Head/brain injury is common. In 2013-14 in UK there were 162,544 admissions for head injury. That equates to 445 every day, or one every three minutes.
A blow to the head is going to impart energy both directly and indirectly but only cause injury if protective mechanisms are overcome.
Injury to the brain is conventionally classified as primary (immediate) or secondary (delayed). It is now recognised that this is an over-simplification, but the concept is still helpful for clarifying clinical priorities. Primary injury is the result of energy transfer to the brain at the moment of injury, and its severity reflects the amount of energy transferred. Secondary injury is due to complications, such as intracranial bleeding or brain swelling, and can be made worse by systemic complications of trauma such as a blocked airway, inadequate breathing, or major blood loss causing shock.
The severity of the injury is determined by the Glasgow Coma Score which stratifies patients according to how well their brains are working after head injury with Mild (GCS14-15), Moderate GCS 9-13) and Severe (GCS 3-8). A patient is by definition in a coma with a GCS of 8 or less, unable to protect their own airway.
The natural defences to injury are the coverings of the head-hair, scalp, skull, meninges and CSF. All of these help dissipate the energy imparted by absorbing energy and reducing the force acting on the brain. The brain is submerged in CSF within the arachnoid membrane and the floating of the brain helps protect against direct trauma but can cause a contra-coup injury.
Extra protection to the head is provided by vehicle design (crumple zone, seat belts, airbags etc), helmets
The amount of direct energy needed to break the skull has been calculated to be 3500N but is directly related to the speed the mass/weight is moving. A static pin is used to purposefully break the outer table of the skull at 60 lb/sq inch to secure the head for brain operations.
Rotational/shearing energies can cause severe injury without breaking the skull and the CSF suspension may exacerbate this tendency. Some of the frequent causes of damage are the relative motion of the brain with respect to the skull (brain retarded or set into motion subsequently by the skull); striking and bouncing of the parenchyma against inner skull protrusions; cavitation phenomena induced by negative pressures (volumetric or compression–tension damage); rupture of bridging veins, axonal fibers and vascular tissue (shear damage).
This concept of a mechanism for brain damage following trauma relates to the transient negative pressures generated in CSF opposite the point of impact causing tissue damage often greater than the contusional injury. Cavitation inception occurs when the local pressure falls sufficiently far below the saturated vapor pressure, a value given by the tensile strength of the liquid at a certain temperature, causing local pressure and tempersture change.
1. Biomechanics of traumatic brain injury
Tamer El SayedComput. Methods Appl. Mech. Engrg. 197 (2008) 4692–4701
Basic study of brain injury mechanism caused by cavitation.