PURPOSE: To experimentally examine the effect of increased corneal stiffness on Goldmann applanation tonometry (GAT) and Tono-Pen (Reichert, Inc., Depew, NY) measurements of intraocular pressure (IOP) in a canine eye model. METHODS: Twenty globes were recovered from 10 dogs with no known diseases. For each dog, corneal stiffening was induced in one eye with glutaraldehyde/phosphate buffered saline (PBS) immersion while the other cornea was immersed in PBS only. Acoustic impedance was measured before and after treatment in all eyes. After treatment, IOP was measured by GAT and Tono-Pen at true pressures of 10, 15, 20, 30, and 40 mm Hg. The corneas were then dissected for uniaxial tensile testing. The GAT/Tono-Pen readings, corneal stiffness (measured by ultrasound and tensile tests), and corneal thickness were compared between the two groups. The correlations between GAT/Tono-Pen readings and corneal stiffness were evaluated. RESULTS: Acoustic impedance significantly increased after glutaraldehyde treatment (P < 0.01). Secant modulus at 1% strain was significantly higher in corneas treated with glutaradehyde/PBS than those treated with PBS only (P < 0.01). GAT and Tono-Pen readings were significantly higher at all pressure levels (P < 0.001) in the eyes with corneal stiffening. Both corneal acoustic impedance and secant modulus were significantly correlated with GAT/Tono-Pen readings at all pressure levels (P < 0.01). CONCLUSIONS: This study provided experimental evidence that corneal stiffening significantly increases GAT and Tono-Pen readings in canine eyes. Noninvasive ultrasound measurement of acoustic impedance may be used to evaluate corneal stiffness and improve the accuracy of clinical measurements of IOP.
PURPOSE: To experimentally examine the effect of increased corneal stiffness on Goldmann applanation tonometry (GAT) and Tono-Pen (Reichert, Inc., Depew, NY) measurements of intraocular pressure (IOP) in a canine eye model. METHODS: Twenty globes were recovered from 10 dogs with no known diseases. For each dog, corneal stiffening was induced in one eye with glutaraldehyde/phosphate buffered saline (PBS) immersion while the other cornea was immersed in PBS only. Acoustic impedance was measured before and after treatment in all eyes. After treatment, IOP was measured by GAT and Tono-Pen at true pressures of 10, 15, 20, 30, and 40 mm Hg. The corneas were then dissected for uniaxial tensile testing. The GAT/Tono-Pen readings, corneal stiffness (measured by ultrasound and tensile tests), and corneal thickness were compared between the two groups. The correlations between GAT/Tono-Pen readings and corneal stiffness were evaluated. RESULTS: Acoustic impedance significantly increased after glutaraldehyde treatment (P < 0.01). Secant modulus at 1% strain was significantly higher in corneas treated with glutaradehyde/PBS than those treated with PBS only (P < 0.01). GAT and Tono-Pen readings were significantly higher at all pressure levels (P < 0.001) in the eyes with corneal stiffening. Both corneal acoustic impedance and secant modulus were significantly correlated with GAT/Tono-Pen readings at all pressure levels (P < 0.01). CONCLUSIONS: This study provided experimental evidence that corneal stiffening significantly increases GAT and Tono-Pen readings in canine eyes. Noninvasive ultrasound measurement of acoustic impedance may be used to evaluate corneal stiffness and improve the accuracy of clinical measurements of IOP.