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