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Katherine Knaus, AnnMarie Hipsley, Silvia Blemker; How do changes in scleral stiffness with age affect ciliary muscle function in accommodation?. Invest. Ophthalmol. Vis. Sci. 2017;58(8):2062.
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© ARVO (1962-2015); The Authors (2016-present)
Changes to the accommodative mechanism (AM) with age have been well documented, including increased sclera stiffness (SS) that may affect ciliary muscle (CM) function. How this contributes to presbyopia is not well understood, due to the difficulty of measuring the AM in vivo and studying these changes in isolation, particularly the activation dynamics of the CM fiber groups: circular (C), radial (R), and longitudinal (L). The goal of this study was to create a physics-based computer simulation to elucidate the biomechanical effects of SS on CM function and the AM. We hypothesize that altered SS, consistent with aging, will reduce CM deformation, increasing the required CM activation required for normal function of the AM.
We created a three-dimensional finite element computer model of the AM that includes the CM, sclera, lens, zonules, and other connected structures (fig1A). Dimensions and material properties for the model were taken from young adult human experimental measurements published in literature. Accommodation was simulated by applying activation to the CM fiber groups. CM activation levels in healthy accommodation were determined by matching model predictions to published CM deformations. To validate the model, predictions of lens deformation were compared with published lens shapes during healthy accommodation. Varying SS and CM activation were simulated and the corresponding effect on CM deformation was predicted.
The computational model predictions of lens deformation in normal accommodation compare favorably with experimental data from literature. While maintaining CM activations at young healthy levels, changing SS from 1.6 to 2.8 MPa (consistent with the difference between young and old adults), reduces CM apex thickening by 6% and shortening by 22% (fig1B). Further, to produce CM deformation corresponding to full accommodation, the model with increased SS required increased activation levels in all three CM fiber groups (C=5%, R=14%, L=21%).
Increasing SS reduced CM deformation, which is necessary for normal function of the AM. While the CM is not limited enough to fully account for presbyopia, model results suggest that sclera stiffening does inhibit the AM. To overcome age-related changes in the sclera, the CM must increase its activation above 20% in sections. These results motivate treatments for presbyopia that focus on improving CM function.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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