Booster aged children wearing a safety belt alone (no booster) suffer their most serious injuries to:
The Head: The most common complaint from 4-8 year olds who are not riding in boosters is that the shoulder belt is rubbing their neck. In an attempt to make themselves comfortable, many children remove the shoulder belt – either by placing it under their arm or behind their back, or simply moving to a position with only a lap belt. It is in these scenarios – where the child has nothing holding their upper body – that head injuries are most common.
Without restraint, the upper body rotates around the lap belt in a “jackknife” motion, allowing the head to travel very far forward with a good chance of contacting hard structures in the vehicle, such as the back of the front seat or even the child’s knees. Typical injuries include concussions, cerebral contusions, subdural hematomas, intracerebral hemorrhage, as well as myriad facial injuries. When compared with children appropriately restrained in boosters, those inappropriately restrained were 1.6 times more likely to suffer facial fractures. 2-5 year olds restrained in safety belts were more than 4 times more likely to sustain a significant head injury when compared with their peers riding in child restraints (car seats and boosters.)
The Abdomen: The area approximately from the bottom of the ribs to the hip bones is likely to be injured when the lap belt rides up over the bony pelvis (hip bones), concentrating the crash forces over the soft abdomen. The risk for these injuries increases when a child slouches or misuses the shoulder belt. Slouching places the lap belt on the abdomen even before a crash. Without a shoulder belt across your chest and shoulder bones (when you ride with a lap-only belt, or place the shoulder belt behind the back or under the arm) your body will double over itself in a crash, but since you bend at your waist (near your belly button) instead of at your hip bones, the lap portion of the belt rides up over the hip bones and into the abdomen.
A large study on thousands of crashes found no cases of abdominal injuries among 4-to-8-year olds riding in boosters. Those 4-8 year olds who were suboptimally restrained – using a safety belt when they should have been using a booster – were more than 3 times more likely to suffer an abdominal organ injury than those optimally restrained (in a booster seat). The presence of bruises on the abdomen or flanks is strongly associated with underlying intra-abdominal injuries – specifically those to the spine, bowel, and bladder.
Four mechanisms for the lap belt riding up into the abdomen have been described in the literature, each with a different root cause and pattern of injuries. It should be noted that the 6 year old crash test dummy (Hybrid III) does not accurately represent the human pelvis-belt interaction as no submarining occurs, so much of this data comes from real-world crashes.
The Spine/Neck: Spinal cord injuries can occur when both the shoulder and lap belt are used, but are more common when the lap belt is the only form of restraint. When the lap belt rides up over the pelvis, applying all of the forces to the spine and abdomen, the thoracolumbar (chest and back) spine is at risk for injury. Not using a shoulder belt (or placing it behind the back or under the arm) increases the loads (forces) in the lumbar spine by as much as 5 times.
Many parents worry that the shoulder belt rubbing the child’s neck might be dangerous – thinking it could lead to spinal cord injury or even decapitation in a crash. These fears are completely unfounded; there is little, if any, evidence in the literature of cervical spine (neck) injuries resulting from poor positioning of the shoulder belt near the child’s neck (in situation’s where the child’s head does not strike anything in the vehicle). A shoulder belt that touches the side of the neck is not likely to cause injury unless the belt is very loose. Cervical spine injuries may occur when no shoulder belt is used – as the child’s head and upper body will travel forward, making the head likely to strike the front vehicle seat or even the child’s knees.