SMILE (small incision lenticule extraction) is a refractive surgery that uses a femtosecond laser to create a refractive lenticule (lens-shaped corneal piece) within the corneal stroma and removes it through a 2–3 mm small incision to correct myopia and myopic astigmatism. It fundamentally differs from conventional LASIK and PRK in that it does not use an excimer laser and is completed with a single femtosecond laser device.
Since its clinical introduction in 2008, it has become one of the most widely performed laser refractive surgeries worldwide, with over 8 million procedures performed globally by the end of 20232). In Japan, it received regulatory approval on March 22, 20231). Initially performed as FLEx (femtosecond lenticule extraction) with a large incision, SMILE with a reduced incision size of 2–3 mm has become the standard technique. Since the first efficacy and safety study by Sekundo et al.13), numerous studies have accumulated, and it is now established as a refractive surgery with high efficacy, safety, and predictability.
The essential feature of this surgery is that it does not create a flap. LASIK first creates a corneal flap, lifts it, and then applies an excimer laser, whereas SMILE removes only the intrastromal tissue without creating a flap. This preserves the structural continuity of the cornea and protects the anterior corneal stroma. Corneal nerve transection is also minimized, resulting in a lower risk of postoperative dry eye compared to LASIK2).
The correction range is up to spherical equivalent of 10 D (myopia ≤10 D, astigmatism ≤3 D) 1). The minimum age is 18 years, and stable refraction is a prerequisite 1). Preoperative corneal topography (including biomechanical evaluation with TBI, CBI, etc.) is mandatory in all cases, and detecting and excluding forme fruste keratoconus is the most critical factor for surgical safety 2).
SMILE uses only a femtosecond laser and removes a lenticule without creating a corneal flap. LASIK creates a flap and applies excimer laser, carrying risks of flap-related complications (dislocation, folds, epithelial ingrowth, etc.). PRK removes the epithelium and applies excimer laser, causing more postoperative pain and longer recovery. SMILE has less pain (similar to LASIK), no flap-related complications, and milder dry eye than LASIK. The incidence of postoperative ectasia is also lower than LASIK (11 vs. 90 per 100,000 eyes), with biomechanical advantages 2). The indication range is within spherical equivalent of 10 D (myopia ≤10 D, astigmatism ≤3 D), which is slightly narrower than LASIK (myopia, hyperopia, astigmatism) 1).
SMILE surgery is indicated for refractive errors (myopia and myopic astigmatism). Patients with the following complaints are candidates.
The following may occur 1–3 days after surgery. They usually resolve within a few days.
Early Postoperative Period (1–7 Days)
Visual recovery: Marked improvement from the next day. Many patients can perform daily activities without glasses from day 1.
Corneal findings: The epithelium at the incision site (2–3 mm) usually heals within 1–2 days. Corneal edema resolves within a few days.
Caution: Excessive eye rubbing can cause epithelial damage at the incision site.
1–3 months after surgery
Stable period: Refraction and visual acuity stabilize during this period. Refractive values stabilize by 6 months after surgery.
Dry eye: Temporary dryness may persist. It improves as corneal nerves recover.
Follow-up: Regular check-ups at 1 month, 3 months, and 6 months after surgery are recommended1).
Postoperative complications (caution required)
DLK (diffuse lamellar keratitis): Overall incidence 0.84%2). Often occurs within 1 week after surgery. Managed with steroid eye drops. Severe cases may require lamellar irrigation.
Corneal ectasia: Global incidence 0.02%2). Strict preoperative screening is essential.
Delayed visual recovery: Overall incidence 1.5%2). Causes include residual refractive error, dry eye, and OBL.
In most cases, you can resume daily activities without glasses the next day. Driving may be possible after vision is confirmed the day after surgery, but follow your doctor’s instructions. Strenuous exercise and contact sports are usually allowed after 1–2 weeks. Activities with infection risk, such as swimming and sauna, are recommended to be avoided for at least 1 month.
Understanding the indications, contraindications, and risk factors for SMILE surgery is essential for safe surgery.
According to the Refractive Surgery Guidelines, 8th Edition 1), the following are absolute contraindications:
According to the KLEx guidelines2):
The following examinations are performed preoperatively to carefully evaluate indications1).
| Examination Item | Purpose | Notes |
|---|---|---|
| Visual acuity (uncorrected and corrected) | Baseline assessment | Best corrected visual acuity also required |
| Refraction measurement (subjective, objective, under cycloplegia) | Determination of correction amount | Cycloplegic refraction is recommended2) |
| Corneal curvature radius measurement | Correction amount / surgical planning | — |
| Slit-lamp microscopy | Ocular surface and anterior segment evaluation | Early detection of keratoconus |
| Corneal topography/tomography | Keratoconus screening | Most important test. Scheimpflug or OCT recommended |
| Corneal thickness measurement (pachymetry) | RST calculation | Foundation of surgical planning |
| Tear film test | Dry eye evaluation | Assessment of risk of postoperative worsening |
| Fundus examination | Assessment of myopic changes | In high myopia, also check the periphery |
| Intraocular pressure measurement | Exclude glaucoma | Low intraocular pressure is contraindicated |
| Pupil diameter measurement (dark room) | Optical zone design | Prediction of night halos |
| Corneal diameter measurement | Surgical planning | — |
| Corneal endothelial cell examination | Preoperative baseline | Required additional test for SMILE1) |
| Corneal biomechanical examination (Corvis, ORA, etc.) | Ectasia risk assessment | TBI, CBI, CRF are useful2) |
Keratoconus is an absolute contraindication; if overlooked, there is a risk of postoperative corneal ectasia. TBI (tomographic and biomechanical index) shows the highest diagnostic accuracy (SUCRA 96.2%), and comprehensive evaluation combined with CBI (83.8%) is recommended2). Three-dimensional evaluation of the anterior and posterior corneal surfaces and corneal thickness using Scheimpflug tomography (Pentacam, etc.) should always be performed.
Guidelines for safe surgical design are as follows2):
Soft contact lenses are usually discontinued 1–2 weeks before. Hard contact lenses (oxygen-permeable, scleral lenses) have a greater impact on corneal shape, so discontinuation for 3 weeks to 1 month or more is necessary. Preoperative examinations must be performed after contact lens discontinuation. This is an essential step to accurately assess corneal shape.
SMILE surgery is performed as a day procedure under topical anesthesia. The standard steps are as follows2):
Total surgery time is about 15–30 minutes for both eyes. The actual laser irradiation time per eye is very short, about 25–40 seconds, placing minimal physical, mental, and time burden on the patient.
Treatment strategy for DLK (diffuse lamellar keratitis) based on the KLEx guidelines2):
| Grade | Frequency | Treatment |
|---|---|---|
| Grade I (peripheral only) | 1.42%2) | Topical steroids (fluorometholone 6–8 times/day) + observation |
| Grade II (extending to central cornea) | 0.29%2) | Intensified topical steroids (prednisolone acetate every hour) |
| Grade III (confluent/focal) | 0.08%2) | High-dose steroids + consider interface irrigation |
| Grade IV (severe) | 0.02%2) | Steroid interface irrigation (mandatory) + systemic steroids |
With appropriate steroid treatment, lesions improve within one week in most cases, and symptoms resolve in about three weeks2).
For undercorrection or refractive regression, enhancement surgery is considered after 6 months postoperatively once refraction is stable1). Options include:
It is essential to confirm that the residual corneal thickness (sum of cap, lenticule extraction cavity, and residual stromal bed) meets the safety standard (RST ≥ 280 μm) when performing enhancement2). The incidence of undercorrection is continuously improving through nomogram optimization and AI-driven predictive models, with recent enhancement rates below 5% at many facilities.
Enhancement surgery is considered if undercorrection, overcorrection, or refractive regression occurs. The decision is usually made after 6 months postoperatively once refraction is stable. The common method is to create a flap on the original cap surface with a femtosecond laser and apply additional excimer laser irradiation. Confirmation of residual corneal thickness is essential.
The femtosecond laser focuses within the corneal stroma, creating interfaces for the anterior and posterior surfaces of the lenticule through plasma formation and fine photodisruption. The corneal stroma (lenticule) sandwiched between these two incision planes is removed, changing the corneal curvature and correcting myopia.
The shape of the lenticule (difference in curvature between anterior and posterior surfaces) is designed according to the amount of refractive correction. Removing a convex lens-shaped stromal piece that is thicker in the center and thinner at the periphery reduces the refractive power of the cornea, correcting myopia.
The minimum lenticule thickness is generally considered to be 15–20 μm or more; if the lenticule is thinner than this, safe extraction may be difficult. For astigmatism correction, the accuracy of limbal marking and eye tracking systems is important to precisely position the asymmetric lenticule shape (axis asymmetry) 6). The optical zone diameter in SMILE is usually set between 6.0 and 7.0 mm, but the KLEx guidelines show that a larger optical zone (≥6.5 mm) results in fewer postoperative higher-order aberrations (especially coma) and better night vision function 2); therefore, a larger optical zone is recommended when corneal thickness is sufficient. However, enlarging the optical zone increases the amount of tissue removed (→ decreased RST), so individualized design considering the balance between biomechanics and visual function is important 2).
SMILE preserves corneal biomechanical properties better than LASIK. A meta-analysis in the KLEx guidelines showed that the decrease in CRF (Corneal Resistance Factor) at 12 months postoperatively was significantly smaller than with FS-LASIK (MD, −1.13; 95% CI −1.36 to −0.90; P < 0.001) 2). The decrease in CH (Corneal Hysteresis) was also smaller than with FS-LASIK (MD, −1.17; 95% CI −1.45 to −0.89; P < 0.001) 2).
The reason for this advantage lies in the cap structure that preserves the anterior stroma (the site with the highest mechanical strength). The cap (anterior stroma) maintains the continuity of the lamellar structure and provides higher biomechanical strength than the LASIK flap 2).
The differences between the SMILE cap and the LASIK flap are: (1) no hinge, maintaining continuity with the cornea around the entire circumference; (2) a closed structure with only a small incision (2–3 mm) under the cap communicating with the outside; and (3) preservation of strong collagen fibers in the anterior stroma. This design also provides the secondary benefit that postoperative flap dislocation due to trauma does not occur 2). The management value of LT index (maximum lenticule thickness/central corneal thickness ratio) ≤ 28% is an important indicator for maintaining this biomechanical advantage; if it exceeds 28%, the reinforcing effect of the cap is exceeded and adverse effects on biomechanics occur 2).
SMILE inserts instruments only through a small incision (2–3 mm) near the corneal limbus, minimizing damage to the sensory nerves (corneal nerve plexus) in the anterior corneal stroma. In LASIK, nerves are severed circumferentially when creating a 360° corneal flap. Therefore, the incidence and severity of postoperative dry eye symptoms are lower with SMILE than with LASIK. Comparative studies of FS-LASIK and SMILE also report that SMILE results in faster recovery of postoperative corneal nerve density and less impact on tear film parameters4).
Longitudinal evaluation of corneal nerve density using in vivo confocal microscopy (IVCM) shows that after SMILE, nerve plexus density recovers to about 70–80% of preoperative levels by 3 months postoperatively, whereas after LASIK, recovery is often only about 40–60% at the same time point4). This difference is thought to correspond to the difference in severity of postoperative dry eye. Complete recovery of nerve density usually takes 6–12 months, and in some patients it may take more than 2 years. In patients with preoperative dry eye, nerve density recovery may be delayed, and aggressive preoperative dry eye treatment may improve postoperative outcomes.
Postoperative corneal ectasia is primarily attributed to reduced corneal biomechanical strength due to insufficient residual stromal thickness (RST) or excessive lenticule thickness, as well as preexisting subclinical keratoconus. Analysis of the KLEx guidelines2) found that 65.5% of ectasia cases had abnormal or suspicious preoperative corneal topography, and 52.3% had RST < 280 μm.
Ectasia after SMILE is rarer than after LASIK (11 vs. 90 per 100,000 eyes)5), but management of ectasia once it occurs is similar to that for post-LASIK ectasia. If progression is confirmed, corneal cross-linking (CXL) is the first-line treatment, and it has been covered by insurance in Japan since 2022.
Opaque bubble layer (OBL) is caused by accumulation of water vapor and carbon dioxide between layers. Maintaining room temperature at 18–25°C and humidity at 30–70%, along with appropriate laser energy settings, are preventive measures2). If OBL is extensive and covers the pupillary zone, it is recommended to wait for complete resolution before proceeding with lenticule dissection. Continuing manipulation forcefully increases the risk of wrong plane dissection.
In a 5-year comparative study by Li et al. (2019), both the SMILE and FS-LASIK groups maintained safety and efficacy, with no significant difference in long-term corneal biomechanical effects 3). The SMILE group maintained good refractive stability even after 5 years. Both procedures showed excellent uncorrected distance visual acuity (UDVA) at 5 years, and good maintenance of best-corrected visual acuity (BCVA) 3).
In a systematic review by Moshirfar et al.5), the incidence of ectasia for PRK, LASIK, and SMILE was calculated as 20, 90, and 11 per 100,000 eyes, respectively. The incidence of ectasia for SMILE was about one-eighth that of LASIK, but the follow-up period for SMILE is still short, and the possibility of underestimation has been noted 5). The same review confirmed that ectasia can occur even in eyes without any known risk factors, indicating the need for further refinement of preoperative screening.
In a prospective multicenter study for US FDA approval of SMILE for myopia with astigmatism reported by Dishler et al.6), the mean residual spherical equivalent at 12 months postoperatively was −0.07 D (±0.38 D SD), and 95.4% achieved UCVA of 20/20 or better, meeting safety and efficacy criteria. For astigmatism correction, the use of limbal marking to correct cyclotorsional error was shown to improve accuracy 6).
In a systematic review and meta-analysis by Song et al.7), comparing astigmatism correction outcomes between SMILE and LASIK, there were no significant differences in correction accuracy, residual astigmatism, or visual acuity. However, in high astigmatism (>2.0 D) cases, cyclotorsional error control in SMILE affected outcomes. The use of limbal marking and eye tracking systems is recommended 7).
Reinstein et al.8) compared the relative corneal tensile strength of PRK, LASIK, and SMILE using a mathematical model. SMILE, with its cap structure that preserves anterior stroma, was shown to retain more corneal strength for equivalent correction amounts compared to LASIK. This theoretical basis is argued to be consistent with the lower ectasia incidence of SMILE 8).
In a 1-year follow-up study by Shetty et al.9), SMILE showed significantly smaller decreases in postoperative corneal biomechanics (CRF, CH) compared to LASIK. This difference became evident at 3 months postoperatively and was maintained at 12 months. It has been suggested that the protection of the anterior stroma may be due to the biomechanical contribution of the cap9).
The AAO Corneal Ectasia PPP10) states that SMILE has a lower risk of ectasia than LASIK and is considered to have a risk profile similar to PRK. However, preoperative screening for forme fruste keratoconus is essential even for SMILE, and comprehensive biomechanical evaluation including CBI and TBI is recommended10).
The international consensus by Gomes et al.11) proposed a definition of ectasia progression (consistent changes in at least two of: anterior corneal steepening, posterior corneal steepening, and corneal thinning), and this criterion is applied to the management of ectasia after refractive surgery including SMILE.
Santhiago et al.12) showed that PTA (percent tissue altered) ≥40% is an independent risk factor for ectasia after LASIK. In SMILE (KLEx), since the cap contributes to corneal strength differently from a flap, there is debate about directly applying the LASIK-based PTA threshold, but management values of LT index ≤28% and RST ≥280 μm are commonly important12).
Meta-analysis of KLEx guidelines confirmed that the high astigmatism group (>2.0D) had significantly more residual astigmatism and lower correction accuracy compared to the low astigmatism group (<2.0D). Correction of cyclotorsional error using limbal marking or triple centration improves accuracy2).
Sekundo et al.13) reported the first efficacy and safety study (6-month results) of femtosecond laser lenticule extraction (FLEx). FLEx is the precursor technology to SMILE, and this study laid the foundation for the later development of SMILE. By reducing the incision size from 7 mm in FLEx to 2–3 mm in SMILE, corneal nerve protection and safety were improved.
In the TFOS DEWS III report by Jones et al.14), multiple studies support that SMILE has a significantly smaller impact on postoperative dry eye symptoms and corneal nerve density compared to LASIK. For ocular surface optimization after refractive surgery, preoperative MGD treatment, diquafosol, and perioperative use of IPL are recommended14).
AI-driven nomogram adjustments combining preoperative corneal biomechanical parameters have been reported to improve refractive prediction accuracy by over 25%2). Future development of personalized nomograms using multimodal data is expected.
The ectasia risk scoring system by Randleman et al.15) is useful for preoperative prediction of ectasia after LASIK and consists of five factors: abnormal corneal topography, low residual stromal thickness, young age, thin cornea, and high myopia. Similar factors increase the risk of postoperative ectasia in SMILE, so this scoring concept can be applied to preoperative screening15).
In addition to SMILE, several femtosecond laser lenticule extraction technologies such as CLEAR (cornea lenticule extraction for advanced refractive correction) and SILK (smooth incision lenticule keratomileusis) are being developed, and further standardization is progressing2). CLEAR is a variant of SMILE that aims to expand indications for hyperopia and presbyopia by improving incision design. SILK is designed to smooth the cut surface and reduce bubble formation, improving ease of lenticule dissection and speed of visual recovery. These new technologies inherit the flap-free and biomechanics-preserving principles established by SMILE while aiming to expand indications, improve accuracy, and reduce complications.
Attempts are being made to reimplant lenticules extracted during SMILE as allogeneic corneal inlays in patients with hyperopia, presbyopia, or keratoconus. Immune reactions may be minimal, but this is still experimental and not yet in general clinical use. In cases where lenticules are reused for hyperopia correction, the principle is to cryopreserve the extracted lenticule and transplant it into an appropriate patient to alter corneal thickness and refractive power, with potential as an alternative to banked corneas. However, long-term safety and efficacy still require accumulation of evidence.
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