Mechanical and structural characterisation of the cranial meninges

The meninges are membranous tissues which are pivotal in maintaining homeostasis of the central nervous system. Despite the importance of the cranial meninges in nervous system physiology and in head injury mechanopathology, our knowledge of the tissues’ mechanical behaviour and structural composition is limited. Improved understanding of traumatic brain injury (TBI) mechanopathology is desirable to improve preventative measures, such as protective helmets, and advance TBI diagnostic/prognostic capabilities. The mechanistic role of the various intracranial tissues during TBI is still a subject of debate, in part due to a lack of experimental data on tissue mechanical, geometrical and structural properties to inform head injury model design. While the mechanical response of the brain and its many subregions have been studied extensively, the meninges have conventionally been overlooked as passive, inert sacs. This thesis focuses on advancing understanding of the cranial meningeal tissues’ mechanical, geometrical and structural characteristics to aid in advancing the biofidelity of head injury models.

The thesis begins with a systematic literature review which identifies a dearth of published studies detailing meningeal mechanical and structural behaviour. Of the limited number of studies published, a wide range of experimental results have been reported with little consensus on how mechanical characterisation data should be collected and reported. Similar results were found for geometrical and structural properties of the tissues.

Initially, porcine tissues are characterised to investigate meningeal mechanical, geometrical and structural heterogeneities. Using a combination of uniaxial tensile testing, Western Blot analysis and histological staining, it was identified that the porcine superior sagittal sinus (SSS), the largest of the dural venous sinuses, is mechanically and structurally heterogeneous to dura mater tissue. Additionally, scanning electron microscopy analysis of the SSS identified layer-specific collagenic alignment, which may explain the observed mechanical heterogeneity. To explore the sensitivity of the meningeal tissues to the effects of ‘subfailure injury’, varying magnitudes of subcatastrophic tensile stretches (i.e. stretches which did not induce tissue failure) were conducted on porcine dura mater tissues. It was found that dura mater tissue underwent subfailure damage at relatively low magnitudes of subfailure stretch, at least when compared to arterial and ligament tissues. In addition, repeated stretching of the dura mater, to mimic repeated TBI, resulted in a decay of peak stresses and shifted the stress-stretch relationship of the tissue, particularly at higher magnitudes of stretch. Finally, biaxial tensile analysis of human dura mater, falx cerebri and SSS tissues identified that these tissues are mechanically heterogeneous, in contrast to the assumption typically employed in FE models that the tissues are mechanically homogeneous. Elastic moduli from the low strain region of the stress-stretch curves are provided as an alternative to the conventional linear elastic model value of 31.5 MPa typically assigned to these tissues. Additionally, a thickness of 0.91 mm for the falx cerebri was identified, which is significantly smaller than the value of 1.5 mm assigned to the structure in FE models. The use of a novel collagen hybridising peptide on the SSS also suggests that this structure is particularly susceptible to the effects of circumferential stretch, which may have important implications for clinical treatment of dural venous sinus pathologies.

Collectively, this research progresses understanding of meningeal mechanical and structural characteristics and may aid in elucidating the behaviour of these tissues in healthy and diseased conditions.