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J. Tang, X. He, J. Liu; Corneal Stiffening Is Significantly Correlated With the Increase of Tonopen Readings in a Porcine Eye Model. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2855.
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Previous studies have suggested that corneal biomechanical properties may affect the accuracy of tonometric measurement of intraocular pressure (IOP). A quantitative ultrasound method developed by Liu et al  provides a non-invasive approach to measure corneal stiffness. The purpose of this study is to experimentally examine the effect of increased corneal stiffness on Tonopen measurements of IOP in a porcine eye model.
Sixteen enucleated porcine globes were obtained within 24 hours postmortem (Sioux-Preme Packing, Inc.). Central corneal thickness (CCT) was measured by using an ultrasound pachymeter (DGH-550BPL, DGH Technology) and corneal aggregate modulus (as a measure of stiffness) was measured by the quantitative ultrasound method. IOP was manometrically controlled by using a saline column adjusted to different levels (10 mmHg, 20 mmHg, and 30 mmHg). At each level, IOP was measured by using a Tonopen (Tono-pen XL, Reichert, Inc.). Corneas were then stiffened by immersion in 1% glutaraldehyde solution for 20 minutes. All measurements were repeated in the stiffened eyes following the same procedure.
A significant increase in corneal stiffness was observed (P<0.001) after glutaraldehyde treatment (corneal aggregate modulus increased from 2.619 ± 0.074GPa to 2.713 ± 0.083GPa). Mean CCT did not change before and after glutaraldehyde treatment (945 ± 78µm vs. 939 ± 53µm, P=0.92). Significant increase in the measured IOP was observed after corneal stiffening. At true IOP level of 10 mmHg, 20 mmHg, and 30 mmHg, the measured IOP was 6.96 ± 1.67mmHg, 15.05 ± 2.67mmHg, and 21.09 ± 3.04mmHg before glutaraldehyde treatment and 24.21 ± 7.84mmHg, 30.73 ± 6.67mmHg and 38.76 ± 6.75mmHg after corneal stiffening. There was a significant positive correlation between the increase in the Tonopen readings and the increase in corneal stiffness (R=0.56, 0.64 and 0.55, P=0.023, 0.008 and 0.028 at true IOP levels of 10 mmHg, 20 mmHg, and 30 mmHg).
This study showed that the Tonopen measurements of IOP were significantly affected by corneal stiffness. Tonopen underestimated IOP in porcine eyes likely due to the low corneal stiffness of fresh porcine corneas. Tonopen overestimated IOP after corneal stiffening in the porcine eye model and the increase in corneal stiffness was correlated with the increase in Tonopen measurements of IOP.1. Liu, J., He, X., Pan, X., and Roberts, C., Ultrasonic Model and System for Measurement of Corneal Biomechanical Properties and Validation on Phantoms, Journal of Biomechanics, 40(5):1177-82, 2007
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