Comparison of Two Cap Thickness in Small Incision Lenticule Extraction: 100μm versus 160μm

Abstract

PurposeTo compare the changes of biomechanical properties, endothelial cell density (ECD), and posterior corneal elevation (PCE) after femtosecond small incision lenticule extractions (SMILEs) with 100μm versus 160μm cap thicknesses.MethodsA total of 12 rabbits were randomly assigned into two groups of 6 each. SMILE was performed at a depth of either 160μm (160-cap group) or 100μm (100-cap group). Corneal biomechanics, PCE, ECD were evaluated pre-operatively, 1week, 1 month, 2 months, 3 months, and 4 months post-operatively by Pentacam, Corvis ST, in vivo confocal microscopy (IVCM) respectively. The Young’s modulus was obtained by strip-extensometry test 4 months after surgery.ResultsAt each time point, the second applanation time (A2T) was similar between the groups with the exception of 4 months after surgery (22.66±0.16 ms in the 160-cap group versus 21.75±0.29 ms in the 100-cap group, p = 0.004). Neither deformation amplitude (DA) nor the first applanationtime (A1T) were significantly different between the two groups. The postoperative posterior surface did not shift forward, the changes of PCE and ECD were not significantly different between the two groups at any observation time. Young’s modulus was higher in the 160-cap group than that in the 100-cap group with no statistical significance (P>0.05). Regression analyses showed that the PCE changes and Young’s modulus were not affected by cap thickness (CT) or residual stromal bed thickness (RBT) (All P>0.05).ConclusionsThe differences of corneal biomechanics, posterior surface elevation, or ECD changes were quite small when using 100μm or 160μm cap thicknesses in SMILE.

Comparison of the posterior corneal elevation and biomechanics after SMILE and LASEK for myopia: a short- and long-term observation

“…It may be easy to believe that ΔPCE or ΔPME was correlated with residual stromal thickness (RST) or AD, as Zhao et al [9] reported that RST was correlated with ΔPCE in their high-myopia group post SMILE, while in the current study, neither ΔPCE nor ΔPME had a significant correlation with RST, which is in accordance with previous studies [4,6,[21][22][23]. The contradiction may be caused by the different degrees of myopia corrected in surgery.…”

“…The possible reason for this posterior shift may vary, namely, corneal swelling [19], the decrease of the IOP, or fibrotic wound healing after surgery, causing disordered morphology and deflection of the optical paths [20]. Another research in rabbits also reported that the posterior corneal surface shifted backward 3 months post SMILE, but the mechanism remained unclear [21]. The change had little clinical significance as it was so minute and within the manufacturer's reported range of error in the Pentacam (± 5.0 μm) [19].…”

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

“…The in vivo biomechanical strength has also been examined with the Corvis ST in SMILE-treated rabbits, comparing 100-and 160-lm cap thicknesses. 22 After adjusting for intraocular pressure (IOP), the study found no significant differences in the Corvis ST parameters at 4 months, suggesting a similar biomechanical performance at 100-and 160-lm cap thickness, when tested under such circumstances. However, the study included a small sample size (n ¼ 12), which may be too small to detect a difference.…”

“…4,22,23 The anterior one-third of the corneal stroma consists of an interwoven arrangement of collagen fibers, while the collagen fibers in the posterior twothirds of the corneal stroma are arranged in distinct lamellae with a predominant vertical and horizontal arrangement. 24 The collagen arrangement seems to be responsible for the depthdependent exponential decrease in the corneal tensile strength, with the 40% anterior stroma being the strongest region, and the 60% posterior stroma being at least 50% weaker.…”

Five-years Results of Small-incision Lenticule Extraction in High Myopia

“…However, some studies have led to different conclusions. He and colleagues compared small differences in rabbit corneal biomechanics resulting from the use of 100 µm versus 160 µm cap thicknesses in SMILE [16]. When the same corneal tissue is ablated, SMILE and FS-LASIK cause no differences in corneal biomechanics [17].…”