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Injury to the lower extremity during an automotive crash is a significant problem. While the introduction of safety features (i.e. seat belts, air bags) has significantly reduced fatalities, lower extremity injury now occurs more frequently, probably for a variety of reasons. Lower extremity trauma is currently based on a bone fracture criterion derived from human cadaver impact experiments. These impact experiments, conducted in the 1960's and 70's, typically used a rigid impact interface to deliver a blunt insult to the 90° flexed knee. The resulting criterion states that 10 kN is the maximum load allowed at the knee during an automotive crash when certifying new automobiles using anthropomorphic dummies. However, clinical studies suggest that subfracture loading can cause osteochondral microdamage which can progress to a chronic and debilitating joint disease.

While the number of deaths from vehicle accidents is declining, probably because of mandatory seat belt laws and air bags, a high frequency of lower extremity injuries from frontal crashes still occurs. For the years 1979-1995 the National Accident Sampling System (NASS) indicates that knee injuries (AIS 1-4) occur in approximately 10% of cases. Patella and femur fractures are the most frequent knee injuries. Current literature suggests that knee fractures occur in seated cadavers for knee impact forces of 7.3 to 21.0 kN. Experimental data shown in a study by Melvin et al. (1975) further suggests that fracture tolerance of the knee may be reduced for an impact directed obliquely to the axis of the femur. The current study hypothesized that the patella is more vulnerable to fracture from an oblique versus an axial impact (directed along the femoral axis), and that the fracture pattern would vary with impact direction.