Bumps in the Road

An evaluation of pediatric biomechanics following motor vehicle accidents

June 19, 2023 Photo

Child passenger safety has evolved significantly in the last two decades, with ongoing improvements still occurring. Even as safety regulations develop, there is currently no federal law standardizing the use of child restraint systems (CRS) across the U.S. Instead, each state and territory determines its own CRS laws.

Currently, there are three main stages of child passenger protection: rear-facing, forward-facing, and booster seats. Each stage is associated with a child’s overall size (height and weight). Many locales also associate their restraint laws to the child’s age, however, age does not take into consideration that the optimal interaction between a child and a CRS depends primarily on the size of the child, rather than the age.

As children grow, their bodies undergo fast changes not only in body proportions, but also in structural and cognitive development. In automotive safety, these changes must all be taken into consideration to ensure a child’s protection. Young children have large and heavy heads in relationship to the rest of their bodies. Because of this, they need assistance and additional support to their heads in the event of an accident to avoid a potential neck injury.

In time, children’s bodies tend to catch up to the size of the head, their musculature develops, and they are able to hold themselves up in a standard seated position. As these outward developments are occurring, their bones, ligaments, tendons, musculature, and brain tissue are also experiencing changes. Bones continue to lengthen and ossify, ligaments gain rigidity, and musculature’s lever points change.

Each stage of child protection is meant to best protect the most vulnerable portion of children as they grow. A properly used forward-facing seat allows for better load distribution along the torso and hips of a child when compared to a standard three-point seatbelt, reducing the load on the smaller clavicles following a frontal vehicle accident while limiting the overall forward excursion the child incurs. A booster seat ensures that the three-point seatbelt engages the child on the proper bony landmarks and not on the soft tissue.

Cognitive developments are also important when considering the transition from one stage of CRS protection to another, and the transition out of child restraint systems. If a child is unable to maintain a proper seated position in the next stage of protection, it does not matter how tall, heavy, or old they are, they are not ready to transition out of their previous stage. Instead, a proper alternative should be sought to ensure the child’s protection. If a child is transitioned to the next step too soon, they will be exposed to an increased risk potential and worse outcomes following a motor vehicle accident.

The Role of a CPST

A certified child passenger safety technician (CPST) is a caregiver’s best resource to ensure a child’s proper restraint and best protection. A CPST has up-to-date training on new safety developments and best protections, and has the ability to educate caregivers on the proper seat installation methods. CPSTs are also trained to identify CRS misuses by examining a seat; evaluating the installation method; and identifying if a restrained child’s size, posture, and behavior are appropriate for the stage of protection used.

After an accident occurs involving a child and a CRS, a forensic expert with CPST training can assist in evaluating the potential role of the CRS in the mechanics of potential injuries. A forensic expert with CPST training can consider all aspects of the child’s protection, including:

•    Was the CRS in use the correct seat for the child’s stature, weight, and development?

•    Was the seat properly installed in the vehicle?

•    Was the child properly harnessed in the seat?

•    Were there any third-party accessories in use at the time?

•    Were there any potential projectiles around the seated area?

•    What are the specific child passenger safety laws to consider in the location of use?

Being able to answer these questions will ensure that the child’s protection was to best practices standards and therefore the best for the child. Following are two case studies where such questions are raised and examined.

Case Study One: Unrestrained and Out of Position

The front plane of a sedan contacted the right side of a pickup. The sedan had two unrestrained minors in the second row. The police report stated that the children were lying down. At the time of the accident, Child A was 6 years old; three feet, 10 inches tall; and weighed 36.4 pounds. Child B was 3 years old and weighed 28.4 pounds.

Following the accident, Child A was found unresponsive in the second-row floorboard and seizing. Child A was airlifted to an emergency room and was diagnosed with polytrauma, spleen laceration, left kidney infraction, intra-abdominal bleeding, and a fracture to a thoracic vertebra. Child B was diagnosed with a hematoma to the left side of the head with no intracranial hemorrhage. The accident occurred in Florida.

Questions: Are the injuries sustained consistent with the dynamics of the accident? Would the injuries have been prevented had the children been properly restrained?

Analysis and conclusions: Florida law requires the use of seatbelts or child restraint devices for all individuals under 18 years of age riding in a vehicle. It also notes that children under the age of five need to be restrained with a child restraint system. Child A’s stature and weight would indicate this child could be properly restrained by either a forward-facing CRS or a booster seat; some forward-facing seats can fit the average seven-year-old child and some booster seats can properly fit an average four-year-old.

Child B’s stature was not recorded in the hospital visit, but they weighed approximately 28.4 pounds. Based on their age and weight, Child B could be properly restrained by either a rear-facing CRS or a forward-facing CRS.

Child passenger safety best practice indicates that children should “max-out” the restraint’s height or weight prior to transitioning to the next stage. Taking this into consideration, a forward-facing seat would be best practice for Child A and a rear-facing seat would be best practice for Child B.

During a vehicle accident, the occupants of the vehicle will move opposite of the principal direction of force. That is, a second-row occupant would move forward toward the vehicle’s first row. Child A sustained injuries focused on the left and back sides of the body. Child A was described to have been lying down and asleep at the time of the accident. While injuries to the spleen and kidney could occur without direct contact, the fracture of the thoracic vertebra suggests there was direct contact to the back by a hard material. The combination of injuries is highly suggestive of Child A laying down with their back facing the front of the vehicle. Following the accident, the child would have had unrestrained forward motion within the occupant compartment and likely contacted the center console located between the driver and front passenger seat.

Had Child A been properly restrained, either with a forward-facing child restraint or a booster seat, their forward motions would have been limited by the interaction with the restraint system. Even when asleep, either child restraint would ensure the child maintained a conventional seated position, which would prevent the interaction with the vehicle’s center console and mitigate the injury risk for a thoracic vertebral fracture.

Child B sustained a contusion to the left side of the head. This injury is consistent with direct contact with the interior of the vehicle following unrestricted motion due to the passenger being unrestrained. Had a CRS been used, Child B’s forward motion within the vehicle’s occupant compartment would have been limited by the interaction with the properly used child restraint and contact with forward structures would have been avoided.

Case Study Two: Rear-End Fatality

A mid-size SUV was rear-ended by a van. At the time of the accident, the SUV had an unrestrained adult female second row occupant, and a rear-facing restrained infant. The adult female sustained abdominal pain, chest wall pain, and facial lacerations following the accident. The infant—9 months old and 19.6 pounds—sustained skull fractures, subdural and subarachnoid hemorrhage, and brainstem contusions. The infant underwent an emergency decompressive craniotomy, but ultimately died. A post-mortem evaluation noted tonsillar herniation and possible hypoxic injury to the child. Emergency room records noted the infant had possibly been unrestrained and ejected from the vehicle. The accident occurred in Texas.

Questions: Was the child properly restrained? Was the child’s cause of death the result of the motor vehicle accident or direct contact with the unrestrained female?

Analysis and conclusions: Texas law indicates that children older than 8 years old or over four feet, nine inches tall can ride in a vehicle without the need for a child restraint system. Texas’ “Click-It-or-Ticket” indicates that, for best protection, infants should be kept in rear-facing seats as long as possible, at minimum until a year old or at least 20 pounds.

Following the accident, photographs of the SUV and CRS were available for review. The CRS showed seatbelt markings along the rear-facing belt path on both sides of the infant carrier base. The CRS harness had been cut to remove the child from the seat. The shoulder portion of the harness was placed on the second available opening. The cut harness demonstrated a spool representative of the size of a small child. The crotch buckle was placed on the first opening.

The seatbelt markings on the CRS indicate it was engaged across the restraint base’s belt path following the accident. This indicates the CRS was properly installed within the vehicle at the time of the accident. The harness location, both on the shoulders and at the crotch buckle, indicates a size similar to a small child, which, in conjunction to the child’s age and weight, indicates this rear-facing infant carrier was a proper restraint for the child. Given the configuration of the CRS and evidence of child’s removal, the child’s ejection was not consistent with the evidence. It was determined that the child had been properly restrained at the time of the accident.

An analysis of the vehicle dynamics was performed. It was noted the SUV experienced a primarily forward-directed change in velocity (delta-V) of approximately 49 miles per hour during the contact with the van. Following the contact, the passengers of the SUV would have moved toward the direction of impact. Since the child was restrained in a rear-facing CRS, the child’s motion, relative to their center of mass, would have been toward the rear of the vehicle.

Even though the accident configuration was a rear-end accident, the rear-facing child would have experienced the equivalent of a frontal collision. The CRS and the child would have rotated along the CRS’s belt path and the rear of the CRS would have been elevated with the child being moved toward the second-row’s seatback. As a result of this motion, the child would have initially experienced forward motions, including head and neck flexion, and direct contact with the second-row’s seatback, then the CRS would have returned to the initial position. The unrestrained passenger’s motion would have been initially backward toward the rear of the vehicle followed by a forward rebound toward the front of the vehicle.

Based on this analysis, the mechanism of injury consistent with the child’s injuries is associated with the accident dynamics and not with contact with the unrestrained passenger. At the time of the accident, the child had been properly restrained and their outcome was directly linked to the severity of the accident rather than to improper restraints or an interaction with the unrestrained passenger.

Children are not small adults. Their bodies have unique mechanical properties that depend on their age and stage of development. Each child is an individual and the optimal solution for their safety in a vehicle may look different from a peer of the same age.

When evaluating a child’s biomechanics following a motor vehicle accident, it is imperative that the engineer understands all aspects of the child’s protections. By performing a complete and unbiased analysis, an expert can help all parties understand the role of the CRS in protecting the child.


About The Authors
Yadetsie N. Zaragoza-Rivera, PhD, CPST

Yadetsie N. Zaragoza-Rivera, PhD, CPST, is a senior consultant and biomedical engineer with Rimkus Consulting Group.  nzaragozarivera@rimkus.com

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