Biomechanical corneal changes induced by different flap thickness created by femtosecond laser

Abstract

OBJECTIVE:To evaluate the impact of the creation of corneal flaps at different thicknesses on the biomechanical properties of swine corneas.METHOD:Twelve swine eyes were obtained to form two groups: 100 µm flap thickness and 300 µm flap thickness. Each eye was submitted to the following examinations: raster topography to investigate corneal curvature alterations, ocular response analyzer to investigate corneal hysteresis change, optical coherence tomography to measure central corneal and flap thickness and sonic wave propagation velocity as a measure of stiffness, before and immediately after flap creation. After flap amputation, surface wave velocity measurements were repeated.RESULTS:Measured flap thicknesses were statistically different for thin and thick flap groups, with an average of 108.5±6.9 and 307.8±11.5 µm respectively. Hysteresis and corneal resistance factor did not change significantly after flap creation in the thin flap group. With thicker flaps, both parameters decreased significantly from 8.0±1.0 to 5.1±1.5 mmHg and from 8.2±1.6 to 4.1±2.5 mmHg respectively. Simulated keratometry values increased in the thick flap group (from 39.5±1 D to 45.9±1.2 D) after flap creation but not in the thin flap group (from 40.6±0.6 D to 41.4±1.0 D). Regarding surface wave velocity analysis, the surgical procedures induced statistically lower results in some positions.CONCLUSION:In the experimental conditions established by this model, thicker flaps presented a greater biomechanical impact on the cornea.

Changes in Biomechanical Properties of the Cornea and Intraocular Pressure After Myopic Laser In Situ Keratomileusis Using a Femtosecond Laser for Flap Creation Determined Using Ocular Response Analyzer and Goldmann Applanation Tonometry

“…13,14 However, these previous reports did not include comparisons of GAT with other IOP measurements. Although the preoperative IOPcc, IOPg, and GAT did not differ significantly in our study, the differences in postoperative IOPcc, IOPg, and GAT were significant.…”

“…12 Recent studies describe changes in biomechanical properties of the cornea after LASIK and flap creation using femtosecond laser. 13,14 Medeiros and colleagues reported that thicker flaps show significant decreases in corneal hysteresis (CH) and corneal resistance factor (CRF), which are not observed with thinner flaps, and that neither the IOPg nor the cornealcompensated intraocular pressure (IOPcc) differ significantly between thick and thin flap creation. However, Medeiros et al 13 did not consider GAT in their comparison of IOPg and IOPcc.…”

Assessing ectasia susceptibility prior to LASIK: the role of age and residual stromal bed (RSB) in conjunction to Belin-Ambrósio deviation index (BAD-D)

“…For example, ectasia has been reported after surface ablation procedures (13) , but is much more common after LASIK, because the lamellar cut has an intrinsic biomechanical impact on the cornea (31) . Such impact varies accordingly to flap thickness (32) . Interestingly, there are reported cases of unilateral keratectasia after LASIK, while the fellow eye remained stable after photorefractive keratectomy (PRK) (33,35) .…”

CT | PDF | VIDEO Corneal Biomechanics: A Paradigm shift in Studying Corneal Pathology

“…22 Case example 1 (Figure 8) demonstrates the role of corneal biomechanics in selection of the refractive procedure • Robust screening of unsuitable patients CH and CRF are significantly reduced after flap creation, Recent Advances with a greater reduction in thicker flaps as compared to thinner flaps. 23 Early assessment of biomechanical properties may enable exclusion of candidates who would otherwise show a normal topography but are at risk of developing ectasia. Case example 2 ( Figure 9) demonstrates the role of corneal biomechanics in identifying the eligibility of a candidate for refractive surgery…”