Biomechanics of the brain

Question 1

Densely vascularised:

Answer 1

70% water

- vascularised more so in grey matter

Question 2

layers

Answer 2

superficial to deep:
scalp > skull > Periosteal dura (attached 2 inner surface of skull) > Meningeal dura >
arachnoid ((closely adhered 2 meningeal dura) > Cerebral vessels: arteries & veins

Question 3

Cerebrospinal fluid

Answer 3

• provides buoyancy for the brain within the skull
• 1500g brain net weight ~50g
• cushioning of mechanical forces

Question 4

Brain tissue mechanical properties

Answer 4

• very soft - deformable
• incompressible - expands radially when compressed
• viscoelastic
• very low permeability: ~4 x 10-12 m4/Ns (Bilston)
at high strain rate >10s-1 injury threshold therefore strains >20%
Tract orientated strain thresholds for injury ~ 6-7%

Question 5

Regional inhomogeneity in structure & behaviour

Answer 5

• white matter behavior is more anisotropic than gray matter
• different degrees of anisotropy exist within the white matter
• correlates with the neurostructural organisation”

brainstem and internal capsule = vertically oriented
• corona radiata = multi-axial
• corpus callosum = L-R orientation

Question 6

Differences in shear modulus in rapid shear testing

Answer 6

CC = more anisotropic than CR and resists shear more in ant. > posterior direction
btwn grey and CR - no significant diff.
grey m and corpus cooll = big diff. with grey m x2 stiffness?E in shear meaning CC is more compliant and will undergo more starin when subjected to shear force.

Question 7

Shearing forces and inertial loading are related to brain mass

Answer 7

smaller brains can tolerate much greater acceleration/deceleration forces - smaller animals

Quadrupeds - the long axes of the brain and spinal cord are parallel
Humans - brain and spinal cord almost at right angles which may increase rotational shearing forces
- brain bulk modulus (compression) = 5 x shear modulus

Question 8

External loading to the head may affect the intracranial contents by:

Answer 8

Direct contact: displacing or deforming the skull or intracranial partitions high focal KE and relatively low cranial momentum

Differential motion between skull / dura and the intracranial contents acceleration/deceleration forces impart large-momentum, rotational,
tensile and shear forces but relatively low kinetic energy

i.e. contents lagging behind the motion of the skull during acceleration of the head (inertial or impulsive phenomena)

Question 9

External loading to the head may affect the intracranial contents by: 2

Answer 9

• Oblique impact most common
• Produces both linear & rotational head kinematics
• Brain is more sensitive to rotation than linear translation
• if head free to move and impact with rapid and oblique force > high rate acc/decl. forces which go to brain = unconsciousness