1. March 6th, 2009 1 The Mechanical Behavior of Orbital Fat in a Finite Element Model of Orbital Mechanics by Frans-Willem Goudsmit

2. March 6th, 20092 Human eye movement To view objects when the head is moving Gaze towards new object of interest that pop up Maintaining gaze on interesting objects Follow objects as they move

3. March 6th, 20093 The human eye Previous mechanical models Need for a new model Finite element principle Construction of the model Results Conclusions

4. March 6th, 20094

5. 5 Koornneef L. Architecture of the musculo-fibrous apparatus in the human orbit. Acta Morpol Neerl-Scan 1977;15:35-64.

6. March 6th, 20096 Tissue interaction

7. March 6th, 20097 What is the relation between the material properties of the orbital fat and the mechanical behavior of the eye and eye muscles? What are the interactions between the moving parts and the orbital fat, in the orbit? Research questions

8. March 6th, 20098 Clinical relevance Orbital traumas, e.g. blow-out fracture

9. March 6th, 20099 Clinical relevance Orbital traumas, e.g. blow-out fracture Orbital tumors

10. March 6th, 200910 Clinical relevance Orbital traumas, e.g. blow-out fracture Orbital tumors Graves disease

11. March 6th, 200911 Clinical relevance Orbital traumas, e.g. blow-out fracture Orbital tumors Graves disease Surgery

12. March 6th, 200912 Clinical relevance Orbital traumas, e.g. blow-out fracture Orbital tumors Graves disease Surgery

13. March 6th, 200913 Previous models Complex tissue interactions are simplified with one single force vector Rotating sphere around a fixed point Exclusion or merger of tissue Simplified geometries

14. March 6th, 200914 Need for a new model A lumped model does not give insight in the complex interactions between the several tissues in the orbit. For full evaluation of the mechanics of the orbital fat a model with six degrees of freedom is needed.

15. March 6th, 200915 Finite element models Schutte S, van den Bedem SPW, van Keulen F, van der Heim FCT, Simonsz HJ. A finite- element analysis model of orbital biomechanics. Vision Research 2006;46:1724-1731.

16. March 6th, 200916 Finite Element Principle

17. March 6th, 200917 Finite Elements in a muscle

18. March 6th, 200918 Construction of a Finite Element Model of Orbital Mechanics Geometries Material Properties Tissue interaction Load cases

19. March 6th, 200919 Construction of a Finite Element Model of Orbital Mechanics Geometries Marien van Ditten Gerard Dunning Sieuwerd Laddé Klaas de Vries

20. March 6th, 200920 MRI-images

21. March 6th, 200921 Obtained surfaces Fifth order NURBS surfaces

22. March 6th, 200922 Finite Element Model 4-node tetrahedron mesh

23. March 6th, 200923 Construction of a Finite Element Model of Orbital Mechanics Geometries Material Properties

24. March 6th, 200924 Material properties Homogenous and isotropic Eye Optic nerve Fat Properties of fat were measured in the past Schoemaker et al., Elasticity, viscosity and deformation of retrobulbar fat in eye rotation. Invest Ophthalmol Vis Sci., 2006 Nov;47(11):4819-26.

25. March 6th, 200925 Material properties Eye muscles are modeled as homogenous orthotropic Muscle contracts along fibers

26. March 6th, 200926 Muscle

27. March 6th, 200927 Muscles Muscle contracts along fibers Direction dependent material properties No available software to model muscle tissue! We need a proper muscle model.

28. March 6th, 200928 Fiber orientation

29. March 6th, 200929 Contraction Contraction with constant volume Muscle contraction is simulated using a thermal expansion coefficient Negative in fiber direction Positive in other two directions

30. March 6th, 200930 Construction of a Finite Element Model of Orbital Mechanics Material Properties Tissue interaction Geometries

31. March 6th, 200931 Tissue interaction Fixed or sliding? Fat and orbital wall Muscles and eye Fat and optic nerve Fat and muscles Fat and eye Muscles and orbital wall Superior oblique and superior rectus muscle Inferior oblique and inferior rectus muscle

32. March 6th, 200932 Tissue interaction Are the interactions between the moving parts and the orbital fat based on sliding or on attachment? Two mechanical models Sliding Tissue attachment Results of horizontal rotation are compared with MRI

33. March 6th, 2009 33 First finite element model of the human orbit including sliding!!

34. March 6th, 200934 Construction of a finite element model of Orbital Mechanics Material Properties Tissue interaction Load cases Geometries

35. March 6th, 200935 Load case Series of loads and displacements to simulate a situation. Initial displacements in the model The outer boundary of the fat Back-end of eye muscles, fat and optic nerve

36. March 6th, 200936 Model vs in-vivo measurements Interpretation of results Validation of the model

37. March 6th, 200937 Load case 1 Pretension of the straight muscles

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39. March 6th, 200939 Load case 2 Contraction of a rectus muscle and relaxation of the antagonist resulting in rotation

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41. March 6th, 200941 Load case 3 & 4 Two forced duction tests Horizontal forced duction Torsional forced duction

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43. March 6th, 200943 Results

44. March 6th, 200944 Muscle paths

45. March 6th, 200945 Results y x

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47. March 6th, 200947 Tissue interaction

48. March 6th, 200948 Results Horizontal forced duction creates a displacement towards the direction of the nose Very soft orbital fat facilitates easy eye rotation Very soft fat gives enough support to the eye to rotate around a virtual point of rotation

49. March 6th, 200949 Conclusions The mechanical behavior of fat and eye muscles can be well described with the finite element model based on the known properties of the orbital fat. As confirmed by comparisons with in- vivo measurements. The predictions of the model can not be entirely validated with the use of a homogenous isotropic material. The eye can not rotate without sliding between the tissues inside the human orbit. Frictionless sliding between interacting tissues facilitates eye movements.