Aging impact on brain biomechanics with applications to hydrocephalus.

Hydrocephalus is a neurological disorder whose clinical symptoms and treatment outcome are correlated with patient age. In Wilkie et al. (2010, A theoretical study of the effect of intraventricular pulsations on the pathogenesis of hydrocephalus. Appl. Math. Comput., 215, 3181-3191), the fractional Zener model was used to investigate the role of cerebrospinal fluid pressure pulsations in the development of hydrocephalus in infants and adults. In this paper, we determine the mechanical parameters of the fractional Zener model for the infant and adult brains using age-dependent shear complex modulus data (Thibault, K. L. & Margulies, S. S. (1998) Age-dependent material properties of the porcine cerebrum: effect on pediatric inertial head injury criteria. J. Biomech., 31, 1119-1126). The displacement of brain tissue under conditions representing the onset of hydrocephalus are then calculated. The infant brain was found to produce tissue displacements that are unphysical for our model geometry and a new boundary condition is proposed to replace the stress-free outer boundary condition used in Wilkie et al. (2010). The steadystate elastic modulus is identified as the parameter of interest in the development of hydrocephalus: it is found to increase from the infant value of 621 Pa to the young adult value of 955 Pa and we hypothesize that it then decreases with age. The low steady-state elastic modulus of the infant brain (and possibly the aged brain) increases the tissue's susceptibility to large deformations and thus to the ventricular expansion characteristic of hydrocephalus.