title: [Changes of asphericity of posterior corneal surface after corneal refractive surgery].
abstract: OBJECTIVE
  To investigate the changes of the asphericity (Q-value) of the posterior corneal surface with different corneal diameters after corneal refractive surgery.
  
  
  METHODS
  One hundred and fifteen myopic eyes of 115 subjects undertaken corneal refractive surgery (59 LASIK and 56 EPI-LASIK) were enrolled in this prospective study. The Q-value of the posterior corneal surface for patients with different corneal diameters (6, 7, 8 and 9 mm) was measured with Pentacin pre- and post-operatively. The correlations between Q-value, Q change (ΔQ), and the mean preoperative spherical equivalent refraction (SE), central corneal thickness (CCT), central ablation depth (AD) and AD/CCT were investigated.
  
  
  RESULTS
  The mean Q-value of the posterior corneal surface in 115 preoperative eyes with corneal diameters at 6, 7, 8 and 9 mm was 0.08 ± 0.16, -0.05 ± 0.13, -0.15 ± 0.12 and -0.26 ± 0.11, respectively. The Q-value had a statistically significant positive shift 1 month after surgery in eyes with 6, 7 and 8 mm corneal diameters (LASIK: t(6mm) = -2.076, P = 0.042; t(7mm) = -2.873, P = 0.006; t(8mm) = -2.180, P = 0.033; EPI-LASIK: t(6mm) = -2.859, P = 0.006; t(7mm) = -3.901, P = 0.000; t(8mm) = -3.510, P = 0.001). The mean Q-value diminished significantly 6 months compared to 1 month after surgery, and had an obvious tendency back to its preoperative level. No statistically significant correlation between Q and SE or CCT was found in the preoperative eyes. Also, no correlation between ΔQ and SE, CCT, AD and AD/CCT was found after surgery.
  
  
  CONCLUSIONS
  The shape of the posterior corneal surface in myopic eyes was more prolate as corneal diameter increased, which was similar to the anterior corneal surface. The posterior corneal surface showed central flattening and peripheral steepening (oblate shift) at early stage either post-LASIK or post-EPI-LASIK, and then returned to its original state over time.
authors: Tianjin Eye Hospital & Eye Institute, Tianjin Medical University, Tianjin 300020, China.
authors: Jie
authors: J
authors: Hou
authors: Yan
authors: Y
authors: Wang
authors: Jing
authors: J
authors: Li
authors: Xiao-yan
authors: XY
authors: Yang
authors: Lin
authors: L
authors: Zhang