Correlation of corneal acoustic and elastic properties in a canine eye model

To examine the correlation between corneal acoustic impedance and Young's modulus in a canine eye model.

Twenty canine globes were recovered from healthy animals. Corneal acoustic impedance was measured in the intact globes using two methods: a quantitative ultrasound spectroscopy method and the reflection amplitude method. The intraocular pressure was maintained at 10 mm Hg during the ultrasound measurements. Corneal strips were then prepared for standard uniaxial tensile tests. Young's moduli at various strain levels and those at a loading level equivalent to that for ultrasound measurements were compared with the acoustic impedance of the same cornea.

The mean acoustic impedance of the canine corneas was 1.72 ± 0.05 MPa · s/m using the quantitative ultrasound spectroscopy method and 1.71 ± 0.04 MPa · s/m using the reflection amplitude method. Young's secant modulus was 1.07 ± 0.48 MPa at 1% strain and 2.01 ± 0.98 MPa at 5% strain, and the tangent modulus was 1.28 ± 0.69 and 3.16 ± 0.71 MPa, respectively. Significant linear correlations between acoustic impedance and Young's modulus (at 1%-5% strains) were found in the measured canine corneas. The correlation remained strong when comparing the two parameters measured under equivalent loading.

This study suggests a potentially strong correlation between corneal acoustic impedance and Young's modulus at low strain levels. If such correlation also exists in the human eye, it may allow the noninvasively determined acoustic impedance to be used as a surrogate for Young's modulus, which is difficult to obtain in vivo.