Comparing the Performance of Stress-Strain Index Versions 1 and 2 in Normal and Keratoconus Patients

The purpose of this study was to compare the performance of the Stress-Strain Index versions 1 and 2 (SSI and SSI2) in healthy patients and patients with keratoconus (KC).

Fifty-two healthy and 104 KC eyes were examined using Scheimpflug-based tomography and air-puff tonometry (Pentacam and Corvis). Correlations between both versions of SSI and age, thinnest pachymetry, and tomographic parameters were assessed. To discriminate KC eyes from healthy eyes, receiver operating characteristic curves were generated to calculate the area under the curve (AUC) for both versions of SSI and compared with a Delong test.

Both versions of SSI showed statistical differences between the healthy group and the group of patients with KC (all P < 0.001). In the healthy group, significant correlations were found between both versions of SSI and age, K1, K2, and Kmax, between SSI and average relational thickness (ART) Max (R2 = 0.23) and between SSI2 and Belin A (R2 = 0.08). In the KC group, significant correlations were found between both versions of SSI and all tomographic parameters (all P < 0.01, except for SSI2 with age P = 0.346); the R2 values in SSI2 were consistently higher than in SSI. AUC for SSI and SSI2 when comparing normal and KC eyes was 0.772 and 0.740, respectively (P = 0.468).

Both versions of SSI correlated with tomographic parameters describing KC severity, but correlations were higher for SSI2 and were not affected by age. Both versions demonstrated the same diagnostic ability for KC.

Both versions of SSI are not interchangeable, SSI2 may be preferred to depict the corneal stiffness.