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Junhua Tang, Jun Liu; Measurement of Scleral Strains under Physiological Loadings using Ultrasonic Speckle Tracking. Invest. Ophthalmol. Vis. Sci. 2011;52(14):6251.
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Scleral mechanical properties may play an important role in affecting the mechanical environment of the optic nerve head. Physiologically, the sclera is subject to both tensile stretch and through-thickness compression due to intraocular pressure (IOP). The purpose of this study is to examine the feasibility of using non-invasive ultrasound to monitor scleral tensile and through-thickness strains under physiological loadings.
Five porcine globes were obtained within 24 hours postmortem. The intact posterior sclera shell was mounted onto a custom-built pressurization chamber. A saline column with a reservoir was connected to the chamber to control the IOP. The IOP was confirmed by a pressure sensor. An ultrasound imaging system (Vevo660, VisualSonics Inc. Toronto) with a 55MHz transducer was employed to acquire B-mode cross-sectional ultrasound images of the sclera at the posterior pole along the circumferential direction. IOP was first maintained at 5 mmHg and gradually increased to 45 mmHg at steps of 5 mmHg. An ultrasound speckle tracking algorithm, validated by simulation results, was utilized to compute the displacement of ultrasound speckles at each level of IOP elevation. Both through-thickness compressive strain and circumferential tensile strain were calculated from the speckle displacement.
The average through-thickness compressive strain at the posterior pole of the porcine sclera was 2.9% ± 1.8%, 4.0% ± 2.2%, 4.9% ± 2.3%, and 5.9% ± 2.7% at pressure of 15 mmHg, 25 mmHg, 35 mmHg and 45 mmHg, respectively. The corresponding circumferential tensile strain was 1.1% ± 0.3%, 1.3% ± 0.7%, 1.4% ± 0.8%, and 1.5% ± 0.9%.
A novel non-invasive ultrasound method was developed to assess scleral mechanical responses under physiological IOP loadings. Our preliminary results showed that in porcine sclera the compressive strains were generally larger and increased more linearly than the tensile strains under the same IOP elevation. Future work will investigate the use of the non-invasive ultrasound method to examine regional viscoelastic properties of the sclera shell.
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