The mean CCT of the 21 canine corneas was 611.9 ± 55.3 μm before all measurements and 623.6 ± 51.6 μm after the tonometric measurements. The small change in CCT suggested a small amount of corneal swelling during the experiments. The mean GAT and Tono-pen readings when the pressure inside the globe (IOP
Man) was manometrically controlled at various levels were presented in
Table 1 and
Figure 2.
Both GAT and Tono-pen underestimated IOP at all pressure levels. Tono-pen had a significantly higher reading than GAT at all pressure levels (the differences were 6.7, 7.3, 6.6, 5.2, and 2.7 mm Hg for IOPMan at 10, 15, 20, 30, and 40 mm Hg, respectively). In the same eye, the GAT and Tono-pen readings were significantly correlated for certain pressure levels but not others. When IOPMan was 15 mm Hg and 20 mm Hg, the two tonometric readings were significantly correlated (Pearson correlation coefficient R = 0.44, P < 0.05; and R = 0.54, P < 0.05, respectively). No significant correlation was found at higher pressures (when IOPMan was 30 or 40 mm Hg; R = 0.04 or −0.1, P = 0.86 or 0.61). There was no correlation between GAT and Tono-pen readings when IOPMan was 10 mm Hg (R = 0.06, P = 0.80), likely because GAT readings were close to zero and thus not reliable at this pressure level in the canine eyes. For an overall correlation analysis including the readings at all pressure levels, the Pearson correlation coefficient between GAT and Tono-pen measurements was 0.96 (P < 0.0001) indicating a strong influence of the range of data in comparing these two tonometric devices.
The measurement errors for GAT (difference between measured IOP and IOP
Man) were shown in
Figure 3. At each level of IOP
Man, there was significant spread across different eyes in both GAT errors (with an average range of 6.2 mm Hg) and Tono-pen errors (with an average range of 6.8 mm Hg). In addition, both GAT and Tono-pen errors increased significantly as the true IOP increased (
P < 0.001, based on the mixed models).
Based on the stress-strain curves obtained from uniaxial tensile tests, secant modulus was calculated as the ratio of stress and strain at a given level of strain. The mean secant modulus was 1.54 ± 0.43 MPa at 1% strain and increased to 3.93 ± 1.35 MPa at 5% strain, demonstrating typical nonlinearity (
Fig. 4). According to the estimation of corneal in-plane tensile stress based on Laplace law and also considering the prestress introduced in tensile tests, the secant modulus at 1% strain seemed a reasonable representation of the tensile stiffness in the range of the loadings relevant to the IOP measurements in this study and thus was chosen as the primary modulus measure for further analysis. It is noted that Laplace law does not predict well the exact stress distribution in the cornea due to inhomogeneous corneal thickness, curvature, and material properties.
A statistically significant Pearson correlation was found between the GAT errors at 30 mm Hg IOP
Man and the 1% secant moduli (
R = 0.49,
P < 0.05;
Fig. 5). At this level of true pressure, the GAT readings appeared to be higher in corneas with higher modulus. No significant correlation was found between GAT errors and the 1% secant moduli at other pressure levels (
R = 0.33, 0.31, 0.26, 0.09; and
P = 0.14, 0.17, 0.26, 0.71, for IOP
Man at 10, 15, 20, and 40 mm Hg, respectively). No statistically significant correlation was detected between GAT errors and the initial CCT (and also the CCT measured after experiments) at all pressure levels (
R = −0.08, −0.15, 0.05, 0.15, 0.28; and
P = 0.72, 0.52, 0.82, 0.53, 0.22, for IOP
Man at 10, 15, 20, 30, and 40 mm Hg, respectively).
No significant correlation was observed between Tono-pen errors at any levels of true IOP and the secant moduli at 1% strain (R = 0.29, 0.16, 0.04, −0.25, −0.04; and P = 0.20, 0.49, 0.85, 0.27, 0.88, for IOPMan at 10, 15, 20, 30, and 40 mm Hg, respectively). No significant correlation was observed between Tono-pen error and initial CCT (and also the CCT measured after experiments) at all pressure levels (R = 0.19, 0.07, 0.13, 0.24, 0.27; and P = 0.41, 0.76, 0.57, 0.29, 0.25, for IOPMan at 10, 15, 20, 30, and 40 mm Hg, respectively).
CCT (initial or measured after experiments) was not correlated to the secant modulus at 1% strain (R = −0.006 or −0.038, P = 0.98 or 0.86).