title: Minor influence of sub-bowman keratomileusis on the posterior corneal surface at early stage.
abstract: OBJECTIVE
  To evaluate the changes in posterior corneal surface (Q value and elevation) measured with Pentacam after femtosecond laser assisted Sub-Bowman Keratomileusis (SBK).
  
  
  METHODS
  The Q values and elevation of posterior corneal surface were determined in 166 myopic/myopic astigmatism eyes of 88 patients using Scheimpflug imaging (Pentacam; Oculus, Wetzlar, Germany), which estimates asphericity and elevation for several areas of cornea analysed (Q value: 6, 7, 8 and 9 mm; elevation: 2, 4, 6 and 8 mm) before 1 and 3 months after SBK. The correlations between the changes of Q value or elevation and the mean preoperative spherical equivalent (SE), central corneal thickness (CCT), central ablation depth (AD), estimated residual bed thickness (RBT) and RBT/CCT ratio were investigated.
  
  
  RESULTS
  Meridian and area differences in Q of posterior surface have been showed. Two major meridians (horizontal/vertical) present the change of significant negative direction before surgery (all p < 0.001), but the alteration of significant positive direction for all post-SBK follow-up visits (all p < 0.05). There was no statistically significant difference in posterior elevation at 2 mm zone (horizontal meridian: p = 0.439; vertical meridian: p = 0.233). Compared with preoperation, minor but significant forward displacements were found in posterior elevation at 4 and 6 mm areas of cornea analyzed (horizontal meridian: all p < 0.001; vertical meridian: p < 0.001, p = 0.024, respectively). However, posterior elevation in two meridians (horizontal/vertical) at 8 mm region was displayed significant backward shift (p = 0.001, p < 0.001, respectively). The Pearson correlation test showed no significant correlation between the changes in the Q values and elevation data in vast majority of areas of cornea analyzed and the SE, CCT, AD, RBT, and RBT/CCT ratio at 1 and 3 months after surgery (all p > 0.05).
  
  
  CONCLUSIONS
  The posterior corneal surface showed central flattening and peripheral steepening at early stage post-SBK. To combine the asphericity with the elevation of the posterior corneal surface can overall and accurately understand the posterior corneal shape and its variations after refractive surgery.
authors.affiliation: Medal Eye Institute, Chongqing, China.
authors.forename: Pisong
authors.initials: P
authors.lastname: Yan
authors.forename: Zhiyu
authors.initials: Z
authors.lastname: Du
authors.forename: Ningling
authors.initials: N
authors.lastname: Wu
authors.forename: Yu
authors.initials: Y
authors.lastname: Zhang
authors.forename: Yincong
authors.initials: Y
authors.lastname: Xu
mesh_terms: D000293:Adolescent; D001251:Astigmatism / Q000473:pathology / Q000601:surgery*; D003315:Cornea / Q000473:pathology / Q000601:surgery*; D019781:Corneal Topography / Q000379:methods*; D005260:Female; D006801:Humans; D020731:Keratomileusis, Laser In Situ / Q000379:methods*; D008297:Male; D008875:Middle Aged; D009216:Myopia / Q000473:pathology / Q000601:surgery*; D010781:Photography / Q000379:methods; D011183:Postoperative Complications / Q000473:pathology; D012189:Retrospective Studies; D055815:Young Adult