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Corneal Topographic Astigmatism (CorT) to Quantify Total Corneal Astigmatism

Noel Alpins, MD, FRANZCO, FRCOphth, FACS; James K. Y. Ong, BOptom, PhD; George Stamatelatos, BScOptom

ABSTRACT 
PURPOSE: To evaluate the performance of corneal topographic astigmatism (CorT) based on total corneal power measurements.
METHODS: Anterior, posterior, and total corneal power measurements of 526 virgin eyes obtained using the CSO Sirius tomographer (Costuzione Strumenti Oftalmici, Scandicci, Florence, Italy) were analyzed. Individual CorTs were created from each set of data. These CorTs were assessed using ocular residual astigmatism (ORA), which quantifies corneo-refractive differences. A low standard deviation of the ocular residual astigmatism (ORAsd) indicates a low variability between corneal astigmatism and refractive cylinder. A low mean of the ORA magnitude indicates a close correlation of refractive cylinder and corneal astigmatism.
RESULTS: The CorT based on total corneal power measurements had an ORAsd of 0.30 diopters (D) and a mean ORA magnitude of 0.53 D. The CorT candidates based on anterior corneal power measurements all had an ORAsd of at least 0.32 D, and the mean ORA magnitudes were all 0.64 D or greater. Both the ORAsd and mean ORA magnitude of the CorT based on total corneal power measurements were significantly less than those of the CorT based on anterior corneal power measurements (both P < .001, as estimated via bootstrapping).
CONCLUSIONS: The CorT based on total corneal power measurements corresponds better, both in variability and closeness, with manifest refractive cylinder than the CorT based on anterior corneal power measurements. This total CorT would be fundamental when planning toric intraocular lenses or limbal relaxing incisions or other corneal astigmatic surgery.
[J Refract Surg. 2015;31(3):182-186.]