Problems in Modeling Orbital Soft Tissue with the Finite Element Method

The human eye is suspended in the orbit, a cavity in the skull that is largely filled with fat (adipose tissue). Fat is a gelatin like substance contained within fat cells surrounded by connective tissue membranes. Fat consists for 80 % of water, therefore it is almost incompressible. Fat supports the eye and eye muscles in the orbit, four rectus muscles pull the eye back on the fat. The eye slides and rotates in Tenon’s capsule, a sack that surrounds the backside of the eye. This sack is filled with synovial fluid, which can also be found in joints in the body. Besides sliding and rotating the eye may translate; eye rotation is not necessarily about a fixed point. This gives the eye six degrees of freedom, three translations and three rotations. The eye is able to rotate for about 50° in left and right gaze. Modeling such a complex three dimensional soft tissue structure gives more insight in functionality of anatomical structures. Such a model can be created with the finite element (FE) method. In the FE method a structure, for example a muscle is subdivided into little cubes. In case of the orbit, first the contours of anatomical structures are determined. These contours are filled with cubes (or elements) such a structure determined by cubes is called a mesh. Once the mesh is created, the elements are assigned material properties like elasticity. Then a simulation of movement of the structure is possible, for example caused by a contraction of an eye muscle. Such a FE model of orbital mechanics was built in 2001 in Delft. This model was able to rotate the eye for 15°, rotations for more than 15° were not possible in the model. It was not clear what caused this limited rotation. An investigation was needed to identify the problem of limited rotation. This study was done to identify problems in the model and seek for possible solutions. The goal was to simulate full eye rotations with the model.Precision and Microsystems EngineeringMechanical, Maritime and Materials Engineerin