Refractive Correction and Biomechanical Strength Following SMILE With a 110- or 160-μm Cap Thickness, Evaluated Ex Vivo by Inflation Test.

Purpose: To examine the refractive correction and corneal biomechanical strength after small incision lenticule extraction (SMILE) by using a 110- or 160-μm cap thickness.

Methods: Thirty-two human donor corneas were allocated into 4 groups, combining one of two cap thicknesses (110 and 160 μm) with one of two spherical corrections (-4 D and -8 D). Each cornea was mounted on an artificial anterior chamber. The chamber pressure was adjusted by an attached 8% dextran media column. The anterior and posterior sagittal 3-mm-diameter curvature (rsag3mm) and the total corneal refractive power (TCRP4mm,apex,zone) were obtained before and after SMILE at a chamber pressure of 15 or 40 mm Hg. The average changes after surgery (Δ = postoperative - preoperative) and at increased chamber pressure (δ = 40 mm Hg - 15 mm Hg) were compared.

Results: A 110-μm cap thickness caused more anterior flattening (Δr15,-8D, 1.02 ± 0.08 mm versus 0.60 ± 0.17 mm), less posterior steepening (Δr15,-8D, -0.19 ± 0.11 mm versus -0.45 ± 0.20 mm), and more myopic correction (ΔTCRP15,-8D, -6.30 ± 0.96 D versus -4.55 ± 1.66 D) than a 160-μm cap thickness for -8 D SMILE (P < 0.034), but not for -4 D SMILE (ΔTCRP15,-4D,110μm, -3.86 ± 1.31 D versus ΔTCRP15,-4D,160μm, -3.57 ± 1.27 D, P = 0.718). After SMILE, increased chamber pressure caused anterior steepening (P < 0.014), which was similar at cap thicknesses of 110 and 160 μm (δr-4D, -0.13 ± 0.14 mm versus -0.09 ± 0.05mm, P = 0.431).

Conclusions: For high myopic corrections, a 160-μm cap caused less anterior curvature flattening and more posterior steepening than a 110-μm cap, and consequently less myopic correction. The inflation test revealed a reduction in the biomechanical strength after SMILE; this was similar when using a 110- or 160-μm cap thickness.