Accuracy of corneal astigmatism correction with two Barrett Toric calculation methods

To compare the prediction error between Barrett Toric calculator and the new online AcrySof Toric calculator which incorporated Barrett astigmatism algorithm in Chinese cataract eyes with normal axial length and anterior chamber depth (ACD).

Prospective case-control study. All the cases had axial length (21-26 mm) with ACD no less than 2.4 mm. Keratometric values were measured by LenSTAR 900. The Barrett Toric calculator was used in group 1. In group 2, SRK-T formula was used to determine the spherical power of the Toric lens, and subsequent calculation of the cylinder type was performed using the new online Alcon Toric calculator. At 1 and 3mo after surgery, a comprehensive subjective optometry was performed. The predicted residual astigmatism calculated by the two calculators was compared with that obtained by postoperative refraction, and the difference was defined as the astigmatism correction error [error of refractive astigmatism (ERA)]. The error magnitude (EM) refers to the algebraic deviation of ERA, and the error vector (EV) indicates the vector deviation of ERA. The influence of the two calculation methods on the correction accuracy of toric IOL was quantitatively analyzed.

The |EM| obtained at 1mo after surgery were 0.21±0.12 D, 0.22±0.18 D in group 1 and group 2 respectively, and correspondingly turned to be 0.19±0.13 D, 0.20±0.19 D at 3mo after surgery, with no statistical difference (P=0.633, P=0.877). The vector analysis showed that |EV| values in two groups at 1mo after surgery were 0.29±0.14@105 (D@angle) and 0.35±0.20@113 (D@angle), respectively, whereas |EV| values 3mo after surgery were 0.27±0.16@86 (D@angle) and 0.32±0.23@102 (D@angle), respectively. The differences between the groups were not statistically significant (P=0.119, P=0.261).

The clinical effect of Barrett Toric calculator has a much more accurate tendency than that of new online AcrySof Toric calculator, but is not evident in cases with normal axial length and normal anterior posterior ratio.