Small incision lenticule extraction (SMILE) is a refractive surgery that uses a femtosecond laser to create a lenticule within the corneal stroma and removes it through a 2-3 mm small incision to correct myopia and myopic astigmatism 1). Since its clinical introduction in 2008, over 8 million procedures have been performed worldwide by the end of 2023 1).
The greatest advantage of SMILE surgery is that it does not require a corneal flap like LASIK 1). There is no risk of flap-related complications, and it provides preservation of corneal nerves and maintenance of good corneal biomechanical properties 1).
SMILE lenticule transplantation is an experimental attempt to reuse the corneal stromal lenticule extracted during this surgery as an allogenic corneal inlay for treating other corneal diseases. The lenticule is pure stromal tissue, and its potential as a transplant material for various refractive errors and corneal diseases is being explored.
Since the SMILE lenticule does not contain corneal epithelium or endothelium, it is thought to reduce the risk of significant immune reaction by the host after transplantation. However, further research is needed on long-term immune reactions.
The symptoms of patients eligible for SMILE lenticule transplantation vary depending on the underlying disease.
This section describes the risk factors for complications of SMILE surgery itself.
The incidence of intraoperative and postoperative complications of SMILE surgery is shown 1).
| Complication | Incidence |
|---|---|
| Incision epithelial damage | 13.6% |
| Opaque bubble layer (OBL) | 5.7% |
| Corneal epithelial defect | 2.8% |
| Difficult lenticule extraction | 2.2% |
Other reported complications include lenticule decentration (1.0%), incomplete extraction (0.9%), and cap perforation or tear (0.5%)1).
The incidence of corneal ectasia after SMILE is reported as 11 per 100,000 eyes, which is lower than that of LASIK (90 per 100,000 eyes) and PRK (20 per 100,000 eyes)2). However, since SMILE has a shorter history than LASIK, differences in follow-up duration should be considered2).
The following examinations are performed to determine the indication for SMILE lenticule transplantation:
SMILE surgery is performed according to the following steps1).
The SMILE lenticule is composed of corneal stroma, primarily consisting of type I collagen. Densely arranged collagen fibrils form a lamellar structure. The absence of epithelium and endothelium is considered an immunological advantage.
Cryopreservation
Method: Stored at -80°C in cryopreservation solution. This is the most common preservation method.
Advantages: Maintains the structure and integrity of collagen fibrils. Sufficient corneal cells survive to maintain transparency.
Challenges: High cost and complex procedure limit clinical application.
Other preservation methods
Cryopreservation: This is being considered as a relatively simple method.
Glycerol preservation at room temperature: Collagen structure is maintained and safety has been confirmed.
Decellularized corneal lenticule: This method involves decellularization followed by storage in preservation solution at room temperature.
Both fresh and cryopreserved corneal lenticules show necrotic and apoptotic cells, but a sufficient number of corneal cells survive in cryopreserved lenticules to maintain a regular stromal collagen structure.
The most common method is cryopreservation (minus 80°C), which maintains collagen structure and transparency. However, cost and procedural complexity are challenges, and alternative methods such as glycerol preservation at room temperature and decellularization are also being investigated.
The principle of SMILE corneal lenticule transplantation lies in changing corneal curvature and increasing corneal thickness by adding tissue to the corneal stroma.
The corneal stroma is composed of type I collagen with a lamellar structure. The SMILE lenticule contains only this stroma, with the epithelium and endothelium removed. After transplantation, the lenticule is incorporated into the host corneal stroma, causing a local change in refractive power.
SMILE maintains corneal biomechanical properties better than LASIK 1). Because no corneal flap is created, the structural continuity of the stroma is preserved.
A 5-year comparative study of SMILE and FS-LASIK found that both procedures are safe and effective, with no significant difference in long-term effects on corneal biomechanics 1).
Corneal lenticule transplantation restores stromal volume and increases corneal thickness, which is also expected to provide structural reinforcement to the thinned cornea.
SMILE lenticule transplantation is being investigated for application in various corneal diseases and refractive errors.
Presbyopia Correction
Concept: Reimplant autologous lenticule into the eye after SMILE to restore stromal volume. Then perform LASIK targeting myopia to establish monovision.
Current status: It has been reported that there was no difference in corneal tissue response between eyes that underwent SMILE + lenticule transplantation + LASIK and eyes that underwent LASIK alone.
Hyperopia Correction
Concept: Change corneal refractive power by lenticule transplantation to correct hyperopia. SMILE has not received FDA approval for hyperopia correction.
Current status: Reported in primate studies and isolated human cases, but still at the experimental stage. sLIKE (small incision lenticule intrastromal keratoplasty) has been proposed as a new procedure.
Keratoconus
Concept: A corneal lenticule is transplanted into a keratoconic eye to reinforce corneal thickness and improve shape.
Current status: Improvement in visual acuity and reduction in corneal curvature have been reported after lenticule transplantation.
Corneal perforation/thinning
Concept: For severe corneal thinning, a corneal lenticule is used as a reinforcing material.
Current status: Successful lenticule transplantation has been reported in a case of corneal thinning due to recurrent pterygium (three recurrences).
Improvement in visual acuity and reduction in keratometry values have been reported after lenticule transplantation in keratoconic eyes. However, these are limited reports, and further research is needed for widespread clinical application.
SMILE lenticule transplantation is currently an investigational procedure and is not a standard treatment available at general hospitals. Further verification of safety and efficacy is required for clinical implementation.
