Mechanism of Injury

Mechanism of injury

This must be assessed before approaching the casualty. In assessing the mechanism of injury you must consider a number of factors and try to piece together what has happened. Consider the forces involved, direction of travel and impacts the casualty has been subjected to, which part(s) of the body are implicated and as a result of this think about what injuries the casualty might have sustained. Ask yourself which underlying organs might be damaged, which direction have forces travelled in?

The law of conservation of energy states that: ‘Energy is neither created nor destroyed, but merely changes form’

A casualty who falls from standing may put their hand out to save themselves. The force of the impact with the ground may fracture their wrist, but that impact energy may also transmit its way up the arm and fracture the collarbone. The energy of the impact with the ground has changed. It hasn’t been lost, but has changed form and transmitted its way up the arm to the collarbone.

Assessing the mechanism of injury can give you an excellent idea of the injuries that might be present in the casualty.


This is how the casualty slows down during an impact or a fall. This can be gradual, which has a lower risk of injury, or sudden which is more dangerous.

In any incident there is more than one impact. A typical example is a road traffic collision. The first impact occurs when one car strikes another car, the second occurs when the casualty strikes the seatbelt and the third impact occurs when the casualty’s internal organs strike their ribs or abdominal wall. Each of these impacts is at the same speed as the initial impact and the forces are transmitted from one impact to the next.

In the above example there is a risk of internal organs being torn, ruptured or bruised. All of these can cause severe internal haemorrhage which may not be visible on the primary survey. This is why it is vitally important to look at, and understand what has happened to the casualty and consider what injuries may have happened.

Rotational forces

This is where a casualty has been involved in an impact that has caused them to turn sharply, or turn upside down. These forces can often cause a shearing effect, which may cause spinal injury by turning the head at a different rate to the body. Internal organs may also be affected by these shearing forces which pull blood vessels or organs from fixed points, or twist and stretch them, causing them to tear. Typical examples of rotational forces are car collisions where the car spins or rolls over or falls from horses, motorcycles or bicycles, where the casualty might twist as they fall, or twist upon impact with, or after striking the ground.

Blunt trauma

Caused when an object strikes the casualty, but doesn’t puncture the skin. Typical incidents are car collisions, (typically: car v. pedestrian or a side impact car v. car) falls from horses, motorcycles, bicycles etc. or assaults. The ‘blunt’ object can be a car, the ground, a fence post, cycle handlebars or in cases of assault, an object such as a baseball bat.

The force from blunt trauma pushes the internal organs aside and squashes them into a smaller space. This may cause the organs to tear or burst. As the blunt object releases its force, everything returns back to normal. This effect is called temporary cavitation.

Penetrating trauma

Caused by an object puncturing the skin. This can be a knife, fence railing, bullet or any other sharp object.

Measuring blood glucose could be viewed as a very minor form of penetrating trauma!

A sharp object can make a lasting hole inside the body as it tears its way through tissues and organs. This effect is called permanent cavitation.

Effects on organs

The body’s organs respond differently to high impact trauma. Hollow organs tend to burst, and solid organs tend to leak.


Divide into groups of 3 or 4. On a flipchart, list the internal organs of the abdominal cavity and mark which organs are hollow, and which are solid.

Relate this to where organs are situated in the abdomen and emphasise why it is important to know where the organs are and whether they are solid or hollow. Then relate this to understanding why it is important to know where the casualty has been struck so that consideration can be given to what body structures might have been damaged by a given impact.

Solid organs Hollow organs

Liver Gall bladder

Pancreas Small and large intestine

Spleen Stomach

Kidneys Bladder

Blood vessels


Patterns of injury

When someone falls from a horse, they are likely to put their arm out to save themselves. This means that the force of the impact is transmitted through their arm, and a fractured wrist, fractured or dislocated elbow or fractured collarbone are all likely injuries.

In a side impact car accident forces may cause spinal injury, pelvic or femoral injury on the side of impact.

Reading an incident scene as described above allows you to estimate what the injuries might be by establishing the pattern of injury.

What injuries might someone have suffered if they jump out of a first floor window and land on their feet?

(Answer: possible # ankles, knees, femurs # or dislocation of the hips, # pelvis, # spine as it is compressed by the force of the landing, possibly a # base of skull.)