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B. Costin, X. He, J. Liu, W. J. Dupps, Jr.; A Comparison of Two Ultrasonic Systems for Non-Invasive Measurements of Corneal Biomechanical Properties Before and After Collagen Cross-linking. Invest. Ophthalmol. Vis. Sci. 2008;49(13):665.
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To report the use of two distinct, non-destructive ultrasound systems to measure the biomechanical properties of porcine corneas before and after collagen cross-linking, and to compare the results obtained from each.
The surface wave elastometer (Sonic Eye, PriaVision, Menlo Park, CA) measures ultrasonic time-of-flight across a 4.5 mm span of corneal surface by direct application of the probe to corneal tissue. The model-based ultrasound system utilizes plane wave interrogation in combination with a mechanical model to non-invasively determinate corneal biomechanical properties including corneal thickness, density, and stiffness (Liu, 2007). The measurement involves submersion of the tissue in a liquid bath and does not involve direct contact with the cornea. 10 fresh porcine globes were obtained and the epithelium was denuded. Each globe was immersed in a 15% dextran solution to deturgesce the corneas. A continuous intravitreal pressure of 15 mm Hg was maintained by direct infusion. Replicate measurements were obtained first with the surface wave elastometer and then with the model-based ultrasonic system. Finally, the entire corneal surface of each globe was immersed in 8% gluteraldehyde solution for thirty minutes to stiffen the cornea and then each measurement was repeated.
The mean speed of sound (through thickness) measured by the model-based ultrasound method was 1523±28 m/s before cross-linking and 1575±39 m/s after. The mean speed of sound (in the collagen lamellar plane) using the surface wave elastometer was 95±25 m/s before cross-linking and 139±28 m/s after. Both techniques demonstrated significant stiffness increases after cross-linking by paired t tests (p = 0.007 and 0.03 for the model-based and surface wave measurements, respectively). Correlation coefficients between the two stiffness measurement techniques were not significant (r = 0.5 before and after cross-linking, p>0.05).
Two non-destructive ultrasound techniques for measuring corneal properties were independently capable of measuring increases in stiffness in the same whole-globe tissue samples after cross-linking. Surface wave and through-thickness measurements are complimentary and differ in repeatability, sensitivity to depth-dependent property changes, and requirements for corneal contact or fluid coupling.
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