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.

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. 1/17/2019 GESAC, Inc

Presentation Road Map Description of the model Parametric study Discussion of results Conclusions Limitations Work in progress 1/17/2019 GESAC, Inc

The Model LS Dyna model consists of orbit, fatty tissue, extra-ocular muscles, sclera, retina, and vitreous. Oribt modelled as rigid shell Fatty tissue is either viscoelastic or elastic 8 noded brick elements Extra occular muscles are spring and dampers in parallel Sclera is thick shell elements Retina was a thick shell elements Vitreous is solid brick 8-noded brick elements 1/17/2019 GESAC, Inc

Model Description Key dimensions Contact definitions 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 Data for sclera and muscle length obtained from Eric Powers’ thesis at VA Tech. Scaled for infants after consultation with Levin. Oribt surface nodes tied to sclera nodes. Allows shear but cannot detach. 1/17/2019 GESAC, Inc

Data for Prescribed Motion Shaking experiment with Aprica 2.5 dummy. Click on picture to see baby being shaken. Large AVI file 1/17/2019 GESAC, Inc

Prescribed motion Applied motion - rotational oscillations Represents average data from shaking tests with Aprica 2.5kg dummy Angular acceleration data obtained from tess. So, was angular Displacement. Angular velocity was obtained by differentiating angular displacement. Average peak velocity was plotted with time and average duration. Pulse is repetitive, same amplitude and duration. Positive – Negative angular motion was 80-90 degrees in each Direction. This is typical. 1/17/2019 GESAC, Inc

Eye ball with muscles 1/17/2019 GESAC, Inc

The Model – Vitreous and Retina Attachment Attachments were made in the back between vitreous and retina. This is because most blood vessals occur there. Attachments were also made in the front. The sclera surround the Vitreous. Picture horizontally from front [left ] to back Top of the eye is on top of the picture. 1/17/2019 GESAC, Inc

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. 1/17/2019 GESAC, Inc

Simulation Matrix Case Vitreous Fat V.E. Fluid Elastic 1 2 3 4 5 6 Vitreous BK, G0 & Gi in MPa V.E. BK=0.7 PR=0.49 G0=0.014 Gi=0.012 BK=7.0 G0=0.14 Gi=0.12 Fluid VC=0.5 VC=0.3 VC=0.1 Fat E, BK, G0 & Gi in MPa Elastic E=0.047 V.E. (Viscoelastic) Material properties obtained from Powers thesis. Vitreous is a viscous gel. Fat could be elastic or viscoelastic. It turned out that VE model of Fat resulted in better control of motion but did not make much Difference in the strains under similar excitation. For the fluid model the vitreous and retina are attached all nodes. PR = Poissons ratio is close to 0.5 or imcompressible. 1/17/2019 GESAC, Inc

Results Results of the simulation study are discussed Stresses on retina Stresses on the sclera 1/17/2019 GESAC, Inc

Stress on Retina – Case 2 Stress on one of the elements in case 2 1/17/2019 GESAC, Inc

Stress on Retina – Case 3 Stress on one of the elements in case 3 1/17/2019 GESAC, Inc

Stress on Retina – Case 5 Stress on one of the elements in case 5 1/17/2019 GESAC, Inc

Simulation – Case 2 Click on picture to see animation of simulation results. Large AVI file. 1/17/2019 GESAC, Inc

Simulation – Case 5 Click on picture to see animation of simulation results. Large AVI file. 1/17/2019 GESAC, Inc

Stress Distribution on Sclera– Case 2 1/17/2019 GESAC, Inc

Stress Distribution on Retina – Case 2 1/17/2019 GESAC, Inc

Stress Distribution on Vitreous – Case 2 1/17/2019 GESAC, Inc

Stress Distribution on Sclera – Case 5 1/17/2019 GESAC, Inc

Stress Distribution on Retina – Case 5 1/17/2019 GESAC, Inc

Stress Distribution on Vitreous – Case 5 1/17/2019 GESAC, Inc

Comparison of Maximum Stress on Retina Case 1 2 3 4 5 6 Maximum Stress (MPa) 0.01448 0.01476 0.00719 0.13899 0.13918 0.08775 * Fat Vitreous Elastic V.E. Fluid * The calculation reaches an infinite loop at 0.94 sec, when case 5 has a maximum of 0.0964 1/17/2019 GESAC, Inc

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. 1/17/2019 GESAC, Inc

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. 1/17/2019 GESAC, Inc

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. 1/17/2019 GESAC, Inc

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. 1/17/2019 GESAC, Inc

Limitations - 3 Effect of variation of mesh size, integration intervals and integration procedures have not been fully evaluated. 1/17/2019 GESAC, Inc

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. 1/17/2019 GESAC, Inc

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. 1/17/2019 GESAC, Inc

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. 1/17/2019 GESAC, Inc

Component models under development - 1 Sclera and cornea Choroid Pars plana and pars plicata Retina 1/17/2019 GESAC, Inc

Component models under development - 2 Vitreous Lens Orbit Fat 1/17/2019 GESAC, Inc

Component models under development - 3 Extra-ocular muscles Eye assembly 1/17/2019 GESAC, Inc

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! 1/17/2019 GESAC, Inc

THANK YOU 1/17/2019 GESAC, Inc