Impact of Flap Thickness on Refractive Outcomes and Corneal Biomechanics Following Myopic Femtosecond Laser-Assisted LASIK

<b>Background/Objectives</b>: Femtosecond laser-assisted LASIK (FS-LASIK) is currently the most commonly performed procedure for the correction of myopia and myopic astigmatism. However, it inherently weakens the biomechanical integrity of the cornea due to flap creation and stromal ablation. This prospective study aimed to compare refractive and corneal biomechanical parameters after myopic FS-LASIK with different flap thicknesses and to identify parameters that may influence the change in corneal biomechanics after surgery. <b>Methods</b>: A total of 246 eyes were enrolled and divided into two groups based on flap thickness: 110 µm (n = 129) and 140 µm (n = 117). All procedures were performed using a femtosecond LDV Ziemer laser and standardized ablation profiles with similar ablation depths. Visual acuity, refractive outcomes, and corneal biomechanical parameters-corneal hysteresis (CH) and corneal resistance factor (CRF)-were assessed preoperatively and during a 6-month follow-up using the Ocular Response Analyzer (ORA). Multivariate regression analysis was used to identify predictors of biomechanical change. <b>Results</b>: The groups did not differ in preoperative values of the mean refractive spherical equivalent, keratometry, central corneal thickness, CH and CRF. At 6 months, both groups achieved comparable refractive outcomes, with no significant differences in uncorrected or corrected distance visual acuity, efficacy index and safety index. However, the thicker flap group exhibited significantly greater reductions in CH (-2.89 vs. -2.04 mmHg, <i>p</i> < 0.05) and CRF (-3.61 vs. -2.77 mmHg, <i>p</i> < 0.05), as well as greater biomechanical weakening per micron of ablation. Multivariate regression identified anterior weighted biomechanical index (AWBI) and flap thickness as the strongest predictors of CH reduction, while flap thickness, residual stromal bed thickness, ablation depth, and central corneal thickness contributed to CRF changes. <b>Conclusions</b>: While FS-LASIK with both flap thicknesses achieved equally effective visual outcomes, thicker flaps were associated with significantly greater biomechanical weakening. Flap thickness had a stronger influence on corneal biomechanics than ablation depth. These findings support consideration of flap thickness in surgical planning to optimize corneal biomechanical stability.