Thorax Injury Mechanisms

In the rail vehicle interior there are two main injury mechanisms.

• Direct impact from a blunt object producing rib fractures and penetration into the thoracic or upper abdominal cavity causing internal organ damage, associated with standing unrestrained and/or forward facing seated occupants impacting seat back tops and grab handles

• High accelerations to the whole thorax area producing internal organ contusions and damage produced in a rearward facing seated occupant

Due to the relatively low decelerations produced in rail collisions the blunt object impacts to forward facing seated occupants are far more prevalent and severe than the acceleration injuries in rearward facing occupants.

Blunt object impact chest injuries in frontal impact can be divided into skeletal and soft tissue injuries, which have been summarised in terms of type and AIS severity Appendix 1, Table 24.

Major Injuries And Their Severity To The Thorax

In terms of rib fracture this has been directly related to the amount of chest compression.  The ribs failing under excessive bending with failure occurring on the tensile side of the rib. However, the rate of deflection can also affect the number of rib fractures as the stiffness of the front of the chest can increase with support offered by the soft tissue organs, which decreases with the rate of deflection.  Cadaver impact tests have shown that a chest compression of 20% produced the onset of rib fracture, while a compression of 40% producing serious multiple rib fractures or flail chest.  In lateral impacts the chest compression is also directly related to rib fracture, however, as the rib cage is stronger laterally with a stiffer impact response to deflection, rib failure is lower with a compression of 17% (35% half chest width) producing multiple rib fractures.

Although flail chest constitutes a serious injury; injury or lesions, to the soft tissue organs, the viscera, and blood vessels, cause the majority of chest injury fatalities.  The majority of these are contusions and haemorrhaging produced by rupture of the organs and associated veins and arteries, or ‘burst’ failures caused by increased internal fluid pressures.  These are not just produced by direct chest compression, as many of the organs are free to move within the thoracic and upper abdominal cavities.  These injuries are dependent on the rate and duration of intrusion and there is no relationship between the severities of injuries and the number of rib fractures.

Sharp object impacts producing penetrating lacerations are in general less severe than the multiple injuries produced in severe blunt object impacts except where the penetrations actually puncture the organs causing loss of function or severe haemorrhaging. These being consistent with stabbing and bullet wounds with penetrating wounds over 5 cm.  As sharp objects capable of producing such large penetrations are unlikely in a rail vehicle interior unless interior features fracture producing sharp exposed parts of the structure, sharp object injuries will only be treated as minor AIS 1-2 injuries.