title: Alignment effects in videokeratography of keratoconus.
abstract: OBJECTIVE
  To determine the effects of corneal apex decentration on videokeratography in subjects with keratoconus. We controlled corneal apex decentration by first locating the cone apex position and then shifting the fixation position by known amounts relative to the videokeratograph axis.
  
  
  METHODS
  We evaluated the effect of keratoconus apex position on videokeratography by measuring 28 eyes (17 subjects) at 4 corneal positions: the vertex normal, apex, and 1 mm or 2 mm above the apex.
  
  
  RESULTS
  When compared to the apex alignment values, the radius of 1 mm (mean difference: 0.27 mm; SD +/- 0.20 mm) and 2 mm (mean difference: 0.82 mm; SD +/- 0.50 mm). The corneal toricity was greater at apex alignment than at the 1 mm (mean: -0.35 D; SD +/- 1.76 D) and 2 mm (mean: 0.62 D; SD +/- 3.23 D) decentrations.
  
  
  CONCLUSIONS
  Decentration of the corneal apex in keratoconus produces significant errors in videokeratography when using the standard alignment rather than alignment with the apex. When there is a decentered cone apex, videokeratograph power errors are greater with the standard (sagittal) power map than with the instantaneous (tangential) power map. The standard power map produces values for the apex position that are about 50% too high and values for the apex power that are from 10 to 40% too low. Most cone shapes have significant toricity that is often not detected with standard videokeratograph alignment.
authors.affiliation: School of Optometry, University of California, Berkeley 94720-2020, USA.
authors.forename: J S
authors.initials: JS
authors.lastname: Chan
authors.forename: R B
authors.initials: RB
authors.lastname: Mandell
mesh_terms: D000293:Adolescent; D000328:Adult; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D007640:Keratoconus / Q000473:pathology* / Q000503:physiopathology; D008875:Middle Aged; D009887:Ophthalmoscopy / Q000379:methods*