There are three forms of automobile belt-type restraints: lap belts; shoulder (diagonal) belts and three-point belts (lap plus shoulder belt). Lap belts were
the first form of restraint offered, becoming standard in automobiles in 1964. They are still found in older vehicles and in the back seats of some newer vehicles. All new vehicles use the three-point belt. As of December 1997, it was estimated that 69% of motor vehicle occupants use safety belts.7
Lap belts are effective in reducing mortality and morbidity, predominantly by preventing ejection of the driver and passengers at the time of a crash. In frontal collisions, they prevent impact of the head of the driver or passenger with the windshield frame and reduce injuries that would be caused by impacting the steering wheel and dashboard. On frontal impact, the head and chest will jackknife over the belt such that the driver's head can impact the steering wheel and the front passenger's head the dashboard. Both can incur serious and fatal injuries. Lap belts also prevent rear passengers from impacting or being propelled over the front seats.
While successful in reducing mortality and injury, on occasion, a lap belt can also cause injury. If the belt is worn too high (above the pelvis), the body can violently jackknife, producing compression fractures of the lumbar vertebrae; transverse fractures of the vertebral bodies; as well as fractures of the pedicles, transverse processes and lamina of the lumbar vertebrae.811 The vertebral injuries are caused by severe flexion combined with shearing forces. Soft tissue injuries produced by lap belts consist of contusions and lacerations of the duodenum, jejunum and ileum and lacerations of the spleen and pancreas. In intestinal injuries, the lacerations are on the anti-mesenteric side of the bowel. While all of the aforementioned injuries occur from wearing the lap belt too high, such injury can still take place if it is worn properly through a phenomenon called "submarining," where, at impact, the pelvis sinks down into the seat and slides under the belt. Most abdominal and spinal injuries, however, are caused by wearing the lap belt too high.
Injuries to the small intestine, colon and lumbar spine, in the plane of the lap belt, are referred collectively to as the "seat belt syndrome." Contusions and abrasions on the abdominal wall, in association with this syndrome, and caused by the belt, are called the "seat belt sign."11
Use of the shoulder restraint without the lap belt can produce fractures of the cervical, thoracic and lumbar spine; fractures of the ribs and sternum and injuries of the larynx, liver, spleen and kidney.10 With combined use of lap and shoulder belts(three-point restraints), the benefits of the lap belt are augment by the benefits of the shoulder belt. Impaction with the steering wheel and dashboard are prevented. However, the three-point restraint still can produce injuries such as rib fractures (single more likely than multiple), fractures of the clavicles, and sternum and cervical spine fractures.9,10
While lap-shoulder belts can produce injuries, such hazards are outweighed when one realizes that they reduce fatalities for drivers of passenger cars by 45%.7 The part played by seat belts in saving lives is further evidenced in their ability to prevent individuals from being ejected from vehicles — 75% of occupants ejected from passenger cars are killed.12 Only 1% of occupants who are restrained are ejected compared with 22% of unrestrained occupants.
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