title: Posterior corneal curvature changes after undersurface ablation of the flap and in-the-bed LASIK retreatment.
abstract: 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.
  
  
  METHODS
  Nonrandomized, comparative, interventional study.
  
  
  METHODS
  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 microm using micropachymetric optical coherence tomography, whereas with further ablation of the bed, the residual bed thickness (RBT) would have been <250 microm, or <55% of the pre-LASIK central pachymetry. In another 23 eyes, RBT allowed the planned ablation for a calculated post-retreatment RBT exceeding 250 microm, >55% of the pre-LASIK central pachymetry.
  
  
  METHODS
  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 follow-up.
  
  
  METHODS
  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 microm) than conventional enhancement eyes (353.0+/-41.5 microm) (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 > or =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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Ophthalmology, University Clinic, University of Navarra, Pamplona, Spain. mjmaldonad@unav.es
authors.forename: Miguel J
authors.initials: MJ
authors.lastname: Maldonado
authors.forename: Juan C
authors.initials: JC
authors.lastname: Nieto
authors.forename: Maite
authors.initials: M
authors.lastname: Díez-Cuenca
authors.forename: David P
authors.initials: DP
authors.lastname: Piñero
mesh_terms: D000328:Adult; D003315:Cornea / Q000000981:diagnostic imaging / Q000473:pathology*; D003319:Corneal Stroma / Q000601:surgery*; D019781:Corneal Topography; D006801:Humans; D020731:Keratomileusis, Laser In Situ / Q000379:methods*; D009216:Myopia / Q000503:physiopathology / Q000601:surgery*; D011446:Prospective Studies; D012086:Reoperation; D013524:Surgical Flaps; D041623:Tomography, Optical Coherence; D014463:Ultrasonography; D014792:Visual Acuity / Q000502:physiology