SIA Vector Analysis and Keratometry Prediction in Temporal SICS with 350 µm Scleral Incision Depth

Abstract

Objectives The magnitude and axis of surgically induced astigmatism depend on the sclera-corneal tunnel dimensions. Often, the incision depth is subjective, and standardization is rarely emphasized. This study attempts to evaluate a fixed scleral incision depth of 350 µm at 1-week post-op. The aim is to determine the SIA vector and centroid using the Pythagorean theorem and keratometric measurements. Materials and methods This study was conducted from January to July 2025 and recruited 42 patients at a tertiary health institution. All patients received scleral incisions to a depth of 350 µm after preoperative auto-keratometry measurements. Preoperative astigmatism was classified into WTR, ATR, and oblique types. Results The overall mean age was 67.14 (± 9.23) years, with 27 (64.29%) males and 15 (35.71%) females. A positive correlation (r = 0.93, 0.69) was found for pre- and post-operative keratometry. T-test for pre- and post-operative vertical keratometry revealed statistical significance (p < 0.0009). The mean SIA magnitude and axis were 2.05 (± 1.75) D and 77.94º (± 59.68º), respectively. The Wilcoxon signed-rank test showed statistical significance for SIA magnitude (p=0.0003). The centroid was at 22.3º. A 28.57% conversion rate from ATR to WTR was observed, with 71.43% remaining unchanged postoperatively. High prediction accuracy was observed for preoperative horizontal and vertical keratometry (R2 values of 0.93 and 0.74, respectively). Discussion This work adds a unique dimension to the literature by explicitly linking a fixed tunnel depth to the SIA vector and keratometric prediction. These results provide a platform for future research exploring scleral incision depth as a determinant of astigmatic outcomes. Conclusions This study finds a statistically significant difference in vertical keratometry with moderate SIA magnitude. Astigmatic axis transition was unchanged in most patients, likely due to a standardized scleral incision depth. Centroid demonstrated a minimal SIA magnitude on horizontal keratometry. An excellent postoperative keratometry prediction was determined.