SENSE AND SENSIBILITY OF IMPLANTING LOW POWER TORIC IOLS

To evaluate how refractive precision of low power toric IOL (less than 1.5 D) results compare to higher power torics. Special focus on measured astigmatism data versus nomograms based on keratometry.

Private eye clinic in Germany

From a previously published data set of 888 eyes implanted with Hoya Vivinex IOLs, 212 eyes were selected that had an astigmatism of 1.0 D or greater and had both full sets of IOLM700 biometry and CASIA2 anterior segment OCT data available. The predicted residual cylinder was calculated with 10 different methods based on IOLM700 keratometry, “true” Keratometry including posterior surface, CASIA2 simK (anterior curvature only) or realK (both curvatures). The difference vector between manifest postoperative cylinder and predicted cylinder was then calculated. Three subgroups were analyzed: T0-2, T3-5 and T6+ toric IOLs. In this real world setting, induced astigmatism as well as misalignments are included in the prediction error.

CASIA2 realK yielded a mean difference vector of 0.44 D overall, 0.47/0.42/0.53 in low/mid/high power TIOLs. In contrast, standard keratometry without nomograms yielded 0.62 overall, 0.77/0.54/0.64 D in subgroups, while trueK yields 0.55 overall, 0.68 / 0.48 / 0.60 D subgroups. The well known Abulafia nomogram results in 0.54 D overall, 0.59 / 0.49 / 0.67 D in subgroups. It will overcorrect keratometry by ≈ 50% in our cohort as the centroid moves from 0.38 D x 90° to 0.19 D x 0°. Other methods will be presented in detail.

Keratometry struggles with low astigmatism. CASIA2 realK (including both curvatures) yielded the best overall results in all groups. However, the centroid is shifted towards the 90° axis by ¹/₈ of a diopter indicating a slight non-corneal effect that is not accounted for. All nomograms based solely on keratometry are less precise. IOLM700 trueK proved to be superior to standard K, equal to nomograms but inferior in CASIA2 realK, especially in low cylinder powers. However, averaging realK and trueK does not improve on the centroid, but does reduce measuring noise and improves mean DV. In low cylinder powers, the diagnostics are the determining source of error, good results can only be expected with meticulous technique. Cylinder powers from 1.5 to 3.0 D will have the lowest prediction errors with all methods examined.