Small incision lenticule extraction (SMILE) techniques: patient selection and perspectives

Figures & data

Table 1 Absolute and relative contraindications for SMILE

Absolute contraindications to SMILE	Relative contraindications to SMILE
Pre-existing corneal ectasia (keratoconus, PMD)	Age ≤21 years
Unstable refractive error	Mild/treated ocular surface and tear film disorders
Exposure keratopathy	Epithelial and basement membrane corneal dystrophies
Ocular surface and tear film disorders such as active uncontrolled ocular allergy, severe dry eye, blepharitis	Systemic immunodeficiency
Pregnancy and breastfeeding	Controlled diabetes mellitus
One-eyed patient	Past ocular herpes infection
Uncontrolled glaucoma or uveitis	History of keloid formation Autoimmune disorders

Abbreviations: PMD, pellucid marginal degeneration; SMILE, small incision lenticule extraction.

Table 2 Laser parameters set during SMILE in various treatment modes

Laser parameter	Expert modea	Standard modeb	Fast modec
Pulse energy	140–200 nJ	130 nJ	170 nJ
Energy offset (1 offset = 5 nJ)	28–40	26	34
Track distance	Lenticule and cap cuts: 4.5 ± 0.5 µm	Lenticule and cap cuts: 3 µm	Lenticule and cap cuts: 4.5 µm
	Lenticule and cap side cuts: 2 ± 0.5 µm	Lenticule and cap side cuts: 2 µm	Lenticule and cap side cuts: 2 µm
Spot distance	Lenticule and cap cuts: 4.5 ± 0.5 µm	Lenticule and cap cuts: 3 µm	Lenticule and cap cuts: 4.5 µm
	Lenticule and cap side cuts: 2 ± 0.5 µm	Lenticule and cap side cuts: 2 µm	Lenticule and cap side cuts: 2 µm

Notes:

a The expert mode allows the surgeon to vary the “fast” mode settings within a certain range.

b The standard settings are preset by the manufacturer.

c The fast settings are set by the application specialist and may vary from surgeon to surgeon.

Abbreviation: SMILE, small incision lenticule extraction.

Table 3 Cap and lenticule treatment parameters for small incision lenticule extraction using VisuMax femtosecond laser system

Treatment parameter	Range
Cap parameters
Cap thickness	100–160 µm
Cap diameter	6–9.6 mm
Incision position	0°–359°
Incision width	2–5 mm
Cap side cut angle	45°–135°
Lenticule parameters
Lenticule diameter	5–8 mm
Transition zone	0.10 mm for CYL 0 for SPH
Minimum lenticule thickness	10–30 µm
Lenticule side cut angle	90°–179°

Abbreviations: CYL, cylindrical; SPH, spherical.

Figure 1 Femtosecond laser application in small incision lenticule extraction.

Notes: (A) Infrared illumination to confirm centration after docking. (B) Lenticule cut (posterior lamellar plane) created in an outside-in manner. (C) Cap cut (anterior lamellar plane) created in an inside-out manner. (D) Cap side cut created at the final step.

Figure 2 Two concentric rings visible after femtosecond laser application, with the outer ring signifying the cap cut (red arrow) and the inner ring signifying the lenticule cut (yellow arrow).

Figure 3 Lenticule dissection and extraction.

Notes: (A) Cap side cut opened with hooked instrument. (B) Anterior lamellar plane delineated on the left-hand side. (C) Posterior lamellar plane delineated on the right-hand side. Meniscus sign confirms the identification of lenticule edge. (D) Anterior plane was dissected first. (E) Posterior plane dissection. (F) Lenticule extracted via microforceps.

Table 4 Intraoperative complications observed during small incision lenticule extraction

Complication	Pathophysiology	Prevention	Management	Outcomes
Suction loss	Low-suction VisuMax system with soft docking	Surgeon experience Avoid difficult orbital anatomy	Depends on stage of suction loss – re-dock to complete procedure or convert to excimer laser ablation (PRK/LASIK)	Satisfactory visual outcomes in majority
Black spots	Debris/air bubbles entrapped between contact lens and cornea		Clean contact lens/ocular surface	Satisfactory visual outcomes Increased adhesions during dissection
OBL	Accumulation and transient opacification of cavitation bubbles in intrastromal interface		Wait for OBL to disappear/gently massage out from interface	Delayed visual recovery – eventual satisfactory visual outcomes Increased higher order aberrations and decreased contrast sensitivity
Cap lenticular adhesions	Lenticule mis-dissection – posterior plane dissected before anterior plane	Intraoperative signs – meniscus sign, shimmer sign, and white ring sign	ASOCT to identify dissection plane. Sinskey hook-assisted dissection. Modified strippers and Y-shaped tip instruments	Increased likelihood of irregular interface, cap and side-cut tears, retained lenticule, epithelial defects
Retained lenticule	Difficult lenticule dissection/forceful extraction	Surgeon experience	Retreat – surface ablation/LASIK/circle pattern of VisuMax laser	Partially retained lenticule fragments – irregular astigmatism Completely retained lenticule – satisfactory visual outcomes after retreatment
Cap tears Side cut tears	Excessive surgical manipulation, forceful lenticule dissection, and extraction	Avoid rough dissection and bulky instruments	Frequent instillation of artificial tears Bandage contact lens in cap tears and extensive side-cut tears till re-epithelialization	Delayed visual recovery Satisfactory visual outcomes in majority Cap tears may lead to stromal scarring and irregular astigmatism
Epithelial defects	Surgical trauma Frequent instillation of topical anesthesia	Avoid frequent instillation of topical anesthesia Surgeon experience	Frequent instillation of topical artificial tears Bandage contact lens in large defects	Satisfactory visual outcomes in majority Interface haze may occur with central defects

Abbreviations: ASOCT, anterior segment optical coherence tomography; LASIK, laser-assisted in situ keratomileusis; OBL, opaque bubble layer; PRK, photorefractive keratectomy.

Figure 4 Opaque bubble layer.

Figure 5 Black spots.

Figure 6 Suction loss during lenticule cut.

Table 5 Management of intraoperative suction loss based on the stage of suction loss

Stage of suction loss	Management
<10% lenticule cut	Re-dock and restart the procedure
	Re-center using infrared light or clear central bubble
>10% lenticule cut	Convert to surface ablation/LASIK (same sitting/later date)
Lenticule side cut	Re-dock and repeat from lenticule side cut
	Decrease lenticule diameter by 0.2–0.4 mm
Cap cut	Re-dock and repeat from cap cut
Cap side cut	Re-dock and repeat only cap side cut
	Decrease cap diameter by 0.2–0.4 mm

Abbreviation: LASIK, laser-assisted in situ keratomileusis.

Figure 7 Cap lenticular adhesion with a completely retained lenticule as seen on the anterior segment optical coherence tomography.

Note: The posterior lamellar plane (lenticule cut) is dissected first (red arrows), and the lenticule is stuck to the overlying cap with a faintly visible anterior lamellar plane (yellow arrows).

Table 6 Intraoperative signs to prevent and identify cap lenticular adhesions

Intraoperative sign	Description	Application
Meniscus sign	Meniscus-shaped gap between the inner ring (diameter of lenticule cut) and the lenticule edge created by slightly pushing the lenticule edge away from the surgeon during creation of the posterior lamellar channel	Meniscus sign clearly visible during posterior plane dissection but not during anterior plane dissection Relation of frilled lenticule edge with surgical instruments acts as a guide to identify the correct plane of dissection
Shimmer sign	Bright reflex observed around the dissecting instrument	Reflex is not visible during dissection of anterior plane and helps to identify the correct plane of dissection
White ring sign	Light reflex from the lenticular side cut, which may be better observed using oblique external illumination in darker irides	Position of the white ring relative to the instrument is useful in identifying the dissection plane, as the white ring is posterior to the instrument during anterior plane dissection and anterior to the instrument during posterior plane dissection

Figure 8 Meniscus sign (yellow arrow).

Figure 9 Side-cut tear due to forceful lenticule extraction.

Table 7 Modifications of surgical technique of small incision lenticule extraction

Technique	Modification	Advantages	Limitation
Chung’s swing technique^Citation42	Posterior plane dissected – dissector lifted and swung at upper margin of lenticule to introduce it in anterior plane – lenticule cap interface dissected in a fan-shaped fashion	Simple and fast technique Significant decrease in the total surgical duration Less postoperative pain and conjunctival injection	Risk of lenticule tears, edge tears, and cap tears
Continuous curvilinear lenticulerrhexis technique^Citation43	Anterior plane dissected – lenticule grasped with forceps and extracted in a continuous, circumferential manner, without separation of the posterior plane	Easy learning curve Decreased surgical time	Potential risk of lenticule tear in thin lenticules Not recommended with OBL and uneven laser scanning
Lenticuloschisis^Citation47	Lenticule is directly peeled off from the surrounding stroma and extracted with a microforceps without initial lamellar dissection	Simplified technique Smoother interface Better visual quality Early visual recovery	Lenticule tears may occur in cases with dense opaque bubble layer, which leads to excessive adhesions
Lenticule irrigation^Citation48	0.2 mL of BSS is injected into the small incision after identifying the anterior and posterior layers of the lenticule	Minimize corneal epithelial damage and maintain the stability of the tear film Enhanced visual recovery	BSS may introduce interface debris or hydrate stroma
Hydroexpression^Citation49	BSS-assisted expression of the lenticule is performed after dissecting the lenticule from the surrounding stroma	Smooth delivery of lenticule with the minimal use of instruments. Simple and useful for novice surgeons. Useful in increased adhesions and thin lenticules	BSS may introduce interface debris or hydrate stroma
Push-up technique^Citation34	Instrument with a Y-shaped tip used to engage lenticule edge between the two limbs of the tip and enhance it by pushing it up from the stromal bed	Easy identification of lenticule edge	Needs specialized instrument
iOCT-assisted dissection^Citation44,Citation50	The dissection planes appear moderately reflective on iOCT after femtosecond laser delivery and become hyperreflective after complete dissection. The use of iOCT can simplify the identification of lenticule edge and dissection plane	Real-time dynamic visualization of the surgical process and instrument–tissue interactions	Increased surgical time
Sequential segmental dissection^Citation51	Anterior plane is dissected first followed by the central part of the posterior plane. Lenticular side cut (peripheral rim) is dissected at the end in a segmental and sequential manner	Useful in thin lenticules Faster	Lenticular mobility may not allow completion of dissection of final segments

Abbreviations: BSS, balanced salt solution; iOCT, intraoperative optical coherence tomography; OBL, opaque bubble layer.

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