Abstract

PurposeTo assess depth-dependent corneal displacements in live normal subjects using optical coherence elastography (OCE).MethodsA corneal elastography method based on swept-source optical coherence tomography (OCT) was implemented in a clinical prototype. Low amplitude corneal deformation was produced during OCT imaging with a linear actuator-driven lens coupled to force transducers. A cross-correlation algorithm was applied to track frame-by-frame speckle displacement across horizontal meridian scans. Intra-measurement force and displacement data series were plotted against each other to produce local axial stiffness approximations, k, defined by the slope of a linear fit to the force/displacement data (ignoring non-axial contributions from corneal bending). Elastographic maps displaying local k values across the cornea were generated, and the ratio of mean axial stiffness approximations for adjacent anterior and posterior stromal regions, ka/kp, was calculated. Intraclass correlation coefficients (ICC) were used to estimate repeatability.ResultsSeventeen eyes (ten subjects) were included in this prospective first-in-humans translational study. The ICC was 0.84. Graphs of force vs. displacement demonstrated that, for simultaneously acquired measurements involving the same applied force, anterior stromal displacements were lower (suggesting stiffer behavior) than posterior stromal displacements. Mean ka was 0.016±0.004 g/mm and mean kp was 0.014±0.004 g/mm, giving a mean ka/kp ratio of 1.123±0.062.ConclusionOCE is a clinically feasible, non-invasive corneal biomechanical characterization method capable of resolving depth-dependent differences in corneal deformation behavior. The anterior stroma demonstrated responses consistent with stiffer properties in compression than the posterior stroma, but to a degree that varied across normal eyes. The clinical capability to measure these differences has implications for assessing the biomechanical impact of corneal refractive surgeries and for ectasia risk screening applications.

Highlights

  • The cornea is a unique tissue that effectively conserves its shape under a variety of physiological stresses while acting as a mechanical barrier to the ocular interior and maintaining transparency to visible light

  • The anterior stroma demonstrated responses consistent with stiffer properties in compression than the posterior stroma, but to a degree that varied across normal eyes

  • The cornea’s complex anisotropic composition and demonstrated heterogeneity in material properties argues for the development of measurement techniques that can resolve spatial property differences.[16]. Available devices such as the Ocular Response Analyzer (ORA, Reichert Technologies, Depew, NY, USA) and the Corvis ST (Oculus Optikgerate GmbH, Wetzlar, Germany) measure the corneal deformation response to a non-contact air puff stressor, and while both devices continue to advance our understanding of corneal biomechanical behavior in many clinical scenarios, neither is designed to assess spatially variant intracorneal property differences.[17]

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Summary

Introduction

The cornea is a unique tissue that effectively conserves its shape under a variety of physiological stresses while acting as a mechanical barrier to the ocular interior and maintaining transparency to visible light. The collagen fibrillar architecture is a major determinant of corneal cohesive and tensile strength,[3,4,5] both of which are greater in the anterior stroma where significantly more oblique collagen branching and interweaving are present.[5,6,7] The non-fibrillar matrix components contribute to the cornea’s biomechanical behavior and may play a role in ectatic diseases as well as the response to corneal crosslinking.[8, 9] Corneal hydration status impacts biomechanical property measurements,[10,11,12] as do environmental and pathological factors such as aging,[13] diabetes,[14] and eye-rubbing.[15] The cornea’s complex anisotropic composition and demonstrated heterogeneity in material properties argues for the development of measurement techniques that can resolve spatial property differences.[16] Commercially available devices such as the Ocular Response Analyzer (ORA, Reichert Technologies, Depew, NY, USA) and the Corvis ST (Oculus Optikgerate GmbH, Wetzlar, Germany) measure the corneal deformation response to a non-contact air puff stressor, and while both devices continue to advance our understanding of corneal biomechanical behavior in many clinical scenarios, neither is designed to assess spatially variant intracorneal property differences.[17]

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