Next-generation femtosecond laser for correction of myopia with or without astigmatism

Next-generation femtosecond laser for correction of myopia with or without astigmatism Smooth incision lenticular keratomileusis (SILK^TM) is performed using the next-generation ELITA^TM femtosecond laser system, which is designed for tissue bridge-free incision. At a showcase symposium sponsored by Johnson & Johnson Vision, Professor Mahipal Sachdev of Centre For Sight Group of Hospitals in New Delhi, India, principal investigator of ELITA’s recent phase III trial, shared his experience in using ELITA for both flap and flapless refractive correction procedures, highlighting swift recovery, improved visual acuity and patient satisfaction with the new system.

New technology delivers precision and accuracy
Compared with existing systems, ELITA femtosecond laser has the shortest pulse duration of approximately 150 fs, lowest pulse energy of 50 nJ, and smallest spot size of 1 μm. (Figure 1A) ELITA’s ultrafast delivery system (10 MHz laser repetition rate and 8,000 Hz resonant scanner) enables contiguous placement of pulses, producing tissue bridge-free surfaces requiring minimal to no dissection. [DOF2023RF4002 ELITA™ Femtosecond Laser device description]  (Figure 1B) The combination of shorter pulse duration, tighter spot spacing (mean, 1 μm) and smaller spot size eliminates tissue bridges, resulting in faster and smoother cuts.

A digital encoder allows precise laser pulse placement and depth, while a moving objective facilitates uniform optical performance across the entire treatment area. (Figure 1C) Post-docking centration and cyclotorsion adjustment help to minimize higher-order aberration and improve accuracy for astigmatism treatment.

ELITA’s computer-monitored vacuum provides strong holding force, which minimizes suction loss during treatment. Its two-piece patient interface facilitates surgeon’s fine control of the eye during docking. In addition, automatic Z-calibration compensates for any thickness and tilt variations of the patient interface, ensuring consistent treatment depth, while the system’s flat applanation aids precise laser pulse delivery across the entire treatment zone.

Novel biconvex lenticule shape
As with other lenticule extraction procedures, during SILK, the anterior and posterior planes of the lenticule are dissected sequentially. However, the extracted lenticule has a unique biconvex shape, chosen for its smallest antero-posterior dimension, which may better preserve corneal asphericity and, potentially, leads to better visual outcomes. [EuroTimes 2023(Suppl_Feb;4-5): A Corneal Refractive Update on Lenticule Extraction Technology and Outcomes]

“A biconvex shape leads to fewer collagen fibres being cut, thus allowing greater preservation of corneal biomechanical strength and faster nerve regeneration,” explained Sachdev. “Symmetrical and matching anterior and posterior surfaces are also intended to minimize folds in Bowman’s layer, leading to better quality of vision. In addition, this special shape enables easy lenticule removal with minimal risk of edge tearing.” [Sachdev M, ELITA Innovation Showcase, 24 May 2023, Hong Kong]

Suitable for flap and flapless procedures
ELITA is designed for both flap and flapless refractive correction procedures. Flap parameters can be customized in terms of diameter (range, 7–10 mm), flap thickness (range, 90–150 μm), flap side cut angles (range, 30–150˚), and bed and side cut energy (range, 55–90 nJ/pulse).

The accuracy of corneal flap thickness created with ELITA for laser-assisted in situ keratomileusis (LASIK) was evaluated against iFS laser in a prospective, single-site, open-label, randomized clinical study involving 50 patients, where study eyes were treated with ELITA and fellow eyes received iFS. [Sachdev M, ELITA Innovation Showcase, 24 May 2023, Hong Kong]

All flaps were planned to 110 μm, and their thickness was measured using anterior segment optical coherence tomography (AS-OCT) scans taken at 1 week, 1 month, and 3 months after LASIK. Correlations between baseline corneal pachymetry and mean keratometry values and flap thickness were also evaluated at these time points.

“This early feasibility study demonstrated excellent accuracy of ELITA compared with iFS in creating flaps for LASIK,” reported Sachdev. “Both systems produced consistent flap thickness, with standard deviations of ±5.6 μm for ELITA and ±6.5 μm for iFS at 3 months. However, ELITA flaps rated better in terms of both stromal bed quality and ease of flap lift in the majority of cases.” (Figure 2)

“ELITA produces fast and smooth cuts, and induces minimal tissue disruption in the stroma by utilizing the smallest, shortest, and lowest-energy femtosecond laser pulse currently available,” emphasized Sachdev.

No correlation was found between baseline corneal thickness or mean keratometry and achieved flap thickness, with r² values <0.07 for both parameters at all time points.

ELITA-assisted SILK procedure
Another study evaluated ELITA-assisted SILK intrastromal lenticule removal procedure, which generates a biconvex lenticule shape, for correction of myopia with or without astigmatism. This prospective, multicentre, phase III, open-label trial recruited 85 adults with uncorrected visual acuity (UCVA) of 20/40 or worse, with or without astigmatism up to -12 DS and -6 DC with sum of sphere and cylinder between -1.00 D and -12.00 D using minus cylinder convention. [ELITA Femtosecond Laser Device Manual 0155-3006]

Intended correction was emmetropia for all eyes (n=158). Visual outcomes were measured at 1 day, 1 week, 1 month, 3 months, 6 months, 9 months, and 12 months after the operation. [Sachdev M, ELITA Innovation Showcase, 24 May 2023, Hong Kong]

“UCVA was 20/20 or better in 85 percent of eyes at 1 week after the operation, increasing to 96 percent of eyes at 3 months. At 9 months postoperation, UCVA was 20/20 or better in 94 percent of eyes, increasing to 100 percent at 12 months,” reported Sachdev. “The best spectacle-corrected visual acuity was 20/20 in 100 percent of eyes at all postoperative time points.”

Furthermore, binocular visual recovery was fast, with 89 percent and 100 percent of patients achieving UCVA of 20/20 or better on day 1 and 1 week after the operation. All patients had binocular UCVA of 20/20 or better at 6 months postsurgery. Of note, 54 percent of eyes achieved monocular UCVA of 20/16 at 3 months, which increased to 64 percent at 6 months.

At 6 months, there was no loss of visual acuity in any of the SILK-treated eyes, while 46 percent had ≥1 line and 8 percent had 2 lines of improvement in corrected distance visual acuity.

Importantly, patients were highly satisfied with their vision after SILK. All patients reported being happy that they underwent the procedure, 98 percent of patients were satisfied with how quickly they saw improvement in their vision, and 96 percent of patients would recommend SILK to family or friends.

Mean manifest refraction spherical equivalent (MRSE) was -0.32 D at 3 months and -0.33 D at 6 months after the operation. The standard deviation of MRSE was <0.25 D at all time points, demonstrating very low variability. At 9 months postoperation, 90 percent of eyes had MRSE within ±0.50 D of intended value, which increased to 94 percent at 12 months.

In addition, SILK provides highly accurate cylinder correction. At 3 months after the operation, 72 percent, 95 percent and 100 percent of eyes were within ± 0.25 D, ± 0.50 D and ± 1.00 D of manifest refraction cylinder target, respectively.

Practical recommendations and personal impressions
“SILK treatment requires keratometry values for optical zone detection and correct lenticule depth,” explained Sachdev. “Before starting the procedure, the surgeon needs to mark the 0˚ and 180˚ axis of the cornea at slit lamp. Marking the visual axis assists centration prior to applying the suction ring. To ensure easy dissection, the anterior lens is identified by dissecting a band 2–3 mm parallel to the entry resection. The entry incision needs to be tented toward the ceiling at approximately 45˚. The surgeon may wish to use bullet-nose or flat anterior instruments for correct plane identification.”

“My personal impression is that SILK lives up to its name, as there were only minor or no tissue adhesions, and all cases [in our study] had complete lenticule removal, with no cold spots, uncut areas or bridges,” said Sachdev. “Furthermore, none of the cases had any difficulty with the entry cut, and plane identification was relatively easy. Corneal biomicroscopy was unremarkable for all day 1 and week 1 postoperative visits. The procedure achieved accurate refractive correction with tight outcomes and standard deviation for both sphere and cylinder. Observed fast visual recovery led to great patient satisfaction.”