Loss of the Steep-Axis Flattening Effect in Sutured Clear Corneal Incisions: A Single-Surgeon Vector Analysis

Purpose: Corneal incisions are conventionally assumed to flatten the incision meridian, and steep-axis incision placement has been advocated to reduce pre-existing astigmatism. We aimed to evaluate whether steep-axis clear corneal incisions actually flatten the incision meridian in a single-surgeon series in which the main incisions were closed with a suture. Methods: This retrospective study analyzed 345 right eyes that underwent phacoemulsification by a single surgeon using superior (100°) or temporal (190°) clear corneal incisions (2.20 or 2.75 mm), each closed with a single 10-0 nylon suture removed at 1 week. Surgically induced astigmatism (SIA) was calculated by vector analysis and decomposed into a flattening component on the incision meridian and a torque component on the orthogonal meridian. The main analysis was focused on steep-axis incisions, defined as a preoperative steep meridian within ±20° of the incision meridian (superior, n = 96; temporal, n = 66). Results: In steep-axis incisions, the mean flattening effect was negative rather than positive, indicating steepening rather than the intended flattening (superior -0.17 ± 0.56 D, p < 0.001; temporal -0.20 ± 0.73 D, p = 0.059). The perpendicular torque exceeded the flattening effect in all subgroups (superior 0.50 D; temporal 0.40 D), and the SIA meridian deviated from the incision meridian by 52-55° on average. The centroid SIA for the 2.75 mm superior incision was 0.50 D at 41°, consistent across astigmatism types. Individual SIA magnitude was highly variable (0-2.04 D). White-to-white corneal diameter was the only independent biometric predictor of SIA magnitude (β = -0.182, p = 0.027). Conclusions: In this single-surgeon series of sutured clear corneal incisions, steep-axis incisions did not produce the expected flattening of the incision meridian, and the SIA acted predominantly as torque with a mean steepening effect. Whether the routine suture contributes to this pattern remains to be confirmed by direct sutured-versus-sutureless comparison.