Late model passenger cars and light trucks incorporate occupant protection systems with airbags and knee restraints. Knee restraints have been designed principally to meet the unbelted portions of FMVSS 208 that require femur load limits of 10-kN to be met in barrier crashes up to 30 mph, +/- 30 degrees utilizing the 50% male Anthropomorphic Test Device (ATD). In addition, knee restraints provide additional lower-torso restraint for belt-restrained occupants in higher-severity crashes.

An analysis of frontal crashes in the University of Michigan Crash Injury Research and Engineering Network (UM CIREN) database was performed to determine the influence of vehicle, crash and occupant parameters on knee, thigh, and hip injuries. The data sample consists of drivers and right front passengers involved in frontal crashes who sustained significant injuries (Abbreviated Injury Scale [AIS] ≥ 3 or two or more AIS ≥ 2) to any body region.

At the time of the analysis, the database included 138 occupants who were involved in frontal collisions, and eighty-one of these occupants sustained one or more AIS 2+ injuries to the knee, thigh, and/or hip (KTH). Most of these KTH injuries occurred for crash severities that are equal to, or less than, those used for current federal regulation and consumer information crash testing. The percentage of drivers sustaining KTH injuries is higher than for right-front passengers, and most KTH injuries occurred with little or no rearward intrusion of the toepan or lower instrument-panel/knee restraint.

The data show that a higher percentage of belt-restrained occupants with airbags and knee restraints sustained hip injuries than belt-restrained occupants in vehicles without airbags and knee restraints.

Comparison of the percentage of unbelted occupants sustaining hip injuries in vehicles with and without airbag/knee-restraint systems could not be made because the CIREN database does not contain sufficient numbers of unbelted occupants in vehicles without airbags.

Hip injuries occurring in UM CIREN frontal crashes are predominately acetabular fractures. While hip injuries are not directly life threatening, they are generally far more disabling and costly in the long term than thigh and knee injuries. These disabling hip injuries are more frequent among taller male drivers, and are nearly equally distributed among adult occupants of all ages. Although hip injuries occurred more frequently to unbelted occupants, many belt-restrained occupants also sustained hip fractures. Hip injuries tend to occur on the side of the body towards the damaged area of the vehicle front end and/or the principal direction of force.

The results of this study suggest that the current 10-kN limit on femur loads in federal testing may not be sufficient to address hip-injury risk in real-world frontal crashes. They also suggest a need for a better understanding of the biomechanics of hip injuries and injury tolerance from forces generated by knee loading transmitted through the thigh, and a need for improved biofidelity and instrumentation of the ATD knee, thigh, hip complex.