road traffic collisions - vehicle occupants
Car crash - Andy Warhol
Source: Modern Art Obsession
Injuries are sustained from striking structures such as the steering wheel, dashboard or windscreen, as well as vehicular parts that have intruded into the occupants' space, including parts of the engine, roof or door pillars etc.
Milroy and Clark (2000) provide an overview of the patterns of injury sustained by vehicle occupants, and note that there is a predominance of frontal impacts, tending to the off-side of the vehicle, and that an unrestrained occupant experiences a fairly predictable sequence of events during such an impact;
- the driver is thrown forwards and upwards;
- the legs contact the front fascia, and the chest/ upper abdomen contact the steering wheel
- the driver's body pivots across steering wheel, continues upwards, with the head bending forwards and striking the windscreen or corner pillar
- the front seat passenger impacts the fascia (legs) and the windscreen (head);
- rear seat passengers are thrown forwards and strike the seats/ occupants in front (head, chest, abdomen injuries)
Seatbelts reduce forward and upwards movements, but head and limb injuries can still be sustained from contact with the steering wheel.
Additional rotational forces act in side-impacts, and door panels intrude into the occupants' space, causing injury to the person on that side.
Significant head injuries are seen in over 50% of fatalities, whilst 90% have at least some surface injury, including abrasions and lacerations from striking vehicular parts, intruding components or from contact with the ground after ejection from the vehicle.
Small angled abrasions and superficial lacerations, with or without glass fragments still embedded, can be seen following impact against window glass (found in door or rear windscreen glass, rather than the front windscreen, which is usually made from laminated glass which cracks at the site of impact).
Windscreen safety glass shattering pattern Source: wikipedia
Over 50% of occupants have skull fractures, and in frontal collisions, these tend to reflect impact speeds of over 30 mph (48 km/h), although they can occur at lesser impact speeds in side-impact collisions.
Cervical spinal injuries include atlanto-occipital disruption leading to instantaneous death, and are probably caused by a combination of extension, rotation and posterior movement of the head on the spine (rather than acute flexion).
Seatbelt marks may be seen (faint bands of bruising running diagonally across the chest), and there may be an abrasion across the clavicle. These marks can indicate which seat the occupant was in at the time of impact, if this subsequently becomes an issue.
Source: Fiona Fahy, Trauma Service, Children's Hospital at Westmead
(see also Life in the Fastlane.com case example of sternal fracture with overlying seatbelt-related injury)
Rib fractures occur commonly in occupant fatalities (>70%), and are seen more often in drivers (possibly due to the proximity to the steering wheel). These may be associated with pulmonary contusions and/ or lacerations.
The heart is injured in 25%, with contusions and lacerations being common. The aorta may be ruptured, often at the origin of the descending thoracic portion, distal to the left subclavian artery orifice.
traumatic aortic transection - chest xray (AP) (Radiopedia.org case 9279 Dr Hani Alsalam)
traumatic aortic transection CT (Radiopedia.org case 9279 as above)
Crass et al (1990) noted that the traditional view regarding the mechanism of aortic rupture from rapid deceleration was a combination of traction, torsion and hydrostatic forces, but proposed an alternative mechanism - the 'osseous pinch'.
They suggested that the aortic isthmus was effectively 'pinched' between the spine and the anterior thorax - including the manubrium sternum, clavicles and first ribs - during chest compression as a result of rapid deceleration.
Liver injury, ranging from superficial surface laceration to complete disruption, are slightly more common in drivers than passengers.
Pelvic injuries, including separation of the symphysis pubis (with or without fractures of the superior and inferior rami on either side) occur more frequently in front seat occupants, and during side impacts. Fractures of the femur accompany impact against the dashboard or steering column.
Seatbelts reduce the fatality risk during a collision by 42% and airbags by 18%. When combined, the risk is reduced by 47%.
Injuries from airbags, designed to inflate within a few miliseconds of impact and cushion the occupant from vehicular structures, include abrasions to the face, neck and chest, minor friction burns to the upper limbs and eye injuries, including retinal detachment.
Excessive neck hyperextension is also thought to be responsible for cervical spine injuries (including atlanto-occipital dislocation, C1-C2 dislocation, and C1 and/ or C2 fractures) and skull ring fractures. (See Shkrum (2002) and Sato et al (2002) for an overview of airbag related injuries).
road traffic collisions - pedestrians
Pedestrian bumper injury causation (lower limb injuries from impact with car bumper/ fender)
Source: Technology Magazine News
Pedestrians are more vulnerable to injury than the relatively protected vehiclular occupant, and children under 15 years account for nearly 40% of all pedestrian casualties (Clark and Milroy 2000).
68.5% of injured pedestrians are struck by the front of the vehicle, whilst 28% are struck by a side impact.
The extent of injury will depend upon the nature of the vehicle, its speed and the site of contact - injury severity increases in proportion to the weight and speed of the vehicle.
Pedestrian injury distribution. Source: wikipedia
As with vehicle occupant injury, a fairly predictable sequence of events occurs following a pedestrian being impacted by the front of a vehicle, involving 5 categories of post-impact trajectories; (Adapted from Clark and Milroy 2000)
Primary impact - struck by bumper (height varies but is between 41-51cm above ground in European cars) then front edge of bonnet (71-76cm above ground), person then rotated, and head/ shoulders/ chest thrown against bonnet, windscreen or its surround (secondary impact), person continues forwards and falls off front of braking vehicle, striking road surface (tertiary impact)
Primary impact injuries - Lower leg (bumper), upper leg/ pelvis (bonnet),
Secondary impact injuries - head/ shoulders/ chest (windscreen),
Tertiary impact injuries - head/ body (road surface)
Applicable with impact by flat fronted vehicles (buses, lorries etc) - victim thrown forwards onto the road surface
Primary impact injuries against head/ body, Secondary impact injuries (impact against road surface), and multiple injuries (run over by other vehicles)
Person struck by front corner of vehicle and is carried over the wing, falling to the ground at the side of, or behind, the vehicle
Head/ body (impact from wing and road surface)
Occurs at high speeds, or when accelerating at impact - person slides up windscreen after primary impact, over the roof and onto road surface behind vehicle
Primary, secondary and tertiary impact injuries
High speed impacts - forces on lower body cause body to somersault into air (no secondary impact)
Primary and tertiary
Injuries to the surface of the body range from minor abrasions and bruises, to lacerations and degloving.
For example, broad grazes represent contact abrasion with road surfaces, and are often contaminated with grit.
Imprint abrasions/ intradermal bruises may outline the striking surface, and allow subsequent comparison with a suspect vehicle / and or component. Abrasions may also retain trace materials such as paint fragments, of potential evidential importance.
Tyre tread pattern
Lines of bruising occur where the skin has been forced into the grooves of a tyre, whilst that skin in contact with the raised edges remains unmarked. Such marks should be photographed with a scale to allow for subsequent comparison with suspect tyres.
Injuries sustained when a vehicle runs over a pedestrian include 'undercutting' or degloving injuries, where large flaps of skin are produced, particularly on the limbs. The hinge point indicates the direction of travel. If the skin is not lacerated, a pocket of blood and iquified fat can accumulate under its surface.
Multiple linear striae (stretch abrasions) may be seen on the skin of the abdomen, indicating 'over-stretch' of that skin, for example on the anterior abdomen from an impact from behind the victim.
Injuries to the legs from the bumper/ bonnet of the vehicle can present as bruises, abrasions, lacerations and compound fractures. Deep bruising of the calf muscles can occur without surface injury, and must be sought during autopsy.
Transverse hinge fractures are common and, in general, the higher the speed of impact, the more severe the head injuries.
Rib fractures and lung contusions are common, whilst aortic ruptures are not as commonly encountered as in vehicle occupants.
Severe injuries are seen following 'run over', with multiple fractures and underlying visceral disruption and fragmentation.
Toro et al (2005) found that there were substantial differences in the distribution of injuries suffered by pedestrians, bicyclists and motor vehicle occupants. A higher rate of head injuries such as skull fractures, extradural/ epidural haemorrhage, subdural haemorrhage, brain contusion and injuries of the lower extremities are seen among pedestrians and bicyclists, for example, compared to vehicle occupants.
Thoracic trauma, such as traumatic aortic rupture, haemothorax, and abdominal injuries, such as liver rupture, however, were dominant in motor vehicle occupants. (For a review of trauma to adult bicyclists, see Rosenkranz and Sheridan (2003)).
Determining the speed of impact
Determining the speed of impact is an issue that is often raised in pedestrian impacts, and the frequency and severity increases with increasing speed. Zivot and Di Maio (1993) noted that amputation of a limb or transection of the torso was associated with high speed impacts (>55 mph or 88 km/h) but, in general, the level of injury varies considerably, and such a determination is best left to traffic investigators, who determine speed by studying the vehicle concerned and tyre marks etc.