Biomechanical Basis of Traumatic Brain Injury

Youmans Neurological Surgery Ch.324

• Clinical classification of brain injuries• Mechanism of injury• Mechanistic cause of head injuries• Injuries and their mechanisms

Outline

Clinical classification of brain injuries

• skull fracture– may or may not involve damage to the underlying brain

• focal injury– generally limited to a well-circumscribed region– contusions to the cortex and subdural, epidural, and

intracerebral hematomas• penetrating injury• blast injury

Clinical classification of brain injuries

• diffuse brain injury– Mildest form : involve only alterations in neural excitability,

neurotransmission, or long-term changes in receptor dysfunction and associated disabilities : concussion

– prolonged coma without a mass lesion and involves some degree of structural derangement at the microscopic level

– Diffuse axonal injury

Mechanism of injury• Static or quasi-static loading• Dynamic loading

Static or quasi-static loading• Uncommon• Slowly, over times longer than 200 msec• Squeezing or crushing (seen in earthquakes, building

collapses, or machinery accidents)• fractures at the vault or basilar skull region• common that consciousness is preserved 

Dynamic loading• More common• Rapidly, duration less than 50 msec• Impulsive

– the head is set into motion indirectly by a blow to another body region

– inertial force,brain move within the skull• Impact

– combination of contact force and inertial (head motion) force– Contact phenomena(head not moving)

• > 2 inch localize skull bending• Shockwave localized change in pressure,distortion,injury

small hemorrhage and contusion• More common in children ; flexible, lower calcification

Distortion or strain

Distortion or strain• Compressive strain

– Compressive– Book/cylinder/table

• Tension strain– elongation

• Shear strain– Force applied along surface– Playing card

• Dilational strain(volumetric strain)– Pressure applied all exposed face

Mechanistic causes of head injuries

• Contact injuries• Head motion(inertial) injuries• Blast induced brain injuries

Contact injuries• Local contact effect

– Skull : Linear and depressed skull fracture,Basilar skull fracture, – EDH : associated with skull fracture or without skull fracture– Coup lesion : beneath the side of lesion

• Direct injury to the brain,surface vessel• High negative pressure from skull rapid snaps back to place

Contact injuries• Remote contact effects

– Vault fracture away from the impact site, basilar skull fracture, contrecoup

– Thick portion of the skull, relatively broad object– Stress wave radiate in 3D

• Formation of intermediate coup contusion• Scatterd ICH, traumatic intracerbral hematoma

– Global change in skull shape(infant and developing children) : • localized change in pressure small petechiae • fluctuation in intracranial volume brain herniation

Head motion(inertial injuries)• Acceleration-deceleration injuries• Brain tissue strain

– Differential movement of the skull and brain : localized strain of surface

– Strain within the brain parenchyma : : widespread disturbances in brain function and structure cerebral concussion and DAI

Type of head acceleration

Type of head acceleration• Translational acceleration

– Uncommon– Center gravity of brain move in straight line– Focal injuries(contrecoup, intracerebral, subdural hematoma )

• Rotational acceleration– Brain rotation without center gravity of brain moving– Highly injuries– High surface strain,deep surface strain

• Angular acceleration– Most common,compound translational and rotational– Neck anatomy– Most damaging (except : skull fracture and EDH)

Determinants of Acceleration injury

• Not only on the type of acceleration• The magnitude of acceleration propagational to the

amount of strain delivered to the brain• First : Brief acceleration c extremely high acceleration• Second : Slightly longer

– Strain restricted to periphery– skull, dura surface, brain periphery and vessel

• Third : Increase even more – Strain propagate to deeper– DAI

Blast-induced brain injuries• Temporary altermental status or confusion immediately

after blast• Primary bTBI

– Rapidly expanding wave,rapid rise and fall• Secondary bTBI

– Penetrating and non-penetrating injuries that occur when high velocity projectile/fragment impact the head

• Tertiary bTBI– Primary bTBI cause the victim to collide with fixed or mobile

object

Injuries and their mechanisms• Skull fracture

– Linear fracture• Contact effect secondary to impact

– Depressed fracture• Small(< 2 inch), hard, impact

– Basilar fracture• Impact or propagation of stress wave

Injuries and their mechanisms• Focal injury

– Epidural hematoma• More complex case of linear skull fracture• Head motion and inertial do not cause

– Coup contusion• Under impact point• Skull rebounding

– Contrecoup contusion• Cavitation effect : negative pressure at opposite loading point cause damage + small bubble turn to normal pressure, small bubble rupture

Injuries and their mechanisms– Intermediate coup contusion

• Impact generate shock wave– Intracerebral hematoma

• Impact• acceleration

– Tissue tear hemorrhage(microhemorrhage)• Inertial or head motion• Superior frontoparietal white matter, corpus callosum,

centrum semiovale, periventricular white and gray matter, internal capsule, basal ganglia

Injuries and their mechanisms• Diffuse brain injury

– Cerebral concussion• Inertial loading,not form contact phenomena effect• Angular rotation

– Diffuse axonal injury• Angular rotation