Incision meridian and the direction of surgically induced astigmatism after phacoemulsification: a single-surgeon vector analysis

Aims: To compare the keratometric surgically induced astigmatism (SIA) of two clear corneal incision strategies (120° oblique for monofocal cases, 180° temporal for trifocal cases) using Alpins double-angle vector analysis. Methods: In this retrospective single-surgeon study, 130 eyes of 106 patients undergoing phacoemulsification [2.4-mm incision, in-the-bag intraocular lens (IOL)] were grouped by strategy, not randomization: 120° oblique with a monofocal IOL (n=110) and 180° temporal with a trifocal IOL (n=20). SIA from anterior keratometry at 1 week and 1 month was computed by the double-angle vector method; the primary outcome was SIA magnitude at 1 month. Groups were compared with the Mann-Whitney U test, with generalized estimating equations (GEE) for inter-eye dependency. Results: Demographics, biometry and preoperative corneal cylinder [0.95±0.50 vs 1.05±0.51 diopters (D); p=0.69] were comparable. Mean SIA magnitude did not differ at 1 week (0.56±0.34 vs 0.60±0.27 D; p=0.62) or 1 month (0.48±0.30 vs 0.53±0.24 D; p=0.30); GEE adjustment for inter-eye dependency was concordant (p=0.39). The 1 month centroid was 0.24 D at 32° (oblique) and 0.17 D at 73° (temporal), in each case close to the meridian orthogonal to the incision; flattening effects were +0.24±0.35 D and +0.14±0.44 D. Components did not differ between groups (all p≥0.31). Conclusion: Both incisions produced low-magnitude keratometric SIA of approximately 0.5 D at 1 month, with no significant magnitude difference before or after adjustment for inter-eye dependency; the principal difference was directional. Each centroid lay close to the meridian orthogonal to its incision, consistent with a small flattening effect at the wound. These surgeon-specific values provide practical inputs for toric power calculation.