Background: It is unclear whether post-operative errors after toric intraocular\nlens implantation would be more amenable to pre-operative correction\nwith a fixed adjustment or a correction ratio that scales with the magnitude of\npre-operative astigmatism. Purpose: To investigate the effect of pre-operative\nanterior corneal astigmatism orientation on outcomes of toric intraocular\nlens implantation in a large population. Methods: A retrospective cohort\nstudy of 625 patients undergoing refractive lens exchange through a superior\nclear corneal incision with Oculentis M-Plus toric intraocular lens implantation\nat an Optical Express, Inc. located in the United Kingdom and Ireland.\nPatients were stratified by axis of astigmatism on automated keratometry as\nwith-the-rule, against-the-rule, or oblique. Analysis of visual acuity and refractive\noutcomes was performed using American National Standards Institute\n(ANSI) guidelines on astigmatic corrections with non-vector as well as\nvector analyses. Analysis was limited to one eye per patient. Results: Patients\nwho had with-the-rule (WTR) astigmatism, compared with oblique and\nagainst-the-rule (ATR), had higher vector magnitudes of surgically induced\nrefractive correction (2.89D, 2.55D, 2.42D; p < 0.001), and larger correction\nratios (1.18, 1.02, 1.07; p < 0.001). This created a change of astigmatic axis in\nthe with-the-rule cohort from WTR pre-operative astigmatism to an average\nATR post-operative astigmatism. The overcorrection in the WTR cohort averaged\n0.30D. We found that the absolute dioptric overcorrection among\nWTR patients did not differ significantly with magnitude of corneal astigmatism,\nbut the correction ratio was lower among those with higher baseline\ncylindrical errors (p = 0.93 and 0.0002, respectively). Conclusions: Refractive\nlens exchange surgery using toric intraocular lenses overcorrected patients\nwho had with-the-rule astigmatism. Degree of overcorrection did not vary\nwith severity of pre-operative astigmatism. Incorporation of axis of astigmatism\nin lens selection and reduction of astigmatic correction among\nwith-the-rule patients by an absolute value of 0.25D - 0.35D, rather than\nproportional adjustments, may reduce cylindrical over-correction.
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