Biomechanical Evaluation Using Stress-Strain Maps Of Healthy, Keratoconus, And Progressive Kc Eyes

The aim of this study was to investigate the distribution of the stress-strain index (SSI) in the cornea in healthy and keratoconus eyes aged between 18 and 50 years. In addition, the KC group was followed at 5 and 3 months to evaluate changes in SSI maps in progressive and non-progressive eyes.

Department of Ophthalmology, University Hospital Carl Gustav Carus, TU Dresden, Dresden

This prospective observational study included subjects with unoperated healthy eyes and clinical KC, who were measured with Scheimpflug tomography (Pentacam HR) and Scheimpflug-based tonometry (Corvis ST). Elevation maps and SSI values were imported into the SSI Map software (University of Liverpool) to determine the SSI distribution. The main result was the SSI inside the cone area (SSI_in) and the SSI outside the cone area (SSI_out), which were calculated automatically. In addition, KC patients were categorized into two groups (progressive and non-progressive) based on changes in Kmax of more than 1 D. Statistical analysis was performed using ANCOVA and linear mixed models with age, IOP and minimum pachymetry (MCT) as covariates.

Ninety-six healthy eyes and 112 KC eyes were analyzed. SSI and SSI_in were statistically significantly different between healthy and KC eyes (p<0.001), while SSI_out showed no differences without adjustment for covariates (p=0.09). After adjusting for covariates, SSI_in was significantly lower in KC (0.717 (95%-CI: 0.7 - 0.74)) compared to healthy eyes (0.839 (95%-CI: 0.82 - 0.86, p<0.001)), indicating a biomechanically weaker area. SSI, SSI_in and SSI_out showed a significant relationship with the covariates, with the exception of MCT in healthy eyes. SSI_in decreased significantly in the progressive group (N=60) after 5 months and another 3 months, adjusted for MCT (p<0.05). The stable group showed no changes during the follow-up (p>1.0).

The SSI distribution, expressed as SSI maps, showed the local biomechanical weakening of the cornea in KC compared to healthy eyes by lower SSI values within the cone area. Furthermore, the progression of KC, defined by a steepening of the corneal curvature, was associated with a weakening of the tissue within the cone area and not outside the cone area, which confirmed the assumption of localized corneal disease.