Tomographic analysis of total corneal astigmatism and interzonal toric stability

To assess interzonal toric stability across four vector-change thresholds in normal, suspect, and keratoconic corneas. This retrospective study included 12,000 participants/eyes (9,580 normal, 1,696 suspect, and 724 keratoconic). Total corneal astigmatism (TCA) was reconstructed from anterior and posterior keratometry at 3-, 5-, and 7-mm zones using one central corneal thickness value per eye. Vector differences (ΔV) for the 3–5-, 5–7-, and 3–7-mm comparisons jointly defined one of eight patterns at each of four thresholds: 0.25, 0.50, 0.75, and 1.00 D. Pattern 1 indicated interzonal toric stability, whereas Patterns 2–8 indicated interzonal toric instability. Analyses included 8,047 eyes with 3-mm TCA ≥ 1.00 D; regression and modeling comparisons used 0.50 D. Across increasing thresholds, instability decreased from 63.9% to 6.1% in normal, 84.5% to 25.9% in suspect, and 99.0% to 78.8% in keratoconus. At the 0.50-D threshold, instability was observed in 26.7% of eyes for the 3–5 mm comparison, 58.4% for the 5–7 mm comparison, and 94.5% for the 3–7 mm comparison. Pattern 4 was the most frequent unstable pattern in normal eyes (13.2%), while Pattern 8 predominated in keratoconus (67.2%). At 1.00 D, Pattern 4 exceeded Pattern 8 in keratoconus (30.2% vs. 22.5%). At 0.50 D, adjusted odds ratios for instability were 3.77 (95% CI, 3.30–4.31) for suspect eyes and 33.95 (95% CI, 24.26–47.51) for keratoconus versus normal eyes. Full-thickness versus anterior-only modeling reclassified 9.9%, 15.4%, and 4.4%, respectively. Diagnostic differences persisted across thresholds, but absolute instability rates and predominant patterns depended on the thresholds. These findings describe single-examination spatial consistency, not longitudinal stability or postoperative outcomes. Prospective reproducibility and outcome studies are needed before clinical application.