title: Influence of Posterior Corneal Astigmatism on Total Corneal Astigmatism in Eyes With Keratoconus.
abstract: OBJECTIVE
  To measure posterior corneal astigmatism (PCA) and investigate its influence on total corneal astigmatism (TCA) in eyes with keratoconus.
  
  
  METHODS
  Keratometric astigmatism (KA), PCA, and TCA were investigated by means of a dual Scheimpflug analyzer in patients with keratoconus. Vector analysis was carried out with the Næser polar value method.
  
  
  RESULTS
  We enrolled 119 eyes. PCA magnitude averaged 0.77 ± 0.43 diopters (D) and exceeded 0.50, 1.00, and 2.00 D in 73.9%, 21.8%, and 16.8% of eyes, respectively. PCA averaged 0.95 ± 0.48, 0.55 ± 0.28, and 0.70 ± 0.35 D in eyes with with-the-rule (WTR), against-the-rule (ATR), and oblique astigmatism. The steepest posterior meridian was oriented vertically (between 61 and 119 degrees) in 55.5% of eyes, thus generating ATR astigmatism. The difference between the location of the steepest meridian of KA and that of TCA was >10 degrees in 8.4% of eyes. On average, KA overestimated TCA in eyes with WTR astigmatism by 0.16 D and underestimated TCA in eyes with ATR astigmatism by 0.22 D. The PCA power oriented along the steeper anterior corneal meridian averaged -0.83 ± 0.40, -0.40 ± 0.37, and -0.53 ± 0.43 D for WTR, ATR, and obliquely astigmatic eyes, respectively. Linear regression disclosed a statistically significant correlation (P < 0.0001, r = 0.16) between the meridional powers of TCA and PCA.
  
  
  CONCLUSIONS
  In eyes with keratoconus, PCA displays large, variable values and is correlated to TCA. The influence of PCA on TCA cannot be disregarded when planning astigmatism correction by toric intraocular lenses.
authors.affiliation: *Fondazione GB Bietti IRCCS, Rome, Italy; †Department of Ophthalmology, Regions Hospital Randers, Randers, Denmark; ‡Department of Ophthalmology, Ospedale di Stato della Repubblica di San Marino, San Marino; and §Laservision, Bologna, Italy.
authors.forename: Giacomo
authors.initials: G
authors.lastname: Savini
authors.forename: Kristian
authors.initials: K
authors.lastname: Næser
authors.forename: Domenico
authors.initials: D
authors.lastname: Schiano-Lomoriello
authors.forename: Alessandro
authors.initials: A
authors.lastname: Mularoni
mesh_terms: D000328:Adult; D001251:Astigmatism / Q000175:diagnosis / Q000503:physiopathology*; D003315:Cornea / Q000503:physiopathology*; D019781:Corneal Topography; D005260:Female; D006801:Humans; D007640:Keratoconus / Q000175:diagnosis / Q000503:physiopathology* / Q000628:therapy; D008297:Male; D008875:Middle Aged; D012189:Retrospective Studies; D055815:Young Adult