A computational study of the passive mechanisms of eye restraint during head impact trauma.

A finite element model of the eye and the orbit was used to examine the hypothesis that the orbital fat provides an important mechanism of eye stability during head trauma. The model includes the globe, the orbital fat, the extra-ocular muscles, and the optic nerve. MRI images of an adult human orbit were used to generate an idealized geometry of the orbital space. The globe was approximated as a sphere 12 mm in radius. The optic nerve and the sclera were represented as thin shells, whereas the vitreous and the orbital fat were represented as nearly incompressible solids of low stiffness. The orbital bone was modelled as a rigid shell. Frontal head impact resulting from a fall onto a hard floor was simulated by prescribing to the orbital bone a triangular acceleration pulse of 200 g (1962 m/s(2)) peak for a duration of 4.5 ms. The results show that the fat provides the crucial passive mechanism of eye restraint. The mechanism is a consequence of the fact that the fat is incompressible and that its motion is restricted by the rigidity of the orbital walls. Thus, the acceleration loads of short duration cannot generate significant distortion of the fat. In contrast, the passive muscles provide little support to the globe. When the connection between the orbital fat and the eye is absent the eye is held mainly by the optic nerve. We discuss the possible role that this loss of contact may have in some cases of the evulsion of the eye and the optic nerve.