  Vehicle safety systems have substantially decreased motor vehicle
crash-related injuries and fatalities, but injuries to the lumbar spine still
have been reported. Experimental and computational analyses of upright and,
particularly, reclined occupants in frontal crashes have shown that the lumbar
spine can be subjected to axial compression followed by combined
compression-flexion loading. Lumbar spine failure tolerance in combined
compression-flexion has not been widely explored in the literature. Therefore,
the goal of this study was to measure the failure tolerance of the lumbar spine
in combined compression and flexion. Forty 3-vertebra lumbar spine segments
were pre-loaded with axial compression and then subjected to dynamic flexion
bending until failure. Clinically relevant middle vertebra fractures were
observed in twenty-one of the specimens, including compression and burst
fractures. The remaining nineteen specimens experienced failure at the potting
grip interface. Since specimen characteristics and pre-test axial load varied
widely within the sample, failure forces (mean 3.4 kN, range 1.6-5.1 kN) and
moments (mean 73 Nm, range 0-181 Nm) also varied widely. Tobit univariate
regressions were performed to determine the relationship between censored
failure tolerance and specimen sex, segment type (upper/lower), age, and
cross-sectional area. Age, sex, and cross-sectional area significantly affected
failure force and moment individually (p<0.0024). These data can be used to
develop injury prediction tools for lumbar spine fractures and further research
in future safety systems.
