Evaluating Corneal Biomechanics: The Stress-Strain Index
Authors: Padmanabhan, P., Lopes, B.T., Eliasy, A., Abass, A., Vinciguerra, R., Vinciguerra, P., Ambrósio Jr, R., Elsheikh, A.
Journal: Journal of Cataract & Refractive Surgery
Publication Date: Oct 2022
The SSI distribution across the groups. *Nonsignificant difference in pairwise comparisons. Box plot drawn from the first to the third quartile, central line representing the median and the whiskers representing the minimum to maximum values excluding outliers. KC = keratoconus = SSI = stress–strain index; VAE-NT = very asymmetric ectasia normal topography
Summary:
Our eyes are among the most complex organs in our bodies. The cornea, the clear outer layer at the front of the eye, is a critical component responsible for focusing light. Its biomechanical behaviour, or how it responds to stress, plays a crucial role in maintaining good vision. In our latest research, we introduce the stress-strain index (SSI), a method to evaluate the biomechanical behaviour of the cornea in healthy individuals and patients with keratoconus, a condition that causes the cornea to thin and bugles into a cone-like shape.
Our study involved 1,221 patients from clinics in Milan, Italy, and Rio de Janeiro, Brazil. We divided the patients into three groups: healthy corneas, bilateral keratoconus, and very asymmetric ectasia (when one eye has clinical ectasia and the other eye has normal topography). We analysed the SSI distribution across the different groups and assessed its correlation with age, biomechanically corrected intraocular pressure (bIOP), and central corneal thickness (CCT).
We found significant differences between healthy individuals and those with keratoconus, observing a progressive reduction in the SSI across the groups. The SSI was positively correlated with age in all groups except for the severe keratoconus subgroup. Importantly, we did not find a correlation between the SSI and bIOP or CCT in all keratoconus subgroups and very asymmetric ectasia-normal topography groups.
Our findings suggest that the stress-strain index is a promising new method to estimate the corneal material properties in healthy individuals and those with keratoconus. The SSI shows a progressive deterioration as the disease stages advance and is relatively independent of bIOP and CCT, but positively correlated with age.
This research is a significant step forward in understanding corneal biomechanics and may help clinicians better diagnose, track, and treat keratoconus and other corneal disorders in the future.