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Bastian Tonn, Thomas Kohnen, Oliver Klaproth; Impact of corneal thickness and posterior corneal curvature on total corneal astigmatism.. Invest. Ophthalmol. Vis. Sci. 2014;55(13):1556.
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© ARVO (1962-2015); The Authors (2016-present)
Description of total corneal refractive power (TCRP) astigmatism evaluated by ray-tracing using a rotating Scheimpflug camera (Pentacam HR, Oculus).
Retrospective analysis of Scheimpflug camera measurements to determine 4mm-zone TCRP astigmatism by ray tracing, corneal astigmatism from simulated keratometry (Sim-K, 15°-zone), anterior corneal astigmatism (ACA), corneal thickness (CT) and posterior corneal astigmatism (PCA). An analysis of changes with age was performed. The error produced by estimating TCRP astigmatism by using Sim-K was calculated through vector analyses.
3818 eyes of 2233 subjects were analyzed. The mean magnitude of PCA was -0.33 diopter (D). A vertically aligned steep corneal meridian (60 to 120 degrees) was found in 71.2% of eyes for ACA and in 88.8% for PCA. With increasing age, the occurrence of steep ACA meridian aligned horizontally increased from 9.1% (age 20-29yrs) to 31.8% (70-79yrs), while vertical alignment decreased from 77.4% to 48.1% and the steep PCA meridian mostly remained vertically (90.9% to 80.7%). A positive correlation between the magnitudes of ACA and PCA could be shown. The mean vector difference between TCRP and Sim-K was 0.21 at 96° and exceeded 0.50 D in 2% of eyes.
TCRP astigmatism cannot be predicted safely by anterior corneal measurements only. Neglecting posterior corneal astigmatism may conclude in visual significant overcorrection or undercorrection in some cases when e.g. toric intraocular lens implantation is performed.
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