Evaluation of Stress-Strain Index (SSI2) in Healthy and Keratoconic

Eliasy, A., Lopes, B.T., Abass, A., Vinciguerra, R., Marcos, S. and Elsheikh, A.,

Investigative Ophthalmology & Visual Science

ARVO Annual Meeting Abstract, 2021

Purpose

To evaluate the performance of a new Corvis ST parameter denoted the Stress-Strain Index (SSI2) as an in-vivo measure of material stiffness of healthy and keratoconic corneas.

Methods

A large parametric study was conducted on realistic ocular finite element models. The input and output parameters were recorded in a database and used to develop an algorithm for SSI2, which involved the central corneal thickness (CCT), the intraocular pressure (IOP) along with a number of the dynamic corneal response parameters provided by the Corvis ST including the integrated inverse radius (IIR) and the Stifness Parameter (SP). The algorithm was validated using data obtained by Vincieye Clinic (Milan, Italy) for 414 healthy participants, 79 forme-fruste keratoconus (FFKC) subjects and 222 patients with confirmed keratoconus. IOP was measured with the Corvis ST (OCULUS Optikgeräte GmbH; Wetzlar, Germany), whose measurements also allowed calculation of the SSI2 values using a custom Matlab code. The results were statistically analysed and correlations were evaluated for SSI2 with CCT and age. Topography data obtained using a Pentacam (OCULUS) were also acquired for the same participants, and analysed using Topographic Keratoconus Classification (TKC) (Chen, JCRS, 2019) to categorise the eyes into healthy, FFKC, mild, moderate and advanced keratoconus. IRB approval in the form of written and informed consent was obtained to use the data in scientific research. The study followed the tenets of the Helsinki Deceleration revised in 2013.

Results

The mean SSI2 was 1.08±0.21, 1.02±0.18, 0.79±0.17, 0.70±0.21 and 0.63±0.15 in healthy participants and patients with FFKC, mild KC, moderate KC and advanced KC, respectively, showing gradual, consistent and significant decreases with keratoconus progression (p<0.05). As expected, the correlation between SSI2 and CCT was not significant in both the healthy population (p=0.552) and the KC population (p=0.205), while the correlation with age was significant in healthy eyes (p<0.05). The correlation with age in KC eyes was not significant (p=0.141) possibly due to the biomechanical changes in the tissue caused by the disease.

Conclusions

The results revealed consistent reductions in SSI2 with keratoconus progression and independence of corneal thickness. This parameter may be used for personalised medicine to optimise procedures such as corneal cross-linking.