Stress Analysis for Scleral Buckling of the Eye

Scleral buckling is a process in which a buckle or band is wrapped around the eye and tightened and is used to treat different eye disorders. The procedure can result in induced myopia by increasing the axial length of the human eye. This study was performed to assess how the application of a scleral buckle of various widths and tightness on eyes with decreased corneal thicknesses affects stresses and strains in the tissue and the anterior-posterior dimension. For this purpose, an axisymmetric finite element model of the eye was created where the mechanical properties of the tissues are assumed to be linearly elastic, the humors as incompressible fluid and the buckle as rigid. The buckles were chosen to have widths of 3, 5 and 7 mm with constrictions of 0.5, 1 and 1.5 mm and the reduced thicknesses of the cornea that were considered are 0, 25 and 50%. The results showed that as the buckle width and tightness increase, the axial length change of the eye increases. The maximum stress is greater for a thinner buckle with greater tightness. Also, the change in corneal thickness has a minor effect on the axial length and maximum stress. For scleral buckle selection, increased buckle width and construction lead to an increase in myopia. Eyes which have thinner cornea due to disease or LASIK procedure for example are more susceptible to this myopic shift than eyes with a normal corneal thickness

Gradient moduli lens models: how material properties and application of forces can affect deformation and distributions of stress

The human lens provides one-third of the ocular focussing power and is responsible for altering focus over a range of distances. This ability, termed accommodation, defines the process by which the lens alters shape to increase or decrease ocular refractive power; this is mediated by the ciliary muscle through the zonule. This ability decreases with age such that around the sixth decade of life it is lost rendering the eye unable to focus on near objects. There are two opponent theories that provide an explanation for the mechanism of accommodation; definitive support for either of these requires investigation. This work aims to elucidate how material properties can affect accommodation using Finite Element models based on interferometric measurements of refractive index. Gradients of moduli are created in three models from representative lenses, aged 16, 35 and 48 years. Different forms of zonular attachments are studied to determine which may most closely mimic the physiological form by comparing stress and displacement fields with simulated shape changes to accommodation in living lenses. The results indicate that for models to mimic accommodation in living eyes, the anterior and posterior parts of the zonule need independent force directions. Choice of material properties affects which theory of accommodation is supported

Characterisation and Modelling of an Artificial Lens Capsule Mimicking Accommodation of Human Eyes

A synthetic material of silicone rubber was used to construct an artificial lens capsule (ALC) in order to replicate the biomechanical behaviour of human lens capsule. The silicone rubber was characterised by monotonic and cyclic mechanical tests to reveal its hyper-elastic behaviour under uniaxial tension and simple shear as well as the rate independence. A hyper-elastic constitutive model was calibrated by the testing data and incorporated into finite element analysis (FEA). An experimental setup to simulate eye focusing (accommodation) of ALC was performed to validate the FEA model by evaluating the shape change and reaction force. The characterisation and modelling approach provided an insight into the intrinsic behaviour of materials, addressing the inflating pressure and effective stretch of ALC under the focusing process. The proposed methodology offers a virtual testing environment mimicking human capsules for the variability of dimension and stiffness, which will facilitate the verification of new ophthalmic prototype such as accommodating intraocular lenses (AIOLs)

A study for accommodating the human crystalline lens by finite element simulation

This paper constructs two finite element models of human crystalline lens and zonules based on published clinical data. Displacement and pressure were applied to study the mechanism of vision accommodation. The simulation results show that, in Model A, under the pull of the zonules, the thickness of the lens decreased linearly, and the lens diameter increased linearly. The optical power of the lens increased as the zonules displacement increased. Furthermore, the pressure had a remarkable influence on the shape of the lens and the optical power. The lens also became thinner and flatter as the pressure increased. The optical power increased when the pressure increased. In Model B, the lens became thicker and optical power increased as the equatorial zonules stretched. It is basically consistent with Schachar's hypothesis. The outcome of this paper proved that the analytical model presented in this paper can be used in the theoretical study of the accommodation mechanism of the lens. (c) 2006 Published by Elsevier Ltd

Finite element modelling of human eye lens

The human lens provides one-third of the ocular focussing power and is responsible for altering focus over a range of distances. This ability, termed accommodation, defines the process by which the lens changes its shape, in response to the movement of ciliary body, to adjust the refractive power. The accommodative ability gradually decreases with age such that around the fifth to sixth decades of life it is lost rendering the eye unable to focus on near objects. Current technologies are unable to effectively restore the requisite optical powers and accommodative ability of a presbyopic eye as the mechanism of accommodation is not fully understood. Plausible explanations, which are contradicted to each other, require definitive supports. Nevertheless, experimental evidences are difficult to obtain from living eye. Computational modelling serves as an alternative solution for the understanding of the physiological process of accommodation. An accurate and detailed model can closely simulate the in vivo behaviour of the eye lens. To date, the relevance of available models to the physiology needs to be further explored. The accuracy of any computational model highly depends on the input parameters. To build up a complete lens model one needs to seek resources from different studies and to assemble parameters of lenses from different subjects, which bring great challenges to this research field. The present work utilizes the Finite Element Analysis as the fundamental approach for investigating the mechanical and optical performances of lens models built at various ages based on input parameters from both in vivo and in vitro studies. The contributions of different ocular parameters to the accommodative loss are investigated i.e. the lens geometries, material properties, capsular thickness, capsular elasticity, zonular angles. Relations between two seemingly contradicting accommodative theories are demonstrated and possible explanations for the presbyopia are proposed