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H.E. Kanngiesser; Dynamic Contour Tonometry Simulation on the Human Cornea using Finite Element Methods . Invest. Ophthalmol. Vis. Sci. 2004;45(13):4437.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Dynamic Contour Tonometry (DCT) is the only direct and non–invasive method to measure Intra Ocular Pressure (IOP) in human eyes. A small concave tip touches the apex of the cornea and forces it into the shape it theoretically achieves when the pressure is the same on both sides of the cornea. A pressure sensor incorporated into the tip measures the IOP precisely with less influence of corneal properties compared to other tonometry principles. We simulated the cornea itself and its behaviour during the measurement to predict the results on different corneas. Instead of using mechanical properties of the cornea presented in the literature, a new specific material model has been developed. Methods: To adapt the behaviour of the model to the living cornea we performed measurements using Applanation Tonometry (GAT) with different applanation diameters. The geometrics of the Gulstrand standardized eye served to generate our parameterized finite element mesh using the B2000 finite element system. To simulate the measurements we performed the following steps: Set up the IOP represented by forces acting perpendicular to the cornea, applanating the cornea with a rigid plane body using increasing diameters of applanation, introducing the precise capillary force (calculations have been presented at ARVO 2003) and calculating the contact forces between rigid body and cornea. The material model was adjusted to achieve the best match between the integral of the calculated contact forces and the measurements. To simulate the behaviour of DCT we performed the same steps but used the concave DCT tip instead of a planar applanation tip. Results: A non linear material with an exponential stress to strain relation σ=C(εαε) describes closely the behaviour of the cornea during applanation (σ: stress, ε: strain caused by the mechanical load). The constants C=10’000N/mm and α=10.5 describe the material properties of the cornea. In contrast to the behaviour of a flat applanation tip, the force distribution between concave DCT tip and cornea is uniform and close to the forces generated by IOP. Conclusions: The finite element model shows how the principle of DCT works on the human cornea with its complex structure. It shows that the force distribution is minimally affected by geometrical variances of the cornea. Calculations of abnormal corneas will reveal the influence of keratoconus, scars and extreme astigmatism.
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