title: Calculations of actual corneal astigmatism using total corneal refractive power before and after myopic keratorefractive surgery.
abstract: OBJECTIVE
  To calculate actual corneal astigmatism using the total corneal refractive astigmatism for the 4-mm apex zone of the Pentacam (TCRP4astig) and keratometric astigmatism (Kastig) before and after photorefractive keratectomy or laser in situ keratomileusis.
  
  
  METHODS
  Uncomplicated 56 eyes after more than 6 months from the surgery were recruited by chart review. Various corneal astigmatisms were measured using the Pentacam and autokeratometer before and after surgery. Three eyes were excluded and 53 eyes of 38 subjects with with-the-rule astigmatism (WTR) were finally included. The astigmatisms were investigated using polar value analysis. When TCRP4astig was set as an actual astigmatism, the efficacy of arithmetic or coefficient adjustment of Kastig was evaluated using bivariate analysis.
  
  
  RESULTS
  The difference between the simulated keratometer astigmatism of the Pentacam (SimKastig) and Kastig was strongly correlated with the difference between TCRP4astig and Kastig. TCRP4astig was different from Kastig in magnitude rather than meridian before and after surgery; the preoperative difference was due to the posterior cornea only; however, the postoperative difference was observed in both anterior and posterior parts. For arithmetic adjustment, 0.28 D and 0.27 D were subtracted from the preoperative and postoperative magnitudes of Kastig, respectively. For coefficient adjustment, the preoperative and postoperative magnitudes of Kastig were multiplied by 0.80 and 0.66, respectively. By arithmetic or coefficient adjustment, the difference between TCRP4astig and adjusted Kastig would be less than 0.75 D in magnitude for 95% of cases.
  
  
  CONCLUSIONS
  Kastig was successfully adjusted to TCPR4astig before and after myopic keratorefractive surgery in cases of WTR. For use of TCRP4astig directly, SimKastig and Kastig should be matched.
authors.affiliation: Department of Ophthalmology, Institute of Vision Research, Eye and Ear Hospital, Severance Hospital, Yonsei University College of Medicine, Seoul, Korea.
authors.forename: Kyoung Yul
authors.initials: KY
authors.lastname: Seo
authors.affiliation: SU Yonsei Eye Clinic, Seoul, Korea.
authors.forename: Hun
authors.initials: H
authors.lastname: Yang
authors.affiliation: B&Viit bright eye center, Seoul, Korea.
authors.forename: Wook Kyum
authors.initials: WK
authors.lastname: Kim
authors.affiliation: Department of Ophthalmology, CHA Bundang Medical Center, CHA University, Seongnam, South Korea.
authors.forename: Sang Min
authors.initials: SM
authors.lastname: Nam
mesh_terms: D000328:Adult; D001251:Astigmatism / Q000175:diagnosis* / Q000601:surgery*; D003315:Cornea / Q000473:pathology* / Q000601:surgery*; D005260:Female; D006801:Humans; D020731:Keratomileusis, Laser In Situ; D054018:Lasers, Excimer; D008297:Male; D009216:Myopia / Q000175:diagnosis / Q000601:surgery; D018903:Photorefractive Keratectomy; D011184:Postoperative Period; D057234:Preoperative Period; D012029:Refraction, Ocular; D055815:Young Adult