title: Why preoperative acuity predicts postoperative acuity in wavefront-guided LASIK.
abstract: OBJECTIVE
  To critically evaluate the following clinical wisdom regarding custom (wavefront-guided) laser in situ keratomileusis (LASIK) that subjects with better-than-average best-corrected visual acuity (BCVA) before surgery have a greater risk of losing BCVA postoperatively than do subjects with worse-than-average BCVA before surgery.
  
  
  METHODS
  High contrast BCVA was measured once before and 3 months after custom LASIK in one eye of 79 subjects. Preoperative spherical equivalent refractive error ranged between -1.00 and -10.38 D. The sample was divided into one of two subsamples: eyes that had better-than-average preoperative BCVA (<-0.11 logMAR) and eyes that had average or worse-than-average preoperative BCVA (≥-0.11 logMAR). Controls were implemented for retinal magnification and for the statistical phenomenon of regression to the mean of the preoperative acuity measurement.
  
  
  RESULTS
  On average, for the entire sample, moving the correction from the spectacle plane to the corneal plane increased letter acuity 4.7% (1 letter, 0.02 logMAR). For each subsample, the percentage regression to the mean was 57.24%. After correcting for magnification effects and regression to the mean, eyes with better-than-average preoperative acuity had a small but significant gain in acuity (∼1 letter, p = 0.040) that was nearly identical to the gain for eyes with worse-than-average preoperative acuity (∼1.5 letters, p = 0.002).
  
  
  CONCLUSIONS
  Custom LASIK produced a statistically significant gain in visual acuity after correction for magnification effects. Dividing the sample into two subsamples based on preoperative acuity confirmed the common clinical observation that eyes with better-than-average acuity tend to remain the same or lose acuity, whereas eyes with worse-than-average acuity tend to gain acuity. However, when only one acuity measurement is taken at a single time point and the sample is subsampled nonrandomly, this clinical observation is due to a statistical artifact (regression to the mean) and is not attributable to the surgery.
authors.affiliation: United States Air Force, School of Aerospace Medicine, San Antonio, Texas, USA. michkoe7@yahoo.com
authors.forename: Michelle T
authors.initials: MT
authors.lastname: Aaron
authors.forename: Raymond A
authors.initials: RA
authors.lastname: Applegate
authors.forename: Jason
authors.initials: J
authors.lastname: Porter
authors.forename: Larry N
authors.initials: LN
authors.lastname: Thibos
authors.forename: Steve C
authors.initials: SC
authors.lastname: Schallhorn
authors.forename: Tyson J
authors.initials: TJ
authors.lastname: Brunstetter
authors.forename: David J
authors.initials: DJ
authors.lastname: Tanzer
mesh_terms: D019781:Corneal Topography; D005139:Eyeglasses; D006801:Humans; D020731:Keratomileusis, Laser In Situ; D008875:Middle Aged; D009216:Myopia / Q000503:physiopathology* / Q000628:therapy*; D011184:Postoperative Period; D011237:Predictive Value of Tests; D057234:Preoperative Period; D016896:Treatment Outcome; D014792:Visual Acuity; D055815:Young Adult