To assess whether fall height and impact surface are associated with clinically important traumatic brain injury (ciTBI) in children under two years.

We performed a single-center retrospective cohort study (Tokyo, Japan, 2017–2021) including children under two years with head trauma. We excluded those with conditions affecting ciTBI risk, unreliable clinical histories, or injury mechanisms that did not involve ground impact. The primary outcome was ciTBI; abnormal head computed tomography (CT) findings were secondary. Multivariable logistic regression modeled fall height per 10-cm increment and impact surface (flooring, soft, hard), adjusted for age and exposures. Generalized additive models (GAMs) explored potential nonlinear associations.

Among 3067 eligible children, 1893 were included in the analysis; 44 (2.3%) had ciTBI. After adjustment for age and mutual adjustment of exposures, impact surface type was not significantly associated with ciTBI (soft surface vs flooring: adjusted odds ratio [aOR] 0.30, 95% CI 0.07–1.34; hard surface vs flooring: aOR 1.10, 95% CI 0.52–2.31). Fall height was significantly associated with ciTBI risk (aOR 1.31 per 10 cm increase, 95% CI 1.19–1.45). Similar associations were observed for abnormal head CT findings. GAMs demonstrated an approximately linear increase in risk with increasing fall height, without an inflection point.

In children under two years with head trauma, fall height was associated with ciTBI risk, whereas impact surface type was not. The association between fall height and ciTBI appeared approximately linear across the observed range, suggesting that fixed height thresholds may not fully reflect clinical risk.