title: Estimation accuracy of surgically induced astigmatism on the cornea when neglecting the posterior corneal surface measurement.
abstract: OBJECTIVE
  To evaluate the accuracy of corneal surgically induced astigmatism (SIA) estimation when neglecting the posterior corneal surface measurement.
  
  
  METHODS
  Fifty right eyes undergoing phacoemulsification were measured with a rotating Scheimpflug camera (Pentacam; Oculus Inc., Wetzlar, Germany) both before and after surgery. Clear corneal incisions with one suture were used in the phacoemulsification surgery. The keratometric corneal SIA (KSIA) was derived using the anterior corneal surface measurement and the keratometric index (1.3375) while neglecting the posterior corneal surface measurement. The Pentacam-derived total corneal SIA (PSIA) was derived by vergence tracing and polar value analysis [KP(135) and KP(180)] of the measurements on both corneal surfaces.
  
  
  RESULTS
  The mean arithmetic estimation errors of the KSIA for the PSIA were 0.16 ± 0.32 (-0.52 to 1.14) D for the KP(135), and -0.02 ± 0.30 (-0.75 to 1.29) D for the KP(180). There was a significant difference between the KP(135) components of the KSIA and PSIA. Bivariate analysis revealed a statistically significant difference between the combined means of the KSIA and PSIA. Overall, 24% had either a KP(135) component of the KSIA that differed by > 0.50 D from that of the PSIA or a KP(180) component of the KSIA that differed by > 0.50 D from that of the PSIA. The blurring strength caused by neglecting the posterior corneal measurement was > 0.50 D in 24% of eyes.
  
  
  CONCLUSIONS
  Neglecting the posterior corneal surface measurement may lead to significant deviation in the corneal SIA estimation after phacoemulsification in a proportion of eyes.
authors.affiliation: Department of Ophthalmology, Buddhist Tzu Chi General Hospital, Taichung, Taiwan.
authors.forename: Li-Sheng
authors.initials: LS
authors.lastname: Cheng
authors.forename: Ching-Yao
authors.initials: CY
authors.lastname: Tsai
authors.forename: Ray Jui-Fang
authors.initials: RJ
authors.lastname: Tsai
authors.forename: Shiow-Wen
authors.initials: SW
authors.lastname: Liou
authors.forename: Jau-Der
authors.initials: JD
authors.lastname: Ho
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D001251:Astigmatism / Q000175:diagnosis* / Q000209:etiology* / Q000517:prevention & control; D002386:Cataract; D019781:Corneal Topography / Q000295:instrumentation / Q000379:methods* / Q000592:standards*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D016343:Monitoring, Intraoperative / Q000295:instrumentation / Q000379:methods / Q000592:standards; D018918:Phacoemulsification / Q000009:adverse effects* / Q000379:methods; D011183:Postoperative Complications / Q000175:diagnosis / Q000209:etiology / Q000517:prevention & control; D011184:Postoperative Period; D011300:Preoperative Care; D015203:Reproducibility of Results; D012189:Retrospective Studies