Analysis of Ex- and Endophthalmos with the Delft Finite Element Model for Orbital Mechanics

Abstract

Purpose: : The Delft Finite Element Model for Orbital Mechanics would be well suited to simulate ex- and endophthalmos like Graves’ disease. In this study the model will be tested in order to see if the mechanics of these diseases can be simulated.

Methods: : Orbital pressure in- and decrease were simulated with expansion and deflation of part of the tetrahedra constituting the model. To model the orbital fat we used Herrmann elements. This type of element is used for the modeling of incompressible materials and provides the hydrostatic pressure as an additional result. The hydrostatic pressure was considered the resultant of arterial and venous blood pressure, tissue osmotic pressure and other pressures. For the simulation of ex- and endophthalmos the orbital volume was increased and decreased; first in the muscle, then in the fat and finally in the fat and the muscle. Diseases were simulated with the model: Graves’ disease was modeled by increasing the osmotic pressure and the volume of the muscles and the orbital fat. Orbital tumor was simulated by adding orbital fat volume. Cachexia was simulated by reduction of the retrobulbar fat volume. Finally caroticocavernous fistula was modeled by increasing venous blood pressure.

Results: : Volume and pressure in- and decrease were applied stepwise until the numerical solution could not be reached by calculation. The model allowed for ±50% expansion and deflation. Tissue swelling and deflation caused by changes in volume resulted in ex- and endophthalmos. Adding 50% volume to the orbital fat resulted in exophthalmos of 6 mm. Pressure increase on the optic nerve was considerable, pressures up to 15 mmHg were calculated by the model. Reduction of the retrobulbar fat volume of 50% resulted in endophthalmos of 4 mm.

Conclusions: : Ex- and endophthalmos simulations are possible in the Delft Finite Element Model for Orbital Mechanics and provide insight in the mechanics of various orbital disorders. The model is now extended to simulate the effect of various surgical approaches.

Keywords: eye movements • orbit • extracellular matrix