title: Optical changes of the human cornea as a function of age.
abstract: OBJECTIVE
  To assess the changes of the surfaces and optical properties of the cornea as a function of age.
  
  
  METHODS
  The corneal shape of 407 normal eyes of 211 subjects with ages ranging from 4 to 79 years old was determined by means of Scheimpflug imaging. These data were analyzed by fitting their elevation topographies to a general surface model, which consists of a biconic plus a Zernike polynomial expansion. The analysis includes the computation of the position and orientation of the model in the three-dimensional space to determine the orientation of the optical axis and the apex coordinates.
  
  
  RESULTS
  Both average corneal surfaces show negative conic constant plus higher order aspheric terms Z(0)(4) and Z(0)(6) are significant). These surfaces are misaligned between them and with the line of sight. Such misalignment increases with age as the cornea seems to rotate as a solid body. The apex curvature and the magnitude of the conic constant along the most curved meridian increase as well, but the largest change with age correspond to the aspheric terms Z(0)(4) and Z(0)(6). As a result, the spherical aberration (SA) of the average cornea increases with age at a rate similar to the total SA of the eye.
  
  
  CONCLUSIONS
  The average corneal surfaces are misaligned general aspheres. Corneal SA is higher than total SA, but both SAs increase with age at a similar rate. This confirms that the lens is partially compensating SA and that such compensation is preserved with aging. Misalignment and solid body rotation seem to reduce astigmatism and coma for young and middle-aged corneas.
authors.affiliation: Instituto de Ciencia de Materiales de Aragón, Consejo Superior de Investigaciones Científicas & Universidad de Zaragoza, Facultad de Ciencias, Zaragoza, Spain. rafaelnb@unizar.es
authors.forename: Rafael
authors.initials: R
authors.lastname: Navarro
authors.forename: Jos J
authors.initials: JJ
authors.lastname: Rozema
authors.forename: Marie-José
authors.initials: MJ
authors.lastname: Tassignon
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000375:Aging / Q000502:physiology*; D002648:Child; D002675:Child, Preschool; D003315:Cornea / Q000033:anatomy & histology / Q000502:physiology*; D019781:Corneal Topography / Q000379:methods; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D012029:Refraction, Ocular / Q000502:physiology*; D055815:Young Adult