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K. W. Hollman, M. O'Donnell; Modeling Elasticity Distribution in a Presbyotic Lens. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3832.
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© ARVO (1962-2015); The Authors (2016-present)
There are many theories describing accommodation and presbyopia. Based on recent measurements in our lab and others, we focus on modeling the contribution from general lens elasticity and the elasticity distribution. These measurements show a constantly varying elasticity as a function of radial distance.
We have developed a finite element model that approximates this varying elasticity as nine spherically concentric layers within the lens. This distribution is based on our measurements and optical microscopy of layer geometry. We do not just model physiology but vary parameters in the model to separate the biomechanical effects of elasticity distribution from the increase in composite elasticity.
The model verifies that an average elasticity increase does produce accommodation loss. For a soft average lens with an average Young’s modulus of 0.67kPa, a ciliary muscle force of 0.05N produces a change in optical power of 3.8D. In a hard average lens (Young’s modulus = 4.0kPa) the same force produces a change of only 0.6D. Results also show that a lens with a soft center will accommodate more than a lens with a hard center even though both have the same average elasticity. For a ciliary muscle force of 0.05N, the soft center model produces a change in optical power of 3.2D while the hard center lens with the same average elasticity produces a change of 0.30D. Of course, in actual physiology both mechanisms would simultaneously affect accommodation in addition to mechanisms proposed by other theories.
This new model indicates that the elasticity distribution significantly contributes to accommodation loss. Methods for correcting presbyopia need to address not just average lens softening but the location of the softening.
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