Evaluation of statistical correction strategies for corneal back surface astigmatism with toric lenses: a vector analysis

Abstract

Purpose: To compare actual and formula predicted postoperative refractive astigmatism using measured posterior corneal power measurements and four different empiric posterior corneal astigmatism correction models. Setting: Tertiary Care Center. Design: Single-center retrospective consecutive case series. Methods: Using a dataset of 211 eyes before and after tIOL implantation (Hoya Vivinex), IOLMaster (IOLM) or Casia2 (CASIA) keratometric and front / back surface corneal power measurements were converted to power vector components C0 (0/90°) and C45 (45/135°). Differences between postoperative and Castrop formula predicted refraction at the corneal plane using the labelled parameters of the tIOL and the keratometric or front / back surface corneal powers were recorded as the effect of corneal back surface astigmatism (BSA). Results: Generally, the centroid of the difference shifted towards negative C0 values indicating that BSA adds some against the rule corneal astigmatism (ATR). From IOLM / CASIA keratometry, the average difference in C0 was 0.39 / 0.32 dpt. After correction with the Abulafia-Koch, Goggin, La Hood, and Castrop nomograms it was -0.18 / -0.24 dpt, 0.27 / 0.18 dpt, 0.13 / 0.08 dpt, and 0.17 / 0.10 dpt. Using corneal front / back surface data from IOLM / CASIA, the difference was 0.18 / 0.12 dpt. Conclusions: The Abulafia-Koch method over-corrected the ATR, while the Goggin, La Hood, and Castrop models slightly under-corrected ATR, and using measurements from the Casia2 tomographer seemed to produce slightly less prediction error than IOLMaster 700.