Elasticity, Viscosity and Deformation of Retrobulbar Fat in Eye Rotation

Abstract

Abstract: : Purpose:The eye is pulled on the retrobulbar fat by pretension in the 4 rectus muscles. The sclera slides within Tenon, but this is limited by the optic nerve, deformation of the retrobulbar fat is therefore necessary for enabling eye rotations that excess sliding.We measured the material properties of retrobulbar fat and, with MRI, its deformation dependence upon gaze. Methods:Elasticity and viscosity of retrobulbar fat of 8 calve and 2 rhesus monkey orbits were measured 1–3 h post mortem with a parallel plate rheometer. A prior strain sweep at fixed frequency showed linear viscoelasticity from 0% to 130% deformation. During subsequent frequency sweeps at 5% deformation the lower plate was rotated sinusoidally at fixed strain amplitude at angular frequencies that stepwise decreased from 100 to 0.1 rad/s over 5 min. The pressure exerted on the specimens was chosen near the in vivo retrobulbar pressure. Orbital MRI's were made in 2 volunteers in 7 gaze positions, T1–weighted, 3–D gradient echo scan, in a 1.5 Tesla scanner. Orbital structures were segmented, using the venous vascular tree as marker for fat deformation. Results:Elastic shear modulus of calve retrobulbar fat was 250–500 Pa, of kidney fat 3000 – 5000 Pa, and of monkey retrobulbar fat 500–900 Pa. Viscous shear modulus of calve retrobulbar fat was 80–150 Pa, of kidney fat 1000–2500 Pa, and of monkey retrobulbar fat 300–500 Pa. 3–D deformation of the fat found with MRI was visualized. By describing the displacement of the fat relative to the eye as a function of sclera distance, we could distinguish between sliding of the sclera within Tenon and deformation of the fat. Conclusions:Elasticity of retrobulbar fat is low to suit its function as bearing material, viscosity is low to minimize energy loss during eye rotation. Apart from deformation of the fat, the sclera slides within Tenon not in proportion to eccentricity of gaze. The part of sliding within Tenon increases when increasing the angle of gaze.

Keywords: eye movements • strabismus • anatomy