Background Pediatric emergency physicians in Japan commonly experience cases of trauma resulting from the infant falling from the arms or the back of the parent while the latter is operating a bicycle. However, the clinical features and dynamic mechanisms of these injuries are unclear.

Methods 1. We conducted a retrospective case-series enrolling children younger than 1 year of age who received head injuries. We reviewed the charts of such patients who visited Tokyo Metropolitan Children’s Medical Center between April 2014 and March 2016. We then extracted patients injured by falling from the arms or the back of the parent while the latter was operating a bicycle. 2. We conducted dynamics experiment by recreating the circumstances of the accident using dummies representing a 6-month-old infant being carried on the back of the mother. We assessed the Head Injury Criterion (HIC) score and the maximum impact load on the head of the dummy infant at the time of the fall. In addition, we videotaped the moment of impact on the head using a high-speed camera.

Findings 1. We found eight patients with an injury due to this mechanism, six of whom were sent home with the diagnosis of minor head injury. However, two patients required intensive care, one of whom experienced neurological sequelae. 2. The HIC scores varied 2998 to 6019 or 7.7 to 17.0 times the reference values for 6-month-old infants. The maximum impact load varied from 6984 to 8920N or 2.26 to 3.47 times the estimated reference value (2569N)

Conclusions Our study revealed that the impact on the head of infants due to this accident mechanism far exceeded the reference value for skull fractures and could result in severe head injuries. Preventive strategies including legislation and the development of baby child seats for the safe transport of infants are needed.