Performance of Corvis ST Parameters Including Updated Stress-Strain Index in Differentiating Between Normal, Forme-Fruste, Subclinical, and Clinical Keratoconic Eyes

Purpose This study seeks to evaluate the ability of the updated stress strain index (SSIv2) and other Corvis ST biomechanical parameters in distinguishing between keratoconus with different disease stages, and normal eyes. Design Diagnostic accuracy analysis to distinguish disease stages. Methods 1084 eyes were included and divided into groups of normal (199 eyes), forme fruste keratoconus (FFKC, 194 eyes), subclinical keratoconus (SKC, 113 eyes), mild clinical keratoconus (CKC-I, 175 eyes), moderate clinical keratoconus (CKC-II, 204 eyes) and severe clinical keratoconus (CKC-III, 199 eyes). Each eye was subjected to a Corvis ST examination to determine the central corneal thickness (CCT), biomechanically corrected intraocular pressure (bIOP), SSIv2 and other eight Corvis parameters including the SSIv1, SP-A1, A1T, ARTh, IIR, DAM, DARatio2 and CBI. The sensitivity and specificity of these parameters in diagnosing keratoconus were analyzed through receiver operating characteristic curves. Results Before and after correction for CCT and bIOP, SSIv2 and ARTh were significantly higher, and IIR and CBI were significantly lower in the normal group than in the FFKC group, SKC group and the 3 CKC groups (all P<0.05). There were also significant correlations between the values of SSIv2, ARTh, IIR, CBI and the CKC severity (all P<0.05). AUC of SSIv2 was significantly higher than all other Corvis parameters in distinguishing normal eyes from FFKC, followed by IIR, ARTh and CBI. Conclusion Corvis ST's updated SSI demonstrated superior performance in differentiating between normal and keratoconic corneas, and between corneas with different keratoconus stages. Similar, but less pronounced, performance was demonstrated by the IIR, ARTh and CBI.