Corneal modulus and IOP measurements in canine eyes using Goldmann applanation tonometry and Tono-pen

To experimentally examine the effect of corneal modulus on Goldmann applanation tonometry (GAT) and Tono-pen (Tono-pen XL, Reichert, Inc., Depew, NY) measurements of intraocular pressure (IOP) in a canine eye model.

Twenty-one canine globes were recovered from healthy animals. IOP was controlled at 10, 15, 20, 30, and 40 mm Hg and measured by GAT and Tono-pen following standard protocols. The corneas were dissected and uniaxial tensile tests were performed on corneal strips. The correlation between GAT and Tono-pen errors and corneal secant modulus was evaluated using Pearson correlation coefficients. The influence of corneal thickness and the true pressure was also examined.

At a true IOP of 10, 15, 20, 30, and 40 mm Hg, the GAT readings were 1.1 ± 1.0, 5.1 ± 1.5, 9.5 ± 2.0, 17.3 ± 1.6, and 25.3 ± 1.8 mm Hg, respectively. The corresponding Tono-pen readings were 7.8 ± 1.7, 12.4 ± 1.7, 16.1 ± 1.9, 22.5 ± 2.1, and 28.1 ± 2.2 mm Hg, respectively. The mean secant modulus at 1% strain of the canine corneal strips was 1.54 ± 0.43 megapascal (MPa). Corneal secant modulus was significantly correlated with GAT errors when the true IOP was 30 mm Hg (R = 0.49; P < 0.05). No significant correlation was observed between tonometric errors and corneal thickness. Both GAT and Tono-pen errors increased significantly at higher pressures (P < 0.001).

Both GAT and Tono-pen underestimated IOP in canine eyes. There was preliminary experimental evidence for a correlation between corneal modulus and GAT in the canine eyes and a higher corneal modulus was associated with higher GAT readings at a certain pressure level. The tonometric errors appeared to be pressure-dependent.