title: Fourier analysis of induced irregular astigmatism. Photorefractive keratectomy versus laser in situ keratomileusis in a bilateral cohort of hyperopic patients.
abstract: OBJECTIVE
  To analyze corneal topographic data by Fourier analysis to determine differences in irregular astigmatism following spherical hyperopic correction by photorefractive keratectomy (PRK) or laser in situ keratomileusis (LASIK).
  
  
  METHODS
  Department of Ophthalmology, St. Thomas' Hospital, London, United Kingdom.
  
  
  METHODS
  Thirty-six eyes of 18 patients with moderate hyperopia had LASIK in 1 eye and PRK in the other eye. The flap was cut on a nasal hinge with a Moria LSK One microkeratome. The laser was a Summit SVS Apex Plus with an optical zone of 6.5 mm and a blending zone of 1.5 mm. Corneal topographic data were acquired with a TMS-1 topographer (Computed Anatomy Inc.) preoperatively and 1, 3, 6, and 12 months postoperatively. The ASCII files containing the dioptric power values were extracted and analyzed with custom-written software to extract the Fourier harmonics.
  
  
  RESULTS
  The irregular astigmatism increased in both groups postoperatively, peaking at 3 months and then decreasing over the next 9 months. There was no statistically significant difference between the 2 groups at any time point (P<.05). The change in the topographically derived equivalent sphere showed undercorrection in both groups at all time points. Regular astigmatism showed a marginal statistically significant increase in the LASIK group at 12 months (P =.049).
  
  
  CONCLUSIONS
  Irregular astigmatism, equivalent sphere, and regular astigmatism were not significantly different in the PRK and LASIK groups during the follow-up. Based on the corneal topography, the 2 procedures induced an equal amount of irregular astigmatism.
authors.affiliation: Department of Ophthalmology, St. Thomas' Hospital, London, England, United Kingdom.
authors.forename: Andrea
authors.initials: A
authors.lastname: Sciscio
authors.forename: Christopher C
authors.initials: CC
authors.lastname: Hull
authors.forename: Chris G
authors.initials: CG
authors.lastname: Stephenson
authors.forename: Heather
authors.initials: H
authors.lastname: Baldwin
authors.forename: David
authors.initials: D
authors.lastname: O'Brart
authors.forename: John
authors.initials: J
authors.lastname: Marshall
mesh_terms: D000328:Adult; D001251:Astigmatism / Q000473:pathology / Q000503:physiopathology*; D015331:Cohort Studies; D019781:Corneal Topography; D005260:Female; D005500:Follow-Up Studies; D005583:Fourier Analysis; D006801:Humans; D006956:Hyperopia / Q000601:surgery*; D020731:Keratomileusis, Laser In Situ; D054018:Lasers, Excimer; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D018903:Photorefractive Keratectomy; D011184:Postoperative Period