Tomographic Study Assesses Interzonal Toric Stability Across Different Corneal Zones

A retrospective tomographic study of 12,000 participants and eyes has demonstrated that total corneal astigmatism exhibits interzonal toric instability rates that vary markedly across measurement thresholds while consistently distinguishing between normal, suspect, and keratoconic corneas. The extensive clinical dataset evaluated by the investigators comprised 9,580 normal eyes, 1,696 suspect eyes, and 724 keratoconic eyes. The primary scientific aim of the investigation was to assess interzonal toric stability across four distinct vector-change thresholds within these three defined corneal diagnostic categories. To perform these evaluations under standardized conditions, the researchers focused their core analytical procedures on a specific subpopulation of 8,047 eyes presenting with a 3-millimeter total corneal astigmatism value of 1.00 diopter or greater.

In their methodological framework, the study authors reconstructed total corneal astigmatism by incorporating both anterior and posterior keratometry measurements across 3-millimeter, 5-millimeter, and 7-millimeter optical zones. This reconstruction procedure utilized a single central corneal thickness measurement per examined eye. The researchers evaluated the spatial consistency of corneal toricity across the different zones by calculating vector differences, designated as delta V, across three specific paired interzonal comparisons. These paired comparisons comprised evaluations between the 3-millimeter and 5-millimeter zones, between the 5-millimeter and 7-millimeter zones, and between the wider 3-millimeter and 7-millimeter zones.

The vector differences across the three paired zone comparisons jointly defined one of eight distinct structural patterns at each of four predefined vector-change thresholds, specifically 0.25 diopters, 0.50 diopters, 0.75 diopters, and 1.00 diopter. Under this classification scheme, Pattern 1 served as the sole indicator of interzonal toric stability across the evaluated corneal zones. In contrast, Patterns 2 through 8 were classified as indicating varying expressions of interzonal toric instability. For subsequent multivariable regression analyses and modeling comparisons between the diagnostic groups, the authors established the 0.50-diopter threshold as the primary reference point.

The empirical results revealed that measured rates of interzonal toric instability decreased substantially as the vector-change threshold increased from 0.25 diopters to 1.00 diopter across all three cohorts. Among normal corneas, the observed rate of toric instability fell from 63.9 percent at the 0.25-diopter threshold to 6.1 percent at the 1.00-diopter threshold. In suspect corneas, interzonal instability declined from 84.5 percent at 0.25 diopters to 25.9 percent at 1.00 diopter. Within the keratoconic cohort, instability decreased from 99.0 percent at 0.25 diopters to 78.8 percent at 1.00 diopter. These findings confirmed that diagnostic differences between groups persisted across all four thresholds, even as absolute instability percentages changed considerably.

At the designated 0.50-diopter benchmark, instability rates varied widely across specific zone comparisons, registering at 26.7 percent of eyes for the 3-to-5-millimeter comparison, 58.4 percent for the 5-to-7-millimeter comparison, and 94.5 percent for the 3-to-7-millimeter comparison. Structural pattern distributions also differed markedly according to corneal diagnosis and the applied threshold cutoff. At 0.50 diopters, Pattern 4 represented the most frequent unstable pattern among normal eyes at 13.2 percent, whereas Pattern 8 predominated among keratoconic eyes at 67.2 percent. When the threshold was expanded to 1.00 diopter, the predominant pattern within the keratoconus group shifted, with Pattern 4 exceeding Pattern 8 at 30.2 percent versus 22.5 percent.

Statistical regression at the 0.50-diopter threshold demonstrated that suspect corneas had an adjusted odds ratio of 3.77 for instability, with a 95 percent confidence interval of 3.30 to 4.31, relative to normal corneas. For keratoconic eyes, the adjusted odds ratio for instability reached 33.95, with a 95 percent confidence interval of 24.26 to 47.51, compared to normal eyes. Reconstructing total corneal astigmatism through full-thickness modeling rather than anterior-only measurements resulted in reclassifying 9.9 percent of normal eyes, 15.4 percent of suspect eyes, and 4.4 percent of keratoconic eyes. The researchers emphasized that these findings characterize spatial consistency within a single examination rather than longitudinal stability over time or postoperative surgical outcomes, concluding that prospective reproducibility and clinical outcome studies are necessary prior to clinical application.

Subject of Research: Tomographic analysis of total corneal astigmatism and interzonal toric stability

Article Title: Tomographic analysis of total corneal astigmatism and interzonal toric stability

Article References: Dursun, Ö., Özer, Ö., Doğan, L., Baysal, Z., Eröz, P., & Türkmen, İ. (2026). Tomographic analysis of total corneal astigmatism and interzonal toric stability. BMC Ophthalmology. https://doi.org/10.1186/s12886-026-05522-0

Image Credits: AI Generated

DOI: 10.1186/s12886-026-05522-0

Keywords: corneal astigmatism, keratoconus, corneal tomography, toric stability, ophthalmology, keratometry, corneal thickness, vector analysis, Tomographic, analysis, total, corneal