title: Posterior corneal aberrations and their compensation effects on anterior corneal aberrations in keratoconic eyes.
abstract: OBJECTIVE
  To characterize posterior corneal aberrations in keratoconic (KC) eyes and investigate compensatory effects between anterior and posterior corneal surfaces.
  
  
  METHODS
  The corneal topography of 113 eyes (37 advanced KC, 31 moderate KC, 14 mild KC, and 31 normal eyes) was used to compute the corneal aberrations. The central 6-mm diameter of both anterior and posterior corneal topographies was decomposed into Zernike polynomials. The magnitude and the orientation of each posterior corneal aberration were calculated by vector analysis. The compensation effects between anterior and posterior corneal aberrations were also assessed quantitatively with a linear regression model.
  
  
  RESULTS
  The average higher order RMS wavefront errors for the posterior corneas were 1.04, 0.54, 0.24, and 0.19 microm in the advanced, moderate, and mild KC and normal eyes, respectively. In the advanced KC eyes, posterior corneal coma was oriented in the superior-nasal direction with a mean orientation angle of 75 degrees +/- 19 degrees OD and 78 degrees +/- 20 degrees OS. On average, 22%, 24%, and 14% of the anterior corneal coma were compensated by the posterior cornea in the advanced, moderate, and mild KC eyes, respectively. However, no significant higher order aberration (HOA) compensation effects were found in normal corneas.
  
  
  CONCLUSIONS
  Significantly larger amounts of posterior corneal aberrations and stronger compensation effects were observed in KC eyes than in normal eyes. The uncorrected posterior corneal aberration in KC eyes was substantial and degraded retinal image quality. This may explain the relatively poor visual acuity obtained in eyes with rigid gas permeable (RGP) lenses, which correct only anterior corneal aberrations.
authors.affiliation: Center for Visual Science, Department of Ophthalmology, Institute of Optics, University of Rochester, Rochester, New York 14627, USA. mchen@cvs.rochester.edu
authors.forename: Minghan
authors.initials: M
authors.lastname: Chen
authors.forename: Geunyoung
authors.initials: G
authors.lastname: Yoon
mesh_terms: D000869:Anterior Eye Segment / Q000503:physiopathology*; D019781:Corneal Topography; D006801:Humans; D007640:Keratoconus / Q000503:physiopathology*; D012029:Refraction, Ocular / Q000502:physiology; D012030:Refractive Errors / Q000503:physiopathology*