Biomechanical modelling of the human eye

24. September 2011.

Zoltán Bocskai

Imre Bojtár

18th Inter-Institute Seminar for Young Researchers

Medical partner: Dr. Andrea KerekHonvéd Hospital - Department of Opthamology,

1134 Budapest - Róbert Károly körút 44.

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Content

- Subjects of the research

- Physiology of the human eye

- Geometry of the eye

- Material properties

- Refractive eye surgeries

- The FE model for analyses

18th Inter-Institute Seminar - Z. Bocskai

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Subjects of the Research

• The process of the accommodation (Helmholtz, Schachar)

• The reasons of presbyopia* (changes of geometry and material parameters): causes, symptoms and treatment?

• The effect of refractive eye-surgery (diminish of the cornea)

*presbyopia: Presbyopia usually begins around the age of 40, when people experience blurred near vision during reading, sewing or working at the computer.

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Anatomy of the human eye

The human eye is an organ which reacts to light for several

purposes.

Rod and cone cells in the retina could conscious light perception and vision including color differentiation and the perception of depth.

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Accommodation of the eye

The diameter of the eye: 24-25 mm. The cornea, the aqueous humor, and the vitreous body have 42 D refractivityThe lens refractivity is changeable: 20-24 DThe sum of the four refractive parts: ≈66 D

The light-ray goes through four refractive parts: cornea, aqueous humor, lens, vitreous body.

But only the lens is able to change its refractivity. !

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Accommodation of the eyeHelmholtz (1909) Schachar (1993)

The lens is held in a stateof radial tension by the zonular fibres that connect the ciliary body to the lens equator. During accommodation, contraction of the ciliary muscle causes the tension in the zonular

fibres to reduce and, as a consequence, the lens changes its shape. This change in shape causes the optical power of the lens to increase.

The ciliary body causes tension just several parts of the zonular fibers. In few fibers arise compression.

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The geometrical parameters

The lens:

The lens size is age-related.

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The material properties of the human eyeThe material properties are age-related:with age the elasticity will be more rigid.

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Refractive eye-surgeries

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- PRK (Photorefractive keratectomy)- LASIK (Laser-Assisted in-Situ Keratomileusis)

- IntraLASIK !

Anathomy ANSYS 3D continuum model1018th Inter-Institute Seminar - Z. Bocskai

FEM model

Some details from the complex modelThe geometry of my model

Ciliary bodyScleraCorneaIrisLens cortexCapsuleLens nucleusAqueous humor

Zonula fibers

Vitreous body

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The geometry of the model with fat and muscles

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Frictional connection:- Aqueous humor contacts- Vitreous body contacts- Between lens nucleus and cortex- Between lens capsule and cortex Material properties:- Linear elastic, isotropic except cornea and sclera (Uchio 1999)

Constructed model

Geometry section18th Inter-Institute Seminar - Z. Bocskai

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Three-dimensional model - 3D solid meshingThe muscles are not axi-symmetric.

Fat

Eyeball

Lens & zonulas

Muscles18th Inter-Institute Seminar - Z. Bocskai

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First result of the complex modelLoad: self-weight

Total deformation

Normal stress – X direction

Equivalent stress

Further results are in progress!

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First result of the complex model

Lens equivalent elastic strain

Eyeball - Normal stress – X direction

Sclera - shear stress - XY direction

Further results are in progress!

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Thank you for your attention!