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Monday 4th November

Purpose: To determine a measure of corneal astigmatism that correlates well with manifest refractive cylinder. Methods: 486 right eyes and 485 left eyes were analysed preoperatively on the Zeiss ATLASTM 9000. Twelve right eyes and thirteen left eyes were excluded because more than 10% of the topographic data was missing due to upper lid interference. Measurements were performed between June 2009 and August 2011. Right eye and left eye data were analyzed separately to ensure that observations were independent. Using all the Placido ring data available from topography, an astigmatism value was calculated for each ring followed by a summated vector mean on the total rings resulting in a measurement known as corneal topographic astigmatism – CorT. This was then compared to the refractive cylinder measured subjectively using the ocular residual astigmatism (ORA). The spread in ORA for the CorT was then compared against other measures of corneal astigmatism: simulated keratometry (Sim K), manual keratometry (manK), corneal wavefront (CorW) and paraxial curvature matching (PCM). A study is currently in progress to determine the accuracy of anterior corneal astigmatism versus total corneal astigmatism (incorporating the posterior cornea) using the parameter of CorT and the CSO Sirius topographer. Results: CorT had a better correlation and less spread with manifest refractive cylinder than sim K, man K, CorW and PCM. CorT for the anterior corneal astigmatism versus CorT for total corneal astigmatism will be presented. Conclusion: An alternative measure of corneal astigmatism, known as CorT, corresponds more closely to the manifest refractive cylinder than other measures of corneal astigmatism.