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Raksha Urs, Ronald H. Silverman; Effect of Intraocular Pressure on Corneal Elasticity Measurement using Acoustic Radiation Force. Invest. Ophthalmol. Vis. Sci. 2012;53(14):1510.
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© ARVO (1962-2015); The Authors (2016-present)
Measurement of intraocular pressure (IOP) by applanation techniques is known to be affected by corneal thickness and elasticity. The purpose of this study is to determine the effect of IOP on corneal elasticity values determined from stress/strain relationships resulting from application of acoustic radiation force (ARF) to the cornea.
A cannula was inserted into the optic nerve of intact porcine eye globes (n=5) immersed in 0.9% saline. The cannula was attached to a saline bag, whose height was varied, to vary the IOP. A pressure sensor with digital output (Omega PX154; Omega Engineering, Inc., Stamford, CT) was used to monitor the IOP. Pressure was increased from 5 to 40 mmHg and then decreased to 5 mmHg in steps of 5 mmHg. At each pressure increment or decrement, an ARF scan of the cornea was recorded with a 25 MHz ultrasound transducer (Olympus SN 781943). The ARF scan consisted of 20 pushing pulses applied at 25% duty cycle, at a 1 kHz pulse repetition rate, with imaging impulses interleaved between push pulses, to allow radiofrequency (RF) data acquisition during the push mode. After the push sequence, the cornea was imaged for another 400 ms. RF data were sampled at 400 MHz (12 bits/sample). Displacement of the front and back surfaces of the cornea, during and after the push, was computed with a spline-based algorithm (Viola et al, 2005), and change in corneal thickness and strain were calculated. Strain during ARF application was fit to the Voigt model to determine the elastic modulus at each IOP level.
The average elastic modulus ranged from 20 to 40 kPa. Elastic modulus increased with IOP (p<0.0001), at a rate of 0.15 kPa/mmHg. The slope was higher during the increasing-IOP cycle (0.21 kPa/mmHg; p<0.0001) compared to the decreasing-IOP cycle (0.11 kPa/mmHg; p=0.02).
Elastic modulus of the cornea, determined by ARF, is influenced by IOP, with corneas measuring progressively stiffer with increasing IOP. Stress-strain curves are thus non-linear over the range of IOP values. For ex-vivo porcine eyes, over the range of 5-40 mmHg, the variation is 13-26% of the average elasticity measured over the full range of IOP values. An IOP-based correction is required when corneal elasticity is measured in vivo, with ARF, in potential applications such as detection of keratoconus, and evaluation of corneal cross-linking therapy.
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