A great deal of new research has emerged during the past decade highlighting the importance of the tear film as a refractive surface. Yet, despite what we have learned about its prominence, the tear film is an often-overlooked element in the context of refractive surgery. That is not to suggest that surgeons are ignoring the role of optimizing the ocular surface prior to biometry and before performing a correction. Indeed, on the whole, surgeons do a very good job in improving the health of the cornea and ocular surface in the presurgical period. However, the degree to which some of the procedures we perform affect the biomechanical properties of the tear film may be underappreciated.
Impact on Corneal Nerves
The tear film is the first, and arguably the most important, of the refractive surfaces in the eye. Studies show that the greatest change in index of refraction is between air and the tear film, which means it is doing the most refracting of light of any of the interfaces.1 In dry eye, which is understood to be an immune-mediated inflammatory disease, chronic inflammation occurring after the release of cytokines interrupts the secretomotor nerve impulses.2 A consequence of this activity is a reduction in the quantity and/or quality of tear production.Refractive surgery inherently compounds this phenomenon in an eye with existing dry eye disease. Even in the absence of dry eye, though, transecting the corneal nerves during laser vision correction yields a transient reduction in sensory input.3 However, there may be important differences in the extent to which LASIK and SMILE procedures affect the normal nerve anatomy.
Refractive surgery inherently compounds this phenomenon in an eye with existing dry eye disease. Even in the absence of dry eye, though, transecting the corneal nerves during laser vision correction yields a transient reduction in sensory input.3 However, there may be important differences in the extent to which LASIK and SMILE procedures affect the normal nerve anatomy. The stromal nerve fiber bundles run centripetally and toward the surface, perforating Bowman’s layer and giving rise to the subbasal nerve fiber layer (Figure 1). In an eye undergoing a LASIK procedure, creating the 270° flap cuts through a large portion of the subbasal nerves, while the deeper fiber layers are further disrupted within the photoablation area (Figure 1C).
During a SMILE procedure, on the other hand, the peripheral nerve fibers are resected only where the 60° arcuate incision is placed (thick red line in Figure 1B). While some of the nerve fibers are resected if they rise superficially to perforate the Bowman layer within the area of the created and extracted refractive lenticule, other fibers that penetrate the Bowman layer outside the lenticule area are undisturbed. One conclusion from this is that because fewer nerves are resected during a SMILE procedure compared with LASIK, there are fewer nerves that need to regenerate, yielding a smaller disruption to physiologic tear production that should last for a shorter duration.
Indeed, there are now several lines of evidence supporting the proposed benefits of SMILE in this regard. A study using confocal microscopy and immunohistochemistry in a rabbit model found that the time to recovery of the length and density of corneal nerves was shorter after SMILE compared with LASIK, with evidence of recovery after SMILE as soon as 4 weeks.4 In a separate retrospective case series and study comparison, Reinstein and colleagues evaluated studies investigating corneal sensation preoperatively and at 12 months after SMILE or LASIK. In their analysis, central corneal sensitivity was reduced to a lesser extent and recovered more quickly after SMILE compared with LASIK.5 Recovery of central corneal sensitivity to baseline was reached by 6 months after SMILE. In addition, during this 6-month recovery period, corneal sensitivity was higher than after LASIK. By comparison, the time for recovery to baseline was about 12 months following LASIK. The latter is further substantiated by a literature review showing that central corneal sensitivity exhibited a smaller decrease and a faster recovery after the SMILE procedure compared with LASIK during the first 3 postoperative months.6
Corneal Biomechanics: LASIK vs SMILE
There may be additional benefits to SMILE, again related to the smaller surface incision, with regard to corneal biomechanics.7,8 In a study of ex vivo human eyes, which were subjected to a refractive correction of -10.00 D sphere and -0.75 D cylinder at 0° with a 7-mm zone, using either a 110-μm flap (LASIK) or 130-μm cap (SMILE), stress resistance was found to be better preserved with SMILE compared with LASIK (Figure 2A).7 Furthermore, in mathematical modeling, investigators found that 75% of corneal tensile strength remained after SMILE versus 54% after LASIK (Figure 2B).8
My own experience with the SMILE procedure is similar to what has been noted in the studies mentioned above. I have performed a number of cases of LASIK in one eye and SMILE in the fellow eye. Every one of those patients reported that the SMILE eye felt more comfortable after the procedure, and several patients related that they felt like nothing was done to that eye. Although this is purely anecdotal, the patient experience is nonetheless a crucial component of ensuring patients are happy with their procedure, particularly with the millennial population that is making up an ever-growing percentage of patients seeking laser vision correction.
1. Tutt R, Bradley A, Begley C, Thibos LN. Optical and visual impact of tear break-up in human eyes. Invest Ophthalmol Vis Sci. 2000;41(13):4117-4123.
2. Stern ME, Beuerman RW, Fox RI, et al. The pathology of dry eye: the interaction between the ocular surface and lacrimal glands. Cornea. 1998;17(6):584-589.
3. Belmonte C. Eye dryness sensations after refractive surgery: impaired tear secretion or phantom cornea. J Refract Surg. 2007;23:598-602.
4. Mohamed-Noriega K, Riau AK, LwinNC, et al. Early corneal nerve damage and recovery following small incision lenticule extraction (SMILE) and laser in situ keratomileusis(LASIK). Invest Ophthalmol Vis Sci. 2014;55(3):1823-1834.
5. Reinstein DZ, Archer TJ, Gobbe M, Bartoli E. Corneal sensitivity after small-incision lenticule extraction and laser in situ keratomileusis. J Cataract Refract Surg. 2015;41(8):1580-1587.
6. He M, Huang W, Zhong X. Central corneal sensitivity after small incision lenticule extraction versus femtosecond laser-assisted LASIK for myopia: a meta-analysis of comparative studies. BMC Opthalmol. 2015;24;15:141
7. Spiru B, Kling S, Hafezi F, Sekundo W. Biomechanical Properties of Human Cornea Tested by Two-Dimensional Extensiometry Ex Vivo in Fellow Eyes: Femtosecond Laser-Assisted LASIK Versus SMILE. J Refract Surg. 2018;34(6):419-423.
8. Reinstein DZ, Archer TJ, Randleman JB, Mathematical model to compare the relative tensile strength of the cornea after PRK, LASIK, and small incision lenticule extraction. J Refract Surg. 2013;29:454-460.