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J. Tang, X. He, J. Liu; Effect of Ocular Tissue Viscoelasticity on Intraocular Pressure Elevation Under Injection at Different Rates. Invest. Ophthalmol. Vis. Sci. 2010;51(13):588.
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© ARVO (1962-2015); The Authors (2016-present)
Ocular rigidity extracted from the pressure-volume relationship has been proposed as a measure of the elastic properties of the eye globe. It has been known that both cornea and sclera exhibit time dependent viscoelasticity. The purpose of this study is to experimentally examine the effect of viscoelasticity of cornea and sclera on intraocular pressure change under stimuli of volume change at different rate.
Three human globes from three donors were obtained within 48 hours postmortem. Central corneal thickness (CCT) was measured by ultrasound pachymeter (DGH, 550 Pachette2, DGH Technology). The globes were placed in an eye holder and a pressure sensor (Omega PX154; Omega Engineering, Inc.) was connected to the aqueous chamber using a 24-gauge needle. Output of the pressure sensor was digitized by an A/D converter (MP35; Biopac Systems, Inc.) at a sampling rate of 500Hz and displayed in a PC. An 18-gauge needle was inserted from the limbus into the vitreous chamber and connected to a syringe pump (NE500 OEM Pump, New Era Pump Systems, Inc.). Each globe was first perfused with saline at a constant pressure of 10 mmHg for 1 hour. Stimuli of Intraocular volume increase of 100ul were then introduced at three different rates (8ml/min, 0.2ml/min, and 0.002ml/min) and the corresponding IOP curves were monitored. IOP was brought back to 10mmHg and stabilized for around 10minutes before each stimulus.Corneal strips (6 mm*15 mm) and scleral strips (6 mm*15 mm from anterior sclera, along the nasal-temporal direction) were prepared by dissection. The stress relaxation tests were performed by applying 1% step strain for 1000 seconds. Instantaneous modulus and equilibrium modulus were calculated at 0.01 second and 600 second, respectively.
The instantaneous modulus of cornea and sclera were 1.72 ± 0.20MPa and 4.40 ± 1.22MPa, respectively. The equilibrium modulus of cornea and sclera were 0.65 ± 0.08MPa and 2.19 ± 0.69MPa, respectively. The IOP increased by 14.56 ± 2.12mmHg, 14.71 ± 2.92mmHg, and 13.76 ± 2.03mmHg for 100ul injection under 8ml/min, 0.2ml/min and 0.002ml/min injection rate, respectively. The IOP elevation curves were different for three globes under injections of the same rate. Most significant difference in magnitude of IOP elevations was observed under 0.2ml/min injection rate.
The results suggest that the difference in the biomechanical characteristics of the cornea and sclera may underline the difference in IOP elevation profile under injection.
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