Driver Lower Extremity Response and Injury with Knee Airbag Deployment

Lower extremity injury is the leading AIS2+ injury for drivers involved in frontal crashes. While there has been steady improvement in vehicle crash test performance over the past two decades, the risk of lower extremity injury remains virtually unchanged.
As one potential injury countermeasure, knee airbags and inflatable knee bolsters have been purportedly designed to mitigate lower limb forces and to improve overall occupant safety by providing early restraint for the pelvis and lower torso. Recent studies, however, suggest an increased injury risk of the tibia/fibula and the foot/ankle with knee airbags. This thesis aims to investigate the lower extremity response and injury with knee airbag deployment, and to identify any adverse effects the knee airbag imposes on lower extremity.
The thesis examines the lower extremity response for drivers in nominal driving posture, under dynamic condition with the presence of an Inflatable Knee Bolster (IKB). A total of seven unbelted sled tests (40 km/h) and eight belted sled tests (56 km/h) were performed with a 50th percentile Hybrid-III dummy to investigate the IKB effects on the lower limb. For the unbelted tests, the average tibia index (TI)increased by 34.6% and 33.3% for tests with IKB on the left upper (1.10±0.10) and left lower tibia(0.97±0.09), respectively, when compared to tests with standard knee bolster(left upper TI: 0.82±0.11; left lower TI: 0.73±0.05). Additionally, the right upper tibia index increased by 20.3% (from 0.64±0.10 to 0.77±0.04), while the right lower tibia index decreased by 26.5% (from 0.86±0.12 to 0.63±0.04). Similar results were found for the belted tests with IKB, where a 9.1% increase in risk of AIS 2+ tibia shaft fractures (from 11.8% to 20.9%) was predicted with IKB deployment.
The study also investigated the effects of the knee airbag on the lower limb injury risk for out-of-position drivers. A total of eleven static knee airbag deployment tests were performed using a 5th percentile female Hybrid-III dummy outfitted with either the original Hybrid-III lower extremities or the 5th percentile THOR-FLx. Baseline tests were performed with FMVSS 208 seating specifications, and a design of experiment for out-of-position conditions was developed with multiple factors including knee-to-instrument panel distance, knee-to-knee distance, and foot placement. The results predicted a 40.6% increase in risk of AIS 2+ tibia shaft fractures for out-of-position drivers (Ave. 53.6%) relative to in-position drivers (Ave. 13.0%).
This thesis indicates that inflatable knee bolsters can increase the likelihood of drivers sustaining tibia/fibula injuries in frontal crashes. In addition, the dummy lower extremity responses recorded in static deployment tests suggest a high risk of lower extremity injury for both in-position and out-of-position small female drivers during knee airbag deployment. While this thesis covers a limited set of crash conditions and knee airbag designs, the results indicate the need for improved design of knee airbag for better protection of lower limb.