Characterization of Corneal Biomechanical Properties and Determination of Natural Intraocular Pressure Using CID-GAT

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 E(qs). The calculated IOP(NC) from the Imbert-Fick's Law using the corneal moduli E(qs) was compared to the natural IOP(N), measured using pressure sensor inserted into the eye. RESULTS: Test results showed that IOP-dependent corneal modulus E(qs) is a primary confounding factor in IOP calculation. The average elastic modulus E(qs) 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 E(qs) into IOP(NC) calculation showed that IOP(NC) are in good agreement with reference IOP(N) (slope = 0.999, r = 0.939, P < 0.001). CONCLUSIONS: The IOP-dependent corneal modulus E(qs) is a primary confounding factor in IOP calculation. A modified CID-GAT procedure to obtain natural cornea-independent IOP(NC) 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.