New method of quantifying corneal topographic astigmatism that corresponds with manifest refractive cylinder

PURPOSE: To derive a method of quantifying corneal topographic astigmatism (CorT) that accurately represents manifest refractive cylinder. SETTING: Private practice, Melbourne, Australia. DESIGN: Retrospective study. METHOD: Axial power measurements obtained using topography in right eyes and left eyes were analyzed. For each Placido ring, an astigmatism value was calculated. The ring astigmatism values were combined via vector summation to create a new measure termed CorT. This parameter was assessed against other commonly used measures of corneal astigmatism using the ocular residual astigmatism (ORA) and its standard deviation (SD) on how closely each measure matched manifest refractive cylinder. The flat meridian of the CorT can also be used to conceptually divide the cornea into 2 hemidivisions and a Con value subsequently calculated for each hemidivision of the cornea. RESULTS: The CorT was assessed against other commonly used measures of corneal astigmatism using the ORA (0.62 diopters [D] +/- 0.33 [SD]) and had better correlation with manifest refractive cylinder than manual keratometry (K) (ORA 0.68 +/- 0.38 D), simulated K (ORA 0.70 +/- 0.35 D), corneal wavefront (ORA 0.74 +/- 0.36 D), and paraxial curvature matching (ORA 0.85 +/- 0.480). The SD of the ORA for CorT was significantly less than the other measures of astigmatism (P<.001). CONCLUSIONS: An alternative measure of corneal astigmatism, known as Con, corresponded better to manifest refractive cylinder than other commonly used measures. A hemidivisional CorT can also represent the nonorthogonal asymmetrical astigmatism in irregular corneas.