Calculating positions

The Emmetropia Verifying Optical (EVO) formula, a new intraocular lens formula, compares favourably to more conventional formulas now in use, reported Tun Kuan Yeo, Tan Tock Seng Hospital, Singapore, at the XXXV Congress of the ESCRS in Lisbon, Portugal. “EVO is based on the theory of emmetropisation. Cornea growth is mainly completed at infancy, and the majority of eye growth occurs in the posterior segment. For a specific corneal power, there is a specific axial length and effective lens position (ELP) to achieve emmetropia. Thus, as axial length differs from the emmetropic axial length, there should be a corresponding change in ELP,” he explained. Dr Yeo presented the results of a retrospective study conducted in his hospital in which 817 eyes underwent cataract surgery. Parameters were measured with the IOLMaster v5.4. Three types of IOL were used in the study: the ZCB00 (n=471), the Z9000 (n=190) and the AAB00 (n=156). The goal was to evaluate and validate the new EVO IOL formula by comparing it to existing IOL formulas: the Barrett Universal II (BUII), RBF, Haigis, Holladay I, SRK/T and Hoffer Q formulas. The EVO, BUII and RBF used ULIB or recommended lens constants, while lens constant optimisation was performed separately for each IOL for Haigis, Holladay, SRK/T and Hoffer Q formulas. Parameters included axial length, K-value and anterior chamber depth. Lens thickness was not used in this study. The EVO formula works with SRK/T ULIB A constant values. The predicted refraction for each formula was subtracted from the postoperative refraction to obtain the error in prediction. “The EVO formula is suitable for all axial lengths and Ks,” said Dr Yeo. The data indicates that it performs similarly to the BUII and RBF formulas, and is statistically better than the Haigis, Holladay I, SRK/T and Hoffer Q formulas. EVO had the lowest median absolute error (0.239) and mean absolute error (0.298), followed by BUII (0.250 and 0.302) and RBF (0.250 and 0.309). 
Spherical equivalent refractive outcomes using the EVO formula were as follows: 82.0% within 0.5D of target refraction, 94.7% within 0.75D and 98.0% within 1.00D. The formula stood out in its calculation in long (>26mm; n = 105) and short (<22.5mm; 
n = 95) eyes. “Highly myopic eyes demonstrated no significant hyperopic error compared to using the older formulas, whereas highly hyperopic eyes showed no significant myopic error,” said Dr Yeo. Tun Kuan Yeo: tun_kuan_yeo@ttsh.com.sg