Injuries to the Hip Joint in Frontal Motor-Vehicle Crashes: Biomechanical and Real-World Perspectives

Rupp, J., Schneider, L. 2004. Injuries to the Hip Joint in Frontal Motor-Vehicle Crashes: Biomechanical and Real-World Perspectives. The Orthopedic Clinics of North America. Vol. 35 p. 493-504.

Injuries to the hip, thigh, and knee in frontal motor-vehicle crashes typically occur when the knees of drivers or right-front passengers strike the lower instrument panel, which is also called the knee bolster. Early impact biomechanics research explored the fracture tolerance of the whole knee-thigh-hip (KTH) complex by impacting the knees of seated human cadavers at high speeds using a ballistic mass with a flat rigid impact surface. These tests produced primarily knee and distal femur injuries, which was assumed to indicate that the fracture tolerance of the hip is higher than that of the knee or femur. The results of these tests were used to develop and justify the injury criterion used in Federal Motor Vehicle Safety Standard (FMVSS) 208 to regulate vehicle design to mitigate the risk of knee, thigh, and hip injuries in frontal motor-vehicle crashes. FMVSS 208 states that that all passenger cars and light trucks sold in the United States must be designed so that the axial force in a midsize male crash test dummy femur does not exceed 10 kN in a 48-kph full-frontal impact into a non-deformable barrier.

With recent advances in frontal crash protection due to increased use of three-point belts and the implementation of frontal-impact airbags in all cars and light trucks, the lower extremities have surpassed the head and chest as the most likely regions of the body to sustain clinically significant injuries in frontal collisions. Of all lower extremity injuries sustained in frontal crashes, KTH injuries are the most frequent and costly. Of all KTH injuries, hip injuries are the most frequent, occurring at a rate of approximately 14,000 per year. In addition, a large percentage of hip fractures and dislocations are sustained by drivers and right-front passengers of FMVSS 208 compliant vehicles in crashes that are less severe than the 48-kph full-frontal impact specified in FMVSS 208. While these hip injuries are not life-threatening, they are of substantial concern to automotive safety engineers and clinicians because of the high potential for long-term impairment.

These findings, along with a lack of hip tolerance data in the biomechanical literature, generated renewed interest in determining injury tolerance of the hip to frontal knee impact loading. A new biomechanical research program was, therefore, initiated to determine the fracture tolerance of the hip relative to that of the knee and thigh for knee impact conditions that are representative of those resulting from frontal impacts of late-model airbag-/knee-restraint-equipped vehicles. Results show that the hip is the weakest part of the KTH complex under these loading conditions, and that hip flexion and adduction from the neutral driving posture significantly reduce hip fracture tolerance. These findings are being used to develop new KTH injury criteria to replace the current 10-kN maximum femur force criterion.