Is there really a "cushion effect"?: a biomechanical investigation of crash injury mechanisms in the obese.

The objective of this study was to document the motion and potential injury mechanisms of obese occupants in frontal car crashes compared to a control group of nonobese occupants in controlled laboratory impacts. Eight cadavers were divided into obese (n = 3) and a nonobese (n = 5) groups and exposed to a 48 km/h impact. High speed digital video documented the motion of the belted subjects. Compared to the nonobese cohort, the obese exhibited a characteristically different set of motions. As expected, the obese (heavier) subjects experienced greater maximum forward displacement (excursion) before their motion was arrested by the restraint. In addition, the obese exhibited a different distribution of excursions among body segments. The primary difference between the cohorts was substantially larger hip excursion in the obese (452 +/- 83 mm vs. 203 +/- 42 mm, P < 0.01), which was the proximate cause of a tendency of the obese cadavers' torsos to pitch forward less during impact. Some of the published epidemiology can be elucidated by the results reported here. The increased hip excursion and concomitant decreased torso pitch may reduce the risk of the head striking some component of the vehicle interior. Furthermore, the reclined torso during belt loading may increase the risk of rib and pulmonary trauma because the load is concentrated on the compliant and vulnerable lower thorax and less on the stiff upper ribs and clavicle. The lower extremities also experience increased excursion as a result of this hip excursion, and thus an increased risk of a hard contact and resulting injury.