title: Impact of Posterior Corneal Surface on Toric Intraocular Lens (IOL) Calculation.
abstract: OBJECTIVE
  To quantify the impact of posterior cornea on toric IOL calculation accuracy using Placido-topography of anterior corneal surface and Scheimpflug measurements of corneal thickness.
  
  
  METHODS
  Three-hundred seventy-nine non-selected eyes undergoing cataract surgery with non-toric intraocular lens (IOL) implantation were measured with TMS-5 (Tomey, Japan), IOLMaster (Zeiss, Germany) and Lenstar (Haag-Streit, Switzerland). Anterior, posterior and total measured corneal astigmatisms were compared with astigmatisms from postoperative refraction by calculating vector differences.
  
  
  RESULTS
  The average absolute vector difference between anterior astigmatism and total astigmatism combining the measurements of anterior and posterior cornea was only 0.3 ± 0.2 D, with a median of only 0.27 D, but a maximum of 1.5 D. Measurements of anterior cornea alone show a systematic difference from refractive cylinder of 0.3-6 D at 90, 0.38 D at 89° and 0.28 D at 91° (IOLMaster, Lenstar and anterior TMS5), whereas the total TMS5 cylinder differs on average by only 0.14D at 81° from the refractive cylinder. With-the-rule (WTR) corneal astigmatism is slightly reduced and against-the-rule (ATR) astigmatism slightly increased on average when posterior corneal surface is taken into account additionally. This could also be confirmed by the calculation of an average pachymetry of all eyes in which the thinnest central part shows an ellipsoidal shape with horizontally long axis.
  
  
  CONCLUSIONS
  Measurements of posterior cornea have on average only a small but significant impact on the outcome of toric IOL calculation, however, they are nevertheless recommended to detect outliers in which corneal irregularities (e.g. beginning keratokonus) may be overlooked.
authors.affiliation: University Eye Hospital , Mainz , Germany , and.
authors.forename: Paul-Rolf
authors.initials: PR
authors.lastname: Preussner
authors.forename: Peter
authors.initials: P
authors.lastname: Hoffmann
authors.forename: Jochen
authors.initials: J
authors.lastname: Wahl
mesh_terms: D001251:Astigmatism / Q000175:diagnosis; D001699:Biometry; D003315:Cornea / Q000473:pathology*; D063171:Corneal Pachymetry; D019781:Corneal Topography; D006801:Humans; D019654:Lens Implantation, Intraocular; D007910:Lenses, Intraocular; D055095:Optics and Photonics; D018918:Phacoemulsification; D012189:Retrospective Studies
keywords: Astigmatism; keratoconus; posterior corneal surface; toric IOL calculation; vector differences