Computer modeling of ocular injury in infants exposed to acceleration

N. Rangarajan, S. Kamalakkannnan, T. Shams. GESAC Inc, Boonsboro, MD.

Alex Levin, MD, MHSc, FAAP, FAAO, FRCSC. Sickkids Hospital, Toronto.Carole Jenny, MD, MBA.

Brown University, Providence, RI.

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Objective

• To develop a finite element model of the infant eye.

• To evaluate the level of stresses and strains when the head is exposed to an acceleration pulse.

• Model to be exercised with acceleration pulses obtained from Aprica 2.5 kg infant dummy shaking tests.

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Presentation Road Map

• Description of the model• Parametric study• Discussion of results• Conclusions• Limitations• Work in progress

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The Model

• LS Dyna model consists of orbit, fatty tissue, extra-ocular muscles, sclera, retina, and vitreous.

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Model Description

• Key dimensions– Sclera, diameter = 20 mm – Rectus muscle length = 50 mm

• Contact definitions– Surfaces for orbit/fat/sclera/retina tied

together– Nodes on retina tied to vitreous surface

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Data for Prescribed Motion

Shaking experiment with Aprica 2.5 dummy. Click on picture to see baby being shaken. Large AVI file

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Prescribed motion

• Applied motion - rotational oscillations– Represents average data from shaking tests with

Aprica 2.5kg dummy

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Eye ball with muscles

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The Model – Vitreous and Retina Attachment

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Parametric Study

• Parametric study was conducted to evaluate the effect of variation in material models and properties of vitreous and fat on the maximum stress and stress distribution.

• Six cases were simulated.

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Simulation Matrix

Case 1 2 3 4 5 6

Vitreous

BK, G0 & Gi in MPa

V.E.BK=0.7PR=0.49G0=0.014

Gi=0.012

V.E.BK=0.7PR=0.49G0=0.014

Gi=0.012

V.E.BK=7.0PR=0.49G0=0.14

Gi=0.12

FluidVC=0.5PR=0.49

FluidVC=0.3PR=0.49

FluidVC=0.1PR=0.49

Fat

E, BK, G0 & Gi in MPa

ElasticE=0.047PR=0.49

V.E. (Viscoelastic)BK=0.7

PR=0.49G0=0.014

Gi=0.012

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Results

• Results of the simulation study are discussed – Stresses on retina – Stresses on the sclera

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Stress on Retina – Case 2

Stress on one of the elements in case 2

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Stress on Retina – Case 3

Stress on one of the elements in case 3

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Stress on Retina – Case 5

Stress on one of the elements in case 5

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Simulation – Case 2

Click on picture to see animation of simulation results. Large AVI file.

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Simulation – Case 5

• Click on picture to see animation of simulation results. Large AVI file.

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Stress Distribution on Sclera– Case 2

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Stress Distribution on Retina – Case 2

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Stress Distribution on Vitreous – Case 2

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Stress Distribution on Sclera – Case 5

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Stress Distribution on Retina – Case 5

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Stress Distribution on Vitreous – Case 5

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Comparison of Maximum Stress on Retina

* The calculation reaches an infinite loop at 0.94 sec, when case 5 has a maximum of 0.0964

Case 1 2 3 4 5 6

Maximum Stress (MPa)

0.01448 0.01476 0.00719 0.13899 0.13918 0.08775*

FatVitreous

ElasticV.E.

V.E.V.E.

V.E.V.E.

V.E.Fluid

V.E.Fluid

V.E.Fluid

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Conclusions - 1

• Stress on retina and sclera accumulates (increases) as the shaking continues for certain material models for vitreous, e.g. viscoelastic or fluid.

• Maximum stress occurs around the vitreous-retina contact area both in front and back.

• Property of vitreous has great effect on maximum stress and stress distribution.

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Conclusions - 2

• When using viscoelastic material for vitreous, smaller bulk modulus value shows a clearer stress accumulation effect. Rate of change of stress is more evident.

• When using fluid for vitreous, the viscosity coefficient does not show significant effect on maximum stress, stress accumulation effect, and stress distribution.

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Limitations - 1

• Current model does not include several important structures,such as lens, choroid, ciliary body, and cornea.

• Definition of distribution of fat and orbit geometry was approximated.

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Limitations - 2

• Input motion was purely rotational at centre of the orbit.

• All model input data were obtained from literature, material data not verified by experimentation.

• Material property data were scaled and appropriateness of scaling has to evaluated.

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Limitations - 3

• Effect of variation of mesh size, integration intervals and integration procedures have not been fully evaluated.

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Limitations - 4

• All the material models used in this study are currently available in LS-Dyna material library. It may be necessary to develop new material models to fully describe the material used in this model.

• This is a preliminary study to provide a qualitative picture of what happens within the infant eye under repeated motion and results should be interpreted with caution.

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Work in Progress - 1

• Study influence of additional features such as lens, choroid and ciliary body on model response.

• Develop a more accurate orbit geometry and fatty tissue distribution.

• Change center of motion to head CG. Both linear and angular motion will be used as input.

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Work in Progress - 2

• Evaluate effect of mesh size change.• Evaluate effect of meshing axis.• Examine response under purely linear

deceleration like a typical frontal crash pulse. This will be an indirect method of validation of the model.

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Component models under development - 1

Sclera and cornea

Choroid

RetinaPars plana and pars plicata

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Component models under development - 2

Fat

Lens

Orbit

Vitreous

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Component models under development - 3

Extra-ocular muscles Eye assembly

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Acknowledgements

• This work was supported by Aprica, inc. Japan. We thank Aprica project managers, Ms. P. Kawasaki and Dr. R.Bigge, MD, PhD.

• Dr. Levin, MD provided invaluable guidance and a push when needed!

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THANK YOU