Biomechanical implications of low-height falls in toddlers

An in-depth scientific article published in the July 2010 issue of Journal of Neurosurgery: Pediatrics, entitled Biomechanics of the toddler fall during low-height falls elucidates the impact of head injuries caused by low-height falls in toddlers, using an anthropomorphic biomechanical surrogate. Authors are Nicole G. Ibrahim, PhD, and Susan S. Margulies, PhD, researchers in the Department of Bioengineering at the University of Pennsylvania in Philadelphia. The article is posted online at: http://thejns.org/doi/full/10.3171/2010.3.PEDS09357.

Of the estimated 1.5 million people treated for head injuries every year at U.S. hospital emergency rooms, nearly 380,000 are children ages 4 and younger. Blunt head trauma is commonly encountered by pediatric neurosurgeons and emergency physicians. In children ages 4 and younger, traumatic brain injury (TBI) is the primary cause of fall-related death and severe injury. Although there is a large body of epidemiological work published on falls and injury outcomes, these studies have been greatly limited by a number of factors. "Clinical studies of children injured from falls suggest that fall height and impact surface contribute to injury severity, but there is a lack of agreement on critical fall height and impact conditions, which is why a study in a more controlled environment is crucial," stated Dr. Margulies.

This laboratory recently developed a biofidelic anthropomorphic surrogate for the 6-week-old infant. The current research builds on the infant work utilizing an 18-month-old toddler dummy to investigate the biomechanics of falls in an age group that commonly incurs head injuries from low-height falls. In the toddler surrogate, body weight, body length, and neck stiffness were increased relative to the infant surrogate, including nearly doubling the skull thickness and developing a fused skull to simulate closed sutures and fontanels. The total weight of the dummy was 11 kg to mirror the weight of a 50th percentile 18-month-old toddler.

In this study, the researchers specifically looked at angular motion rather than linear translational head accelerations, because according to existing research, angular motion is more closely associated with TBI. Angular acceleration of the head has been shown to cause stretching and shearing of the underlying vascular and white matter tissue, leading to common clinical manifestations of pediatric TBI such as subarachnoid hemorrhage and diffuse axonal injury.

To simulate falls, the dummy was dropped from 1, 2, and 3 ft heights onto two surfaces (carpet pad and concrete). A total of 60 drops were performed, resulting in 53 successful drops from three heights onto two surfaces.

A typical drop consisted of initial head impact followed by rapid deceleration. An ANOVA was utilized to evaluate the influence of contact surface material and drop height, as well as directional differences in head acceleration response for each drop condition. The following select results were noted:

•Surprisingly, little to no rebound was observed in any scenario, indicating that there was no reversal of direction of the head after the rapid deceleration.

•In drops from 1, 2, and 3 ft onto carpet pad and concrete, the majority of head motion occurred in the sagittal and horizontal directions. This suggests that when contacting the surface, the head mostly rebounds up, perpendicular to the impact surface and rotates horizontally, creating horizontal acceleration.

•Subsequent analysis showed that concrete drops produced significantly higher impact forces than the carpet pad from 1 and 3 ft. On both surfaces, 3-ft drops resulted in higher impact forces than those at 1 ft.

Toddler versus Infant Findings

When comparing the drop tests for the toddler surrogate to the drop tests for the infant surrogate, several key differences were noted:

•Toddler head accelerations were more than double those of the infant from the same height onto the same surface, likely contributing to the higher incidence of loss of consciousness reported in toddlers.

•The impact force was significantly higher in the toddler, but did not appear to change with height. In the infant, the impact force increased with height.

•Due to its greater thickness, the authors estimated that the toddler skull should be able to withstand 4.6 times more force than the infant skull.

"The differences is angular acceleration, estimated peak impact force, peak-to-peak change in angular velocity and impact duration in the toddler versus the infant can be attributed to the toddler's larger head and torso mass, stiffer neck, and thicker fused skull. Due to the greater head acceleration in the toddler, we expect that toddlers will incur greater neurological impairment than infants when there is direct head impact from falls. The findings provide an important step to better understanding the mechanisms of falls in this age group, and offer a starting point for improving head injury prevention in playgrounds, day care centers, and other toddler-specific settings," concluded Dr. Margulies.