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R. Shah, W. Dupps, Jr.; The Relationship Between Corneal Stiffness Measurements and Intraocular Pressure. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1241.
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To investigate the relationship between corneal stiffness and intraocular pressure (IOP) in porcine globes. In addition, we compare the accuracy of two clinical techniques for measuring IOP that rely on corneal deformation.
IOP was directly controlled and continuously measured by vitreous cannulation. In 10 non-scalded porcine globes, intra-vitreal IOP was increased from 5 to 35 mmHg in 10-mmHg increments. At each pressure, three replicate trans-corneal IOP measurements were obtained with a Tono-pen XL and Pneumatonometer. At each step, a handheld system designed to measure ultrasonic wave propagation time between two fixed-distance surface transducers was used to measure central corneal stiffness (Sonic Eye, PriaVision, Inc.). Linear regression was used to describe the relationship between sonic wave velocity and IOP, and differences in trans-corneal and direct IOP measurements were assessed.
Measurement error averaged 0.3 ± 2.4 mmHg for Tono-pen and 2.1 ± 1.9 mmHg for Pneumatonometer across all pressure levels. The Pneumatonometer signficantly overestimated IOP at pressures above 15 (p ≤ 0.007). Across all ten eyes, the relationship between corneal stiffness and intravitreal IOP had a significant linear component (Figure 1). A stratified analysis comparing the 5 highest and 5 lowest Tono-pen and Pneumatonometer measurements at each IOP level demonstrated insufficient variance to detect a predictive relationship between corneal stiffness and trans-corneal IOP measurement error.
Central corneal ultrasonic wave velocity, an indicator of corneal stiffness, increases with IOP. The slope of this relationship is greatest between 5 and 15 mmHg. Accordingly, measurements of corneal stiffness, including Young’s modulus, must be reported with reference to the IOP at which they were obtained. Conversely, the dependence of corneal stiffness measurements on IOP may prove useful in improving the accuracy of IOP measurement when corneal biomechanical properties introduce error. The Tono-pen provided slightly more accurate IOP measurements than the Pneumatonometer in porcine globes.
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