title: Estimating residual stromal thickness before and after laser in situ keratomileusis.
abstract: OBJECTIVE
  To determine the factor(s) that influences measurement of residual stromal thickness (RST) after laser in situ keratomileusis (LASIK) surgery.
  
  
  METHODS
  Clinical office-based excimer laser refractive surgery center.
  
  
  METHODS
  In this retrospective comparative interventional case study of 6235 eyes, ultrasonic corneal pachymetry was performed immediately before and after flap creation and immediately after laser ablation in the primary procedure and after 647 enhancements. Differences in the methods for calculating RST were compared statistically.
  
  
  RESULTS
  Using the RST measured at enhancement as the actual RST, measurements of RST immediately after laser ablation underestimated residual thickness due to laser-induced stromal dehydration and microkeratome effects (P<.001). Estimates of RST using a "standard" or estimated flap thickness were less accurate predictors of residual thickness (P<.001) than use of the theoretical laser resection with a measured flap thickness (RST-4) (P =.78) or a modified flap thickness subtracted from the postoperative corneal thickness (RST-8) (P =.98), which provided the best RST estimates.
  
  
  CONCLUSIONS
  Before LASIK, the best means of estimating RST is to subtract the theoretical laser resection obtained from the laser computer and the expected flap thickness normally obtained with a given microkeratome system from the preoperative central corneal thickness. After LASIK, the most accurate means of calculating RST is to subtract the original flap thickness from the postoperative central corneal thickness.
authors.affiliation: Gordon Binder Vision Institute, San Diego, California, USA.
authors.forename: Gerald
authors.initials: G
authors.lastname: Flanagan
authors.forename: Perry S
authors.initials: PS
authors.lastname: Binder
mesh_terms: D000328:Adult; D000368:Aged; D003319:Corneal Stroma / Q000473:pathology*; D006801:Humans; D020731:Keratomileusis, Laser In Situ; D008875:Middle Aged; D008953:Models, Anatomic; D011184:Postoperative Period; D012044:Regression Analysis; D012189:Retrospective Studies; D013524:Surgical Flaps