Mechanisms Of Injury

With the discussion of trauma and its associated injuries, consideration and understanding to the mechanism must be achieved to properly care for the patient and the injuries that are present. These injuries result from energy transfer based on the laws of motion and energy. Since energy cannot be created or destroyed and objects have both stationary and mobile energy, it can be deduced that the force generated in a traumatic event is transferred to the patient resulting in various soft tissue and osseous injuries. This transfer of energies leaves two types of cavities. On initial impact, the force pushing the tissue and the cellular particles create a temporary cavity. Once the forces have dissipated, the tissues will recoil back to their previous position depending on the amount of elasticity and compliance they possess. This process occurs in a fraction of a second. A permanent cavity is noticed when the tissue has the inability to recoil due to destruction or crushing. Both these cavities occur due to two main types of mechanisms: blunt and penetrating.2

Blunt Trauma

Blunt trauma leads to compression, shear, and overpressure injuries. Compression injuries occur due to the force crushing the cells that it comes into contact with. Shear injuries appear from an organ accelerating or decelerating at a different velocity than the surrounding tissues or cavity. The stretching and rupturing of surrounding tissues when excessive pressure is placed on the tissue cause overpressure injuries.2

Causes of blunt trauma are categorized into motor vehicle crashes, pedestrian injuries, falls, and assaults. Motor vehicle crashes are associated with frontal, lateral, rear, off-center, rotational, and ejectional collisions.2 Pedestrians struck by a motorized vehicle sustain injuries that are associated with multiple mechanisms and multiple points of force. The initial impact is followed by a fall on another impact point. These types of mechanisms potentially lead to many occult injuries; therefore, a high index of suspicion should be maintained. Falls occur from various heights, leading to various amounts of force on impact and variable injury patterns. Once again, a high index of suspicion needs to be maintained for occult injuries. Assaults come in a myriad of types and sizes, but are generally more localized than the other mechanisms mentioned. Injuries are isolated to the area of impact, for the most part. It must be kept in mind that any of these mechanisms can overlap into penetrating trauma (Fig. 2-2), depending on if an object violated the exterior barrier to enter the soft tissue in the process of the blunt trauma.

Figure 2-2 Combined blunt and penetrating mechanisms of injury in a patient who was impaled by a metal fence post. The post is seen entering the upper abdomen and exiting the left flank.

Penetrating Trauma

Penetrating trauma encompasses those of bullet/missile injuries and stab injuries from various objects (Fig. 2-3). These injuries lead to direct tissue destruction, various sizes of temporary cavities, and a more permanent cavity. The amount of tissue destruction is dependent on the type of object, its size, and the velocity at which the object was traveling. The density of the tissue the object interacts with will also affect cavity formation. As the object moves through the tissue, energy is transferred to the tissue. The tissue in contact with the object is crushed and destroyed while the surrounding tissue is stretched all along the path of the object until the force is dissipated and the tissue recoils.2

Stab injuries will have very small temporary cavities created as the object is entering the tissue, but will have a permanent cavity dependent on the size of the object and the velocity it was traveling when entering the tissue planes. Bullets and projectile missiles, on the other hand, will create larger

temporary cavities and obvious permanent cavities with greater secondary trauma. The size of the frontal area of the projectile is a major factor in the size of the cavity it creates. The larger the area in contact with tissue, the larger the cavity formed, both permanent and temporary. The projectile can create a temporary cavity of 20-25 times that of its frontal area. Therefore, as a projectile encounters a tissue and becomes deformed, a larger frontal area will be created which will lead to larger cavities. Other factors that will increase the projectile's frontal area are its ability to tumble through the tissue due to changes in momentum between the front and the back of the projectile; its ability to fragment on encountering tissue; and its ability to explode on tissue contact.

Generally, penetrating injuries are classified as low-velocity or highvelocity injuries. Most stab injuries and handguns are low-velocity injuries due to the projectiles traveling at speeds less than 450 m/s. Projectiles with speeds greater than 450 m/s are considered to cause high-velocity injuries. Kinetic energy is proportional to the square of the projectile's velocity. Thus, higher-velocity projectiles cause much greater injuries, mostly due to temporary cavitation and secondary tissue destruction.2

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