Neck Injury Criteria

Neck Dynamic Properties

The majority of neck injuries are caused by indirect loading produced by inertial loads being transferred from the torso to the head, following head impact, or head to the torso, following torso impact or high acceleration.  As the neck comprises of skeletal vertebrae joined by cartilage ligaments and muscles the range and type of motions produced are complicated, being combinations of translations and rotations in all three dimensions.  The overall motion is also dependent on the direction of the resultant force and therefore impact location and direction on the head, or acceleration direction on the torso.

The injury criterion could be based on two mechanisms:

• Displacement and rotation maximums which would predict soft tissue damage and possible spinal cord injuries.  However this would be very difficult to accurately predict without a very good simulation of detailed neck motions both overall and between the individual elements.  For example the shear translation between cervical vertebrae and its effect on the total motion

• Axial, shear and bending moment forces measured at specific points on the neck using a good simulation of the overall neck dynamic properties and knowing how these are linked to specific types of injury

With the lack of detail simulations of neck internal motions, and only requiring total neck motions in respect to the loads, then the force based injury criteria is the better system to use.

Neck Injury Criteria

The neck injury criteria are divided into 3 main load types.

• Neck axial loads – tension and compression
• Neck longitudinal shear force
• Neck longitudinal bending moment – flexion (head forward) and extension (head back)

As the upper two vertebrae, C1 and C2, are considerably weaker than the rest of the vertebrae, C3- C7, and the potential injuries more severe, then loads are measured at the top the neck, atlanto-occipital condyles, at its junction with the cranium.  As biomechanical information on the loads produced at the lower part of the neck, C7 to T1, vertebrae is generated, then separate or combined injury criteria could be defined.

The neck injury criteria are:

• Neck Upper Tensile Load. (KN)
• Neck Upper Compressive Load. (KN)
• Neck Upper Longitudinal shear force (same for fore and aft loads). (KN)
• Neck Upper Longitudinal extension bending moment. (Nm)
• Neck Upper Longitudinal flexion bending moment. (Nm)

However as very short duration loads produce very small displacements then the injury severity will increase with the length of time a load is applied producing a force time injury tolerance curve for both axial loads and shear forces.  However, further biomechanical research is required to be able to produce different force time tolerance curves for the 4 different injury classification levels.

Application

The neck injury criteria should be applied to both the upper (C1/C2) and lower (C6/C7) vertebrae to assess the overall potential injury for the cervical spine.  The criteria should be higher in the upper spine for which tolerance levels have been proposed from biomechanical data. Further research work is required to propose tolerance levels for the lower cervical spine. The neck injury criteria should be used for all impact scenarios where the occupant undergoes a rapid acceleration, including rear-facing seats.

Evaluation Techniques

Crash test dummies/ physical simulations – 6 axis load cells are already available for both the upper, head to neck interface, and lower, neck to torso interface, neck on the HIII frontal dummy.  These load cells can also be installed in the BioSID dummy but are not yet available for the EuroSID dummy.

Computer simulations – Loads can be analysed at both top and bottom of the neck in both MADYMO and Dyna3D final element dummy models.