Abstract
Purpose:
The intraocular pressure (IOP) measured using Goldmann Applanation Tonometry (GAT) is confounded by individual corneal properties. We investigated a modified method that removes the confoundment by incorporating corneal properties into the Imbert-Fick's law is investigated.
Method:
Porcine eyes were pressurized between 10 and 40 mm Hg using a manometer. The eyes were indented using a flat cylindrical indenter. A modified corneal indentation device (CID) procedure was used to obtain the corneal moduli Eqs. The calculated IOPNC from the Imbert-Fick's Law using the corneal moduli Eqs was compared to the natural IOPN, measured using pressure sensor inserted into the eye.
Results:
Test results showed that IOP-dependent corneal modulus Eqs is a primary confounding factor in IOP calculation. The average elastic modulus Eqs is 0.173 ± 0.018 MPa at 20 mm Hg, and increases with IOP at a linear rate of 0.0066 MPa per mm Hg (r = 0.997, P < 0.001). Incorporation of individual Eqs into IOPNC calculation showed that IOPNC are in good agreement with reference IOPN (slope = 0.999, r = 0.939, P < 0.001).
Conclusions:
The IOP-dependent corneal modulus Eqs is a primary confounding factor in IOP calculation. A modified CID-GAT procedure to obtain natural cornea-independent IOPNC is developed and verified in this study. The CID-GAT IOP modification may be used in place of conventional GAT when the confounding effects in eyes with atypical cornea (e.g., laser-assisted in situ keratomileusis [LASIK] thinned) are significant.
Translational Relevance:
Confoundment from corneal properties results in IOP measurement errors. The study showed that the CID-GAT method can significantly reduce the confounding corneal errors.
The good agreement between IOP
NC and reference IOP
N is dependent on the ability of the CID to characterize the quasistatic
Eqs of the cornea. Asejczyk-Widlicka et al.
20 reported a corneal elastic modulus of 0.05 to 0.24 MPa in the IOP range from 12 to 25 mm Hg on porcine eyes ex vivo.
20 Inflation tests were conducted in their study, but the rates were not specified. Elsheikh et al.
3 performed inflation tests on human and porcine eyes. In their tests, the eyes were quasistatically loaded to set pressure and with similar loading conditions to our study. The corneal quasistatic tangent moduli determined in their inflation tests of porcine eyes (dashed line) are in the same range of results as the present study shown in
Figure 7.
The confounding effects of corneal properties in GAT IOP measurement were examined quantitatively by Liu et al.
8 They investigated the IOP elevation in porcine eyes after glutaraldehyde treatment and found that the corneal modulus increased 1 MPa for every 5 mm Hg change in IOP.
21 Our results are in line with their model over the tested range of pressure (
Fig. 8).
The confounding effect of the geometric factor
Display Formula\({K_g}\), corneal center thickness, corneal radius, and
Eqs on IOP are shown in
Figures 10 to
13, respectively. The variation between the IOP
N and IOP
GAT indicates the dependencies of GAT measurement on these corneal properties. Comparison (
Fig. 14) showed that the standard deviation (SD = 0.11) of IOP
NC from IOP
N was significantly smaller than that (SD = 0.32) of the IOP
GAT from IOP
N. More than 80% of IOP
NC were within 10% error of IOP
N while IOP
GAT generally deviated from IOP
N by 50%.
The CID-GAT method was designed to characterize the IOP
N by accounting for the effects of individual-specific corneal biomechanical properties and corneal geometries on IOP
N. Changes in corneal curvature (
Rc), thickness (
t), and corneal elastic properties (
Eqs) were accounted for in
Equation 6. The corneal elastic properties (i.e., the elastic modulus of the tissue) are known to increase with aging and IOP.
22–25 The curvature and corneal modulus may change in subjects with keratoconus, and the corneal thickness may increase in subjects with edematous corneas. These confounding effects from aging or illnesses are readily accounted for by the CID-GAT method in
Equation 6.
In conclusion, the confounding effect of corneal properties has a great impact on GAT IOP measurement. The effect can be reduced significantly using the CID-GAT method with an updated treatment for corneal resistance in the Imbert-Fick's Law. The updated calculation of IOPNC using corneal modulus obtained from the CID showed good agreement with IOPN, the natural IOP of the eye. The CID-GAT method to calculate IOPN may be of particular relevance and use for subjects with corneas having abnormal biomechanical properties (e.g., aging effects, refractive surgeries, swelling, keratoconus, and so forth).