Neck Injury Mechanisms

Cervical Spine Injuries

There are 3 major types of injury to the cervical spine.

• Fracturing of the cervical vertebrae and damage / rupturing of the ligaments and soft tissue linking the vertebrae together

• Damage to the spinal cord due to shearing and bending between the cervical vertebrae, physical stretching of the spinal cord or direct impingement by fractured vertebrae

• Direct impact damage to the soft tissues containing the trachea with larynx, oesophagus and major arteries and veins to the head and brain.

The first two are intrinsically linked as both are caused by excessive bending, shear or axial displacements produced by indirect loading on the neck from relative displacements between the head and the torso.  These types of neck injuries have their own specific problems.

• Any damage to the spinal cord, particularly in the atlas (C1) and axis (C2), can be fatal, or lead to permanent paralysis

• The loads generated in the neck are a complex combination of bending, shear and axial forces

• The vertebrae have many different fracture mechanisms from complete ‘burst’ fractures under high axial loads to avulsion fractures of the vertebrae processes.

The indirect loads can be generated by relative head to torso displacements which are produced by two main mechanisms.

• If head is rapidly decelerated on impact with a rail vehicle interior surface the torso continues to move and is decelerated by forces generated within the neck. A typical example of this would be following a head impact on the seat back in front, in unidirectional seating.  Potentially this produces the most severe injuries due to the higher relative mass of the torso.

• The head is accelerated rapidly via the neck in a rear impact with a low seat back with no head support producing neck hypertension or whiplash.

Frontal Loading Mechanism

The frontal head impact mechanism produces the most severe injuries due to the potentially higher indirect loads and a more varied direction of loading with the possibility of vertebrae and spine dislocation.  The second, rear impact mechanism with neck. hyperextension, produces mainly soft tissue damage which, although not life threatening, can lead to long term disability.

In the first front loading mechanism, severe bending, fracturing and dislocations of the vertebrae impinges directly on to the spinal cord.  In a minor loading case it can produce reversible and irreversible damage to individual nerves causing the person to loose sensory and motor control of specific parts of the body.  At its worst, it can cause complete damage and dislocation of the spinal cord with the person losing total control of their entire body.

Unfortunately this is at its most severe at the upper cervical spine where severe fracturing and dislocation of the C1 and C2 vertebrae, severs the spinal cord where it forms the brainstem and, is usually fatal, with AIS 6.  The severity of the injury reduces the lower down the cervical spine the dislocation takes place. So if the dislocation occurs below C5, control of the thorax and abdomen is maintained but sensory and motor control of all the limbs is lost, causing quadriplegia or paraplegia, AIS 5.

Minor fractures damage which do not impinge on the spinal cord, soft tissue, muscle and ligament, are not life threatening AIS 2-3, but can lead to long-term disability and discomfort.

In terms of injury mechanisms to the cervical spine these are nearly always due to indirect loading produced by relative translations and rotations between the head and the torso.  The Figure 15 overleaf defines the significant loading directions with the injuries produced by specific loading types and combination of loading types.  In the rail environment the majority of these indirect neck injuries will be produced by the head / face impacting a seat back or partition and the torso being decelerated through the neck, producing a complex combination of bending and axial forces.  These forces being determined by the relative trajectories of the head and torso, angle of the impacting surface and/or impact location on the head or face.  Figure 15 shows some examples of different loading situations and possible injuries which may be applicable to the rail environment.

The two major impact injury mechanisms being:

• Compression-flexion (impact on the head with the head bent forwards) producing vertebrae wedge and burst fractures and spine dislocations

• Compression-extension (impact on face with the head in neutral or bent back) producing fractures of the spinal processes, damage to the load bearing surfaces and spine dislocations

Any lateral bending or lateral rotation of the head immediately prior to impact of course can complicate both of these.

The only other common injury mechanisms is tension-extension where the torso is accelerated rapidly forwards and the head is accelerated via the neck producing hyperextension or ‘whiplash’ type injury as in rear facing seats with no or poorly designed head restraint.  This is normally associated with soft tissue injury although the actual injuries that cause the long-term disability and discomfort are still unresolved.

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Figure 15 Shows Some Examples Of Different Loading Situations And Possible Injuries

            Unidirectional low seat back head impact on upper seat edge

            Open bay head impact on to seat back/ standing/ exposed seating

            Rear Impact with low seat back

Frontal Neck Injuries

The head and the torso usually protect the neck, so direct impacts to the front of the neck are relatively rare.  However in the rail environment with low seat backs and standing occupants these potentially can occur.  There is very little literature concerning such injuries both from blunt objects, which if severe can cause direct damage to the trachea, AIS 3, Larynx, AIS 2 - 4, and Oesophagus, AIS 3 and sharp objects which can cause severe lacerations and penetrating injuries to the Carotid artery, AIS 4-5. and jugular vein, AIS 3.