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Trauma basics
Basic trauma crit care
Traumatic shock
Trauma in elderly
Abdomen & pelvis
Chest injuries
Fat embolism
Head injury
Larynx & trachea
Spinal injury

Mechanisms of injury

Injury occurs as a result of cavitation, over-pressure, compression and shear injury


  • When an object hits a body, tissue particles are knocked from their position and crash into other tissue particles and a cavity is formed

  • Two cavities are created:

    • Permanent: seen when the patient is examined

    • Temporary: exists for a fraction of a second - expands rapidly at the time of impact but tissue particles that are displaced subsequently return to their original or near-original positions

  • Effects of cavitation depend on tissue involved. Less marked in elastic tissue, like muscle which spring back into shape. More marked in less elastic tissues, like solid intra-abdominal organs - tend to shatter  


When body cavity is compressed at a faster rate than surrounding tissue the walls of the cavity stretch (± rupture) like the walls of a balloon that is squeezed. Such an effect occurs in frontal impact motor vehicle accidents when the steering column hits the anterior abdominal wall causing the diaphragm to rupture.

Shear injury

Occurs when one body part or organ continues to move after a structure to which it is attached stops moving eg in frontal impacts where posterior thoracic wall and attached descending aorta stop moving but heart, ascending aorta and aortic arch continue to move forward. Results shear injury at point at which descending aorta is attached to spine


Compression forces are similar to laying an organ on a solid steel table and hitting it with a hammer. Contact between the hammer and the organ results in compression and crushing of cells. (Some cavitation will occur in adjacent tissue).

Blast injury

3 components:

  1. Primary injury is due to burns from the blast and injury from the pressure wave that follows - produces overpressure to all gas-filled or hollow organs, eg sinuses, tympanic membranes, lungs, or gastrointestinal tract

  2. Secondary injury is due to flying objects

  3. Tertiary injury due to victim being knocked - results in similar injuries to ejection from a vehicle or a fall from a height

Frontal impact motor vehicle accident

Vehicle and its occupants are moving at same speed until the collision. At this point the vehicle decelerates. However any unrestrained occupants continue to move at the same speed until their motion is slowed by collision with dashboard or steering wheel or in the case of backseat passengers by the back of the front seat. As occupants move forward they do so in two different patterns:

  • Up and over. Head is lead point. As it hits windscreen or seat it comes to a stop but torso continues to move, producing cervical spine injuries, until it too comes to a halt due to impact on the steering wheel, dashboard or seat. Results in injuries to:

    • head

    • cervical spine

    • torso (abdomen > thorax)

  • Down and under

    • If the tibia is major point of impact: femur overrides tibia, dislocating knee. Damage to popliteal vessels

    • If the femur is major point of impact

      • # shaft of femur OR

      • Pelvis overrides head of femur ̃ posterior dislocation of hip ± # acetabulum

      • Injury to torso (thorax > abdomen)

Rear impact motor vehicle accident

  • Relatively few injuries

  • Cervical spine injury is greatest risk

Lateral impact motor vehicle accident

2 components

  • Intrusion into passenger compartment causing injury to:

    • Arm

    • Proximal femur

    • Lateral chest wall

    • Shoulder

  • Lateral movement of vehicle

    • Shear injury to:

      • Aorta - result of lateral acceleration of the descending aorta, due to its attachment to the vertebral column, and relative lack of initial movement of the heart and aortic arch

      • Kidney

      • Spleen

    • Centre of gravity of the head is anterior and superior to the pivot point of the head on the neck. When the torso is accelerated laterally this results in the head being flexed laterally and rotated towards the impact: tends to open and then rotate and dislocate the vertebrae, finally locking the facets on the contralateral side to impact.

Further reading

McSwain NE. Kinematics of trauma. In 

©Charles Gomersall, April, 2014 unless otherwise stated. The author, editor and The Chinese University of Hong Kong take no responsibility for any adverse event resulting from the use of this webpage.
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