Corvis ST enables in vivo assessment of the dynamic corneal deformation response and corneal biomechanical properties. Corvis ST software version 1.6r2503 was used in this study. The dynamic corneal deformation response incited by a uniaxial air pulse in Corvis ST involves the compression of the extracellular matrix (ECM) and the sliding of corneal collagen lamellae.
32 Corvis ST captures high-speed corneal deformation images by using Scheimpflug technology.
22 The air pulse emitted from the device forces the normally convex cornea inward into a state of concavity. As the cornea moves inward through the first applanation, it reaches its highest concavity. This oscillation phase is followed by the returning phase in which the cornea moves outward through a second applanation before returning to its natural convex shape. The Corvis ST outputs several intercorrelated parameters that can generate analytical problems such as multiple comparisons and multicollinearity.
22,33 We selected Corvis Biomechanical Index (CBI) and Corvis Biomechanical Factor (CBiF) as indicators of corneal biomechanical stability. CBI was calculated using a logistic regression algorithm combining different dynamic corneal response parameters obtained from Corvis ST to measure corneal biomechanical stability
34 and optimize the discrimination of keratoconus
34 and subclinical keratoconus
35 from normal corneas. However, CBI is based on a nonlinear model.
34 The recent introduction of CBiF has facilitated the linear and standardized assessment of corneal biomechanics.
36 CBiF reflects the overall biomechanical stability of the cornea in a linear manner.
36 Lower CBiF and higher CBI indicate decreased corneal biomechanical stability.
34,36 E-stage was defined by classifying CBiF into five stages (E0–E4) to rank the severity of corneal destablilization.
37 CBiF values ≥5.94 were designated as stage E0. An interval spacing of 0.58 was defined for CBiF gates of each E-stage.
37 Lower CBiF values indicated higher E-stages.
37 Biomechanically corrected IOP was calculated from finite element simulations that were least affected by corneal properties.
38 CCT was measured using Corvis ST Scheimpflug imaging, which has excellent repeatability and accuracy compared with ultrasound pachymetry.
39