Ocular Pulse Elastography: Imaging Corneal Biomechanical Responses to Simulated Ocular Pulse Using Ultrasound.

Purpose: In vivo evaluation of corneal biomechanics holds the potential for improving diagnosis and management of ocular diseases. We aimed to develop an ocular pulse elastography (OPE) technique to quantify corneal strains generated by naturally occurring pulsations of the intraocular pressure (IOP) using high-frequency ultrasound. Methods: Simulated ocular pulses were induced in whole porcine and human donor globes to investigate the effects of physiologic variations in baseline IOP, ocular pulse amplitude, and frequency on corneal strains. Ocular pulse-induced strains were measured in additional globes before and after UVA-riboflavin-induced corneal crosslinking. The central cornea in each eye was imaged with a 50-MHz ultrasound imaging system and correlation-based speckle tracking of radiofrequency data was used to calculate tissue displacements and strains. Results: Ocular pulse-induced corneal strains followed the cyclic changes of IOP. Both baseline IOP and ocular pulse amplitude had a significant influence on strain magnitude. Variations in pulse frequency within the normal human heart rate range did not introduce detectable changes in corneal strains. A significant decrease of corneal strain, as quantified by the OPE technique, was observed after corneal crosslinking. The extent of corneal stiffening (i.e., strain reduction) seemed to correlate with the initial strain magnitude. Conclusions: This ex vivo study demonstrated the feasibility of the OPE method to quantify corneal strains generated by IOP pulsation and detect changes associated with corneal crosslinking treatment. Translational Relevance: Integrating in vivo measurement of IOP and ocular pulse amplitude, the OPE method may lead to a new clinical tool for safe and quick biomechanical evaluations of the cornea.

Corneal Hydration Control during Ex Vivo Experimentation Using Poloxamers.

Purpose: The purpose of this study was to develop an effective treatment method using poloxamers to restore and maintain physiological hydration in postmortem porcine and human corneas during ex vivo experimentation, and to compare corneal inflation response with or without treatment.Materials and Methods: Corneal buttons obtained from whole globes (n = 30 porcine, n = 8 human) were treated with various concentrations of poloxamer 188 (P188, a synthetic macromolecule surfactant) for 24 hrs to identify the concentration that would return the cornea to near-physiological hydration (i.e. H = 3.2). Whole globes (n = 12 porcine, n = 16 human) were also used to monitor central corneal thickness (CCT) during deswelling treatment. Inflation testing from 5 to 30 mmHg was performed in the porcine globes and a subset of human globes to characterize the mechanical response of the cornea after treatment.Results: Physiological hydration was obtained after 24 hrs immersion in 3.25% P188 for porcine corneas and 4.25% P188 treatment for human corneas. CCT was stabilized and returned to physiological levels after 24 hrs of treatment in 3.25% P188 in porcine (891 ± 66 µm) and 4.25% P188 in human (574 ± 34 µm) whole globes. Corneal axial strains at 30 mmHg were significantly larger at physiological hydration than in swollen cornea in both porcine (-6.42%±1.50% vs. -3.64%±1.05%, p = .004) and human (-2.85%±0.09% in vs. -1.53%±0.27%, p = .031) eyes.Conclusions: Our results suggest that P188 treatment was effective in restoring and maintaining near physiological corneal hydration during ex vivo testing, and hydration appeared to significantly impact corneal inflation response in both porcine and human eyes.

Mechanical Deformation of Human Optic Nerve Head and Peripapillary Tissue in Response to Acute IOP Elevation.

Purpose: To measure the deformation of the human optic nerve head (ONH) and peripapillary tissue (PPT) in response to acute intraocular pressure (IOP) elevation. Methods: The ONH and PPT of 14 human donor globes were imaged with high-frequency ultrasonography during inflation testing from 5 to 30 mm Hg. A correlation-based speckle tracking algorithm was used to compute tissue displacements, and the through-thickness, in-plane, and shear strains were calculated by using least-squares strain estimation methods. The ONH and PPT were segmented along the anterior-posterior direction and the nasal-temporal direction. Regional displacements and strains were analyzed and compared. Results: The ONH displaced more posteriorly than the PPT in response to an acute IOP increase. Scleral canal expansion was minimal but correlated with ONH posterior displacement at all IOP levels. Through-thickness compression was concentrated in the anterior of both the ONH and the PPT. Shear was concentrated in the vicinity of the canal with higher shear in the peripheral ONH than the central ONH and higher shear in the PPT near the scleral canal than that further away from the canal. Conclusions: High-resolution ultrasound speckle tracking showed a displacement mismatch between the ONH and the PPT, larger compressive strains in the direction of IOP loading in the anterior ONH and PPT, and higher shear strains in the periphery of ONH in response to acute IOP elevation in the human eye. These findings delineate the deformation patterns within and around the ONH and may help understand IOP-associated optic nerve damage.