Biomechanical Investigation of Airbag-Induced Upper-Extremity Injuries 973325

The factors that influence airbag-induced upper-extremity injuries sustained by drivers were investigated in this study. Seven unembalmed human cadavers were used in nineteen direct-forearm-interaction static deployments. A single horizontal-tear-seam airbag module and two different inflators were used. Spacing between the instrumented forearm and the airbag module was varied from 10 cm to direct contact in some tests. Forearm-bone instrumentation included triaxial accelerometry, crack detection gages, and film targets. Internal airbag pressure was also measured. The observed injuries were largely transverse, oblique, and wedge fractures of the ulna or radius, or both, similar to those reported in field investigations. Tears of the elbow joint capsule were also found, both with and without fracture of the forearm.

Forearm fracture occurrence was analyzed with respect to time of airbag deployment, distal forearm speed, airbag-module-to-forearm spacing, bone mineral content, and upper-extremity mass. Forearm fractures occurred within 5 to 10 ms of the onset of airbag pressure, as indicated by crack detection gage output. Time of fracture was found to coincide with local reductions in resultant distal forearm acceleration, and local plateaus of distal forearm speed. Peak distal forearm speed ranged from 9 to 21 m/s, and decreased approximately 30 percent when initial airbag-module-to-forearm spacing was increased from direct contact to 2.5 cm. A distinct division between fracture and nonfracture cases was found at about 15.2 m/s peak, and 11.7 m/s average, distal forearm speed. Distal forearm speed, fracture incidence, and fracture severity were typically reduced as initial airbag-module-to-forearm spacing was increased. Additionally, under direct-contact conditions, fractures occurred in bones having mineral contents less than 1.03 g/cm, but not in bones having greater mineral contents. Both distal forearm speed and mineral content were found to be strongly related to the mass of the upper extremity. Therefore, the potential for forearm fracture was also found to be strongly related to the mass of the upper extremity.

The results of this study suggest that airbag-module-to-forearm spacing has a substantial influence on airbag-induced upper-extremity injuries sustained by drivers. Increased initial spacing between the airbag module and the forearm reduces the incidence and severity of forearm fractures caused by direct interaction with the airbag. Furthermore, increased initial spacing reduces the speeds ultimately achieved by the distal forearm, thereby potentially mitigating flailing injuries. The results also suggest that a simple airbag aggressivity-assessment tool for the prediction of forearm fracture might be based upon peak or average distal forearm speed of a biofidelic, surrogate arm.