Background Seatbelt use, alone and in conjunction with an airbag, is associated with lower risk of mortality, blunt abdominal trauma and kidney injury in motor vehicle crashes (MVCs). However, the effect of these protective devices on risk of severe liver injury is not well characterised.

Methods This retrospective cohort study included patient admissions with liver injuries from MVCs from the National Trauma Data Bank (NTDB), collected from 2010 to 2015 in the USA. We examined associations between injury severity and seatbelt use and airbag presence individually and in the presence of additive interaction. Secondary outcomes were mortality, complications and discharge disposition.

Results We analysed 55 543 records from the National Trauma Data Bank. In adjusted analysis, seatbelt use alone was protective against severe (AAST VI or above) hepatic injury (risk ratio (RR) 0.79, 95% CI 0.75 to 0.84), while airbag presence alone was not (RR 1.05, 95% CI 0.8 to 1.12). The joint association of seatbelt use and airbag presence with injury severity was greater than seatbelts alone (RR 0.74, 95% CI 0.70 to 0.79), with 13% of the joint lower risk attributable to interaction (95% CI 3% to 24%). The adjusted mortality risk of those without protective devices (10.3%, n=2297) was nearly double that of patients who used a seatbelt in conjunction with a present airbag (5.3%, n=699, p<0.001).

Conclusions Seatbelts are associated with lower liver injury severity and are more protective with airbags present, while airbags without seatbelt use were not protective against severe injury among patients with liver injury.