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Abhijit Sinha Roy, William Dupps, Cynthia Roberts; Comparison of Biomechanical Effects of Small Incision Lenticule Extraction (SMILE) and Laser in situ Keratomileusis (LASIK): A Finite Element Analysis Study. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1633.
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© ARVO (1962-2015); The Authors (2016-present)
LASIK creates a flap in the anterior corneal stroma, followed by ablation of the exposed stroma with an excimer laser. A newer procedure, small incision lenticule extraction (SMILE), does not create a flap and uses a femtosecond laser to remove a lenticule having a shape corresponding to the programmed refractive correction. It is hypothesized that since a flap is avoided in SMILE, the mechanical stresses in a post-SMILE cornea more closely approximate those of an idealized cornea with unaltered material properties than in a comparable LASIK correction.
Finite element models of myopic surgery using patient-specific corneal geometry and an anisotropic, collagen fiber-dependent formulation were constructed for LASIK and SMILE. Surgical parameters, the magnitude of myopic correction, flap thickness in LASIK and depth of lenticule creation in SMILE were varied. Further, since the corneal stroma is generally stiffer in the anterior region, two sets of models with uniform and depth-dependent material properties were constructed. A geometry-matched model with unaltered material properties from the preoperative state but with post-operative geometry (including thickness) was used for comparing the magnitude and distribution of von Mises stresses in SMILE and LASIK models.
We observed the stress distribution in the post-SMILE simulations to be similar to that of the geometry-matched model since much of the stiffer anterior stroma is preserved. In contrast, LASIK consistently reduced the stress in the flap and increased the stress in the residual stromal bed compared to the postoperative geometry-matched model. Further, increase in the thickness of the flap or depth of lenticule creation resulted in a greater amount of increase in the stress in residual stromal bed of the LASIK model compared to the SMILE model.
SMILE may present less biomechanical risk to the residual bed of susceptible corneas than comparable corrections involving LASIK flaps. Corrections at greater stromal depths in SMILE may be possible with less relative risk of ectasia than a comparable LASIK correction.
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