Fig 1.
OPE measurement setup showing a subject sitting in front of an anti-vibration table with head secured into a chin-and-head rest mounted on the table.
The ultrasound probe is mounted on a holder whose XYZ position can be adjusted by an operator. Ultrasound B-mode images and RF data of the cornea are collected along the nasal-temporal cross-section centered at the cornea apex.
Fig 2.
A. Raw and band-pass filtered cCAD curves from one OPE measurement in a KC patient showing eye motion in raw cCAD which was effectively removed by filtering. B. CAD for each measurement was calculated as the average trough (blue circle) to peak (blue triangles) distance for three automatically selected cycles.
Fig 3.
High-frequency ultrasound B-mode images of the central 5.7 mm of the cornea from a normal subject (left) and a grade-4 KC patient (right), along with their corresponding thickness and curvature maps obtained by Pentacam.
Table 1.
Summary statistics for age, IOP, OPA, Kmax, thinnest pachymetry, CCT, CAD, and CSI in 20 normal subjects (40 eyes) and 20 keratoconus patients (35 eyes).
Fig 4.
Boxplots showing the comparison of A. corneal axial displacement (CAD) and B. corneal stiffness index (CSI) in normal and keratoconus subjects. Average values for each group are marked with ×.
Fig 5.
Correlation of CAD and CSI to KC grade (N: normal).
A. CAD increased significantly at higher grade (p = 0.002). B. CSI decreased significantly at higher grade (p = 0.011). The red lines mark the average value within each grade.
Fig 6.
Both CAD (A) and CSI (B) show strong bilateral symmetry in both normal and KC eyes.
Fig 7.
Scatter plots of CAD (top), CSI (bottom) vs. Kmax, thinnest pachymetry, and IOP.
These plots indicated potential trends of correlation between CSI and Kmax or thinnest pachymetry and a significant correlation between CSI and IOP (p = 0.003) in KC eyes (red dots) but not in normal controls (blue dots). CAD was not correlated with Kmax, thinnest pachymetry, or IOP in either group.
Fig 8.
ROC curves of the OPE-measured CAD and CSI, as well as thinnest pachymetry and Kmax in all measured subjects.
Thinnest pachymetry and Kmax had an AUC greater than 0.9, while CAD and CSI had an AUC between 0.7–0.8.
Fig 9.
Comparison of stress-strain relationships in normal and KC corneas.
Within similar levels of stress induced by physiological IOP, KC corneas have lower stiffness (smaller slopes, indicated by red lines) but experiences more drastic changes in stiffness when stress/IOP changes. Normal corneas have higher stiffness but smaller changes in stiffness when stress changes.