This review article reviews the current state of the technique, updated clinical results,1 experimental studies, and, finally, presents some of the challenges that need to be addressed by new technologies.
Depending on the method used to access the lenticule, ReLEx can be split into FLEx, in which a laser-assisted in situ keratomileusis (LASIK)-like flap allows surgical removal of the lenticule, and small incision lenticule extraction (SMILE) in which a small incision (approximately 2–4 mm in length) is created for manual lenticule extraction. A blunt spatula is used to break any remaining tissue bridges after the laser treatment, and thelenticule is removed with a pair of forceps (see Figure 1). For further details on the surgical approach, please refer to Sekundo et al.,2 Shah et al.,3 and Vestergaard et al.4
In contrast to LASIK, ReLEx is a one-laser approach, where the critical laser treatment is performed on the intact cornea rather than on exposed corneal stroma. Consequently, the potential variability associated with the excimer laser photoablation is avoided. In addition, the minimally invasive SMILE treatment has several theoretical advantages over flap-based treatments, including little trauma to the corneal surface, less corneal denervation, and better biomechanical strength due to an almost intact anterior stroma. Since the first introduction of ReLEx, the repetition rate of the VisuMax laser has been increased from 200 to 500 kHz, and the settings for laser spot size, energy, and distance have been optimized, changes that may have had a significant impact on the clinical outcome after surgery. Furthermore, the flap-based FLEx represents an evolutionary step before SMILE and is today primarily used as an introductory step for new ReLEx surgeons. Due to these changes, this review focuses primarily on studies concerning SMILE.
Currently, the VisuMax allows myopic corrections up to –10 diopters (D) spherical equivalent (SE) correction, with an astigmatic component of up to 5 D. Hyperopic treatments are not available at the moment, although one study has reported on the outcome of hyperopic FLEx.5 The VisuMax laser is Conformité Européenne (CE) marked and is currently being evaluated in clinical studies for the approval of SMILE by the US Food and Drug Administration (FDA).
Overall, ReLEx has been reported to have high refractive predictability in moderate and high myopia. In the largest report to date on SMILE in 670 myopic eyes 3 months after surgery (preoperative SE refraction was –7.2 D) the mean error in SE refraction was –0.25 ± 0.44 D, with 80 % of eyes within ± 0.50 D and 94 % within ± 1.0 D.6 We recently extended this evaluation to the first 1,574 eyes 3 months after SMILE and found a similar mean error of –0.15 ± 0.50 D with 77 % of eyes within ± 0.50 D and 95 % within ± 1.0 D.7 Other reports on SMILE3,4,8,9 and 500 kHz FLEx10–14 have found similar refractive outcomes in smaller numbers of patients. Most studies have included patients with moderate and high myopia, while the refractive predictability in treatment of low myopia (less than 2 D) has not been evaluated thoroughly or compared with results after FS-LASIK or photorefractive keratectomy (PRK).
The refractive stability after SMILE has not been extensively investigated. However, in one study on 279 eyes with high myopia, refraction was found to be stable from 1 to 3 months after surgery, although a minor regression of –0.15 D was observed during the first month.4 Another study on 54 eyes found no regression during the first 6 months after surgery.8 Similarly, no regression has been found during the first 3–6 months after 500 kHz FLEx10,11,13,14 or for 1 year after 200 kHz FLEx.15,16 A prospective, randomized, paired-eye study comparing SMILE and FLEx documented no significant regression between 1 week and 6 months. The procedures were similar in terms of safety, efficacy, predictability, and stability, suggesting that the presence or absence of lifting the flap does not significantly affect these visual and refractive outcomes.17
Interestingly, the refractive predictability after SMILE has been found to be unrelated to the degree of the attempted myopic correction.6 This is in contrast to excimer-based treatments, which show decreasing precision with increasing myopic correction.18 Furthermore, other parameters including preoperative corneal power, patient age, and gender have beenfound to have limited impact on the refractive outcome after SMILE.6