Distribution of posterior corneal astigmatism and its effect on the calculation of Toric intraocular lens

Abstract

Objective To analysis the distribution of posterior corneal astigmatism (PA) and evaluate the influence of keratometric astigmatism (KA) and total corneal astigmatism (TCA) on the calculation of Toric intraocular lens (Toric IOL) in patients with age-related cataract (ARC) and high corneal astigmatism. Methods An observational study design was adopted.Pentacam was used to measure 200 eyes of 181 patients with ARC and KA>0.75 D in Affiliated Hospital of Nantong University from August 2016 to April 2017.KA, PA, TCA and anterior corneal astigmatism (AA) were recorded.The astigmatism magnitude and axis of PA was studied.The subjects were divided into astigmatism with the rule group, astigmatism against the rule group and oblique astigmatism group according to the axis of AA.The correlations of decomposition values between PA and AA or KA and TCA in each group were analyzed by Pearson linear correlation analysis.The difference of decomposition values between KA and TCA in each group was compared by paired sample t test.The type and axis of Toric IOL were calculated by online formula according to KA and TCA.This study followed the Declaration of Helsinki and written informed consent was obtained from each patient prior to any medical examination. Results The mean astigmatic magnitudes of PA was -0.32 D×93.1°.PA exceeded 0.5 D in 22 eyes (11%). The steepest posterior corneal meridian was vertically aligned in 163 eyes (81.5%). The decomposition value KP(0) and KP(45) of PA were positively correlated with those of AA (r=0.480, P<0.001; r=0.251, P<0.001). The mean astigmatic magnitudes of KA and TCA were 1.44 D×89.6° and 1.32 D×89.5° in astigmatism with the rule group, 1.39 D×153.4° and 1.71 D×154.4° in astigmatism against the rule group and 1.13 D×122.0° and 1.24 D×124.2° in oblique astigmatism group.53 eyes (69.7%) had KA higher than TCA in astigmatism with the rule group.82 eyes (87.3%) had KA lower than TCA in astigmatism against the rule group; 20 eyes (66.7%) had KA lower than TCA in oblique astigmatism group.There were significant differences in KP (0) between KA and TCA in different astigmatism groups (all at P<0.001). The calculated Toric IOL type were inconsistent in 85 eyes(42.5%) and the calculated axis were inconsistent in 176 eyes (88.2%). Conclusions In patients with high corneal astigmatism, the astigmatism type of PA is mostly astigmatism against the rule.Ignoring the PA can lead to deviation of Toric IOL type selection and axis placement in some patients.For patients who cannot measure PA or TCA, the type of Toric IOL should be adjusted appropriately. Key words: Cataract; Posterior corneal astigmatism; Total corneal astigmatism; Keratometric astigmatism; Toric intraocular lens