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Keyton Clayson, Elias Pavlatos, Jun Liu; 3D Inflation Strains in Porcine Corneas. Invest. Ophthalmol. Vis. Sci. 2016;57(12):2388. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To measure the 3D deformation of the cornea using high-resolution ultrasound speckle tracking, and to evaluate the presence of shear during inflations of the cornea.
Seven porcine globes were tested within 72 hours post-mortem. Whole globes were immersed in a 10% dextran solution for 1 hour to reduce swelling, then secured to a custom-built holder and immersed in 0.9% saline. The anterior chamber was infused with Optisol GS (Bausch and Lomb) via a column system to control intraocular pressure (IOP). The globes were preconditioned with 25 pressure cycles from 10 to 12 mmHg, then equilibrated at 10 mmHg for 1 hour. Testing involved raising the pressure to 11 mmHg and 12mmHg consecutively, with a 15 minute equilibration time at each pressure. For each step, a 55 MHz ultrasound probe with an imaging window of 5.5 mm was aligned along the nasal-temporal direction of the cornea, and consecutive 2D frames of radiofrequency data were acquired at 14 µm steps scanned over 2 mm in the superior-inferior direction to form a volume centered on the corneal apex. The 3D principal strains (ε1, ε2, ε3), max shear, and volume ratio were calculated using an ultrasound speckle tracking technique (Cruz Perez et al, ABME, 2015). Volumetric strain maps and vector plots were visualized in Paraview (Kitware Inc).
The average principal strains, max shear, and volume ratio in porcine corneas are summarized in Table 1. Representative maps of the principal strains and principal vector directions are shown in Figure 1. During corneal inflation, the principal strain of largest magnitude was through-thickness compression (ε3), while the other two principals (ε1, ε2) were tensile and largely in-plane. Significant shear with a magnitude larger than all principals was observed. The volume ratios close to one confirmed minimum swelling and near-incompressibility during inflation.
3D ultrasound speckle tracking showed that the cornea experiences compressive, tensile, and shear strains during IOP increase. The predominance of shear deformation during pressure increase suggests that weakness in resisting shear may be involved in corneal ectasia and warrants future investigations.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
Table 1: Mean ± SD of principal strains and maximum shear during inflation (n=7).
Figure 1: Maps of principal vector directions (top) and principal strains (bottom) in a representative cornea (A-P: anterior-posterior, S-I: superior-inferior, N-T: nasal-temporal).
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