6/20/2023 0 Comments Itrace ray tracing![]() The authors received research support from Ziemer Group. Selecting toric intraocular lenses based on anterior corneal measurements could lead to overcorrection in eyes that have with-the-rule astigmatism and undercorrection in eyes that have against-the-rule astigmatism. Ignoring posterior corneal astigmatism may yield incorrect estimation of total corneal astigmatism. Anterior corneal measurements underestimated total corneal astigmatism by 0.22 180 and exceeded 0.50 D in 5% of eyes. The magnitudes of anterior and posterior corneal astigmatism were correlated when the steeper anterior meridian was aligned vertically but not when it was aligned horizontally. With increasing age, the steep anterior corneal meridian tended to change from vertical to horizontal, while the steep posterior corneal meridian did not change. The steep corneal meridian was aligned vertically (60 to 120 degrees) in 51.9% of eyes for the anterior surface and in 86.6% for the posterior surface. The mean magnitude of posterior corneal astigmatism was -0.30 diopter (D). The study analyzed 715 corneas of 435 consecutive patients. Vector analysis was used to assess the error produced by estimating total corneal astigmatism from anterior corneal measurements only. Total corneal astigmatism was calculated using ray tracing, corneal astigmatism from simulated keratometry, anterior corneal astigmatism, and posterior corneal astigmatism, and the changes with age were analyzed. Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, USA. To determine the contribution of posterior corneal astigmatism to total corneal astigmatism and the error in estimating total corneal astigmatism from anterior corneal measurements only using a dual-Scheimpflug analyzer.
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