title: The effect of posterior corneal flat meridian and astigmatism amount on the total corneal astigmatism estimated from anterior corneal measurements.
abstract: BACKGROUND
  To evaluate the effects of posterior corneal astigmatism and the absolute flat meridian difference between anterior and posterior corneal surfaces (AMDAnt-Post) on the estimation of total corneal astigmatism using anterior corneal measurements (simulated keratometry [K]).
  
  
  METHODS
  Ninety-nine eyes of 99 healthy participants were enrolled. Anterior, posterior, and total mean corneal power, cylinder power, flat meridian, and vector components J0, and J45 measured by a dual Scheimpflug camera were analyzed. The correlation between the posterior corneal cylinder power, AMDAnt-Post, and the difference in the cylinder power between simulated K and total cornea (cylinder power differenceSimK-Tot) were evaluated.
  
  
  RESULTS
  The cylinder power differenceSimK-Tot was positively correlated with the posterior corneal cylinder power (rho = 0.704 and P < 0.001) and negatively correlated with AMDAnt-Post (rho = -0.717 and P < 0.001). In the multivariate linear regression analysis, anterior corneal J0 was strongly associated with the posterior corneal cylinder power and the AMDAnt-Post. When corneal J0 had a positive value, the cylinder power of simulated K tended to be larger than the total corneal cylinder power. In comparison, the opposite trend was presented in eyes with negative anterior corneal J0. When anterior corneal J0 was larger than 1.0 or smaller than -0.9, the errors from estimating the total corneal cylinder power using anterior corneal measurements tended to be larger than 0.25 D.
  
  
  CONCLUSIONS
  Posterior corneal astigmatism should be considered for more accurate corneal astigmatism predictions, especially in eyes with anterior corneal astigmatism greater than 2.0 D of with-the-rule astigmatism or greater than 1.8 D of against-the-rule astigmatism.
authors.affiliation: Department of Ophthalmology, Korea University College of Medicine, Seoul, South Korea.
authors.forename: Youngsub
authors.initials: Y
authors.lastname: Eom
authors.forename: Su-Yeon
authors.initials: SY
authors.lastname: Kang
authors.forename: Hyo Myung
authors.initials: HM
authors.lastname: Kim
authors.forename: Jong Suk
authors.initials: JS
authors.lastname: Song
mesh_terms: D000293:Adolescent; D000328:Adult; D001251:Astigmatism / Q000175:diagnosis*; D058452:Axial Length, Eye / Q000473:pathology; D003315:Cornea / Q000503:physiopathology; D003430:Cross-Sectional Studies; D004728:Endothelium, Corneal / Q000473:pathology*; D019573:Epithelium, Corneal / Q000473:pathology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies