Injuries to the head involve the scalp, skull, brain and the nerves and blood vessels entering and exiting the skull. Many studies have documented that these head injuries are frequent, are commonly severe and are as large a cause of death and of disability as are injuries to the rest of the body combined (Gennarelli et al., 1994). The head is the most commonly injured body region and accounts for a large part of resulting impairment and disability.
Biomechanical Issues: A considerable amount of research into the nature and causation of head injuries and a large degree of consensus has been achieved over the basic biomechanical questions. This agreement can be summarised as follows:
• The many different types of head injuries have been described and classified (Graham et al., 2002).
• General agreement has been reached on the severity and the importance of the various types of head injuries regarding the potential for death and disability. These are adequately described in the Abbreviated Injury Scale (Gennarelli et al., 1982, Abbreviated Injury Scale 2005).
• A general understanding exists of the underlying mechanisms that cause altered brain function post injury (Ommaya et al., 2002).
• Frequencies of single and combinations of head injuries have been identified in various injury-producing circumstances.
• The biomechanics of head injuries are reasonably well understood from a qualitative perspective (Goldsmith 2001).
• Regarding the mechanisms and levels of stress or strain that cause injury (i.e., the biomechanical tolerances), there is general agreement regarding scalp injuries and some types of skull injuries.
• There have been estimates from limited sources regarding mechanical tolerances for all head injury globally and for various specific levels of diffuse brain injury (concussions and prolonged traumatic coma) (Gennarelli et al., 2003).
• The applicability of the HIC is limited in understanding certain types of brain injury and there is a need to develop more suitable injury assessment functions.
Head injury criterion
The Head Injury Criterion (HIC) is a measure of the likelihood of head injury arising from an impact. The HIC can be used to assess safety related to vehicles, personal protective gear, and sport equipment.
Normally the variable is derived from the acceleration/time history of an accelerometer mounted at the centre of gravity of a dummy’s head, when the dummy is exposed to crash forces.
It is defined as:
Where t1 and t2 are the initial and final times (in seconds) of the interval during which HIC attains a maximum value, and acceleration a is measured in g's (standard gravity acceleration). Note also the maximum time duration of HIC, t2 - t1, is limited to a specific value, usually 15 ms.
This means that the HIC includes the effects of head acceleration and the duration of the acceleration. Large accelerations may be tolerated for very short times.
At a HIC of 1000, one in six people will suffer a life-threatening injury to their brain (more accurately, an 18% probability of a severe head injury, a 55% probability of a serious injury and a 90% probability of a moderate head injury to the average adult).
NECK INJURIES
Cervical spine distortion (CSD) injuries play a major role in car-to-car collisions worldwide. The high rate of CSD represents not only an economic burden but also a medical challenge. A particular characteristic of so-called “whiplash” injuries is that they can occur in car crashes at low velocity (Krafft et al., 2002). During such an event, vehicle occupants are prone to hyperextension of the neck, particularly at the level of the C6-C7 cervical vertebrae (Kaneoka et al., 1999).
The incidence rate of neck injuries remains high despite the belief that hyperextension of the cervical spine would not occur as long as the occupant is using a head restraint (Lovsund et al., 1988, Olsson et al., 1990, Ono and Kaneoka, 1997). This strongly suggests that factors other than the improper use of head restraints must be involved as a cause of neck injuries. For example, current seat systems including the head restraint are not adequately designed to prevent or mitigate neck injuries.
The potential for long-term impairment, including paraplegia and quadriplegia, is always inherent in injuries to the spine and particularly to the spinal cord. Of all spinal segments, the cervical spine is the region most frequently injured. As the head and the neck form one functional entity, head loading often also implies neck loading and almost always vice versa.
In a rear impact, the occupant is subjected to various forces which tend to differ among individual occupants due to differences in seat position and seat cushion stiffness which are presumably related to the incidence of neck injuries (Lovsund et al., 1988, Olsson et al., 1990, Ono and Kaneoka, 1997, Hell et al., 2003). While research has focused on the relationships between neck muscle responses, motions of cervical vertebrae and injuries to intervertebral discs and articular surfaces, detailed information about these relationships, ranging from relatively minor neck injuries to those resulting in impairment, are still not well understood.
Severe (head-contact) cervical injuries occur to unbelted car occupants not only in rear impacts but also in frontal, lateral and oblique impacts. Half of all minor neck injuries occur in frontal impacts (Hell et al., 2003). The vast majority of cervical spine injuries, however, are minor soft tissue AIS 1 injuries. These injuries, while not associated with overt structural injury to the cervical spine or the central nervous system, are both a common and potentially debilitating injury.
In fact, they are the most frequently occurring injuries in automobile collisions and more often to females than to males (Bunketorp et al., 2004, Jakobsson and Norin, 2004). Thus, soft tissue neck injuries are a major concern in road traffic.
Injury Mechanisms: The mechanisms of the so-called “whiplash” injury have not been clearly understood, and the relationship between the objective physical/medical observations and the subjective symptoms remains unclear (Carlsson et al., 1985, Schrader et al., 1996).
The human neck is a complex structure consisting of skeletal frames, ligaments, blood vessels, muscles and soft tissues such as nerves with diverse strengths. Their forms tend to change continually and in a potentially injury-producing situation, neck muscle strength can depend upon level of consciousness. An impact is transmitted directly to each vertebra or dispersed through the soft tissues, then transmitted to the lower torso. The influence of a head impact on the neck differs significantly depending on the direction of the impact and the orientation of the neck when the impact occurs.
In other words, the neck injury mechanism is roughly classified by the direct transmission of the head impact to the neck (Yamada 1970) and by the inertial head motions around the neck acting as a pivot (Society of Automotive Engineers 1986).
Specific injury mechanisms related to cervical vertebrae are flexion (bending), compression, extension (tension), rotation (torque) or shear force (Figure 1). In general, injuries to the lower vertebral region result from flexion or extension whereas an injury such as a Hangman-type fracture or Jefferson-type fracture, again depending upon the orientation of the neck, results from a shear force. An intervertebral disk or vertebral anterior aspect is likely to be injured by flexion, and the bending moment tends to be greater than in the case of extension. With flexion, however, the impact load against the vertebra changes as the chin contacts the chest (chin-chest impact). In the latter case, the so-called “whiplash” injury may occur without a direct impact to the head. A typical example of such impacts is a vehicle rear-end collision. In the initial stage of a rear-end collision, the occupant’s spinal column is rounded by the seatback reaction force, then straightened upward, causing the torso to move upward along the seatback at the same time. As the head remains in the initial position due to the inertia, an axial compression force is applied to the lower cervical vertebra due to the straightening of the spinal column and the upward motion of the torso. A shear force is then applied to the lower cervical spine due to the collision between the seatback and the upper portion of the torso, resulting in head retroflexional rotation around the lower cervical vertebra acting as the pivot. The phenomena created from the initial impact, the resulting motions of the spinal column and torso, and the retroflexional rotation of the head are becoming more complex due to the specific detail of head restraint installation in recent years.
Generally, the impact time zone during which a neck injury occurs is in the order of several-tens to several-hundred seconds from the moment of impact. The injury severity also depends on the duration of the impact. In some cases, the duration that causes the “whiplash” can be as long as 200ms. These are important considerations to a better understanding of these minor but frequent and sometimes debilitating neck injuries.
Injury Criteria and Tolerance to Injuries: Early work by Yamada (1970) on physical properties and strengths of biological materials such as cervical vertebrae, intervertebral disks and ligaments has contributed significantly to the field of impact biomechanics. Table 1 summarises experimental data on tensile, compression and torsional strengths of vertebrae and the intervetebral discs.
These data are fundamental to the field of impact biomechanics and often referred to as useful data in various other fields. The injuries that occur in line with the flexion, compression, torsion and/or shear forces applied to the cervical vertebrae described above are classified (Society of Automotive Engineers 1986) and shown in Table 2.
Mertz and Chou (1976) proposed neck injury tolerance curves in 1976 based on experimental data from volunteers and cadavers. These curves determine the limits for torque, flexion angle and extension angles relative to the occipital condyle.
The test data of axial load, shear force and bending torque are summarized and shown in Table 3.
Moreover, Mertz (1990) also proposed neck injury criteria based on dynamic experiments conducted using cadavers, and accident simulations using Hybrid III dummies. The proposed tolerance level to the occipital condyle torque is 190 Nm for the forward flexion, and 57 Nm for the backward extension. These injury criteria are applied at present to the evaluation of neck injuries in automobile safety evaluation tests using the Hybrid-III crash test dummy. Injury criteria values for axial compression, tension and shear force are shown in Figure 2.
It should be noted, however, that the data obtained with these injury criteria values are applicable mainly to experiments and studies on head inertia loads. These values do not provide injury criteria for human biological impact responses where human heads are subjected to direct impacts (Xu et al., 2000). The values being used at present as the injury criteria (Federal Register 2000) are listed in Table 4.
Recently, quite extensive studies on minor neck injury (i.e., “whiplash”) caused by head inertial impacts were conducted in addition to the above, and the “whiplash” injury evaluation parameters and the criteria are proposed as shown in Table 5 (IIWPG 2001).
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