Comparison of Stress Distribution Patterns in Keratoconus and Healthy Eyes

Abstract

Purpose : To compare stress distribution patterns in keratoconus (KCN) and normal (NRL) corneas and the relationships between areas of greatest curvature, thinnest pachymetry and high vs low stress.

Methods : Geometric Corneal Stress Factor (GCSF) was defined according to the hoop stress formula without intraocular pressure (IOP), R/2t, where R is radius of curvature and t is thickness. GCSF was calculated on 66 KCN eyes of 40 subjects and 310 NRL eyes of 155 subjects who were prospectively enrolled and received Pentacam Tomography (Oculus, Wetzlar, Germany). Custom software was developed and used to identify the 2mm diameter zone of greatest curvature (AvgK_Max), 2mm diameter zone of thinnest pachymetry (Pach_Min), 2mm zone of greatest GCSF (Stress_Max), 2mm zone of lowest GCSF(Stress_Min). Average maps were created for KCN and NRL cohorts with central AvgK_Max (center located within 3.0 mm diameter central zone), Inferior-Nasal AvgK_Max and Inferior-Temporal AvgK_Max. Paired T-tests were performed between locations of AvgK_Max and Stress_Max and locations of AvgK_Max and Stress_Min as well as between locations of Pach_Min and Stress_Max and Pach_Min and Stress_Min. Univariate linear regressions were performed with significance threshold of p < 0.05 for all analyses.

Results : In NRL, greatest stress is significantly associated with both thinnest pachymetry and greatest curvature, while in KCN, lowest stress is significantly associated with greatest curvature. The range of greatest stress in KCN is 6.50 – 15.27 with a mean of 8.82 +/-1.70 and the range of greatest stress in NRL is 5.85 – 8.45 with a mean of 7.07 +/-0.44. The range of lowest stress in KCN is 4.97 – 7.42 with a mean of 6.40 +/-0.54 and the range of lowest stress in NRL is 5.07 – 7.42 with a mean of 6.14+/-0.44. There is a significant difference in greatest stress between KCN and NRL, with KCN being higher than NRL. There is also a significant difference in lowest stress between KCN and NRL, with KCN being higher than NRL.

Conclusions : The hoop stress formula shows greater stress with either lower thickness or flatter curvature (greater R). In NRL, corneal stress distribution is driven by thickness, with greatest stress at lowest pachymetry and greatest curvature. However, in KCN, corneal stress distribution shifts such that it is driven by curvature with lowest stress at greatest curvature.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.