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Benjamin Cruz Perez, Junhua Tang, Hugh Morris, Richard Hart, Xueliang Pan, Jun Liu; Through-thickness Measurement of Porcine Sclera Deformation under Biaxial Loading Using High Resolution Ultrasound. Invest. Ophthalmol. Vis. Sci. 2013;54(15):68.
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Mechanical properties of the posterior sclera influence the stresses and strains experienced by the optic nerve head. This study aims to characterize the through-thickness deformation of sclera under biaxial loading using high-resolution ultrasound speckle tracking.
Nine porcine globes were obtained within 24 hrs post-mortem. A 7 mm square sample from the supero-temporal region of the posterior sclera was prepared and mounted biaxially along the circumferential and meridian directions using hooks and suture. The samples were immersed in a saline solution during all tests. Preconditioning was achieved using the built-in sine frequency sweep in the Bose biaxial testing system (ElectroForce Planar Biaxial TestBench, Bose Corporation, Eden Prairie, MN, USA) with a load range from 1 to 18 grams. Specimens were then equilibrated for 15 min at 1 gram preload and ramped biaxially to 25 increasing force levels from 2 to 18 grams simulating the physiological intraocular pressure range of 5 to 45 mmHg. Two ramps were performed to image the deformations within a cross-section of the sclera along both axes with a 55 MHz ultrasound imaging system. Each sample was allowed to equilibrate at a preload of 1 gram for 15 minutes between ramps. A speckle tracking algorithm (Tang & Liu, J Biomech Eng 2012, 134(9)) was used to compute the internal strains of the scanned cross-sections. The average strain in a pre-defined region of interest was calculated for each specimen.
Sclera demonstrated a heterogeneous pattern of strains through thickness and a typical hyperelastic behavior during biaxial loading. The circumferential strains were 2.69±0.72%, 2.97±0.81%, and 3.24±0.86%, and the meridian strains were 3.34±0.61%, 3.62±0.64%, and 3.38±0.63% at stress levels of 20, 25, and 30 kPa, respectively. The circumferential strains were significantly lower than the meridian strains at these stress levels (P<0.05).
Ultrasound speckle tracking was used to characterize the mechanical responses of sclera during biaxial loading. Porcine posterior sclera displayed heterogeneous and anisotropic responses with a significantly stiffer response along the circumferential direction than the meridian direction. Future studies combining finite element modeling are needed to derive mechanical properties of the sclera.
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