Something to SMILE about. Is small incision lenticule extraction ready to become the gold standard in laser refractive surgery? Yes

Millions of laser in situ keratomileusis (LASIK) procedures have been performed for over 30 years with consistent and impressive results, making LASIK one of the most successful surgeries and the refractive procedure of choice for millions of patients [1]. Although LASIK has led to the improvement of vision and quality of life [2] of millions of patients, this ‘ride’ has not been without problems and bumps along the way. Symptoms and sequelae associated with dry eye and the possibility of corneal ectasia have been reduced with the help of advanced technology for screening (such as Pentacam) [3]; although reduced, these risks remain [4].

Since its launch in 2008, small incision lenticule extraction (SMILE) has created a lot of interest as it promised the possibility of mid to high dioptre myopia correction without even the creation of a flap [5]. In SMILE, a small lenticule is removed from the relatively weak mid stroma, while the strongly interweaving collagen fibres of the anterior stroma that provide the corneal structural integrity remain intact [6] (Fig. 1).

Fig. 1. Surgical steps of SMILE.

A: Creation of the SMILE lenticule. B, C: Separation of lenticule from corneal stroma. D: Removal of lenticule.

Since this lack of flap creation and further ablation by the excimer laser does not disturb the epithelium, Bowman layer and anterior stroma, SMILE does not cause the structural weakening associated with LASIK and hence has a reduced risk of post-operative ectasia a dreaded complication of LASIK especially in higher corrections. SMILE has mainly, but not solely, been used in mild myopia; in fact, SMILE has received FDA approval for myopic corrections of ≤–10.00 D with or without astigmatism ≤–5.00 D. Studies with mid to long-term results on low and high myopic corrections are slowly emerging [7].

As far as the ectasia risk is concerned, in their systematic review, Moshirfar et al. [8] found that SMILE had the lower rate for post-operative ectasia at 0,011% vs LASIK at 0.09% and PRK at 0.02%.

Post-operative dry eye disease is also a major concern in LASIK. The corneal nerve plexus extends from the periphery located in the anterior third of the cornea and then radially penetrates Bowman’s layer towards the centre, where it branches vertically and horizontally between Bowman’s and the epithelium. Due to its anatomic location, this nerve plexus is particularly damaged by LASIK during the flap creation (either with microkeratome or femtosecond laser) and the use of the excimer laser. On the other hand, SMILE only minimally disrupts the corneal nerve plexus, leading to significantly less dry eye and less patient discomfort in the early and late post-operative period. In fact, a number of studies demonstrated that tear film break-up time was significantly shorter and tear osmolarity significantly higher in the LASIK group compared to SMILE and similarly, patient-reported dry eye symptoms, post-operative pain and discomfort were significantly less in SMILE compared to LASIK. In their study, Denoyer et al. [9] found that, at 6 months postoperatively, the LASIK group had a considerably higher incidence of dry eye disease than the SMILE group. Also, the SMILE group demonstrated better results in corneal nerve density, number of long fibres, and corneal nerve branching. Likewise, Xu et al. [10] found superior results in dry eye parameters in SMILE at 6 months postoperatively. In their 3-year follow-up study, Elmohamady et al. [11] found that SMILE took half the time of LASIK (3 vs 6 months) to recover from dry eye symptoms. Meta-analyses such as Kobashi et al. and Cai et al. [12, 13] have found a lower rate of dry eyes, ocular surface irritation and corneal innervation as well as faster recovery of corneal sensitivity in SMILE than in LASIK.

While in eyes with low to mid refractive corrections with SMILE, DED still occurs in the early follow-up period, this tendency is gradually resolved over the following months and usually at 12 months postoperatively, the dry eye disease metrics have returned to preoperative values [14]. When examining eyes with higher refractive corrections, the existing literature is more limited. In their study, Wang et al. [15] evaluated dry eye diseases in 90 eyes with spherical equivalent refractive error ≥−6.00. They reported that DED values after SMILE returned to normal at 12 months postoperatively. Moreover, while SMILE outperformed FS-LASIK at 6 months, this difference was nullified at 12 months.

Overall, SMILE has achieved visual outcomes that are comparable, if not better, to LASIK. Ganesh et al. in their study demonstrated that at 3 months after surgery, 96% of patients in the SMILE group achieved an uncorrected vision of 20/20 or better with 12% at 20/16 or better, compared with 88 and 4%, respectively, for femto-LASIK [16]. Also, when comparing the quality of vision, it is important to take the importance of contrast sensitivity and induced higher-order aberrations (HOA) into consideration. Ganesh et al. found SMILE superior to LASIK in regards with both contrast sensitivity and HOA. In fact, the increase of HOA was more than double in the femto-LASIK group compared to the SMILE group, leading to more glare and worse night vision. Likewise, Liu et al. [17] and Gyldenkerne et al. [18] showed that LASIK induces significantly greater rates of spherical aberrations in the 6- and 5-mm zones, respectively.

SMILE is an evolving technique and new strategies and approaches are emerging. Although SMILE has not yet been approved for the treatment of hyperopia, current results have been promising. In their study Pradhan et al. reported the visual and refractive outcomes of hyperopic SMILE at 12 months after surgery and showed promising results [19].

An additional advantage of SMILE originates from patient’s perceptions and preferences. SMILE is a minimally invasive surgery that uses a microincision of 2–3 mm to remove the lenticule which is a far cry from the 6.5 mm optical zone area in LASIK. As a result, patients are more likely to proceed with an operation that offers a much smaller surgical intervention to their cornea. The smaller incision and lack of flap also frees the patients from post-operative restrictions such as wearing protective glasses for contact sports or avoiding these types of activities all together [20].

Lastly, the advantages of SMILE extend beyond the current uses of the respective patient. Corneal lenticules taken from SMILE patients are a valuable human tissue with a vast potential for future applications. The removed lenticules have been used as a tectonic graft [21] and have recently started being used as an inlay for presbyopia correction [22].

Although the results of LASIK over the past 30 years have been impressive, the uptake of SMILE over the past 10 years has been growing rapidly. Further research is needed to study the long-term results of SMILE and improve the predictability and effectiveness of this procedure for the correction of hyperopia (Table 1).

Table 1.

Advantages of SMILE over LASIK.

Retains anterior stroma → Lower risk for ectasia

Less corneal nerve damage → Less dry eye symptoms

Less HOA, better CS → Better visual outcomes

Corneal lenticule → Use as tectonic graft

Corneal lenticule → Potential use as an inlay in hyperopia + presbyopia

Minimally invasive → More easily accepted by patients

HOA high-order aberrations, CS contrast sensitivity.

Author contributions

MT: original idea, data collection, reference check, review of the manuscript. IG: data collection, reference check, review of the manuscript. IT: reference check and review of the manuscript. NZ: reference check and review of the manuscript. SJ: image editing and review of the manuscript.

Competing interests

The authors declare no competing interests.