Characterizing Corneal Biomechanical Properties Using Dynamic Optical Coherence Elastography
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Purpose: Optical coherence elastography (OCE) quantifies the tissue’s biomechanical properties through mechanical loading and imaging the tissue response using optical coherence tomography (OCT). Current techniques evaluating corneal stiffness do not account for the influence of key physiological factors on the measured corneal biomechanical properties and either require contact, or create global deformations masking the localized variations: the hallmark of corneal ectasias, e.g., keratoconus. To implement OCE in the cornea, we developed a micro air-pulse stimulator that provides non-contact, dynamic, spatially localized, tissue stimulation. This dissertation determines a) the acute effects of tissue hydration and UV riboflavin cross-linking (CXL) treatment on the corneal ultrastructure, and evaluates the corneal biomechanical properties determined using OCE due to the effect of b) hydration and CXL treatment, c) deep stromal cross-linking treatments and d) in vivo application. Methods: a) Ex vivo de-epithelialized rabbit corneas (n=11) were cross-linked instilling 0.1% riboflavin solution for 30min across the whole cornea and UV irradiation (365nm, 3mW/cm2) to only the temporal half-region for 30min while instilling riboflavin and processed for light and transmission electron microscopy. Corneal thickness and collagen fibril separation computed as the average radial inter-fibrillar distance from the sampled fibril cross-sectional electron micrographs were recorded. b) OCE imaging was performed using phase-sensitive OCT imaging to quantify the tissue deformation dynamics resulting from a spatially discrete, low-force air-pulse (150μm spot size; 0.8ms duration; <10Pa (<0.08mmHg)). The time-dependent surface deformation is characterized by a viscoelastic tissue recovery response, quantified by an exponential decay constant—relaxation rate (RR). Higher RR is consistent with increased stiffness. Hydration influence was determined (n=10) instilling 0.9% saline every 5min for 60min and 20% dextran for another 60min. Measurements were made every 20min to determine central corneal thickness (CCT) and RR. Hydration and CXL effects were determined by obtaining OCE measurements on cross-linked corneas using isotonic (n=6) and hypertonic (n=7) riboflavin. c) OCE measurements were performed (n=10) at: the de-epithelialized stromal surface, 2/3rd corneal depth post-trephination, and after deep stromal cross-linking treatment. Rose bengal green light cross-linking (RGX) using 0.1% rose bengal solution for 20min (n=5) and 10min green light irradiation (565nm, 0.25W/cm2) and CXL treatment (n=5) was performed in the deep stroma. d) In vivo OCE was performed on anesthetized Dutch belted rabbits (n=20) recording within-session (IOP: 10, 20, 30, 40mmHg) and between sessions RR measurements before and after animal re-positioning (10mmHg). Results: a) Corneal thickness decreased significantly (−56%) after CXL treatment. Anterior collagen fibril spacing decreased significantly in the paired CXL treated region (−23%) showing that acute CXL treatment-induced changes are not only tonicity-driven. b) Corneal thickness was positively correlated (R2=0.9) with stiffness. CXL treatment using isotonic riboflavin (CCT: −1%) produced stiffer corneas (higher RR: +10%). However, CXL treatment using hypertonic riboflavin (reduced CCT: −31%) produced a tonicity-driven stiffness decrease that offset the expected stiffer material properties due to CXL treatment, resulting in no significant change in corneal material properties (RR: +6%). c) Deep stromal RGX (RR: +22%) and CXL (RR: +44%) treatments showed significantly stiffer corneas. d) In vivo RR showed excellent measurement precision for within and between session measures. Conclusion: OCE is a promising technique to quantify the corneal biomechanical properties while preserving the intact corneal shape and structure. We demonstrate the influence of hydration, and the modifications due to cross-linking treatments on the corneal ultrastructure and biomechanical properties using OCE methods. The observed excellent measurement precision is critical for in vivo application of OCE in clinical settings. Further development and future application of OCE to derive corneal material properties will allow us to quantify the magnitude of ectatic diseases, the effectiveness of CXL treatment and follow changes over time.