Parametric studies of optic chiasmal compression biomechanics using finite element modelling

A tumour of the pituitary gland can compresses the chiasm from below and can cause bitemporal hemianopia. This is a type of partial blindness caused by damage to the crossed nerve fibres which carry the information from the temporal visual field. So far, the reason why chiasmal compression selectively damages the crossed nerve fibres is still unclear. In vivo experiments have been performed to investigate the mechanical behaviours of the optic chiasm, but none of them has provided detailed stress/strain distributions due to the complexity of the experiment. In this paper, three-dimensional finite element models were built in ANSYS to investigate the stress distribution in the optic chiasm when the chiasm was elevated by a pituitary tumour that grows beneath it. Optic nerve fibre models were also established to investigate the difference in resulting stress distributions in the crossed and uncrossed fibres. Parametric studies were conducted to measure the dependence of the resulting stress fields on alterations in the optic nerve sheath thickness, elastic modulus of the pial sheath and nerve tissue, and angles between nerve fibres. The study showed that changes of all these parameters above can affect the stress value in the chiasm significantly. Although marked absolute differences were found in these models, the stress distributions showed similar trends in all conditions. It was found that the central aspect of the chiasm always bore higher stresses than the peripheral aspect of the chiasm. In the nerve fibre models, for a given load the average stress increased with increasing crossing angle between nerve fibres.