Human Lumbar Spine Injury Risk in Dynamic Combined Compression and Flexion Loading.

Anticipating changes to vehicle interiors with future automated driving systems, the automobile industry recently has focused attention on crash response in novel postures with increased seatback recline. Prior research found that this posture may result in greater risk of lumbar spine injury in the event of a frontal crash. This study developed a lumbar spine injury risk function (IRF) that estimated injury risk as a function of simultaneously applied compression force and flexion moment. Force and moment failure data from 40 compression-flexion tests were utilized in a Weibull survival model, including appropriate data censoring. A mechanics-based injury metric was formulated, where lumbar spine compression force and flexion moment were normalized by specimen geometry. Subject age was incorporated as a covariate to further improve model fit. A weighting factor was included to adjust the influence of force and moment, and parameter optimization yielded a value of 0.11. Thus, the normalized compression force component had a greater effect on injury risk than the normalized flexion moment component. Additionally, as force was nominally increased, less moment was required to produce injury for a given age and specimen geometry. The resulting IRF may be utilized to improve occupant safety in the future.

Failure tolerance of the human lumbar spine in dynamic combined compression and flexion loading.

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 simultaneous and out-of-phase combined axial compression and 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 lumbar spine segments with three vertebrae (one unconstrained) and two intervertebral discs (both unconstrained) were pre-loaded with axial compression (2200N, 3300N, or 4500N) and then subjected to rotation-controlled 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. Failure tolerance varied within the sample and were categorized by the appropriate data censoring, with clinically relevant middle vertebrae fractures characterized as uncensored or left-censored and potting-grip fractures characterized as right-censored. Average failure force and moment were 3290N (range: 1580N to 5042N) and 51Nm (range: 0Nm to 156 Nm) for uncensored data, 3686N (range: 3145N to 4112N) and 0Nm for left-censored data, and 3470N (range: 2138N to 5062N) and 101Nm (range: 27Nm to 182Nm) for right-censored data. These data can be used to develop and improve injury prediction tools for lumbar spine fractures and further research in future safety systems.

Comparison of injuries in multiple and single event crashes.

Objective: Compare injuries for occupants in multiple event (ME) crashes where a less severe event preceded a more severe event to occupants in similar single event (SE) crashes. Methods: Occupants in ME crashes from NASS-CDS years 2000-2015 where the most severe event occurred subsequent to a less severe event were matched to occupants in SE crashes where the SE was similar to the most severe event in the ME crash. Occupants were matched based on occupant, vehicle, and crash characteristics and were compared across 21 detailed body regions using conditional logistic regression.Finite element (FE) simulations were performed with human surrogate models (detailed GHBMC and Hybrid III) and in both low- and high-speed conditions (n = 8 total simulations). At each speed, the crash simulations with both human body models reproduced a common multidirectional ME crash scenario, where the second impulse was more severe and similar to the SE impulse. Relative injury risk was assessed, and ME versus SE were computed and compared to those from the field data. Results: 1,663 ME occupants were matched to 3,217 SE occupants. ME occupants had higher MAIS2+ and MAIS3+ injury risk, and showed directionally higher injury risk in all but one body region. Eleven out of the 27 injury groups had higher injury risk in ME (false discovery rate (FDR)<0.1; all p-values < 0.0427). Increased injury risk was seen in some injuries to the head, thorax, lumbar spine, shoulder, and lower extremity (odds ratios >1.54).In FE simulations, ME displayed larger anterior and lateral displacement compared to SE. Head and thorax injury risk was increased in ME simulations by up to 5-fold. The detailed GHBMC and Hybrid III exhibited different kinematics and injury risk across all simulations, as did low- and high-speed conditions. Conclusions: The field data and FE simulations suggest that a first, less severe crash event results in occupants having greater injury risk when they are involved in a second, more severe crash event than if they were involved only in the second event. Several factors could cause this increase in injury risk, such as improper interaction with safety systems and airbags after the first event renders the occupant out-of-position.