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Fig. 58. Short-statured driver wearing a lap-shoulder belt. Abrasions were noted on the left neck. Transected ends of left carotid artery (arrows).

(obesity, pregnancy) cause the lap restraint to be situated on the abdomen (502,564, 565). A shoulder restraint on the neck of a short individual can cause injury (Fig. 58; ref. 515). Seat belt injuries occur in all age groups and are seen with two-point (lap) and three-point (lap-shoulder) belts (510). Adult seat belts do not provide adequate protection for children (530,566).

Cutaneous injuries are indicative of seat belt use and loading (Figs. 21 and 59). Seat belt use is also determined by evidence of loading marks on the belt fabric (567). Skin trauma, as a result of inappropriate placement of the belt, constitutes a "seat belt sign" and raises the possibility of internal trauma (500,501,515,518,519,521,547,558,568). The cutaneous injuries vary and are not always associated with visceral injury (500,501). Conversely, the absence of external trauma—for example, in a heavily clothed occupant—does not exclude underlying injury (Fig. 60; refs. 499, 501, 514, 515, and 568). Severe abdominal visceral injuries are more likely with lap-only restraints compared with lap-shoulder belts (569).

Proposed mechanisms of intra-abdominal injury caused by seat belt compression include:

1. Avulsion of fixed structures (e.g., duodenal-jejunal junction, jejunum at ligament of Treitz, terminal ileum, transverse-sigmoid colon junction, areas of fibrous adhesion) owing to deceleration.

2. Compression of viscera against the spine.

3. Closed-loop intestinal perforation from localized pressure.

4. Shock wave effects on hollow and solid viscera from increased intraperitoneal pressure (505,515,516,518,521,570).

Fig. 59. Front seat passenger. Cutaneous injuries (abrasions) on right neck caused by shoulder belt.
Fig. 60. Lap-belt injury. (A) Lower abdominal wall hematoma seen on abdominal incision. Only small scattered cutaneous bruises were observed in the vicinity. (B) Transected loops of small bowel.

Lap-belt loading tends toward hollow viscus injury (509,513).

Lumbar fractures ("Chance fractures") from seat belt loading are thought to be caused by a lap belt improperly positioned at the umbilicus, allowing hyperflexion of the spine (505). Fractures involve the vertebral body and posterior elements, typically of the second, third or fourth lumbar vertebra (500,510,533). Compression fractures also occur in nonbelted occupants (533). Axial loading during vehicle rotation, multiple impacts, and "pancake" landings have been associated with lumbar fractures (500,533).

The leading cause of fetal demise in a motor vehicle crash is maternal death (571). The best protection for the mother is the use of a restraint system that reduces interior contacts, especially in lower-severity crashes (538,542,571,572). Fetal demise does occur in lower-severity crashes, and there can be minimal maternal injury (539,572-576). Seat belt loading of the abdomen raises pressure in the gravid uterus (539). Uterine distortion causes the less elastic placenta to tear, leading to abruptio placentae (571). Notably, the spectrum of injuries observed in restrained pregnant women—uterine rupture, abruption of the placenta, and fetal injury—are seen in unrestrained females (571,572). Associated maternal pelvic and femur fractures occur (577).

Anterior rib fractures, owing to shoulder belt loading, are distributed asymmetrically compared with those caused by air bag loading (133,504,578).

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