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Christopher Chiu, Sangarapillai Kanapathipillai, Padmaja Sankaridurg, Arthur Ho; Feasibility of a simple in vivo scleral tensometer: a finite element model. Invest. Ophthalmol. Vis. Sci. 2019;60(9):4373. doi: https://doi.org/.
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It has been demonstrated that reduced scleral rigidity may play a role in axial elongation that leads to myopia development and progression (McBrien, 2003; Sergienko & Shargoroqska, 2012). To test such a hypothesis, an in vivo method is required to quantify the mechanical properties of the sclera. We used a finite element (FE) model to evaluate the feasibility of an in vivo scleral tensometer based on the principle of applanation tonometry.
A simplified model for scleral tensometry was constructed using Autodesk Inventor 2016 comprising the whole eye globe clad with the sclera without the cornea, and an applanation tensometer probe. The globe was modelled as a hemisphere (15 mm radius) and uniform scleral thickness (0.5 mm). For FE modelling (ANSYS Workbench 19), the material property of the sclera was assumed to be isotropic, homogenous and linear elastic. Three scleral moduli were tested (15 MPa to 25 MPa; Poisson’s ratio=0.49). To reduce computation cost, the globe was divided into two regions for mesh control with the higher density meshing near the region of applanation. The probe was assumed to be undeformable. A range of pressure loads (10 mmHg to 20 mmHg) were applied normal to the inner surface of the globe to model intraocular pressure (IOP).To model scleral applanation, an initial step displacement of the probe was increased until the area of applanation matched the target applanation diameter (3.06 mm) and the resultant displacement (0.2 mm) was used throughout the remainder of the study.The FE model was run to obtain probe force required for applanation under the combinations of different scleral moduli and IOPs.The approach taken for FE modelling of scleral applanation was validated against a series of measurements on squash balls.
The figure below plots the force required to applanate to a fixed target diameter for different scleral moduli and IOPs. It appears that resolution of scleral modulus with this technique can be down to a few MPa even with the influence on applanating force within a relatively normal range of IOP.
While the model requires further development and improvement to more realistically simulate in-eye conditions, as well as considerations for practical implementation constraints, this model demonstrates, as a first approximation, the potential for an applanation scleral tensometer.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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