Purchase this article with an account.
Jun Liu, Junhua Tang, Richard Hart, Cynthia Roberts, Paul Weber, Xueliang Pan; Through-thickness variation of human scleral strains in response to IOP elevation measured by ultrasound speckle tracking. Invest. Ophthalmol. Vis. Sci. 2013;54(15):77.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
Scleral mechanical properties are important factors influencing the mechanical states of the optic nerve head (ONH). This study aimed to examine the through-thickness variations of strains experienced by human posterior sclera during intraocular pressure (IOP) elevations.
Eleven human globes from 7 donors (age from 57 to 73 yo) were tested. Scleral shells were prepared and mounted onto a custom-built inflation chamber. An ultrasound probe (55MHz, Vevo660, Visualsonics) was employed to acquire 2 cross-sectional scans of the posterior sclera near the ONH along the meridian and circumferential directions. Prior to measurements, each globe was preconditioned with 5 cycles of inflation from 5 to 45 mmHg in 60 seconds. Six minutes were allowed for recovery. IOP was then gradually increased from 5 to 45 mmHg at steps of 2.5 or 5 mmHg and the ultrasound radiofrequency signals were acquired at each step. The tangential strains were calculated using a speckle tracking algorithm described previously (Tang & Liu, J Biomech Eng 2012, 134(9)). Each scleral cross-section was divided into three layers (outer, middle and inner) with equal thickness, and the average strains within each layer was calculated and compared.
The average tangential strains in the outer layer were 0.20±0.08%, 0.30±0.11%, 0.35±0.13%, and 0.40±0.13% in the outer layer, 0.21±0.10%, 0.36±0.13%, 0.44±0.13%, and 0.52±0.13% in the middle layer, and 0.21±0.12%, 0.38±0.17%, 0.47±0.20%, and 0.56±0.23% in the inner layer, at pressures of 15, 25, 35 and 45 mmHg, respectively. Consistently lower tangential strains were found in the outer layer than either the inner or middle layer at pressure levels of 30, 35, 40, and 45 mmHg (all P's <0.05, paired t-tests, Fig.1).
Although tangential strains were small (<0.6%) in average within the physiological range of IOP (< 45 mmHg), significant through-thickness variations were found in the human posterior sclera. This result suggests the need to examine the strains throughout the thickness in order to fully characterize the sclera’s response to IOP elevations. The implications and underlying mechanisms of larger strains observed in the inner layers of the sclera need further investigations.
This PDF is available to Subscribers Only