Mats Y Svensson

The present work addresses the problem of neck injuries following rear-end car collisions. Biomechanics and sites of the injuries have been investigated. A new dummy neck has been developed and used to study the influence of different seat properties on occupant kinematics.

Anaesthetised pigs were exposed to swift extension-flexion motions of the cervical spine. Pressure pulses in the central nervous system with magnitudes of up to about 150 mmHg (2.0 kPa) were registered during these neck motions. These pressure magnitudes seem to depend on the velocity and direction of the angular motion between adjacent vertebrae. Evans Blue dye conjugated to albumin was injected into the blood system and injuries to the spinal ganglia, particularly in the lower cervical region were found at histopathological examination. These injuries could explain many of the symptoms that are typical for patients with an AIS 1 neck injury sustained in rear-end collisions. The measured pressures are believed to be a possible cause of these injuries.

In order to enable better assessment of the neck motion during staged rear-end collisions at low impact-velocities (<20 km/h), a new dummy-neck was developed. It consists of seven cervical and two thoracic vertebrae connected by pin-joints. Data from volunteer tests found in the literature were used for validation.

A Hybrid III-dummy equipped with the new dummy-neck was used in two series of rear-end collision sled tests at 5 km/h and 12.5 km/h pre-impact sled-velocity. In the first series, five types of front-seats (bucket seats) and three types of rear-seats from passenger cars were tested. Seemingly similar seats displayed large differences as to the head-neck kinematics. In the second series, one of the front-seats was modified and tested again. A head-restraint with a flat vertical front surface and with its top well above the level of the head centre of gravity was attached to the seat-back. Two different head to head-restraint gaps were tested in combination with different stiffnesses of the seat-back frame and the lower seat-back cushion as well as different depths of the upper seat-back cushion.

The horizontal head to head-restraint gap proved to have the largest influence on the head-neck motion during the impact. The maximum head-torso displacement increased with increasing head to head-restraint gap. An increased stiffness of the seat-back frame resulted in slightly increased maximum head-torso displacement, but in combination with a stiffening of the lower seat-back cushion and deeper upper seat-back cushion, the result was a clear reduction of the head-torso displacement. The results indicate that minor changes to existing car-seats might radically improve the protection against neck injuries in rear-end collisions.

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