Differences in Corneal Biomechanical Behavior between Normal and Keratoconic Corneas using In-vivo Optical Coherence Elastography

Abstract

Purpose : To compare spatially resolved corneal biomechanical properties in normal and keratoconic corneas (KC) of human subjects using optical coherence elastography (OCE).

Methods : A previously described OCE technique (Ford et al, JBO 2011) was implemented in a clinical prototype. The study was approved by the IRB and informed consent was obtained. Corneal displacement was achieved using a flat lens attached to a linear actuator with rapid sampling rate force transducers. A cross-correlation algorithm was applied to track frame-by-frame intrastromal speckle motion. Regional displacements for the anterior and posterior stroma were plotted in a force vs. displacement graph. A displacement/force relationship was defined by dividing the 1st order term of a linear fit of the slope for the anterior (N_a) and posterior cornea (N_p) so that K=N_a/N_p. Elastographic maps were generated based on the regional displacement/force variable in each region of the corneal stroma. K values were compared using the Mann-Whitney U test.

Results : Twenty-three eyes were analyzed overall: 14 eyes from 8 normal patients and 9 KC from 7 patients. The overall intraclass correlation was 0.91 (95% CI: 0.77-0.96), indicating good test-retest reliability. The mean K for the normal eyes group was 1.129 ± 0.067 and for the KC group was 0.988 ± 0.089 (p < 0.01). This data is presented in more detail in Table 1, along with tomographic variables for each patient. Figure 1 shows a Pentacam scan, an elastography map and the force vs. displacement graph for one normal and one KC patient.

Conclusions : OCE revealed stiffer behavior in the anterior stroma than in the posterior stroma in normal in vivo corneas. Of note, K values were significantly lower in KC and, on average, were less than 1, indicating a relative loss of native anterior stromal stiffness in the diseased corneas. Several KCs demonstrated an inversion of the normal property gradient. This finding is consistent with histopathological evidence of Bowman-level abnormalities and regional loss of interlacing fibrils in KC. OCE may be a useful tool for detecting early changes in spatial biomechanical properties in a clinical setting

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.

 

Descriptive values for curve fit and relationship between anterior and posterior stroma in normal and keratoconic corneas

 

Elastography maps and force vs. displacement graphs. Left: normal patient. Right: subject with keratoconus.