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J. Liu, X. He, C. Roberts, X. Pan; Ultrasonic Detection of Changes in Corneal Biomechanical Properties Associated With IOP Elevation. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1856.
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Previous studies indicated that corneas may stiffen at an elevated IOP. However, these studies were performed on corneal strips or buttons that had altered structural and mechanical boundary conditions from corneas in intact eyes. We have developed an ultrasonic system that is capable of non-invasive characterization of corneal biomechanical properties. The goal of this study is to utilize the ultrasonic system to examine the changes in corneal properties in response to elevated IOP in fresh, enucleated porcine eyes.
Normal porcine eyes (three pairs) were obtained and immersed in warm Dexol-equivalent media during the measurements. The anterior chamber of the eye was connected to a liquid column using a 25G needle for pressure adjustment. The IOP was monitored by a pressure sensor (Omega Px154) and a digital monitor (Omega DP 25B). For each pair of the eyes, one eye was used as the treatment eye, and the other, control. For treatment eyes, IOP was increased from 10 mmHg to 50 mmHg with an increment of 10 mmHg. For control eyes, no IOP changes were introduced. After each pressure adjustment, the eyes were given 20 min for equilibrium. Both treatment and control eyes were measured by the ultrasonic system at the same time point. A broadband transducer (10 MHz, XMS, Panametrics-NDT) was used to send ultrasonic pulses to interact with the corneas. The reflected signals were recorded by a 500 MHz/8-bit digitizer (Acqiris, DP105). A computer algorithm was developed to reconstruct corneal properties including thickness, density and stiffness (i.e., aggregate modulus), by analyzing the reflected ultrasonic signals.
All pressurized eyes demonstrated an increase in stiffness as IOP increased from 10 to 50 mmHg. The contralateral control eyes, however, all presented a decrease in stiffness over time. Corneal thicknesses increased significantly in both groups, suggesting postmortem swelling of corneas in intact, enucleated eyes. The decrease of corneal stiffness in control eyes is likely associated with swelling. The pressured eyes had similar swelling, yet demonstrated an increase in corneal stiffness, indicating that stress-stiffening introduced by IOP elevation was the dominant factor.
This study demonstrated the feasibility of using a non-invasive ultrasonic system to detect and characterize the changes in corneal biomechanical properties associated with IOP elevation in intact eyes.
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