title: [The effect of the flap thickness on forward shift of posterior corneal surface in excimer laser surgery correcting high myopia].
abstract: OBJECTIVE
  To investigate the influence of flap thickness of laser in situ keratomileusis (LASIK) and subepithelial keratomileusis on the forward shift of the posterior corneal surface in high myopia
  
  
  METHODS
  It was a case-control study. In this study, 151 right high myopic eyes of patients [spherical equivalent range - 6.00 to - 10.00 diopter, 46 subepithelial keratomileusis eyes (epithelial sheet group), 64 LASIK eyes with 110 microm flap thickness (thin flap group) and 41 LASIK eyes with 130 microm flap thickness (normal flap group)] were evaluated preoperatively and postoperatively with the Pentacam to determine the shape change in the posterior corneal surface. Changes in posterior shape were calculated by comparing refraction and elevation of posterior corneal surface in the part corresponding to the pupil center preoperatively and postoperatively.
  
  
  RESULTS
  The mean estimated residual bed thickness (RBT) of epithelial sheet eyes, thin flap eyes and normal flap eyes was (362 +/- 23), (320 +/- 25) and (300 +/- 21) microm, respectively. The RBT difference between epithelial sheet group and thin or normal flap group was statically significant (F = 85.685, P <0.001) and the difference between 2 LASIK groups was also significant (P < 0.05). There was no significant difference between three groups in ablation depth (P > 0.05). The mean posterior corneal refraction and elevation change of epithelial sheet eyes, thin flap eyes and normal flap eyes in the pupil center was (-0.12 +/- 0.14)D and (0.9 +/-1.8) microm, ( -0.20 +/- 0.22)D and (1.3 +/- 1.6) microm, (-0.21 +/- 0.18)D and (1.5 +/- 1.5) microm, respectively. A significant correlation between posterior elevation and refraction change was present in all 3 groups. The amount of posterior corneal elevation change correlated significantly with the ablation depth in thin flap group and normal flap group (r = 0.265, 0.311; P <0.05).
  
  
  CONCLUSIONS
  In treating high myopia, both subepithelial ablation and LASIK surgery induce tiny forward shift of posterior corneal surface when the RBT was thick enough. Deeper ablation or thicker corneal flap may increase the trend of forward shift of posterior corneal surface.
authors.affiliation: Department of Ophthalmology, Eye Ear Nose & Throat Hospital, Fudan University, Myopia Key Lab of Health Ministry, Shanghai, China.
authors.forename: Hao
authors.initials: H
authors.lastname: Zhou
authors.forename: Xing-Tao
authors.initials: XT
authors.lastname: Zhou
authors.forename: Ren-Yuan
authors.initials: RY
authors.lastname: Chu
authors.forename: Jin-Hui
authors.initials: JH
authors.lastname: Dai
authors.forename: Xiao-Mei
authors.initials: XM
authors.lastname: Qu
mesh_terms: D016022:Case-Control Studies; D003315:Cornea / Q000033:anatomy & histology*; D016039:Corneal Transplantation; D006801:Humans; D049008:Keratectomy, Subepithelial, Laser-Assisted; D020731:Keratomileusis, Laser In Situ; D009216:Myopia / Q000601:surgery*; D013524:Surgical Flaps