When faced with a pedestrian fatality, the pathologist needs to address other issues in addition to determining the cause of death:
• Was the pedestrian upright when struck?
• Is there evidence of intoxication? One study indicated that intoxicated pedestrians sustain more severe injuries (661).
Frontal impacts are the most common type of collision involving pedestrians (662).
Various kinematic trajectories occurring in frontal motor vehicle-pedestrian collisions have been described (Fig. 71; refs. 656, 662, and 663).
Initial vehicle impact with an upright adult pedestrian can lead to "bumper" fractures, typically of the tibia and fibula (664-666). Fractured bone ends typically splay outward, stretching and even tearing overlying skin (Fig. 72). The impact site on the opposite side may not be visible, but soft tissue hemorrhage is revealed by skin incision (Fig. 73; ref. 665). The distance from the sole of the foot to the fracture site is an approximate indication of bumper height at the time of impact. Radiographs confirm visible or palpable fractures and also demonstrate fractures that are inapparent externally. The fracture may consist of a wedge-shaped bone ("butterfly" fracture or "Messerer's wedge;" see Fig. 74). The apex of the wedge has been described as pointing in the direction of vehicle travel, but the opposite has been observed (667-669). Femur fractures occur from hood or ground contact (670). Amputation is possible (664,665,671). Bumper fractures can be absent, affect one leg in more than one site, or
Fig. 74. Pedestrian struck by car. Wedge-shaped fracture of tibia (arrow). (Courtesy of Dr. E. Tweedie, London Health Sciences Centre, London, Ontario, Canada.)
involve both legs (672). Leg impact in an upright individual can cause "bone bruises" demonstrated in cross-sections of the fibular and tibial epiphyses (665,666). Knee injuries can arise from flexion/hyperextension and dislocation, which lead, respectively, to condylar compression of the tibia and avulsion of knee ligaments. Bone bruises caused by the former mechanism are large and central, whereas avulsion leads to smaller more peripheral injury. Ankle-joint injuries (e.g., malleolar fractures), arising from pronation, supination, or flexion, also indicate an upright position (669). Like knee-joint injuries, the mechanism and consequent location of ankle-joint trauma is dependent on the direction of impact (665,666,669). Other indications of an upright position include traces of car paint on the legs and abrasions on shoe soles (664). Abrasions on toes may indicate forceful contact within a shoe following leg impact
Secondary impact with the hood edge or headlight assembly can cause pelvic and hip injuries (Fig. 22; ref. 670). Cutaneous "stretch" abrasions in the inguinal-femoral area are indicative of these fractures (Fig. 76). The wrap trajectory is most common (Fig. 71; refs. 656 and 673). When a decelerating vehicle hits an adult pedestrian it causes the upper body to bend over, striking the hood (Fig. 77 and 78). When the vehicle stops, the pedestrian hits the ground (674). Children, because of a lower center of gravity, frequently have head injury because of contact with the hood edge (Fig. 79; refs. 656, 675, and 676). Children tend to run out between parked cars or are backed over in the home driveway (675,677). The roof vault trajectory is associated with the most severe pedestrian injuries. A vehicle traveling at high speed lifts a pedestrian onto the hood, onto or over the windshield, roof, and even the trunk (Fig. 80; ref. 656).
Forward projection most commonly occurs in children and in adults hit by trucks and vans (656,673,674,678). Compared with pedestrians struck by passenger cars, those struck by sports utility vehicles, light trucks, and vans at lower impact speeds (<30 km/h or 19 mph) were more likely to have severe injuries and die.
Head injuries are common in pedestrian cases (673,674,676). Hyperextension of the neck causes upper cervical spine dislocation (atlanto-occipital level, C1-C2, C2-C3) and isolated brainstem injury (Fig. 81; ref. 679 and 680). Associated tears of basal blood vessels (e.g., basilar artery) can occur (680). Hemorrhage into the lower insertions of the scalene and sternocleidomastoid muscles has been shown to correlate with the direction of force (681). The pathologist must be aware that brainstem lacerations
(e.g., pontomedullary tears) can be subtle. Vigorous tugging of the brain, after removal from the cranium, can create an artifactual tear at this site.
A study of frontal motor vehicle-pedestrian collisions determined relationships between impact velocity and the resulting injury pattern (682). A correlation between a minimum impact velocity and the occurrence of certain injuries has been observed: spinal fractures (27.5 km/h or 17 mph); thoracic aortic rupture combined with a thoracic spine fracture (63 km/h or 39 mph); inguinal skin tearing (66 km/h or 41 mph); and dismemberment
of neck, trunk, or limb (98 km/h or 61 mph). Leg fractures from bumper impact occurred in the range of 18.5 to 142 km/h (12 to 88 mph) but was absent in some cases (highest impact velocity = 67 km/h or 42 mph).
During the autopsy, police investigators may document various body measurements that will assist them in determining the victim's center of gravity and consequent trajectory.
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