Solid–fluid interaction in force and energy transmission in shaken baby syndrome

A problem presented at the UK MMSG Loughborough 2008.

Presented by:
Dr Donal McNally (Institute of Biomechanics, University of Nottingham)
LR Band, BS Brook, RJ Dyson, GW Jones, D McNally

Problem Description

Current injury thresholds for shaken baby syndrome are empirical combinations of angular velocity and angular acceleration based on single cycle events. These thresholds are nearly impossible to exceed by shaking a baby, yet injuries are reported resulting from such abuse. In reality, shaking is a multicycle event, with motion and loading of the brain coupled through a fluid layer of cerebrospinal fluid (CSF) and there is the possibility for energy accumulation and resonance.

The study group is asked to consider simple models for a brain within a skull separated by a layer of fluid which can be cyclically accelerated linearly and rotationally, in order to create create new, realistic injury thresholds that incorporate the cyclical nature of the injury.

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Study Group Report

During the study group we investigated two different mechanisms for brain damage during shaken baby syndrome: those caused by the elastic deformation of the brain, and those caused by the presence of the viscous CSF layer leading to rupture of bridging vessels.

We first consider the brain to be an incompressible linearly-elastic structure; in this case the motion of shaking can be decomposed into a longitudinal and a rotational part, which may then be considered separately. The effect of the thin layer of CSF within this model is captured by allowing tangential slip along the brain-skull boundary, whilst enforcing zero penetration. We investigate which of the decomposed motions will cause the largest deformation, and calculate the frequencies at which resonance within the brain will occur.

We then consider the dynamics of the CSF layer. Representing the brain and skull as square, rigid bodies with the brain contained within the skull, surrounded by a thin layer of CSF, we analyse the fluid dynamics of the CSF layer to find the variations in CSF thickness caused by oscillations of the skull. This is turn can be used to investigate whether the bridging vessels which attach the brain to the skull are likely to rupture.

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