The Effect of Anterior Corneal Astigmatism Orientation on Toric Intraocular Lens Outcomes

Background: It is unclear whether post-operative errors after toric intraocular lens implantation would be more amenable to pre-operative correction with a fixed adjustment or a correction ratio that scales with the magnitude of pre-operative astigmatism. Purpose: To investigate the effect of pre-operative anterior corneal astigmatism orientation on outcomes of toric intraocular lens implantation in a large population. Methods: A retrospective cohort study of 625 patients undergoing refractive lens exchange through a superior clear corneal incision with Oculentis M-Plus toric intraocular lens implantation at an Optical Express, Inc. located in the United Kingdom and Ireland. Patients were stratified by axis of astigmatism on automated keratometry as with-the-rule, against-the-rule, or oblique. Analysis of visual acuity and refractive outcomes was performed using American National Standards Institute (ANSI) guidelines on astigmatic corrections with non-vector as well as vector analyses. Analysis was limited to one eye per patient. Results: Patients who had with-the-rule (WTR) astigmatism, compared with oblique and against-the-rule (ATR), had higher vector magnitudes of surgically induced refractive correction (2.89D, 2.55D, 2.42D; p < 0.001), and larger correction ratios (1.18, 1.02, 1.07; p < 0.001). This created a change of astigmatic axis in the with-the-rule cohort from WTR pre-operative astigmatism to an average ATR post-operative astigmatism. The overcorrection in the WTR cohort averaged 0.30D. We found that the absolute dioptric overcorrection among WTR patients did not differ significantly with magnitude of corneal astigmatism, but the correction ratio was lower among those with higher baseline cylindrical errors (p = 0.93 and 0.0002, respectively). Conclusions: Refractive lens exchange surgery using toric intraocular lenses overcorrected patients who had with-the-rule astigmatism. Degree of overcorrection did not vary with severity of pre-operative astigmatism. Incorporation of axis of astigmatism in lens selection and reduction of astigmatic correction among with-the-rule patients by an absolute value of 0.25D - 0.35D, rather than proportional adjustments, may reduce cylindrical over-correction.