Analysis of Corneal Biomechanical Properties in Different Keratotopographic Patterns of Keratoconus.

Abstract

Purpose: To analyze the frequency of main keratotopographic patterns at the 1^st, 2^nd, and 3^rd stages of keratoconus and investigate corneal biomechanical properties across different patterns.

Methods: The study comprised two stages. The first stage was computational-experimental, where we utilized COMSOL Multiphysics® software (COMSOL AB, Stockholm, Sweden) to mathematically model corneal mechanical behavior under intraocular pressure and pulsed air jet action in both normal and keratoconic conditions. The second stage was the clinical phase, during which we retrospectively analyzed the examination results of both healthy subjects and patients with keratoconus. In total, the study included 256 eyes (256 subjects). Among them, 174 eyes (174 healthy individuals) had normal corneas with different refractions, and 82 eyes (82 individuals) had stages 1, 2, and 3 of keratoconus based on Amsler-Krumeich classification. The keratotopographic characteristics of the participants were assessed using Sirius (Schwind, Germany) and Pentacam AXL (OCULUS Optikgeräte GmbH, Germany) keratotomographs, while the corneal biomechanical properties were studied using noncontact tonometry with the Corvis ST device (OCULUS Optikgeräte GmbH, Germany). The study focused on evaluating corneal stiffness index values in the central optical zone under various keratotopographic patterns. In addition, it compared the elastic coefficient values c₁, c₂, and c₃ for the model of hyperelastic behavior of corneal material outside the keratoconus zone. Furthermore, the study examined the values of reduced stiffness zone characteristics, including the stiffness reduction factor ψ_max, effective radius R_k, and the ratio of maximum and minimum strain intensity in keratoconus of different stages.

Results: The mean age of the patients in the study was 30.16 ± 8.31 years, with 166 men and 90 women participating. The study revealed typical keratotopographic patterns in the examined keratoconus stages, as well as their occurrence frequency. In the 1^st stage of keratoconus, the pattern of asymmetric astigmatism was noted more frequently (in 75% of cases). In the 2^nd stage, the pattern with an ectasia zone in the lower cornea was observed in 80% of cases, and in the 3^rd stage, a centrally located keratotopographic ectasia pattern was found in 42% of cases. Comparative analysis showed that the greatest decrease in stiffness parameter (SpA1) and stress-strain index occurred in the central pattern (by 64% and 46%, respectively), while the least decrease was observed in local corneal radius reduction in the lower cornea (by 42% and 33%, respectively).

Conclusions: The decrease in strength properties in keratoconus occurs in a local area, the size and degree of which are determined by the disease stage. The indicated biomechanical parameters are consistent regardless of the shape and localization of keratotopographic patterns but are relevant to diagnostic specifications using the Corvis ST pneumotonometer, which assesses corneal properties in the apical zone.