Stromal Curvature, Power and Corneal-Stromal Curvature Ratios From a Hybrid AS-OCT in Eyes With Keratoconus.

Background: To characterise stromal curvature and curvature ratios in keratoconus (KCN) using anterior segment OCT, and to evaluate the implications of using single-, two-, and three-surface refractive models for corneal power estimation in ectatic eyes.

Methods: Retrospective observational study. Anterior segment OCT measurements (MS-39, CSO) were analysed. Anterior, stromal, and posterior curvature radii were computed across five concentric zones (2.0-6.0 mm) using a floating best-fit sphere, and curvature ratios were subsequently derived: anterior-to-stromal (ASR), stromal-to-posterior (SPR), and anterior-to-posterior (APR). Corneal power was calculated using one-, two-, and three-surface models. KCN severity was classified according to the Belin ABC grading stage and ASR, SPR, and APR were stratified accordingly.

Results: Data from 944 keratoconic eyes were analysed. Peripheral zones (6.0 mm) exhibited reduced variability in curvature measurement compared to central zones (3.0 mm). Differences between simplified (one- and two-surface) and three-surface power models correlated moderately with increased APR and SPR values. ASR, SPR, and APR all increased progressively with advancing ABC grade.

Conclusion: In advanced keratoconus, three-surface modelling yields different corneal power estimates versus simplified models in KC; prospective outcome studies are needed to assess clinical impact. Stromal curvature and its derived ratios provide novel structural metrics that change with KCN severity. Curvature ratio increase -especially APR and SPR- reflects posterior steepening and anterior-posterior decoupling, with possible implications for staging and surgical planning.