Objective:To analyze LASIK retreatment-induced changes in the posterior corneal curvature(PCC) with undersurface ablation of the flap(UAF) and in-the-bed techniques.Design:Nonrandomized,comparative,interventional study.Participants:Forty-six eyes with a residual spherical equivalent refraction between-0.37 and-2.75 diopters(D) and astigmatism between 0.0 and-1.25 D were included prospectively.In 23 eyes,the calculated postenhancement flap thickness exceeded 150 μ m using micropachymetric optical coherence tomography,whereas with further ablation of the bed,the residual bed thickness(RBT) would have been< 250 μ m,or< 55% of the pre-LASIK central pachymetry.In another 23 eyes,RBT allowed the planned ablation for a calculated post-retrea-tment RBT exceeding 250 μ m,>55% of the pre-LASIK central pachymetry.Intervention:Eyes with insufficient RBT for further ablation underwent UAF retreatment,whereas those with adequate RBT received conventional in-the-bed LASIK retreatment.Examinations were performed before retreatment and 3 and 6 months postoperatively.No eye was lost to followup.Main Outcome Measures:Micropachymetry,Orbscan II scanning-slit PCC data,and visual acuity(VA) .Results:The groups did not differ in age,intraocular pressure,or retreatment ablation depth,but the UAF eyes had a lower mean pre-retreatment RBT(270.7± 25.4 μ m) than conventional enhancement eyes(353.0± 41.5 μ m) (P=0.001) .Eyes undergoing UAF had no significant change in PCC,whereas eyes undergoing conventional retreatment had an increase in the posterior corneal power within the central 3-mm zone(P=0.008) 3 months after retreatment.No significant changes occurred thereafter.The amount of change in posterior corneal power within the 3 mm central zone from before to after retreatment differed significantly between the groups(mean difference,0.135 D;95% confidence interval,0.022-0.248 D;P=0.02) .No keratectasia developed clinically,and no retreated eye lost or gained ≥ 2 lines of best-corrected VA.Six months after retreatment,the efficacy and safety indices for the UAF procedure were 0.96 and 1.01,respectively,and 1 and 1.06 for conventional LASIK enhancement.Conclusion:Undersurface ablation of the flap retreatment appears to have less potential for changing the posterior corneal surface than conventional LASIK enhancement and can help reduce the likelihood of retreatment-induced keratectasia. Objective: To analyze LASIK retreatment-induced changes in the posterior corneal curvature (PCC) with undersurface ablation of the flap (UAF) and in-the-bed techniques. Designing: Nonrandomized, comparative, interventional study. Participants: Forty-six eyes with a residual spherical equivalent refraction between-0.37 and-2.75 diopters (D) and astigmatism between 0.0 and-1.25 D were included prospectively. In 23 eyes, the calculated postenhancement flap thickness exceeded 150 μm using micropachymetric optical coherence tomography, and with further ablation of the bed, the residual bed thickness (RBT) would have been <250 μm, or <55% of the pre-LASIK central pachymetry. Another 23 eyes, RBT allowed the planned ablation for a calculated post-retrea-tment RBT exceeding 250 μm,> 55% of the pre-LASIK central pachymetry. Prevention: Eyes with insufficient RBT for further ablation underwent UAF retreatment, those with adequate RBT received conventional in-the-bed LASIK retreat. Examine were performed before retreatment and 3 and 6 months postoperatively. No eye was lost to follow up. Main Outcome Measures: Micropachymetry, Orbscan II scanning-slit PCC data, and visual acuity (VA). Results: The groups did not differ in age, intraocular pressure, or retreatment ablation depth, but the UAF eyes had a lower mean pre-treatment RBT (270.7 ± 25.4 μm) than conventional enhancement eyes (353.0 ± 41.5 μm) (P = 0.001) .Eyes undergoing UAF had no significant change in PCC, while eyes undergoing conventional retreatment had an increase in the posterior corneal power within the central 3-mm zone (P = 0.008) 3 months after retreatment. No significant changes occurred after the amount of change in posterior corneal power within the 3 mm central zone from before to after retreatment differed significantly between the groups (mean difference, 0.135 D; 95% confidence interval, 0.022-0.248 D; P = 0.02) .No keratectasia developed clinically, and no retreated eye lost or increased ≥ 2 lines of best-corre cted VA.Six months after retreatment, the efficacy and safety indices for the UAF procedure were 0.96 and 1.01, respectively, and 1 and 1.06 for conventional LASIK enhancement. Conlusion: Undersurface ablation of the flap treatment appears to have less potential for changing the posterior corneal surface than conventional LASIK enhancement and can help reduce the likelihood of retreatment-induced keratectasia.