title: Accuracy of corneal astigmatism estimation by neglecting the posterior corneal surface measurement.
abstract: OBJECTIVE
  To evaluate the accuracy of corneal astigmatism estimation by neglecting the posterior corneal surface measurement.
  
  
  METHODS
  Prospective, observational study.
  
  
  METHODS
  The right eyes of 493 subjects were measured with a rotating Scheimpflug camera (Pentacam; Oculus, Wetzlar, Germany). The keratometric corneal astigmatism (KA) was obtained by using the anterior corneal surface measurement and the keratometric index (1.3375) while neglecting the posterior corneal surface measurement. The Pentacam-derived total corneal astigmatism (PA) was derived by doubled-angle vector analysis of the astigmatisms on both corneal surfaces.
  
  
  RESULTS
  The mean arithmetic and absolute estimation errors of the KA magnitude for the PA magnitude were -0.06 +/- 0.28 diopters (D) (range, -0.59 to 0.91 D) and 0.24 +/- 0.16 D (range, 0 to 0.91 D), respectively. The mean arithmetic and absolute estimation errors of the KA angle for the PA angle were -0.6 degrees +/- 12.7 degrees (range, -69.9 degrees to 83.4 degrees) and 7.4 degrees +/- 10.3 degrees (range, 0 degrees to 83.4 degrees), respectively. Among all eyes, 142 eyes (28.8%) had either a KA magnitude that differed by > 0.50 D from the PA magnitude or a KA angle that differed by > 10 degrees from the PA angle. For the 282 eyes with a KA magnitude exceeding 1.0 D (that are candidates for intraoperative correction of a preexisting astigmatism during cataract surgery), 29 eyes (10.3%) had either a KA magnitude that differed by > 0.50 D from the PA magnitude or a KA angle that differed by > 10 degrees from the PA angle.
  
  
  CONCLUSIONS
  Neglecting the posterior corneal surface measurement may lead to significant deviation in the corneal astigmatism estimation in a proportion of eyes.
authors.affiliation: Department of Ophthalmology, Taipei Medical University Hospital, Taipei, Taiwan.
authors.forename: Jau-Der
authors.initials: JD
authors.lastname: Ho
authors.forename: Ching-Yao
authors.initials: CY
authors.lastname: Tsai
authors.forename: Shiow-Wen
authors.initials: SW
authors.lastname: Liou
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001251:Astigmatism / Q000175:diagnosis*; D002648:Child; D003315:Cornea / Q000473:pathology*; D003941:Diagnostic Techniques, Ophthalmological / Q000295:instrumentation; D005260:Female; D006801:Humans; D008297:Male; D008433:Mathematics; D008875:Middle Aged; D010781:Photography / Q000379:methods; D011446:Prospective Studies; D015203:Reproducibility of Results; D055815:Young Adult