Injury risk function for the small female wrist in axial loading.

Previous experiments with human cadavers and side airbags revealed the potential for wrist injuries as a result of the hand becoming entrapped in the handgrip. The purpose of this paper was to develop an injury tolerance for the small female wrist that may be used in the design phase of side airbags in order to reduce the risk of wrist injuries resulting from side air bag deployment. Small female cadaver upper extremities were used to develop the wrist tolerance as a conservative estimate of the most vulnerable section of the driving population. The energy source was a pneumatic impactor that was configured to match the force onset rate, impulse, and peak force in order to simulate the load profile of a deploying side airbag. A total of 17 (n=17) axial impact experiments were performed on the wrists of small female cadavers. Post-test necropsy revealed that 9 of the 17 tests resulted in wrist injuries. The injury patterns were identical to those observed from cadaver tests with side airbags and included fractures of the scaphoid (AIS 2), lunate (AIS 1), distal radius (AIS 3), and distal ulna (AIS 2). Using the injury outcome as the binary variable, a logistic regression analysis was performed. When mass scaled to the fifth female, the analysis produced an injury risk function that predicts a 50% risk of injury at a wrist load of 1700 N (P=0.0037). Risk of injury was found not to be dependent of subject bone mineral density (P=0.49), age (P=0.99), mass (P=0.31), and stature (P=0.69). Based on the similarities in impact load profile and observed injury patterns between the impactor tests and the side airbag tests, it is suggested that the injury risk function will accurately predict the risk of wrist injuries in the automobile crash environment.

The axial injury tolerance of the human foot/ankle complex and the effect of Achilles tension.

Axial loading of the foot/ankle complex is an important injury mechanism in vehicular trauma that is responsible for severe injuries such as calcaneal and tibial pilon fractures. Axial loading may be applied to the leg externally, by the toepan and/or pedals, as well as internally, by active muscle tension applied through the Achilles tendon during pre-impact bracing. The objectives of this study were to investigate the effect of Achilles tension on fracture mode and to empirically model the axial loading tolerance of the foot/ankle complex. Blunt axial impact tests were performed on forty-three (43) isolated lower extremities with and without experimentally simulated Achilles tension. The primary fracture mode was calcaneal fracture in both groups. However, fracture initiated at the distal tibia more frequently with the addition of Achilles tension (p < 0.05). Acoustic sensors mounted to the bone demonstrated that fracture initiated at the time of peak local axial force. A survival analysis was performed on the injury data set using a Weibull regression model with specimen age, gender, body mass, and peak Achilles tension as predictor variables (R2 = 0.90). A closed-form survivor function was developed to predict the risk of fracture to the foot/ankle complex in terms of axial tibial force. The axial tibial force associated with a 50% risk of injury ranged from 3.7 kN for a 65 year-old 5th percentile female to 8.3 kN for a 45 year-old 50th percentile male, assuming no Achilles tension. The survivor function presented here may be used to estimate the risk of foot/ankle fracture that a blunt axial impact would pose to a human based on the peak tibial axial force measured by an anthropomorphic test device.