underwent ELITA SILK with diverse surgical steps, pulse energies, scan overlaps, and surgical methods. Flaps created with the iFS Advanced Femtosecond Laser and ELITA systems were also evaluated. The disrupted interface thickness was determined by imaging corneal layers at different depths through the interface with confocal microscopy and counting layers with elevated backscattered light via computer program–assisted, subject-matter-expert visual judgment with blinding.

Results: The disrupted interface thickness for ELITA SILK was 25 ± 3 µm; for the ELITA flap, it was 25 ± 2 µm; and for the iFS flap, it was 32 ± 3 µm. Factors influencing the total ELITA SILK disrupted interface thickness included laser pulse energy (0.11 µm/nJ; P < 0.01), scan overlap (5 µm; P < 0.01), and mechanical manipulation (7 µm; P < 0.01). Varying surgical techniques for mechanical manipulation resulted in a difference in disrupted interface thickness of 4 µm (P < 0.01).

Conclusions: The ELITA SILK disrupted interface thickness was less than that of the iFS flap and similar to that of the ELITA flap. Assessing disrupted interface thickness identified factors influencing the quality of the corneal interface with SILK.

Translational Relevance: The disrupted interface thickness, a new method for measuring corneal damage, has been used to quantify the potential effects of various refractive surgery factors on surgical outcomes.

Introduction

Since the introduction of lasers in ophthalmology, femtosecond lasers have been widely employed to treat myopia, hyperopia, and astigmatism in a procedure known as femtosecond-laser in situ keratomileusis (FS-LASIK). In FS-LASIK, a femtosecond laser is used to create the corneal flap, which is lifted by the surgeon, exposing the stroma for ablation with an excimer laser for the reshaping of the cornea and correction of the refractive error.^1,2 

Keratorefractive lenticule extraction (KLEx) is a new femtosecond-laser–assisted corneal refractive procedure. In KLEx procedures, the femtosecond laser creates incisions to dissect a lens-shaped tissue, followed by a short entry incision, through which the lenticular tissue is separated and extracted from the cornea by the surgeon, resulting in the correction of refractive errors.^2,3 KLEx is a technique that forgoes the necessity for a corneal flap and excimer laser ablation, allowing for some advantages compared to FS-LASIK, such as increased stromal biomechanical strength from the lack of flap side cuts,^2,3 reduced surgical steps from the lack of excimer laser usage,² and avoidance of all flap-related complications.⁴ 

Despite the myriad advantages attributed to KLEx, a significant drawback is its prolonged visual acuity recovery time when compared to FS-LASIK, which is ascribed to the occurrence of corneal haze.^3,5–7 Previous clinical study resulted in compromised visual quality in the immediate postoperative period after small incision lenticule extraction (SMILE) procedures, suggesting that it may take 3 months or more for complete visual recovery.⁸ Similarly, another clinical study has reported that the SMILE eyes exhibited an elevated corneal backscatter for at least 3 months post-surgery, possibly linking visual acuity recovery time with corneal backscatter.^3,9 

Corneal backscatter measured by a confocal microscope increases when disruptions or roughness in the tissue are present, as backscattered light intensity at that stromal position increases.^9,10 The disrupted corneal tissue interface thickness can be measured by determining tissue thickness with increased corneal backscatter. In a previous study, the elevated corneal backscatter for SMILE was observed at 30 µm above and 30 µm below the lenticule extraction position, indicating that the thickness of the disrupted interface is at least 60 µm and significantly thicker than FS-LASIK.⁹ The disrupted interface thickness is a key characteristic quantifying the collateral tissue damage from the surgery. 

The ELITA Femtosecond Laser System (Johnson & Johnson Surgical Vision, Milpitas, CA) is a recently developed laser platform designed to perform smooth incision lenticular keratomileusis (SILK), a novel KLEx procedure.¹¹ In contrast to the iFS Advanced Femtosecond Laser (Johnson & Johnson Surgical Vision), ELITA utilizes lower laser pulse energy, a higher pulse repetition rate, and more contiguous laser spots to achieve a tissue-bridge–free dissection, resulting in less tissue damage and faster visual recovery.^11,12 The disrupted interface thicknesses of the ELITA SILK procedure and the ELITA flap procedure are unknown, as are the variables affecting this thickness. 

A novel method for evaluating corneal tissue disruptions, along with a study on the variables that affect the disruptions, is of significant relevance to both clinicians and researchers. In this study, we measured and compared the disrupted interface thickness of ELITA SILK with those of ELITA flap and iFS flap, and we also examined variables such as a single laser scan versus full (multiple) laser scans, laser pulse energy, surgical techniques and tools, and ex vivo tissue types. 

Methods

Preparation of Eye Samples

This study involved 71 ex vivo porcine eyes and six human cadaver eyes. Ex vivo porcine eyes (Animal Technologies, Tyler, TX) were preserved immediately and placed in saline and ice within 24 hours of harvest. Any extraocular tissues were removed from each eye, and the epithelial layer of each eye was scraped off using a corneal knife. Each eye was inspected thoroughly to exclude eyes with any signs of pretreatment corneal damage. Phosphate-buffered saline solution was injected into the vitreous chamber to maintain consistent intraocular pressure. The eyes were then placed in a custom-made eye holder with the cornea centered and exposed. Human cadaver eyes (Saving Sight, Kansas City, MO) with no injury and visually clear cornea were preserved in ice and received within 72 hours from death. The cadaver eyes underwent identical preparation procedures as the porcine eyes. 

Femtosecond Laser Treatment

The ELITA Femtosecond Laser System was used for the SILK procedures. The ELITA system has a pulse duration of about 150 fs, pulse repetition rate of 10 MHz, and spot diameter of about 1 µm (full width at half maximum), and it uses a contiguous laser scan pattern with a mean spot center-to-center separation of about 1 µm.¹¹ For all treatments, the system was set to correct a manifest refraction of –4.00 diopter (D; sphere) and –1.00 D (cylinder) × 180° (cylinder axis), with –0.65-D sphere nomogram adjustment and –0.10-D cylinder nomogram adjustment. Lenticule treatments were programmed with a 110-µm anterior depth, 3.0-mm entry width, 60° entry side cut angle, and 2.0-µm scan steps. Unless specified otherwise, laser pulse energies for cutting lenticular surfaces were set to 40 nJ. During the testing of individual scans or lenticular surfaces, either the laser pathway was deliberately obstructed or the laser activation foot pedal was engaged and released at predetermined intervals to achieve the desired posterior or anterior laser scans. 

The ELITA and iFS systems were used for flap creations. Both lasers were set to create a flap with a 9.0-mm flap diameter and 110-µm flap depth. For iFS flap treatment, the bed energy was calibrated using an optical power meter to output 700 nJ (i.e., 0.70 µJ), which is a typical energy for iFS flap bed cuts. For ELITA flap treatment, the bed energy was set to 40 nJ and 50 nJ for porcine eyes and 50 nJ for cadaver eyes. 

Surgical Technique and Tools

To study the effects of tissue manipulation, two different techniques and tools were used for tissue separation and extraction. First, a one-tool technique was utilized in which the paddle-shaped separator (Reinstein Lenticule Separator; Malosa Medical, Elland, UK) was used to dissect the anterior and posterior surfaces and to extract by dragging out the lenticule. In contrast, the two-tool technique employed the Reinstein dissector or the needle-shaped separator of the Shroff dissector (Shroff Dissector and Opener, Double Ended; Epsilon Eye Care, Mumbai, India) to separate the anterior and posterior surfaces, followed by forceps (SMILE Lenticule Removal Forceps; Duckworth & Kent, Hertfordshire, UK) to extract the lenticule. For all other studies, the lenticular surfaces were first separated using a LASIK spatula (Seibel II Intralase Flap Lifter & Retreatment Spatula Titanium; Katena Products, Parsippany, NJ) and then extracted using forceps. When single scans and single lenticular surfaces were made, the surface of the cornea was gently stroked with the spatula to release any bubbles. Flaps created by ELITA and iFS were lifted using the LASIK spatula. The spatula was inserted under the flap near the hinge and slid across the bed to separate the flap surfaces. After the flap was lifted, the flap was placed back on the host cornea. 

Disrupted Interface Thickness Measurement

After the femtosecond laser treatment, the eye holder was mounted on a confocal microscope fixture ensuring that the cornea was parallel to the imaging plane, as shown in Figure 1A. A laser confocal microscope (HRT3 RCM; Heidelberg Engineering, Heidelberg, Germany) with Heidelberg Eye Explorer 1.9.13.0 was used. It operates with a 63× magnification microscope objective, and the laser wavelength is 670 nm. The confocal microscope has a transverse resolution of 1 µm and a depth resolution less than 1 µm.¹³ To measure the disrupted interface thickness, the confocal microscope takes a series of images at different depths. Each image covers an area of 400 × 400 µm at a fixed depth. The depth step between adjacent images is 1.25 µm. The starting depth and the depth range are set such that the image collection process starts at a depth sufficiently above and ends sufficiently below the interface. A gel bridge (carbomer 0.2%) was created between the microscope objective and the eye. The focus position was reset at the corneal surface. The brightness level was adjusted to ensure sufficient brightness for stromal cell identification, keeping image quality level above 80. Normalization of incident light brightness levels across eyes at same depths was not repeatable, as the initial quality of corneal tissue affects the optical attenuation of the incident light up to the imaging site. 

Figure 1.

 

(A) Custom-made vacuum eye holder with porcine eye mounted on the confocal microscope. (B) Representative confocal images of ELITA SILK at 50 nJ on cadaver eye. Each image covers an area of 400 × 400 µm. Images from progressively deeper layers are placed left to right and then top to bottom at a depth step of 1.25 µm. The depth from the surface (DS) of the cornea is labeled beneath each image. Images with disrupted tissue detected are enclosed with a red line. (C) Representative plot of average pixel brightness intensity at increasing depths for ELITA SILK at 50 nJ on a cadaver eye. The blue line represents corneal tissues without elevated corneal backscatter, and the red line represents corneal tissues with elevated corneal backscatter. Corresponding images at various depths are shown. In this example, the disrupted interface thickness is about 29 µm.

All confocal images were captured within 30 minutes after the laser treatment for each eye. The laser confocal microscope volume scan was used to capture corneal layers with disrupted interface thickness. A single volume scan captured 40 images, spanning 50 µm of corneal tissue depth. For each eye where the laser cut the entire surface or the lenticule, the corneal layers were captured at 16 different locations near the center of the treatment area with a measurement area of 400 × 400 µm per location (spanning a total measurement area of 1600 × 1600 µm). For each eye where the laser cut a single scan, the corneal layers were captured at three different locations near the center of the treatment area (spanning a total measurement area of 400 × 1200 µm). To ensure that the measurement area was approximately centered within the treatment area, a positional calibration technique was employed. The microscope was first aligned by imaging a reference object with a known location (such as the treatment area edge), and then repositioned to the desired location relative to the reference object using the known field of view dimensions (400 × 400 µm) of the microscope. 

When disruptions in the tissue are present, backscattered light intensity at that stromal position increases, resulting in brighter image obtained from the confocal microscope.^9,10 The thickness of the disrupted interface at each location was measured by calculating the difference between the initial corneal depth, where an increase in brightness and the appearance of hazy or unclear edges of the corneal stroma were observed, and the final corneal depth, where the brightness decreased and the edges of the stroma became distinct again, as shown in Figure 1B. The corneal depths with elevated backscatter were determined through computer program–assisted, subject-matter-expert visual judgment with blinding, where, for each image at varying depths, decisions were made as to whether the corneal backscatter intensity increased or corneal stroma became hazy compared to undamaged corneal layers. 

To assist with the visual judgement, a MATLAB program (MathWorks, Natick, MA) was used to plot the average pixel brightness intensity for each image across increasing depths by computing the mean value of the pixel brightness levels across the entire image, as shown in Figure 1C. Corneal layers with an average brightness intensity value percent change from the previous layer of greater than 1% (including the peak) were initially determined to be disrupted interfaces by the computer program. After this computation, the subject-matter expert adjusted the disrupted interface thickness starting and ending points within five adjacent images based on visual judgment of disrupted and undisrupted corneal cells. The disrupted interface thickness measurement was conducted with blinding. The computer program randomly selected the sets of confocal microscope images to be analyzed and displayed the sample name only after all disrupted interface thickness measurements were completed. The subject-matter expert measured the disrupted interface thickness without the knowledge of which samples were being analyzed, reducing potential observer bias. 

Statistical Analysis

Statistical analyses were performed in MATLAB R2021b using two-tailed type 2 Student's t-tests to test the difference between two samples. Linear regression analysis was conducted to test for the statistical linear relationship. Differences were statistically significant for P < 0.05 (95% confidence interval). 

Results

Measured disrupted interface thicknesses are summarized in the Table. Figure 2 shows the measured disrupted interface thicknesses of the iFS flap at 700 nJ (which is the typical energy setting for iFS flap bed cuts), ELITA flap at 40 nJ, and ELITA SILK at 40 nJ. The difference in the disrupted interface thickness between ELITA SILK and ELITA flap was not statistically significant (P = 0.27). ELITA SILK and ELITA flap had disrupted interface thicknesses of 25 ± 3 µm and 25 ± 2 µm, respectively, significantly less than the iFS flap disrupted interface thickness of 32 ± 3 µm (P < 0.01). 

Table.

 

Measured Disrupted Interface Thickness Within 30 Minutes After Treatment

 

Figure 2.

 

Disrupted interface (DI) thicknesses with iFS flap created at a pulse energy of 700 nJ, ELITA flap at 40 nJ, and ELITA SILK at 40 nJ. Each data point (green square) represents the measured DI thickness for a given location. The box plots show the median, 25th percentile, 75th percentile, and full range of minimum and maximum DI values.

Disrupted Interface Change Throughout the SILK Procedure

To evaluate the change in disrupted interface throughout the SILK procedure, we measured the disrupted interface thickness of a single laser scan, a fully scanned but unseparated lenticular surface, a fully scanned and separated lenticular surface, and the surface after lenticule extraction on porcine eyes (Fig. 3). Within a single scan, no statistically significant differences were observed between the disrupted interface thickness at the center and that at the edge (P = 0.23). From a single scan to unseparated surface, the disrupted interface thickness increased by 5 µm (36%; P < 0.01). From unseparated surface to separated surface, the disrupted interface thickness increased by 7 µm (37%; P < 0.01). No statistically significant differences were observed between the disrupted interface thickness of separated surface (either anterior or posterior) and the surface after lenticule extraction (P = 0.31). 

Figure 3.

 

Disrupted interface (DI) thickness changes throughout the SILK procedure: single scan, unseparated surface after full lenticule scans, separated surface, and combined surface after lenticule extraction. Each data point represents the DI thickness measured for a given location at the specified surfaces. The box plots show the median, 25th percentile, 75th percentile, and full DI thickness range.

For anterior lenticular surface, the disrupted interface thicknesses for single scan, unseparated surface, and separated surface were 13 ± 1 µm (n = 3 eyes; nine locations), 17 ± 2 µm (n = 2 eyes; 32 locations), and 23 ± 3 µm (n = 2 eyes; 32 locations), respectively. For posterior lenticular surface, the disrupted interface thicknesses for single scan, unseparated surface, and separated surface were 15 ± 2 µm (n = 3 eyes; nine locations), 22 ± 2 µm (n = 2 eyes; 32 locations), and 28 ± 4 µm (n = 2 eyes; 32 locations), respectively. The differences were statistically significant for single scan (P = 0.045), unseparated surface (P < 0.01), and separated surface (P < 0.01). 

Effects of Pulse Energy

To evaluate the effects of pulse energies, the disrupted interface thicknesses of single scans and after lenticule extraction at 40-nJ, 50-nJ, and 60-nJ energies were measured in porcine eyes (Figs. 4A, 4B). Based on the linear fit for the average disrupted interface thicknesses at the three energies, the disrupted interface thickness of single scans increased by 0.19 µm/nJ (P < 0.01), and the interface after lenticule extraction increased by 0.11 µm/nJ (P < 0.01). The percent increase in single-scan disrupted interface thickness from 40 nJ to 50 nJ (21%) was greater than that from 50 nJ to 60 nJ (5%). The effects of pulse energy on the photodisruption-induced gas bubble formation after laser treatment and before mechanical dissection are shown in Figure 4C. As pulse energy increased, the light scattering of the gas bubble increased, the bubble pattern became less homogeneous, and some bubbles expanded and merged to form a big bubble at the center of treatment. 

Figure 4.

 

(A) Disrupted interface thickness of single scans with varying pulse energies. The box plots show the median, 25th percentile, 75th percentile, and range. The line of best fit (red line) and 95% confidence bounds (dotted red line) are represented. (B) Disrupted interface thickness after lenticule extraction with varying pulse energies. (C) Microscope pictures for porcine eyes immediately after the SILK laser treatment with varying pulse energies.

Effects of Laser Scan Overlap

The radial scanning pattern of ELITA SILK creates overlapping scans in the lenticule center and single scans in the periphery. To test whether the laser-scan overlap is creating an increase in disrupted interface thickness, specific locations with and without laser-scan overlaps on a lenticular surface were evaluated. Scan overlap resulted in an increased disrupted interface thickness of 4 µm (P < 0.01) for the unseparated surface and 3 µm (P < 0.01) for the separated surface. 

Effects of Tissue Manipulation

To assess the effects of tissue manipulation, the disrupted interface thickness was measured after the lenticule was separated and extracted with three varying methods (Fig. 5A). The disrupted interface thickness of the one-tool technique with the Reinstein dissector was significantly greater than that of the two-tool technique with the Reinstein dissector (4 µm; P < 0.01). No statistically significant disrupted interface thickness difference was observed between the two-tool technique with the Reinstein dissector and the Shroff dissector (P = 0.34). The one-tool technique required a much greater force on the cornea than the two-tool technique to extract the lenticule. 

Figure 5.

 

(A) Disrupted interface thickness of SILK with various surgical techniques and tools categorized by method type. The box plots show the median, 25th percentile, 75th percentile, and range. Shown are data points for the one-tool technique with the Reinstein dissector (blue square), two-tool technique with the Reinstein dissector (green square), and two-tool technique with the Shroff dissector (orange square). (B) Disrupted interface thickness of SILK with various techniques and tools categorized by method type and eye number. The box plots show the median, 25th percentile, 75th percentile, and range. Shown are data points for the one-tool technique with the Reinstein dissector (blue square), two-tool technique with the Reinstein dissector (green square), and two-tool technique with the Shroff dissector (orange square).

The one-tool technique had the largest trial-to-trial variability, with trial 1, trial 2, and trial 3 producing disrupted interface thicknesses of 32 ± 3 µm (n = 1 eye; 16 locations), 27 ± 2 µm (n = 1 eye; 16 locations), and 31 ± 2 µm (n = 1 eye; 16 locations), respectively (Fig. 5B). Trial 1 and trial 3 of the one-tool technique had disrupted interface thicknesses significantly greater than those of trial 2 (P < 0.01). 

Effects of Tissue Type

To assess the effects of tissue type, six ex vivo porcine eyes and six cadaver eyes were treated with ELITA flap and ELITA SILK at 50-nJ pulse energy, and the disrupted interface thicknesses were measured under the laser confocal microscope (Fig. 6). The disrupted interface thickness was marginally greater for human eyes than for porcine eyes for both ELITA flap (2 µm; P < 0.01) and ELITA SILK (1 µm; P = 0.02). 

Figure 6.

 

(A) Disrupted interface thickness of ex vivo porcine and human cadaver eyes tested with ELITA flap at 50-nJ pulse energy. Each data point is represented (green square). The box plots show the median, 25th percentile, 75th percentile, and range. (B) The disrupted interface thickness of ex vivo porcine and human cadaver eyes tested with ELITA SILK at 50-nJ pulse energy. Each data point is represented (green square). The box plots show the median, 25th percentile, 75th percentile, and range.

Discussion

A previous clinical study reported that SMILE resulted in elevated corneal backscatter and disrupted interface thickness compared to FS-LASIK.⁹ Our investigation revealed no discernible or intrinsic disrupted interface thickness difference between ELITA SILK and flap creation procedures. In fact, ELITA SILK and ELITA flap exhibited superior disrupted interfaces when compared to the iFS flap. Previous research has shown improved corneal tissue quality after excimer ablation when analyzing metrics such as root-mean-squared roughness.¹⁴ Nevertheless, with our new method, we were able to successfully measure the thickness of disrupted interfaces and detect differences between various femtosecond laser treatments. 

From the standpoint of tissue interface generation, the SILK procedure consists of the following steps (not in sequence of time): (1) the first posterior or the first anterior lenticular scan creates a single-scan tissue interface; (2) the full posterior or the full anterior lenticular surface is incised with multiple laser scans with a preprogrammed laser blanking pattern (blanking means no photodisruption); (3) the anterior and posterior lenticular surfaces are separated by surgeon with a mechanic lenticule dissection instrument; and (4) the lenticule is extracted (Fig. 7). The minimum disrupted interface thickness that can be achieved is that of a single scan. As more scans trace the lenticular surfaces, the disrupted interface thickness increased slightly due to scan overlaps. Mechanical dissection contributes a relatively large increase in disrupted interface thickness throughout the procedure. The disrupted interface thickness of either the posterior or the anterior lenticular surface after mechanical separation is similar to that of the final interface after the lenticule is removed from the cornea, meaning that the upper half of the anterior surface and the lower half of the posterior surface are combined to result in the final disrupted interface. 

Figure 7.

 

Illustration of disrupted interface thickness for different SILK treatment steps. The final disrupted interface thickness is determined by both laser scans and the surgeon's technique when separating and extracting the lenticule with mechanical instruments. The intact, undamaged cornea tissue is shown in yellow, and the disrupted tissue interface is shown in red.

The disrupted interface thicknesses obtained from single scans are comparable to the following estimation based on the maximum radius of the fast-oscillating cavitation bubble generated by the laser induced plasma. In Vogel et al.,¹⁵ a 40-nJ laser pulse, with a plasma threshold at 22 nJ, produced a maximum cavitation bubble radius of around 12.8 µm in water. This pulse energy and the plasma threshold are close to those of ELITA. Because the cavitation bubble size in the cornea is around 85% of that in water,¹⁶ we estimate the maximum cavitation bubble diameter of the oscillating cavitation bubble to be approximately 0.85 × 2 × 12.8 µm ≈ 22 µm in cornea, which produced a disrupted interface thickness of about 14 µm. In addition, we observed energy dependence behavior similar to that in Vogel et al.,¹⁵ where the slope of disrupted interface thickness as a function of pulse energy decreased as the pulse energy increased. The increase of disrupted interface thickness with increased energy is consistent with findings from previous studies, which have reported an escalation in corneal haze and possible prolonged visual acuity recovery time concomitant with heightened pulse energy.^17,18 

The mechanical manipulation, including separation and extraction, contributed significantly to the disrupted interface thickness. The difference in tools had a minimal effect on disrupted interface thickness, but the difference in surgical techniques significantly affected disrupted interface thickness. The greater force that was required to extract the lenticule with the one-tool method resulted in larger disrupted interface thicknesses. There was trial-to-trial or eye-to-eye variability with the one-tool method compared to the two-tool method, suggesting the possibility of a causal interaction between variations in eye characteristics, such as intraocular pressure and biomechanical strength, and tissue manipulation on the disrupted interface thickness.^19,20 

Porcine eyes exhibit distinguishable characteristics compared with human eyes, most notably marked by the absence of Bowman's layer,²¹ different biomechanical properties,²² and smaller eye dimensions.^23,24 Furthermore, in this study, the porcine eyes and human eyes had contrasting tissue harvesting procedures, which could have resulted in differences in tissue quality. These variables are the possible causes of marginal differences in disrupted interface thickness between porcine eyes and human eyes. 

One limitation of this study is that the thickness of tissue with elevated corneal backscatter was measured, but the intensity of the corneal backscatter was not analyzed due to the lack of incident light intensity normalization across eyes. Furthermore, the lack of light intensity normalization made fully automated detection of brightness increases difficult, as each location differed in backscattered light intensity. A future study normalizing the incident light intensity to automatically measure the disrupted interface thickness, total amount of excess corneal backscatter, and maximum corneal backscatter intensity would be of interest. Another limitation is the small eye sample size per testing group despite the large location sample size per testing group. In the future, testing with larger eye sample sizes and confirmation with optical coherence tomography, slit-lamp photographs, and other characterization methods would benefit this study greatly. In vivo measurement of disrupted interface thickness would also provide cellular responses not evaluated in this study. Also, although previous studies suggest that elevated corneal backscatter may affect visual acuity recovery time, the full clinical impact of the disrupted interface thickness is unknown.^3,9 Further clinical studies on the disrupted interface thickness are needed to bridge the gap between the findings of this study and their clinical relevance. 

In conclusion, the disrupted interface thickness of ELITA SILK was less than that of iFS flaps but equivalent to that of ELITA flaps. We identified the changes in disrupted interface throughout the SILK procedure and different variables that affect the disrupted interface. Our future goal is to further optimize ELITA pulse energy, spot separation, and line separation to achieve a minimal disrupted tissue interface thickness that is comparable to that of the FS-LASIK procedure. 

Acknowledgments

The authors thank Mohammad Saidur Rahman, PhD, and Wenzhi Gao, PhD, for their support in this research. 

Supported by Johnson & Johnson Surgical Vision, Milpitas, CA, USA. 

Disclosure: H. Minoguchi, Johnson & Johnson Surgical Vision (E); A. Umar, Johnson & Johnson Surgical Vision (E); H. Fu, Johnson & Johnson Surgical Vision (E)