Abstract
Purpose :
Previous finite element models of orbital mechanics crashed due to extreme deformation of the orbital fat in eye rotation. We solve these problems by using fluid cavities.
Methods :
We used Abaqus v6.14 to model orbital fat as a fluid cavity (a bag filled with a fluid). The walls of the cavity consist of 2D shell elements. The fat was assumed to be incompressible with zero visco-elasticity. The eye, modelled as a rigid ball, was pressed - this force representing the force of 2 rectus muscles - into a bin with one fluid cavity representing the orbital fat. it was assumed that there was no shearing between eye and fluid cavity.
Then we modelled the interface between the sclera and Tenon’s capsule as a double-layered flat fluid cavity (fig 1, top half). We exerted a torque to study the deformation and displacement of the fat during eye rotation. Outcomes were stresses and strains in the cavity walls and translation of the center of the eye.
Results :
We used pre-formed cavities to avoid large deformations and stresses that occurred when the eyeball was pressed into a flat surface. In this model a combined force of 0.16 N for medial and lateral rectus muscles moved the eye 0.4 mm into the fat. The fat deformed with smooth parabolic curves around the eyeball. When changing the forces to 0.06 N on the right side and 0.10 N on the left side, an eye rotation of 6.8 degrees was obtained. The center of the eye displaced 0.1-0.5 mm.
Modelling the orbital fat as a single component resulted in too much fat moving with the eye. This decreased when a cilindrical membrane separating intra- from extraconal fat was added (fig 1, bottom half).
It was possible to rotate the eye within Tenon’s capsule with almost no friction by use of a flat fluid cavity.
Conclusions :
Stable rotation of the eye results from the combined action of pulling muscles and resulting pressure of the fat behind the eyeball (fig. 2).
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.