DeflVol: a model-derived volumetric descriptor of corneal deflection at highest concavity under Corvis ST air-puff loading

Air-puff-induced corneal deformation is inherently three-dimensional, motivating a volumetric descriptor that complements conventional displacement- and area-based metrics. We developed a closed-form, axisymmetric two-sphere model to estimate corneal deflection volume (DeflVol) at the highest concavity (HC) moment from the anterior corneal curvature radius, inverse curvature radius, and deflection amplitude. The model was evaluated using paired Pentacam AXL and Corvis ST measurements from 1,351 young emmetropic or myopic participants. Geometric consistency was assessed against Corvis ST outputs not used as model inputs; a geometry-based area-matching analysis and sensitivity analyses of corneal asphericity and regular anterior toricity were also performed. Predicted deflection area closely tracked measured area (R ² = 0.99, slope = 0.96, mean bias = -0.181 mm²); the modeled overlap chord closely tracked peak distance (R ² = 0.84, slope = 0.95, mean bias = +0.008 mm). DeflVol was 9.48 ± 1.72 μL and correlated strongly with deflection amplitude (r = 0.94, p < 0.001) and deflection area (r = 0.99, p < 0.001). Predicted area was 5.5% ± 1.4% lower than measured area; in the geometry-based area-matching sensitivity analysis, the model-implied DeflVol increase was 7.8% ± 2.2%, or approximately 0.7 μL Physiologically representative asphericity altered DeflVol by less than 1% on average, in 99% of eyes. DeflVol provides a computationally transparent, model-derived three-dimensional descriptor of corneal deformation that showed strong associations with established Corvis ST metrics.