Corneal Stress Distribution Evolves from Thickness-Driven in Normal Corneas to Curvature-Driven with Progression in Keratoconus
Cynthia J. Roberts, Kayla M. Knoll, Ashraf M. Mahmoud, Andrew Hendershot, Phillip T. Yuhas
Abstract
PurposeTo introduce the novel parameter of Corneal Contribution to Stress and compare stress distribution patterns between keratoconus and normal corneas.DesignProspective, Observational, Cross-sectional StudyParticipantsThe study included 66 eyes of 40 subjects diagnosed with keratoconus (KCN) and 155 left eyes from 155 normal control (NRL) subjects.MethodsTomography was obtained to calculate the newly proposed Corneal Contribution to Stress (CCS), defined according to the hoop stress formula without intraocular pressure, R/2t, where R is radius of curvature and t is thickness. CCS maps were calculated from pachymetry and tangential curvature maps. Custom software identified the 2mm diameter zones of greatest curvature (Cspot-max), thinnest pachymetry (Pach-min), greatest stress (CCSmax), and lowest stress (CCSmin). Stress difference (CCSdiff) was calculated as CCSmax – CCSmin. Distances between Cspot-max vs Pach-min, vs CCSmax, and vs CCSmin, as well as between Pach-min vs CCSmax and vs CCSmin were calculated. T-tests were performed between cohorts and paired T-tests were performed within cohorts. Univariate linear regression analyses were performed between parameters and distances. Significance threshold was p < 0.05.Main Outcome MeasuresCorneal stress parameters, corneal features of maximum curvature, minimum thickness, and distances between corneal stress parameters and corneal features.ResultsCCSmax was significantly closer to Pach-min (0.79 ± 0.92) and Cspot-max (2.04 ± 0.85) than CCSmin (3.17 ± 0.38, 2.73 ± 1.53, respectively) in NRL, p < 0.0001, while CCSmin was significantly closer to Cspot-max (1.35 ± 1.43) than CCSmax (2.52 ± 0.72) in KCN, p < 0.0001. Cspot-max (severity) was significantly related to CCSdiff in KCN (p < 0.0001; R2 = 0.5882) with a weak relationship in NRL (p < 0.0080, R2 = 0.0451). Cspot-max was significantly related to the distance from Pach-min to CCSmax (p < 0.0001; R2 = 0.3737) without significance in NRL (p = 0.8011).ConclusionsCorneal stress is driven by thickness in NRL, with greatest stress at thinnest pachymetry and greatest curvature. However, maximum stress moves away from thinnest pachymetry with progression in KCN, and minimum stress is associated with maximum curvature. Severity in keratoconus is significantly related to greater difference between maximum and minimum stress, consistent with the biomechanical cycle of decompensation.