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Austen M. Erickson, Fabian Conrad, Fabrice Manns, Derek Nankivil, David Borja, Esdras Arrieta-Quintero, Raksha Urs, Jean-Marie Parel, Arthur Ho; An Individualised Finite-element Model of Accommodation Based on Direct Measurements. Invest. Ophthalmol. Vis. Sci. 2011;52(14):820.
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© ARVO (1962-2015); The Authors (2016-present)
While finite element models (FEM) have been used to study accommodation, they have used geometric and mechanical property values based on sample-averages. We constructed an individualised FEM of accommodation based on direct measurements of an eye. With this FEM, we used a backward-calculation strategy to estimate the mechanical properties of the lens.
The lens, zonules, ciliary and sclera-ring of a baboon (age=7.8 yrs; PMT 26 hours) was mounted in a lens stretching system and stretched in a step-wise fashion (0.25 mm/step to 2.5 mm radially). At each step, load was recorded by tensometers and images were digitised to obtain lens profile, thickness and equatorial diameter and ciliary ring diameter. Lens contour was extracted, corrected for decentration and tilt then fit with an 11th-order even Fourier series.Using the fitted contour, a 2-D axisymmetric FEM of the lens, assumed to have uniform mechanical properties and near incompressible (Poisson ratio, =0.499), was constructed (189 surface nodes). Meshing the geometry with quadrilateral elements (average edge length=0.05 mm), the FEM was stretched to a maximum of 0.76 mm ciliary displacement over 21 steps (maximum load=3 MPa). Values for the criteria variables (CV; lens thickness and diameter, and zonule length) were extracted and compared to those from measurements. E-modulus of the lens (EL) and capsule (EC) were varied to obtain the best match to the CV. Zonules E-modulus was maintained at 1.5 MPa.
The moduli that returned the best match for lens diameter and zonule length were 0.375 MPa and 1.27 MPa for EL and EC respectively. With the assumed , lens thickness could not be matched with these values requiring instead EL=0.167 MPa. Reducing to 0.3 returned an improved match to the CV.
An FEM customised to an individual eye based on direct measurement has been constructed. For the sample tested, the best match between model and measurements was obtained when the lens was allowed to be compressible.
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