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

March 6th, 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

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

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

March 6th, Tissue interaction

March 6th, 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

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

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

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

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

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

March 6th, 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

March 6th, 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.

March 6th, 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:

March 6th, Finite Element Principle

March 6th, Finite Elements in a muscle

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

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

March 6th, MRI-images

March 6th, Obtained surfaces Fifth order NURBS surfaces

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

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

March 6th, 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):

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

March 6th, Muscle

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

March 6th, Fiber orientation

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

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

March 6th, 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

March 6th, 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

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

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

March 6th, 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

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

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

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

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

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March 6th, Results

March 6th, Muscle paths

March 6th, Results y x

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March 6th, Tissue interaction

March 6th, 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

March 6th, 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.