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Keyton Clayson, Yanhui Ma, Sunny Kwok, Jun Liu; Shear strains in porcine and keratoconic human cornea. Invest. Ophthalmol. Vis. Sci. 2019;60(11):PB0182.
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© ARVO (1962-2015); The Authors (2016-present)
Corneal collagen fiber slippage due to excessive shear is hypothesized as a biomechanical mechanism for keratoconus (Meek et al, IOVS 2015). This study aimed to characterize the distribution of corneal shear strains in normal porcine eyes and keratoconic human donor eyes using high-resolution ultrasound speckle tracking.
Ten porcine whole globes were obtained from a local abattoir and tested within 72 hrs postmortem, and two donor globes from an 84 year old donor with bilateral keratoconus of different severity were obtained from Lions VisionGift (Portland, OR) and tested within 12 hrs of receipt (84 hrs postmortem). Porcine globes were pretreated in a 10% dextran solution, secured to a custom-built holder and immersed in 0.9% saline. Human globes were pretreated in a 5% poloxamer-188 solution overnight to return the corneas towards physiological hydration, and remained in this solution during testing. All globes were preconditioned with 20 pressure cycles from 5-30 mmHg and then subjected to inflation testing from 5-30 mmHg in either 0.5 mmHg (porcine) or 1.0 mmHg (human) steps. Radiofrequency data of B-mode images of the central cornea were acquired using a 50 MHz ultrasound probe (MS700, VisualSonics), and corneal strains were calculated using an ultrasound speckle-tracking technique (Tang and Liu, ASME 2012).
In porcine corneas, shear strain was minimal throughout the imaged corneal cross-section (Fig. 1) and had significantly smaller magnitude than the radial and tangential strains (-0.63%±1.54% vs. -8.59%±1.31% and 3.93%±1.79% at 30 mmHg, all p’s<0.05). In contrast, both the mild and advanced keratoconic corneas had significant strain variations through the corneal thickness, with shear strains similar in magnitude to the radial and tangential strains observed in both globes (Fig. 2).
Strain patterns appear altered in keratoconic cornea as compared to normal corneas, with a more varied response throughout the cornea. Shear strain magnitudes appear to increase in keratoconic corneas, consistent with previous observations of altered collagen organization in the disease. Weakness in resisting shear deformation may be involved in corneal ectasia and warrants future investigation.
This abstract was presented at the 2019 ARVO Imaging in the Eye Conference, held in Vancouver, Canada, April 26-27, 2019.
Fig. 1. Average strain response for porcine corneas (n=10) with strain maps from a representative globe at 30 mmHg.
Fig. 2. Strain maps at 30 mmHg from a bilateral keratoconic donor, with arrows indicating cone location.
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