Air Bag Deployment Injury Patterns

Front air bags supplement seat belts and protect front occupants in moderate to severe frontal crashes. The deployed air bag cushions the front seat occupant's head and chest from contact with the steering wheel or dashboard. An air bag is activated when there is significant deceleration along the longitudinal axis of the vehicle; and most deployments are in frontal crashes (579). The threshold speed for deployment of a "first-generation" air bag in a frontal impact is 11 to 25 km/h (7 to 16 mph [579]). Side and undercarriage impacts can also trigger deployment (Fig. 61; ref. 579).

Most air bag injuries are minor (e.g., facial cutaneous trauma [580,581]). Ocular injuries and facial fractures have been described (580). Upper extremity fractures occur if the driver's arm is positioned over the air bag module while the steering wheel is turned (580). The lower extremities are not protected if the occupant "submarines" under the air bag (582-585).

Because they are pyrotechnic devices, air bags can cause unusual isolated trauma by unique mechanisms (580,581,586). An "out-of-position" occupant—i.e., a driver's chest within 25 cm (10 in.) of the steering wheel—is at risk for injury (579,586,587). There are multiple factors that predispose a front seat occupant being out of position (Table 3; Figs. 62 and 63).

During the "punch-out" phase—i.e., when the torso of the occupant is covering or very close to the air bag module—air bag inflation is blocked (588). If the air bag is partly inflated and further expansion is resisted ("membrane-force" phase), then thora-coabdominal injuries and head and neck injuries occur (Fig. 64; ref. 588). Because forces are being transmitted at high velocity (145 to 328 km/h or 90 to 211 mph), a spectrum of trauma, including blast-type injuries, can result (Table 4; ref. 579).

When assessing whether an air bag contributed to injury causation, the pathologist must realize that, in high-severity crashes, the protective capability of the airbag may be

Fig. 61. The driver died in this far-side collision that triggered deployment of the air bag. The front seat passenger in the target vehicle survived (see Figs. 63 and 64). (Courtesy of Road Safety and Motor Vehicle Regulation Directorate, Transport Canada.)

Table 3

"Out-of-Position" Front Seat Occupant: Predisposing Factors Large physique-obesity

Short stature-adjustment of car seat forward to reach floor pedals

Skeletal conditions (e.g., scoliosis, achondroplasia) Driver < 5 ft. 4 in. (160 cm) tall Other position changes

Occupant of any age improperly seated, reaching, leaning forward

Driver slumped over steering wheel owing to drowsiness, medical condition, intoxication Forward movement of occupant of any age owing to:

• Pre-impact braking

• Preceding minor (< air bag deployment threshold) impact(s) and exacerbated if manual restraint system

° Slack (heavy clothing contributory); slow lock-up

° Relaxation of inertia-activated belt after minor impact followed by air bag deployment in major impact ° Improperly worn; lap belt only Child-safety seat in right front seat

Adapted from ref. 586 with permission from the Journal of Forensic Sciences.

Fig. 62. Minor collision. Deployment of air bag; driver dead at scene, likely slumped over steering wheel because of ethanol intoxication. At autopsy, a laceration of the left main pulmonary artery was found. (Reprinted from ref. 579 with permission from the Journal of Forensic Sciences.)

Fig. 63. Short-statured female driver had her seat positioned forward. (A) The victim was also observed leaning forward to better visualize the icy road. Her vehicle skidded into an oncoming vehicle and the air bag deployed (see Fig. 61). (Reprinted from ref. 579 with permission from the Journal of Forensic Sciences.) (B) Lipstick impression (arrow) on air bag.

Fig. 63. Short-statured female driver had her seat positioned forward. (A) The victim was also observed leaning forward to better visualize the icy road. Her vehicle skidded into an oncoming vehicle and the air bag deployed (see Fig. 61). (Reprinted from ref. 579 with permission from the Journal of Forensic Sciences.) (B) Lipstick impression (arrow) on air bag.

exceeded because of occupant loading through the air bag (586,644). In multiple impacts, an air bag may deploy early and not be available to protect the occupant later in the crash sequence (Fig. 65).

If there are multiple injured occupants in a collision and the identity of the driver is unclear, then examination of the air bag, including swabbing for DNA analysis, assists in this determination (645).

Fig. 64. A postmortem radiograph demonstrated dislocation of C1-C2 vertebrae (atlantoaxial dislocation; arrow) caused by air bag deployment (see Figs. 61 and 63). (Reprinted from ref. 579 with permission from the Journal of Forensic Sciences.)

Table 4

Fatal and Nonfatal Serious Injuries in Drivers and Front Passengers Associated With Air

Bag Deployment: Literature Review

Craniocerebral trauma

Skull fracture(s), nos2 (589-591) Base (nos, "ring," "hinge") ± calvarium2 (579,589,592-595)

Associated vascular tear (cavernous sinus, carotid artery [589,596]) Depressed, nos^ (589) Occiput2 (589,597) Calvarium only^ (598)

Epidural, subdural/subarachnoid/intraventricular hemorrhage2 (589,590,595,597,599-604) Brain injury, head injuries, nos2 (589,591) Maceration^ (589) Cerebral edema2 (589,590,598,601) Closed head injury2 (579,589,598,605) Diffuse axonal injury2 (590,603,606) Cerebral contusion2 (589,590) Cerebral hemorrhage, nos2 (589,594,598) Brain stem-cervical spine trauma Decapitation or neck transection^ (589) Cervical spine injury or neck fracture, nos2 (589,595)

Atlanto-occipital or atlanto-axial dislocation2 (579,589,590,594,595,602,604,606-614) Other fractures-dislocations (C2 to C7)2 (579,589,590,593,594,600,610,615,616)

The Forensic Pathology of Trauma Table 4 (Continued)

Brainstem injuries, nos (595)

Ponto-medullary tear or transection" (602,604,606) Medullary hemorrhage/tear/transection (589) Laceration or transection, nos" (589,593,595) Brainstem or pons contusion/hemorrhage" (589,590,600) Cervical spinal cord injuries, nos" (589,590)

Compression, laceration, or transection" (589,590,594,595,607-609,612) Contusion^ (594) Other neck injuries

Transection or crushing of trachea-larynx" (589,617,618)

Blunt trauma to neck with asphyxia, neck swelling leading to airway closure, retropharyn-geal hematoma, airway compromise owing to cervical spine fractures (589,593,607,619,620) Neck injury, nos (589) Vertebral artery tear" (621) Internal carotid artery dissection (622) Thoracic injuries

Massive intrathoracic hemorrhage (589) Heart injury, nos (589) Laceration of heart or myocardium, nos (589,590,595,623) Laceration of atria/ventricles (579,589,595,600,611,623-626) Laceration of valve(s), nos (589,595) Tricuspid (589,627) Aortic (270,628,629) Contusions (589,600,611,623)

Blunt force trauma to the chest resulting in sudden heart stoppage (589) Lacerations of great vessels Aorta, nos (579,589,595,605) Thoracic, nos (589,611) Ascending thoracic (589,630,631) Arch (589,623)

Descending thoracic-isthmus (602,625) Abdominal (589,632) Aortic dissection (589) Vena cava

Superior (589,623) Inferior" (317,594) Brachiocephalic artery (intima) (625) Pulmonary vein (589) Pulmonary artery (579,607,619,633) Azygos vein laceration (634) Diaphragm rupture (635) Lung injuries

Contusions" (589,590,594,595,600,611) Hemorrhage = blast injury (579) Lacerations (589,611)

(Hemo)-Pneumothorax (589,590,594,600,602,636) Esophageal rupture (637)

Multiple rib fractures—crushing chest injury, flail chest (579,589,593,600,623)c,d

Table 4 (Continued)

Thoracic spine fractures-spinal cord laceration" (589,594,595,615) Abdominal and retroperitoneal injuries

Lacerations of spleen, liver, pancreas, bowel, mesentery (589,593,595,599,600,602,611,623,632,638,639) Avulsion of kidney^ (640) Pregnancy-related injury

Abruptio placentae and fetal death (641) Rupture of placental membranes (642) Fetal trauma including blast injury (576,643)

"Injury described in children also. ^Injury described in children only.

cRib fractures contributing to respiratory failure in 79-yr-old man with lung cancer, belted driver, 8 v = 35 km/h (22 mph).

dA 74-yr-old female, 220 lb (100 kg), 65 in. (163 cm). nos, not otherwise specified.

(Adapted from ref. 586 with permission from the Journal of Forensic Sciences.)

Fig. 65. Multiple minor impacts and deployment of air bag early in crash sequence. Steering wheel deformation owing to loading by driver. Aortic transection. (Courtesy of Road Safety and Motor Vehicle Regulation Directorate, Transport Canada.)

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