Although design defects have not changed much over the last couple of decades, recent medical advances allow crash victims to survive formally fatal injuries.
Seriously brain damaged and paralyzed spinal cord victims’ survival and increased life expectancies require huge sums for lifetime medical expenses and costly functional and psychological problems.
How brain and spinal cord injuries occur - - and methods for eliminating or reducing their severity - - is discussed below.
Severe Brain Injury
Brain injuries are most commonly caused by head impact, inertial loading of the head, and loss of oxygen (hypoxia).
While brain injury from head impact requires direct impact, inertial loading of the brain results from direct or indirect loading like violent flexion/extension in a rear-end crash lacking properly designed head restraints.
This trauma causes diffuse and focal brain injury, the latter involving harm to defined brain regions in which victim experiences skull fractures and brain contusions or epidural or subdural hematomas located beneath - - or opposite to - - impact’s site.
Head contact may cause diffuse axonal injury, i.e., an initial concussion followed by cellular or bleeding damage to multiple brain regions.
Regardless of whether the injury is focal or diffuse, swelling or edema (tissue damage and increased fluid) more life threatening than the injury may occur and horribly injure the brain stem, i.e., the link between the brain and spinal cord.
Either direct head impact or indirect violent head movement induces two types of brain injury: translational and/or rotational movement.
Translational (or linear) motion is the movement in a direct path through head’s center of gravity (CG). Rotational motion occurs when the brain angulates around the head’s CG causing it to violently move in a non-uniform fashion and exposing areas of the brain to injury.
The brain is a viscoelastic organ susceptible to injury due to sensitivity to force, shape of impacting object, acceleration, and rate at which brain is accelerated.
While most closed head injuries occur within a fraction of a second of exposure to acceleration, the head injury’s nature and extent depends upon the acceleration’s rate of onset, directionality and peak.
Severe Cervical Spinal Cord Injury
The cervical spine is made up of vertebrae positioned to produce “lordotic curvature” causing the spine to arch when standing upright.
The lower cervical vertebrae are similar in shape, with size and mass increasing as there is movement toward the thoracic spine.
The commonly acknowledged mechanism causing cervical spine fracture or dislocation —resulting in catastrophic quadriplegia/paraplegia — is axial loading with failure of the spine in a flexion mode.
Considering the cervical spine with the neck in the neutral position, the spine’s normal alignment is with extension because of the lordotic curve. When the head moves forward and the neck flexes forward, the cervical spine is straightened.
With the force exerted from head impact or arrest of the head with the body in motion, along the axis of a straight spine, loading of the spine as a segmented column occurs. When energy input exceeds energy absorbing capacity, intervertebral disc injury, vertebral body fracture, ligamentous disruption or posterior element fracture results. When maximum vertical compressive deformation is reached, acute cervical spine flexion occurs, with fracture or dislocation. The spinal cord is traumatized by impingement of the spinal process.
Because current research places the critical limits on the compressive load’s magnitude at 700 to 1000 pounds, force exceeding this load causes spinal cord injury.
Automotive and Helmet Design and Injury
To properly design a motor vehicle to reduce brain or spinal cord injury, the restraint system and "head strike zone" (e.g., steering wheel, instrument panel, roof, windshield, etc.) must account for head contact, inertial loading of the head from torso restraint, and spinal cord loading.
Thus, manufacturers must set design/injury parameters under foreseeable accident circumstances then test and measure the safety componentry’s performance. Generally, the more shock attenuation provided by the component or system placed between the head/neck and loading force (e.g., helmet, instrument panel, "A" pillar padding, etc.), the less transmitted acceleration and less likely the victim will be injured.
Head and spinal cord injury is also a major cause of disability in sports activities associated with helmets often due to lax and outdated helmet test standards.
If designed correctly, all helmets protect through load distribution and energy absorption. Distribution depends on the stiffness of the shell while absorption is based upon the liner’s deformable properties, i.e., density and thickness.
Unfortunately, many helmet designers fail to provide head and neck protection in impacts with velocity changes exceeding 15 mile per hour often meeting only minimal governmental or industry standards.