April 2011
Volume 52, Issue 14
Free
ARVO Annual Meeting Abstract  |   April 2011
The Role Of Posterior Corneal Astigmatism To Overall Corneal Astigmatism
Author Affiliations & Notes
  • Li Wang
    Cullen Eye Institute, Department of Ophthalmology, Baylor College of Medicine, Houston, Texas
  • Mariko Shirayama
    Ophthalmology, University of Tokyo, Bunkyo-ku, Japan
  • Douglas D. Koch
    Cullen Eye Institute, Department of Ophthalmology, Baylor College of Medicine, Houston, Texas
  • Footnotes
    Commercial Relationships  Li Wang, None; Mariko Shirayama, None; Douglas D. Koch, None
  • Footnotes
    Support  Research to prevent blindness
Investigative Ophthalmology & Visual Science April 2011, Vol.52, 5900. doi:
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      Li Wang, Mariko Shirayama, Douglas D. Koch; The Role Of Posterior Corneal Astigmatism To Overall Corneal Astigmatism. Invest. Ophthalmol. Vis. Sci. 2011;52(14):5900.

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      © ARVO (1962-2015); The Authors (2016-present)

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Abstract

Purpose: : To determine the effect of posterior corneal astigmatism on overall corneal astigmatism measured with the Galilei dual-Scheimpflug corneal analyzer.

Methods: : One hundred thirty-seven eyes of 81 normal subjects who had the Galilei measurements were analyzed. Four different measures of corneal astigmatism were evaluated: (1) corneal astigmatism from simulated keratometry (CA-SimK), (2) corneal astigmatism from total corneal power, which is calculated using ray tracing and combines astigmatism from the anterior and posterior corneal surfaces (CA-TCP), (3) anterior corneal astigmatism, calculated by multiplying CA-SimK by 376/337.5 (CA-front), and (4) posterior corneal astigmatism, obtained from the posterior axial map (CA-back). Using vector analysis, the differences between CA-TCP and CA-SimK were calculated, and the magnitude and meridian of CA-front and CA-back and their changes with age were also assessed.

Results: : The mean vector difference in astigmatism between CA-TCP and CA-SimK was 0.25 D @ 7 degrees, and 41% and 93% of eyes were within 0.25 D and 0.50 D, respectively. The magnitude of CA-back increased with increasing magnitude of CA-front (r=0.53, P<0.01). Vertical orientation of steep meridian (60º to 120º) was found in 57% of eyes on the anterior corneal surface and 89% on the posterior corneal surface. The steep meridian of anterior corneal surface tended to change to against-the-rule astigmatism with increasing age, while that of posterior corneal surface did not change.

Conclusions: : Our result indicates that posterior corneal astigmatism tends to compensate for astigmatism from anterior surface in about 1/2 of eyes, mostly in patients with up to age 60. Ignoring posterior corneal surface might yield an incorrect estimation of total corneal astigmatism.

Keywords: astigmatism • topography • cornea: clinical science 
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