Mechanism of Injury

Context

Understanding the mechanism of injury is crucial to our clinical decision making for trauma patients during the preparation for arrival and assessment upon arrival stage in Emergency Medicine (ED).

One of the benefits of working in the prehospital environment is direct access to the scene. It is crucial that we gather information here which will allow us to paint an accurate picture for our hospital colleagues, make decision about any necessary critical pre-hospital interventions, and guide speed of extrication (if applicable).

In the ED we have access to prehospital clinicians, and relatives which inpatient teams may not. It is important that the information we gather from these sources is carefully documented to help guide ongoing investigation and management for the patient journey.

Epidemiology

Injury patterns by age and gender

With advancing age, Falls become the most common mechanism of injury in major trauma (majority falls < 2 metres).  Violent assault with or without weapons are most common in those under 60, as are injuries from RTCs. Sporting injuries are most common in those under 16. Overall, traumatic injuries occur more in men than women in the UK.

Mechanism of Injury in Major Trauma (ISS>15) by age group^1,2

Gender in UK Major Trauma³

 

Definition

Mechanism of injury is the manner in which a physical injury has occurred. It describes how, with what force, and to which part of the body energy was applied⁴.

This information is often given at the beginning of a handover of a patient between teams for context and provides a clue as to what type and severity of injuries we may expect.

For example: “patient fell 10ft from tree onto head”, or “patient fell from standing height”.

HOWEVER, it is easy to underestimate what injuries may have occurred judging by mechanism alone.

The 2 components of what injuries occur in a trauma are;

1. The mechanism and energies involved
2. The strength of the tissues subjected to the injury

We know that a fall from standing can cause severe multisystem traumatic injuries in the elderly compared with most healthy 25-year-olds who would have little or no injury from the same mechanism.

Patterns of injury can be considered in a number of ways:

Blunt Injury

Injuries that result from direct contact with a blunt object i.e. “something hitting someone or someone hitting something”. There may be overlying bruising, swelling or abrasions but damage to deeper structures may be less apparent, such as contusions, vascular damage, and fractures. Most RTCs, falls from height and horse-riding accidents cause blunt injuries.

Blunt trauma results in fracture, tearing by shear forces, pressure causing “blowout” type injuries, and coup–contrecoup injuries.

Penetrating injury

Any mechanism whereby the body surface is breached by an object leaving a wound.

Examples include stab wounds, gunshots, crossbow wounds, as well as patients being impaled on railings/objects.

In general, handguns, shotguns and hunting rifles are referred to as low-velocity weapons.

(High velocity weapons describes military grade weapons that can inflict greater damage).

Bullets travel in a straight line when stable, though can ricochet. An energy wave is produced as it passes through tissue, creating tearing injuries that extend further into tissues beyond the simple bullet wound track.

Stab wounds typically follow a direct path through the body resulting in damage along the line of the path itself(5), though this rarely happens with an individual in the anatomical position. As such, tracks may not be straight when you examine them.

Crush injury

This occurs when part of the body is subject to prolonged compression. The injury to soft tissues, muscles and nerves may be as a result of the direct trauma or lack of blood flow due to the compression causing distal ischaemia.

This can occur as a result of being trapped under heavy rubble/debris or entrapment in RTCs.

Crush asphyxia, a mechanical cause of hypoxia resulting from external compression in blunt thoracic trauma, may also develop.  

Burns

Burns are tissue damage that result from heat, radiation, chemical or electrical contact. Burns are covered in more depth in other modules. It must be remembered that burns can co-exist with major traumatic injuries. If suggested by the mechanism both should be actively assessed for and treated e.g. explosions can cause burns (see below) alongside other injuries, and vehicle occupants can sustain burns due to subsequent vehicle fire with concomitant injuries from the impact.

Blast⁶

There are 4 mechanisms of blast injury caused by an explosion.

Primary blast injuries are caused by the travelling high pressure air wave and can be subtle and have a delayed presentation, such as tympanic membrane rupture or damage at air/solid interfaces such as alveoli or bowel linings. It may also produce significant injuries such as traumatic amputation or even vaporisation.

Secondary blast injuries are caused by debris that is displaced by the blast wind of the explosion, or materials that may be packed around an improvised explosive device, such as nails, hex nuts, bolts or ball bearings. Secondary blast injuries are more common than primary blast injuries and are the most common cause of mortality in victims of an explosion.

Tertiary blast injuries are caused when the person is displaced through the air and impacts on another object by the blast wind, or when a structure collapses and causes injury.

Quaternary blast injuries are those not included in the above. They can be caused by exposure to the resulting fire, fumes, radiation, biological agents, smoke, dust, toxins and environmental exposure, depending upon the source of the explosion.

Acceleration/deceleration injury

Occurs when a moving person comes to an abrupt stop, but internal body tissues continue to move with forward momentum, often with differential movement. This creates shearing forces and is most commonly seen with injuries within the thoracic aorta, immediately distal to the arch and can range from dissection to complete transection.

Acceleration/deceleration injuries may also be seen at the renal pedicles, the cervical-thoracic spinal junction and between white and grey brain matter⁷.

There are certain injury patterns common to specific mechanisms of injury that can be useful in guiding trauma assessment.

**In trauma patients who are sedated, have received significant prehospital opiate or hypnotic medication, or who are not cognitively ‘intact’ (GCS<15) – a thorough secondary survey must be completed so as not to miss injuries which while not immediately life threatening may result in significant morbidity later e.g. scaphoid fracture. It is important to note such injuries may not show up on a trauma CT. If handing over care of a patient to a colleague or in patient specialty team it is important that this is conveyed – e.g. primary survey complete, secondary/tertiary survey still outstanding.

Fall from height (>2 storeys)

• Skull base fracture
• Spinal compression fractures
• Hip injury
• Tibial plateau fracture
• Pilon ankle fracture- (distal tibia fracture with intraarticular component, and often multiple fragments seen in high energy mechanism)
• Calcaneal fractures

Axial loading [e.g. diving into shallow pool or heavy load falling on head from height]

• Intracranial injury (e.g. subdural, extradural subarachnoid bleeds or contusions)
• Cervical spine injury

Motor vehicle crash (MVC)

Frontal impact

If unrestrained

• Craniofacial injuries
• Hyperextension of the cervical spine
• Atlanto-occipital dislocation
• Rib and sternal fractures
• Deceleration injuries (aortic injury, pedicle injury)
• Posterior hip dislocation
• Femoral fracture + knee dislocation.

In restrained occupants, the pattern of injury will be determined by the pattern and location of the seat restraints.

Lateral

• Craniofacial injuries
• Lateral rotation/flexion of c spine
• Humeral fractures
• Lateral flail chest and lung contusion
• Lateral abdominal compression (liver/splenic lacerations)
• Lateral pelvic compression fracture
• Fractured femur

Rear

• Neck injury from rapid hyper-extension.
• Lower back injury

Injuries will be more severe if the vehicle is fitted with a tow bar and force is transmitted to the passenger shell avoiding rear crumple zones.

Pedestrians

There are typically three impact phases when a pedestrian is hit by a car:

1. Bumper impact: in an adult who is upright, initial impact is usually on the lower limbs causing injuries such as bilateral tib/fib fractures.

2. Windscreen impact: torso and head injuries occur when a pedestrian impacts the body of the vehicle. Adults are more likely to get thrown up onto bonnet of the car due to their height whereas children/shorter patients can get knocked onto the floor.

3. Ground impact: further injuries occur as the pedestrian hits the ground.

Waddell’s triad: pattern of injury which appears in children hit by motor vehicles

• Contralateral head injury
• Ipsilateral intrathoracic or intra-abdominal injury
• Ipsilateral fracture of the femoral shaft.

Motorcyclists

• Posterior fossa fractures (caused when force impacts the chin-piece of a helmet)
• Cervical spine injuries
• Bilateral mid-shaft femoral fractures
• Pelvic fractures
• Bilateral wrist fractures
• Shoulder girdle injuries
• Bladder rupture
• Maxillofacial fractures

 

Mechanism of injury

While assessing the mechanism of injury, gathering information from sources such as witnesses, bystanders, and the surrounding environment is important while concomitantly clinically assessing the patient.

Police have often gained collateral history at the scene and will interview bystanders and therefore a potential source of information to inform mechanism of injury.

A reliable account of the mechanism can be useful in guiding further investigation and index of suspicion for certain injury patterns. However, witness accounts should be treated with some caution if the sole source of information on mechanism⁸.

The following questions should establish mechanism of injury:

1. Where was the patient?

• Was the patient a front seat passenger, a pedestrian on the pavement, a child in rear, on a bike seat carrier etc.?
• The majority (76%) of passenger vehicle occupants killed in 2020 were drivers⁹.
• Pedestrians, cyclists and motorcyclists are “vulnerable road users” as their mortality is much higher if involved in a RTC¹⁰.

2. Where did the energy force originate, and at what speed was force applied?

• How fast was each party going? This can be in mph/kph if known (the speed limit of the road can guide this) but can be descriptive such as “travelling downhill on a road bike”.
• What did they hit/hit them? For example:  travelling at 50 mph and ejected from vehicle after hitting lamppost”.
• In falls: What was the height & landing surface? Did they land on concrete patio or grassy area? Was there any debris or protrusions where they landed (e.g., railings or tree stumps)? Was the fall broken?

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Vehicle speed

Speed of vehicles involved is helpful but often gets confused and drivers or witness reports of speed may not be wholly reliable. If speed of the vehicles are not known then the speed limit of the road can give an indication of speeds that may have been involved.

People often refer to “Combined speed” when handing over patients involved in RTCs, however it is not a useful term and is better to describe the individual speeds of the vehicles involved in descriptive terms.

For example: “this patient was the driver of a car travelling at 60mph which collided head on with a HGV pulling out of a junction”. This gives more information and describes the forces better than “RTC with combined speed of 70 mph”.

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In RTCs: Rural roads have a much higher average speed than urban roads. Rural roads can prove more challenging in nature with blind bends, dips and other distractions. Accidents at lower speeds on urban roads are less likely to result in serious injuries or fatalities¹¹.

3. What protection did the patient have?

In motor vehicle collisions, specific points to note:

• Age & type of vehicle? Vehicles produced in the last 20 years have much better built-in protection than older models. Safety features include airbags, crumple zones, and anti-lock braking systems. Small/light vehicles have less structure to absorb crash energy so forces on occupants will consequently be higher¹².
• Were seatbelts worn by all occupants? Seat belts generally lock when a car is involved in a significant RTC. The fire service will be able to tell you if patients were wearing seatbelts if they have been extricated before your arrival. Document your own observations also i.e. has the belt been cut to extricate?
• There is good evidence that wearing seatbelts decreases the overall risk of major road traffic injuries¹³. There is also evidence that there is an almost 5-fold increase in risk of death for occupants of a car wearing a belt where there are other unrestrained occupants¹⁴.
• Were children in car seat? Proper car seat use has been shown to reduce the risk of injury or hospitalisation by >70% when compared with seat belts or no restraints¹⁵.
• Did airbags deploy? Typically, airbags deploy if the car has an impact above 16 mph in the UK.

For pedal bikes/motorcyclists/horse riders:

• Was protective clothing such as helmet or back protection worn? There is evidence that helmets are protective against brain injury⁹ so the lack of a helmet should increase our index of suspicion for a major head injury.
• What is the damage to that protective equipment e.g. is the helmet split?

4. What injuries have others sustained/other vehicles? 

• Are there other severely injured or dead passengers? Whilst another occupant of the same compartment being dead as a result of their injuries doesn’t necessarily increase the chances of severe injuries in your patient¹⁶, it demonstrates the high energy forces transmitted through the occupants during a crash.
• Rollover: A vehicle (and therefore the occupants) can undergo several impacts at many different angles during a rollover. This increases the potential for serious injury, as well as the possible extrusion of body parts through apertures, resulting in momentary crush damage or complete ejection.
• Intrusion: if there is significant intrusion into the vehicle, the likelihood of entrapment increases.
• Entrapment /if patient could self-extricate: Trapped patients have been shown to have a higher rate of significant injury and mortality¹⁷.
• Windscreen Bullseye: The windscreen is the most commonly struck area by pedestrians when hit by a car. Pedestrians or unrestrained occupants of a car who “bullseye” the windscreen would have hit it with force and as such have a risk of serious intracranial and spinal injury.

Documentation

• For prehospital teams this may mean photos of the scene and injuries that have been reduced or covered. It can be much easier to understand the extent of a mechanism when looking at a photo of the scene as a whole rather than just a verbal description. Be mindful that any images taken may be disclosable in future trials.
• History provided by the patient and witnesses should be clearly documented in the notes.

As the initial assessing clinician, we have unique access to information about mechanism of injury. It is important to use the correct terms in documentation of any injuries found as these could be crucial in any future legal proceedings related to injuries sustained. For example, a laceration describes only a wound created by blunt force, whereas wounds caused by cutting or stabbing actions are incisions/incised wounds. Describing a wound incorrectly could imply a blade was used when it wasn’t, and vice versa.

If you are unsure of the causation, it is better to use general descriptive terms such as “injury”, and “wound” rather than incorrect terminology.

A “wound” is legally defined as a break in the continuity of the skin or mucosal membrane and an “Injury” would refer to things like bruises or swelling [18]

**Where patients are conscious, consent should be gained for any information recorded in the patient record where possible. Whether treating patients in best interests or with their consent, all documentation should follow GMC guidance, ensuring to respect patients’ privacy and dignity, and using only secure devices if electronically stored¹⁹.

• Penetrating injuries made with a sharp object often look smaller due to the elasticity of the tissues.
• If a weapon is seen, leave in place and inform the Police. It is important to avoid contaminating forensic evidence. Do not assume this weapon caused your patients injury [this may inadvertently provide incorrect information to downstream clinicians].
• Knowing patterns of injury, while useful should not supersede or replace clinical assessment, nor radiological investigation.
• Elderly or frail patients can sustain more severe injuries from seemingly low energy mechanisms.
• Remember medical events (e.g. arrythmia, seizures etc) may have preceded or precipitated an apparently traumatic mechanism. Did the patient suffer a seizure or cardiac arrest prior to the car crash, or did the person suffer a traumatic cardiac arrest post impact? Clues from the scene such as non-deployed airbags, minimal exterior damage MAY suggest a medical event. This, of course, does not rule out traumatic injuries but can help guide direction of management.

1. Major Trauma in Older People – 2017 Report.
2. Severe Injury In Children Report – 2019-20. TARN – The Trauma Audit & Research Network. England & Wales. [Accessed July 2023]
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4. Nutbeam T, Boylan M. ABC of Prehospital Emergency Medicine. ISBN: 978-0-470-65488-0 September 2013 BMJ Books.
5. Indiana Department of Health, Mechanism of Injury Manual. https://www.in.gov/health/files/Mechanism_of_injury.pdf [Accessed September 2022)
6. Jorolemon MR, Lopez RA, Krywko DM. Blast Injuries. [Updated 2022 Jul 18]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-.
7. Rivara FP, Grossman DC, Cummings P. Injury prevention. First of two parts. N Engl J Med 1997; 337(8): 543-48.
8. Versteegh S. The Accuracy Of Driver Accounts Of Vehicle Accidents. Centre for Automotive Safety Research (CASR), 2004.
9.  The Insurance Institute for Highway Safety. Fatality facts 2020. passenger vehicle occupants. [Accessed September 2020]
10. Claire E. Baker and others, The relationship between road traffic collision dynamics and traumatic brain injury pathology. Brain Communications, Volume 4, Issue 2, 2022, fcac033
11. Department for Transport. Facts on Road Fatalities, 2015. [Accessed September 2022]
12. Wenzel, Tom. Assessment of NHTSA’s Report “Relationships Between Fatality Risk, Mass, and Footprint in Model Year 2003-2010 Passenger Cars and LTVs”. United States: N. p., 2016. Web. doi:10.2172/1341729.
13. Fouda Mbarga N, Abubakari AR, et al. Seatbelt use and risk of major injuries sustained by vehicle occupants during motor-vehicle crashes: a systematic review and meta-analysis of cohort studies. BMC Public Health. 2018 Dec 29;18(1):1413.
14.  MacLennan PA, McGwin G, Metzger J, et al. Risk of injury for occupants of motor vehicle collisions from unbelted occupants. Injury Prevention 2004;10:363-367.
15.  Safe kids Canada. Child safety good practice guide. Good investments in unintentional child injury prevention and safety promotion. [Accessed September 2022]
16. Serio F, et al. “Death in the Same Compartment” as a Predictor for Injury Severity. iMedPub. Trauma and Acute care. Vol.5 No.1:79, 2020.
17. Nutbeam T, Fenwick R, Smith J. et al. A comparison of the demographics, injury patterns and outcome data for patients injured in motor vehicle collisions who are trapped compared to those patients who are not trapped. Scand J Trauma Resusc Emerg Med 29, 17, 2021. https://doi.org/10.1186/s13049-020-00818-6
18. Ref Legal case: JJC (A Minor) v Eisenhower [1983] 3 WLR 537. Defined wounds and Injury in England and Wales.
19. General Medical Council (GMC). Making and using visual and audio recordings of patients. 2013. [Accessed October 2022]