Modification of Corneal Biomechanics in Keratoconus and Healthy Corneas in Relation to Age, IOP, and Pachymetry in a North African Population: A Case–Control Study

Aims and background:To evaluate the influence of age, sex, intraocular pressure (IOP), and central corneal thickness (CCT) on corneal biomechanical parameters, including stress and strain index version 2 (SSIv2), Corvis biomechanical index (CBI), and Corvis biomechanical factor (CBiF), in keratoconus and normal corneas in a North African population.Materials and methods: Prospective observational case-control study included 100 normal eyes and 107 keratoconus eyes.Each eye was subjected to Corvis ST examination to determine biomechanical parameters, including the new indices SSIv2 and CBiF.Spearman's correlation and a quantile regression model were used to evaluate relationships between biomechanical parameters and age, sex, IOP, and CCT.A linear regression was used to create new biomechanical parameters that are less dependent on IOP and CCT.Results: Intraocular pressure and CCT were the two parameters most strongly associated with biomechanical measures in both the normal and keratoconus groups.In the normal group, the strongest associations were observed between stiffness parameter (SP) and IOP (r = 0.634, p < 0.001), and between tomographic and biomechanical index (TBI) and CCT (r = -0.679,p < 0.001).In contrast, in the keratoconus group, the strongest associations were found between deformation amplitude (DA) and IOP (r = -0.728,p < 0.001), and between CBiF and CCT (r = -0.670,p < 0.001).Age exhibited weak correlations with a limited number of biomechanical parameters in both groups (all r < 0.294).Sex showed no significant influence on any biomechanical parameters in either group.Among the parameters studied, Ambrsio relational thickness (ARTh) and SSIv2 were the least affected by IOP and CCT, respectively.Furthermore, the corrected variables demonstrated substantially weaker correlations with IOP and CCT compared to their original version (all r < 0.244). Conclusion:Our study highlights the significant influence of age, IOP, and CCT on corneal biomechanical indices in both normal and keratoconus eyes.We propose, for the first time, a correction method for combined biomechanical indices aimed at minimizing the confounding effects of IOP and CCT, thereby enhancing their reliability in clinical assessment.Clinical significance: Intraocular pressure and CCT should be considered confounding factors in corneal biomechanical evaluations conducted in clinical practice, such as screening for refractive surgery and keratoconus.