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Elias Pavlatos, Hong Chen, Xueliang Pan, Jun Liu; Corneal strains induced by ocular pulse and larger IOP elevations. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):1100.
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© ARVO (1962-2015); The Authors (2016-present)
To compare corneal strains induced by an ocular pulse of a few mmHg with those during inflation from 5 to 30 mmHg in the same eye to evaluate whether the ocular pulse associated strains could predict the outcome of a standard mechanical testing, i.e., inflation.
Seventeen porcine globes were tested within 48 hours postmortem. Whole globes were secured using a custom-built holder and immersed in 0.9% saline. A 20G needle was inserted into the anterior chamber and connected to a pressure sensor (P75, Harvard Apparatus) to monitor the intraocular pressure (IOP). Another 20G needle was connected to a programmable syringe pump (PHD Ultra, Harvard Apparatus) to control IOP. The globes were preconditioned with 5 pressure cycles from 5 to 30 mmHg and then equilibrated at 16.5 mmHg for 15 minutes. The ocular pulse was simulated by oscillating IOP between 15 and 18 mmHg at 1 Hz for 25 cycles, and ultrasonic scans (radiofrequency data) were saved for the last 5 cycles. After equilibration at 5 mmHg for 15 minutes, the globe was inflated from 5 to 30 mmHg with 0.5 mmHg steps every 15 seconds. Ultrasonic scans were performed at each step. Corneal radial strains were determined using an ultrasound speckle tracking technique (Tang & Liu, J Biomech Eng 2012, 134(9)). For both the ocular pulse and inflation tests, a stiffness index “b” was calculated by fitting the nonlinear relationship between IOP and strain.
For all seventeen globes, the average peak radial strain induced by ocular pulse was 0.13 ± 0.03%. The average radial strain at 30 mmHg in the inflation tests was 3.10 ± 0.72%. The correlation between these peak strains was significant (R=0.671, p=0.003; Figure 1). A representative strain map obtained from ocular pulse is shown in Figure 2. The b-values, more representative of the overall nonlinear relationship, were also significantly correlated (R=0.570, p=0.017).
The strong positive correlation in maximum strain magnitudes and b-values between ocular pulse and inflation tests suggested that these two methods generated correlative biomechanical evaluation of the cornea. While the inflation across a large range of IOPs is difficult to implement in vivo, the naturally occurring ocular pulse could be a feasible alternative to evaluate corneal biomechanics in vivo.
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