Passive biomechanical characterization and modeling of bovine extraocular tissues

Reported mechanical properties of extraocular tissues (EOTs) have been too sparse to model constitutive relationships underlying biomechanical interactions in strabismus. Toward the goal of comprehensive biomechanical characterization of EOTs, two studies have been completed. To create a platform for future finite element (FE) simulations, a human orbit was rendered in 3-D from magnetic resonance images.;The first study describes the nonlinear, history-dependent constitutive relationship of the EOMs using a quasi-linear viscoelastic (QLV) model. From each of the six bovine oculorotary EOMs, longitudinally oriented specimens were taken from different regions and subjected to uniaxial tensile, relaxation, and cyclic loading testing using an automated load cell under temperature and humidity control. Elastic and reduced relaxation functions were fit to the experimental results, from which a QLV model was assembled and compared with cyclic loading data. Predictions of the QLV model agreed with observed peak cyclic loading stress values to within 8% for all specimens.;The second study focused on development of a multi-mode upper convected Maxwell (UCM) model for both orbital connective tissue and fatty tissue. From 20 fresh bovine orbits, 30 samples of connective tissue were taken from rectus pulley regions, and 30 samples of fatty tissues from the posterior orbit. Additional samples were defatted to determine connective tissue weight proportion, as verified histologically. Mechanical testing employed an oscillating disc triborheometer to perform strain sweeps; shear stress relaxation; viscometry; and dynamic tests. From experimental results, two multi- mode UCM models, which accurately described measured properties of both pulley and fatty tissues, were developed. Viscoelastic properties of pulley differ markedly from those of fatty orbital tissues, but both are accurately reflected using UCM models over a wide range of frequencies and strains.;Lastly, three-dimensional rendering of a human orbit was completed. Coordinates for cross sectional areas were extracted from coronal and sagittal MRI scans, along with guiding line required to complete rendering process. The coordinates were input into Solidworks, a computer aided design program, to render the human orbit consists of six rectus muscles, connective tissue and fat. Along with models developed in first two studies, groundwork for FE simulation has been completed.