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Benjamin Cruz Perez, Richard T. Hart, Paul A. Weber, Jun Liu; Influence of Corneal Diameter on GAT Readings and Corneal Inflation. Invest. Ophthalmol. Vis. Sci. 2012;53(14):5043.
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The white-to-white corneal diameter in human eyes varies across subjects. This structural parameter may potentially influence corneal deformation including inflation and bending. The purpose of this study was to numerically simulate the influences of corneal diameter on corneal inflation and the accuracy of Goldmann applanation tonometry (GAT).
The model of the anterior hemisphere of the eye was created using COMSOL finite element analysis software (version 4.2a, Burlington, MA) with two regions representing the cornea and the sclera. The cornea and the sclera were constructed as ellipsoids with varying thickness. Corneal diameter was varied from 9.6 mm to 13.8 mm to represent the natural range in human subjects [Rüfer, et al. Cornea. 2005, 24(3)]. The GAT tip was modeled from current clinical devices using a tip diameter of 3.06 mm. The movement of the GAT tip was prescribed onto an inflated globe until applanation was achieved. A linear elastic material model was used with a Young’s modulus of 1 MPa for the cornea and 5 MPa for the sclera. The Poisson’s ratio for the cornea and sclera were set to 0.49 and the intraocular pressure (IOP) was set to 15 mmHg. The refined mesh was built with very high density in the contact region between the GAT tip and the cornea using quadratic triangular elements with approximately 54,000 degrees of freedom. The models were solved as a series of static problems at each increment of the GAT tip displacement.
Corneal apical rise under a normal IOP (=15 mmHg) was 179 µm for eyes with an average corneal diameter (11.7 mm) and decreased to 168 or 173 µm in eyes with larger (13.8 mm) or smaller corneas (9.6 mm), respectively. Simulated GAT readings were 11.2 mmHg for the larger corneas, 14.9 mmHg for the smaller corneas, and 13.8 mmHg for the average cornea, for a true IOP at 15 mmHg.
The current model showed that corneal apical rise was largest in eyes with average corneal diameter, and decreased in eyes with either larger or smaller corneas. Simulated GAT readings were slightly higher in eyes with smaller corneas and lower in eyes with larger corneas. These results suggest that corneal diameter has an influence on corneal deformation and GAT readings, but the influence of corneal diameter alone may be clinically insignificant. Future studies will implement a nonlinear material model and investigate the interactions between corneal diameter, corneal radius of curvature, corneal stiffness, and true IOP.
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