Smooth Incision Lenticular Keratomileusis (SILK^TM)

The groundbreaking invention of the lasers in the 1960s opened a new paradigm for vision correction. In the early 1990s the laser-assisted in situ keratomileusis (LASIK) procedure was developed and gradually became a major procedure for laser vision correction. The modern LASIK procedure takes two major steps, each of which is performed with a different laser system. First, a femtosecond laser is used to cut a flap on the cornea; second, the exposed corneal stroma is ablated with an excimer laser, followed by repositioning of the flap to cover the ablated area. Excimer laser ablation modifies the radius of curvature of the cornea and corrects the refractive error. Nearly 40 million eyes have undergone this procedure across the world. The advantage of LASIK is that most patients gain a sharp vision shortly after the surgery.

In 2009, Zeiss launched a small incision lenticule extraction (SMILE) procedure. It uses a femtosecond laser to cut a lenticule in the cornea, followed by a 3-mm-long cap opening incision for lenticule extraction.[1,2] This incision is much shorter than the arc length of the LASIK flap side cut. Advantages of lenticule surgery over LASIK include better preservation of corneal biomechanical stability and corneal nerves. This results in reduced risk of corneal ectasia and dry eye. So far, over 10 million eyes have undergone this procedure. In SMILE, the corneal lenticule needs to be separated from the surrounding cornea with the help of a blunt spatula before it can be extracted. This manipulation can compromise the quality of the tissue interface, resulting in delayed visual recovery.[3,4]

Today, four companies have developed femtosecond lasers for performing corneal lenticule extraction procedures and each has a specific name for the procedure. The refractive surgery community tends to agree to call the Kerato-refractive Lenticule Extraction (KLEX) procedure. Fig. 1 illustrates the LASIK procedure and the KLEX procedure.

Figure 1.

Illustration of (a and b) LASIK procedure; (c) Kerato-lenticular extraction (KLEX) procedure

The ELITA femtosecond laser system is an innovative corneal refractive surgical system developed by Johnson & Johnson. Its smooth incision lenticular keratomileusis (SILK^TM) is J&J KLEX procedure for refractive vision correction. The ELITA technology is aimed to provide easier lenticule removal and superior visual outcomes based on the femtosecond laser-tissue interaction physics.

The ELITA femtosecond laser is intended to provide the combined advantages of LASIK and KLEX, i.e., superior visual outcome and better corneal biomechanics. To achieve this goal, the ELITA is designed to use the lowest pulse energy, contiguous laser cut pattern and submicron precision for the control of the cutting trajectory [Fig. 2].

Figure 2.

Illustration of the key ELITA technologies

Table 1 shows a comparison of the key parameters for the current on-market KLEX femtosecond laser systems.

Table 1.

Comparison of KLEX femtosecond laser systems on the market

Company		Johnson-Johnson		Zeiss		Zeiss		Ziemer		Schwind
System		ELITA		VisuMax 500		VisuMax 800		Femto LDV Z8		ATOS
KLEX		SILK		SMILE		SMILE		CLEAR		SmartSight
Laser pulse duration		~150 fs		220–580 fs		220–580 fs		200–500 fs		<295 fs
Pulse repetition rate		10 MHz		0.5 MHz		2 MHz		<10 MHz		Up to 4 MHz
Pulse energy		40–50 nJ		125–170 nJ		125–170 nJ		<100 nJ		80–125 nJ
Spot separation		~1 μm		3–4.5 μm		3–4.5 μm		No data		3–4.5 μm
Laser cut time		~16 s		~30 s		~10 s		~30 s		~30 s
Patient interface		Flat		Curved		Curved		Flat		Curved
Postdocking centration		Allowed		No		No		Allowed		No
Postdocking cyclotorsion		Allowed		No		Allowed		Allowed		Allowed
Lenticule shape		Biconvex		Plano-convex		Plano-convex		Plano-convex		Plano-convex

ELITA SILK procedure introduces for the first time in the industry the biconvex lenticule shape [Fig. 3]. Although the plano-convex lenticule cuts may be implemented differently, the common feature is that it has a uniform anterior depth, such as the flap depth for the LASIK flap and the cap depth for KLEX. Both biconvex and plano-convex lenticule shapes follow the same Munnerlyn formula for refractive vision correction, and they remove the same amount of tissue for a given refractive correction.[4] ELITA chooses the biconvex lenticule shape because of two major reasons. First, it provides mirror symmetric posterior and anterior lenticular surfaces that are expected to match perfectly after lenticule removal, leading to fewer wrinkles in Bowman’s membrane, therefore less light scattering and better visual contrast sensitivity. Second, since the corneal stiffness, collagen fiber density and nerve density decrease from anterior to posterior, the biconvex shape is expected to better preserve corneal strength, leading to fast regeneration of corneal nerves.

Figure 3.

Illustration of ELITA biconvex lenticule shape (left) and the conventional plano-convex shape that is used by LASIK and all other KLEX shapes

To evaluate the visual outcomes following SILK^TM procedure for correction of myopic refractive errors with and without astigmatism, using the ELITA^TM femtosecond laser system, a total of 170 eyes of 85 patients underwent SILK procedure.[6] Preoperative mean refractive spherical equivalent (MRSE) was −4.14 D ± 1.32 D (range −1.38 D to −8.88 D) and the mean cylinder was −0.77 D ± 0.62 D. Intraoperative surgeon ease of lenticule dissection was rated as grade 0 or 1 in 85.3% of eyes (no/only mild dissection needed) [Fig. 4]. Uncorrected distance visual acuity (UDVA) at 1 day, 1 week, 1 month, and 6 months was 20/20 or better in 65.9%, 85.4%, 91.5%, and 96% of eyes, respectively. No eyes lost any lines of CDVA at 6 months [Fig. 5]. The postoperative MRSE was stable over time, ranging from −0.34 D ± 0.24 D at 1 month to −0.33 D ± 0.23 D at 6 months. MRSE predictability (±0.50 D) was 93.5% (129/138) at 3 months and 91.1% (113/124) at 6 months. No serious adverse events were noted.

Figure 4.

Ease of lenticule removal surgeon ratings (n = 169), where 0 = no dissection needed, 1 = mild dissection required (<50% of lenticule), 2 = moderate dissection needed (50–75% of lenticule), 3 = significant dissection needed (76–100% of lenticule), and 4 = unable to dissect

Figure 5.

Cumulative Snellen visual acuity (20/× or better) comparing postoperative (postop) uncorrected distance visual acuity (UDVA) with preoperative (preop) corrective distance visual acuity (CDVA) at each time point

The mean logMAR UDVA was 1.02 ± 0.11 at baseline, which improved to −0.07 ± 0.07 at the 6-month postoperative visit (P < 0.0001). Monocular UDVA was 20/40 or better in 100% of eyes at the 1-week, 1-, 3-, and 6-month study visits, exceeding the primary effectiveness endpoint target of 85% at 1 month. Similar results were observed for binocular UDVA over time, with 100% of eyes achieving 20/40 or better from the 1-week postoperative visit onwards. At postoperative day 1, 65.9% of eyes achieved 20/20 or better monocular UDVA, which increased to 85.4% at 1 week, 91.5% at 1 month, and 96% at 3 and 6 months.

The ELITA laser system was designed to deliver lower energy per pulse compared to other laser systems on the market [Table 1] and to produce continuous tissue-bridge-free corneal lenticules that are easy to remove to enable faster visual recovery. Another unique feature of the ELITA system is the biconvex lenticule shape that creates identical surfaces and tissue matching following lenticule removal. The biconvex shape is expected to cut fewer collagen fibers in the peripheral cornea by cutting deeper into the stroma, thereby potentially producing less opaque-bubble-layer and better preservation of corneal biomechanical strength. Additionally, the biconvex lenticule shape is expected to better preserve corneal nerves (clinical results will be published soon). Due to the use of high-precision digital encoder-guided scanning and the minimal mechanical dissection involved in lenticule removal, the ELITA SILK procedure introduces minimal high-order aberrations compared with LASIK and other KLEX procedures, which is expected to benefit the quality of vision, such as the depth of focus.[5]

Johnson-Johnson started commercializing ELITA for SILK and LASIK flap procedures in January 2023 in selected markets across Asia and Europe. The early outcomes obtained with ELITA are aligned with the multicenter clinical data that were published, and with our overall experience with the device.[6] In our hands, ELITA continues to deliver a fast and easy (almost no dissection) lenticule removal procedure, with an extremely low rate of suction loss, a short learning curve and fast visual recovery. Further developments in software and technology will continue to optimize our refractive outcomes, with the goal to achieve LASIK-like postop day 1 visual outcomes, provide faster nerve regeneration than LASIK and other KLEX procedures, and provide equal or better corneal biomechanics than LASIK and other KLEX procedures.

I feel, SILK™ lives up to its name by delivering smooth incisions with minor or no tissue adhesions. Every case has a complete lenticule removal without encountering cold spots, uncut areas, or bridges, and entry cuts pose no difficulty. Plane identification is relatively easy, and corneal biomicroscopy remains unremarkable during Day-1 and Week-1 postoperative visits. Patients experience fast visual recovery and receive accurate refractive correction, with tight outcomes and standard deviations for sphere and cylinder. Overall, SILK™ achieves great patient satisfaction.

About the author

Professor (Dr.) Mahipal. S. Sachdev

Professor (Dr.) Mahipal. S. Sachdev, Chairman & Medical Director of Centre for Sight is a renowned ophthalmic Surgeon who is recognized for his expertise in the area of Cataract, Refractive & Corneal Surgery, both nationally and internationally. He was honored with Padmashri award on January 26, 2007. In the year 2017, he attained Professorship in Ophthalmology & in year 2020 has been elected as President of All India Ophthalmological Society. He is widely credited to be pioneer in India in propagating the technique of Phacoemulsification for cataract surgery and Lasik Laser for removal of glasses.

In 1996, Professor (Dr.) Mahipal Sachdev stepped out of AIIMS with a dream of creating a world class eye hospital. “Centre for Sight” is an outcome of that.

Professor (Dr.) Mahipal is a visionary and leader. He has always been the torchbearer in the field of eye care pioneering several procedures including laser assisted blade less surgery for cataract and Lasik.

Prof. (Dr.) Mahipal S. Sachdev is a rare combination of a brilliant surgeon, an entrepreneur and above all, a compassionate human being. He has ensured that the Indian Flag remains flying high in the domain of eye care and that the patients in India have access to the best and the latest when it comes to treating their eyes in line with his own mission statement “…. every eye deserves the best”.