10.1 Mechanism of Injury
Key Takeaways
- Mechanism of injury is the working scan the cabin will not give you; blunt shears fixed points, and penetrating tracks plus cavitation hide under tiny holes.
- Name the MVC path—up-and-over, down-and-under, T-bone, rollover, ejection, restraint—and package for the occult injuries that path predicts.
- Falls need height, surface, and first body part; blast is primary through quaternary; reconstruct gunshot and stab trajectory instead of probing.
- Stay on scene only for problems that prevent lift; uncontrolled hemorrhage belongs in a Level I or Level II trauma center, not the closest emergency department, when time and weather allow.
Mechanism of injury is the energy story you reconstruct before the cabin steals your exam. Domain 3.A of the August 2026 Board of Certification for Emergency Nursing (BCEN) Certified Flight Registered Nurse (CFRN) outline tests mechanism of injury (MOI) because helicopter emergency medical services (HEMS) hide what a quiet trauma bay would still let you see. Noise and a vacuum mattress do not change physics. Read the wreck, then package for the injuries it predicts.
Blunt versus penetrating
Blunt trauma transfers kinetic energy through intact or nearly intact skin. Organs tear at attachments, viscera crush against the spine, and vessels shear at fixed points such as the aortic isthmus and mesenteric roots. Penetrating trauma puts a track through tissue. A knife follows a narrow path. A gunshot wound (GSW) adds temporary cavitation: high-velocity rifle rounds damage a cone far larger than the bullet. Do not treat a tiny hole as a tiny injury. In the air you cannot auscultate a slowly filling hemothorax. MOI is the working scan when the cabin is too loud for a second look.
| Pattern | Energy path | Occult injuries to hunt |
|---|---|---|
| Up-and-over MVC | Unrestrained torso over the wheel into glass | Facial, cervical, sternal, myocardial, hollow viscus |
| Down-and-under MVC | Pelvis and femurs under the dash | Femur, acetabulum, posterior hip, pelvis, distal aorta |
| Lateral / T-bone | Near-side door into occupant | Head, cervical spine, flail, aorta, pelvis, liver or spleen |
| Rollover / ejection | Multiple vectors, roof crush, no compartment | Multi-system, axial-load spine, open fractures |
| Restraint / airbag | Belt load plus bag slap | Sternal, Chance fracture, bowel, placental abruption |
| Feet-first fall | Axial stack | Calcaneus, tibia, pelvis, lumbar burst, aortic |
| Primary blast | Overpressure | Tympanic membrane, blast lung, hollow viscus |
Motor-vehicle, motorcycle, ATV, and pedestrian patterns
Up-and-over, down-and-under, T-bone, rollover, ejection, restraint
A motor-vehicle collision (MVC) is not one injury. Ask who sat where, what deformed, whether anyone was ejected, and what the belt and airbag did.
Up-and-over is the unrestrained or poorly belted occupant who leaves the seat toward the windshield and steering column. Expect face, cervical spine, sternum, myocardium, and upper-abdominal hollow viscus. Down-and-under is the occupant who submarines under the dash: femur, posterior hip, pelvic ring, and sometimes a torn distal aorta.
Restraint changes the map. A lap-only belt is a fulcrum: think Chance fracture and bowel perforation. A three-point belt plus airbag reduces ejection; it does not grant a normal exam. Carry belt marks, intrusion, and death of another occupant with the patient after you can no longer see the wreck.
T-bone (lateral) impact loads the near-side chest and pelvis. Driver-side threatens spleen and left chest; passenger-side threatens liver. Lateral force is a classic blunt-aortic setting. Rollover multiplies vectors and adds roof crush; assume axial-load spine. Ejection is a high-energy flag by itself—treat the whole body even if the first blood pressure looks polite.
A motorcycle or all-terrain vehicle (ATV) has no crumple zone. Record helmet use and whether the rider was thrown, pinned, or dumped in a ditch. Handlebar abdominal impact hides small-bowel and pancreatic injury. Pedestrian versus vehicle follows height: an adult takes the bumper at the tibia or knee, then hood, then ground. A child takes the bumper at the pelvis or femur, then is thrown or run over. Waddell's triad—femur, torso, contralateral head—is a pattern, not a clearance.
Falls, blast, projectiles, and crush
Height, surface, and first body part
Ask height, surface (concrete versus dirt or snow), and which body part hit first. Feet-first axial load stacks calcaneus, tibia, pelvis, lumbar burst, and sometimes aorta. Head-first is brain plus cervical spine. American College of Surgeons (ACS) field-triage teaching has long treated adult falls from more than about 20 feet and children from two to three times their height as trauma-center criteria—clinical teaching, not a BCEN-owned cutoff. A short fall onto concrete in an older adult on an anticoagulant can still be a lethal subdural.
Blast, trajectory, industrial crush
Blast injury is four layers:
- Primary: overpressure injures gas-filled organs—tympanic membrane (TM), blast lung, bowel.
- Secondary: fragments, the usual killer.
- Tertiary: the body is thrown (blunt plus crush).
- Quaternary: burns, inhalation, crush syndrome, toxins.
A ruptured TM does not prove blast lung, and an intact TM does not clear the lung. Dalton's law will unmask blast lung on climb when the partial pressure of inspired oxygen (PIO2) falls. For GSW and stab wounds, reconstruct trajectory. A thoracic entry can finish in the abdomen if the patient was bent or seated. Do not probe or pull an impaled object. Mark entry and exit if both exist.
Industrial crush is local and systemic: compartment syndrome, rhabdomyolysis, hyperkalemia, and reperfusion crash when the load comes off. Ask how long the limb was pinned.
Occult injury, scene time, and destination
A noisy cabin hides an evolving abdomen, an expanding neck hematoma, and a cooling foot under a mattress. Use MOI to decide what you recheck after every move.
Scene time versus scoop-and-run is a HEMS decision, not a slogan. Stay for problems that kill before the rotor spins: compressible hemorrhage, an unsecured airway, tension you can decompress. Do not finish a hospital secondary survey in a ditch. Uncontrolled noncompressible hemorrhage belongs in an operating room.
A hypotensive ejected motor-vehicle occupant can reach a community emergency department in 12 minutes or a verified Level I trauma center in 18 minutes. Fuel, weather, and daylight allow either hop. What is the correct destination logic?
An unrestrained driver has a bent steering wheel and a starred windshield. Which occult injury pattern should the flight nurse package for?
A roadside patient has uncontrolled intra-abdominal hemorrhage and a patent airway. What scene-time plan matches helicopter emergency medical services teaching?