title: Astigmatism induced by spherical photorefractive keratectomy corrections.
abstract: OBJECTIVE
  The purpose of the study is to evaluate the induced astigmatism after spherical photorefractive keratectomy on the Summit Omnimed (Summit Instruments, Waltham, MA) and the Nidek EC-5000 (Nidek Co. Ltd, Aichi, Japan) excimer lasers.
  
  
  METHODS
  A total of 4269 eyes of 3289 patients were treated with a 5-mm optical zone using the Summit Omnimed excimer laser and 1825 eyes of 1303 patients treated with the Nidek EC-5000 excimer laser. The final astigmatic refractive outcome was compared with the initial refraction by vector analysis (Alpin and Jaffe method).
  
  
  RESULTS
  Subjective astigmatic refraction for the Summit laser reduced from a mean of -0.39 diopter (D) +/- standard deviation (SD) 0.33 D (range, 0 to -2.50 D) to -0.33 D +/- SD 0.41 D (range, 0 to -3.00 D). Surgically induced astigmatism (SIA) had a mean of 0.42 +/- SD 0.34 D (range, 0 to 2.89 D). Mean SIA increased with increasing preoperative astigmatism by 0.60 D SIA for every 1.00 D of preoperative cylinder. For the Nidek laser, subjective astigmatic refraction changed from a mean of -0.18 D +/- SD 0.21 D (range, 0 to -1.25 D) to -0.30 D +/- SD 0.33 D (range, 0 to -3.00 D). Surgically induced astigmatism had a mean of -0.32 D +/- SD 0.29 (range, 0 to 3.05 D). Mean SIA increased with increasing preoperative astigmatism by 0.47 D SIA for every 1.00 D of preoperative cylinder.
  
  
  CONCLUSIONS
  The authors show that spherical photorefractive keratectomy corrections can induce significant astigmatic change, particularly if a large amount of preoperative astigmatism is present.
authors.affiliation: Optimax Laser Eye Clinics, Manchester, England.
authors.forename: S
authors.initials: S
authors.lastname: Shah
authors.forename: A
authors.initials: A
authors.lastname: Chatterjee
authors.forename: S J
authors.initials: SJ
authors.lastname: Doyle
authors.forename: D A
authors.initials: DA
authors.lastname: Bessant
mesh_terms: D001251:Astigmatism / Q000209:etiology* / Q000503:physiopathology; D006801:Humans; D053685:Laser Therapy / Q000009:adverse effects*; D054018:Lasers, Excimer; D018903:Photorefractive Keratectomy / Q000009:adverse effects*; D011184:Postoperative Period; D012029:Refraction, Ocular; D012086:Reoperation