title: [Quantification of corneal aberrations with an aberration coefficient].
abstract: BACKGROUND
  Abnormal corneal surfaces in keratoconus, contact lens induced corneal distorsion or after refractive surgery are thoroughly described by Zernike polynoms. This study investigated such deviations from the healthy eye by a newly introduced aberration coefficient.
  
  
  METHODS
  300 healthy eyes were measured with videokeratometry. Corneal Zernike coefficients (grade 1. - 8. degrees ) were used to calculate the aberration coefficient. This number was also determined for 113 eyes with keratoconus (videokeratometric stage 1 to 3 - 4), 326 eyes of contact lens wearers (200 soft and 126 hard lenses) and 119 eyes after Lasik surgery.
  
  
  RESULTS
  In the keratoconus group the aberration coefficient was outside normal limits in 99.2 % and correlated well with the illness stage. Corneal distorsions of low extent appeared in 5 % of soft lens wearers without a difference between yearly or disposable replacement mode. The percentage of hard lens-induced distorsions was 39 % (low extent) and 5 % (considerable extent), respectively. Only 16.8 % of eyes after Lasik had a completely normal aberration coefficient; the rest lied between 1.0 - 1.5 (62.2 %), 1.6 - 2.0 (19.3 %) and > 2.0 (1.7 %).
  
  
  CONCLUSIONS
  The aberration coefficient is a helpful tool for keratoconus classification, quality control of contact lens fitting and quantification of corneal aberrations after refractive surgery. Optical aberrations of the cornea after Lasik surgery are more widespread than presumed so far and reach a considerable extent in a fifth of the operated persons.
authors.affiliation: Privatpraxis, 3600 Thun, Schweiz. e.buerki.thun@bluewin.ch
authors.forename: Ernst
authors.initials: E
authors.lastname: Bürki
mesh_terms: D003261:Contact Lenses / Q000009:adverse effects; D003316:Corneal Diseases / Q000175:diagnosis* / Q000209:etiology; D019781:Corneal Topography / Q000706:statistics & numerical data*; D006801:Humans; D007640:Keratoconus / Q000150:complications / Q000175:diagnosis; D020731:Keratomileusis, Laser In Situ; D011183:Postoperative Complications / Q000175:diagnosis; D012016:Reference Values; D012680:Sensitivity and Specificity; D014741:Video Recording / Q000706:statistics & numerical data*