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K. H. Yang and H. Mao
5.3.5 Types of Injury to Be Simulated
Because an FE head model can only be used to calculate the corresponding
responses at the instant of impact, secondary head injury (defined as the pathological, physiological, and biochemical changes to the brain tissue after the primary
injury) is beyond the capability of current FE head models and will not be discussed
in this chapter. A good understanding of the injury mechanism is one of the
most important components when studying injury prevention. Without knowing
the proper injury mechanism and the associated injury threshold, it is not possible
to use an FE head model to predict the type, location, and severity of TBI. As
previously discussed in Sect. 5.2 of this chapter, it is unfortunately that proper injury
mechanisms have not been appropriately established yet.
The mechanism of open- and closed-head injury is quite different. Because skull
and facial bone fractures do not always correlate with brain injury, such fractures
are not simulated in most FE head models. Nevertheless, readers should be aware
that a fractured skull could affect subsequent intracranial response and a skull
fracture is more complicated than a long bone fracture. Gurdjian and Webster [26]
in their stress-coating and strain gauge study showed that there was a momentary
in-bending at the site of impact and out-bending at the adjacent regions with linear
fractures occurring in the out-bended area. Additionally, an instantaneous increase
in intracranial pressure upon impact may contribute to the incident rate of skull
fracture or to the fracture length. Four types of brain injury (cerebral contusion,
DAI, acute subdural hematoma, and subarachnoid hematoma) are discussed in this
chapter.
Figures 5.10 shows two MR images of a cerebral contusion case. Cerebral
contusion is a bruising of the brain surface where haemorrhagic necrosis and acute
brain swelling occur. Contusions usually involve the surface of the brain, especially
the crowns of gyri, and are more frequent in the orbital surfaces of the frontal
lobes and the tips of the temporal lobes irrespective of the site of impact [27,
76]. Most researchers believe that this focal injury is associated with intracranial
pressure generated as a result of high linear acceleration. Positive pressure, typically
associated with the so-called coup injury mechanism, is assumed to be the result
of the moving skull towards the stationary brain, producing a compressive wave
in the brain at the time of impact or direct compression of the brain due to inbending of the skull. Negative pressure, which has been associated with the so-called
contrecoup injury mechanism, is hypothesised to be the result of tension generated
by the skull moving away from the brain that is lagging behind the skull. If pressure
is the underlining mechanism of this coup-contrecoup phenomenon, FE modelpredicted pressure levels can be used to estimate the risk of contusion injury.
Alternately, the negative pressure could be due to a tensile wave that was formed
by the reflection of the original compression wave off the skull. Cavitation (collapse
of a vapor bubble) occurs if negative pressure is lower than the vapor pressure of
water. In this case, negative pressures below atmospheric pressure may be a good
indicator for contusion injury. This cavitation-induced injury mechanism has not
K. H. Yang and H. Mao
5.3.5 Types of Injury to Be Simulated
Because an FE head model can only be used to calculate the corresponding
responses at the instant of impact, secondary head injury (defined as the pathological, physiological, and biochemical changes to the brain tissue after the primary
injury) is beyond the capability of current FE head models and will not be discussed
in this chapter. A good understanding of the injury mechanism is one of the
most important components when studying injury prevention. Without knowing
the proper injury mechanism and the associated injury threshold, it is not possible
to use an FE head model to predict the type, location, and severity of TBI. As
previously discussed in Sect. 5.2 of this chapter, it is unfortunately that proper injury
mechanisms have not been appropriately established yet.
The mechanism of open- and closed-head injury is quite different. Because skull
and facial bone fractures do not always correlate with brain injury, such fractures
are not simulated in most FE head models. Nevertheless, readers should be aware
that a fractured skull could affect subsequent intracranial response and a skull
fracture is more complicated than a long bone fracture. Gurdjian and Webster [26]
in their stress-coating and strain gauge study showed that there was a momentary
in-bending at the site of impact and out-bending at the adjacent regions with linear
fractures occurring in the out-bended area. Additionally, an instantaneous increase
in intracranial pressure upon impact may contribute to the incident rate of skull
fracture or to the fracture length. Four types of brain injury (cerebral contusion,
DAI, acute subdural hematoma, and subarachnoid hematoma) are discussed in this
chapter.
Figures 5.10 shows two MR images of a cerebral contusion case. Cerebral
contusion is a bruising of the brain surface where haemorrhagic necrosis and acute
brain swelling occur. Contusions usually involve the surface of the brain, especially
the crowns of gyri, and are more frequent in the orbital surfaces of the frontal
lobes and the tips of the temporal lobes irrespective of the site of impact [27,
76]. Most researchers believe that this focal injury is associated with intracranial
pressure generated as a result of high linear acceleration. Positive pressure, typically
associated with the so-called coup injury mechanism, is assumed to be the result
of the moving skull towards the stationary brain, producing a compressive wave
in the brain at the time of impact or direct compression of the brain due to inbending of the skull. Negative pressure, which has been associated with the so-called
contrecoup injury mechanism, is hypothesised to be the result of tension generated
by the skull moving away from the brain that is lagging behind the skull. If pressure
is the underlining mechanism of this coup-contrecoup phenomenon, FE modelpredicted pressure levels can be used to estimate the risk of contusion injury.
Alternately, the negative pressure could be due to a tensile wave that was formed
by the reflection of the original compression wave off the skull. Cavitation (collapse
of a vapor bubble) occurs if negative pressure is lower than the vapor pressure of
water. In this case, negative pressures below atmospheric pressure may be a good
indicator for contusion injury. This cavitation-induced injury mechanism has not