Abstract

To reproduce human in vivo accommodation numerically. For that purpose, a finite element model specific for a 29-year-old subject was designed. Once the proposed numerical model was validated, the decrease in accommodative amplitude with age was simulated according to data available in the literature. In contrast with previous studies, the non-accommodated eye condition was the reference configuration. Consequently, two aspects were specifically highlighted: contraction of the ciliary muscle, which was simulated by a continuum electro-mechanical model and incorporation of initial lens capsule stresses, which allowed the lens to become accommodated after releasing the resting zonular tension. The morphological changes and contraction of the ciliary muscle were calibrated accurately according to the experimental data from the literature. All dynamic optical and biometric lens measurements validated the model. With the proposed numerical model, presbyopia was successfully simulated. The most widespread theory of accommodation, proposed by Helmholtz, was simulated accurately. Assuming the same initial stresses in the lens capsule over time, stiffening of the lens nucleus is the main cause of presbyopia.

Highlights

  • Accommodation is a dynamic change in the dioptric power of the eye that allows a change in focus from distant to near objects

  • There is a gradual transition from the outermost longitudinal muscle fibres through the radial fibres to the innermost circular muscle fibres, with some intermingling of the different fibre types.4-­6 Contraction of the entire ciliary muscle as a whole pulls the anterior choroid forward, moving the apex of the ciliary processes towards the lens equator, and serves the primary function of releasing resting zonular tension at the lens equator to allow accommodation.[4,5]

  • As the lens equatorial diameter cannot be obtained by optical coherence tomography (OCT), it was extracted from the MRI work of Kasthurirangan et al.[29] with the value of 9.04 mm being obtained

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Summary

Introduction

Accommodation is a dynamic change in the dioptric power of the eye that allows a change in focus from distant to near objects. This process is achieved by changing the shape of the lens, following contraction of the ciliary muscle, thereby releasing the zonular tension around the lens equator, according to the widely accepted accommodation theory of Helmholtz.[1] Numerous authors have confirmed this theory over time, including Fincham,[2] who demonstrated the ability of the elastic lens to round up after the release of zonular tension, and Glasser and Kaufmann,[3] who showed relaxation of the zonules due to ciliary muscle contraction along with other investigators.[4]. There is a gradual transition from the outermost longitudinal muscle fibres through the radial fibres to the innermost circular muscle fibres, with some intermingling of the different fibre types.4-­6 Contraction of the entire ciliary muscle as a whole pulls the anterior choroid forward, moving the apex of the ciliary processes towards the lens equator, and serves the primary function of releasing resting zonular tension at the lens equator to allow accommodation.[4,5] During contraction of the ciliary muscle, the circular portion of the ciliary muscle tends to increase in thickness, whilst the radial and longitudinal portions decrease in thickness

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