Lower extremity (knee) injury prediction resulting from impact trauma is currently based on a bone fracture criterion derived from experiments on predominantly aged cadavers. Subsequent experimental and theoretical studies indicate that more aged, pathological specimens require higher, not lower, loads to initiate bone fracture. This suggests that a bone fracture criterion based solely on aged specimens may not be representative of the current driving population. In the current study, we sought to determine if cadaver age, physical size, sex, baseline joint pathology, or patellar geometry correlated with fracture load. An analysis was made of data from previous impact experiments conducted on fifteen isolated cadaver knees using a consistent impact protocol. The protocol consisted of sequentially increasing the impact energy with a rigid interface until gross fracture. Gross bone fractures occurred at loads of 6.9±2.0 kN (range 3.2 to 10.6 kN) using this protocol. Regression analyses revealed that fracture load was predicted by only one parameter: patellar geometry. Alternately, we developed a 2-D mathematical model of the human knee to explore parameters that might influence the loads required to cause gross bone fracture. In support of our recent experimental studies using rigid and padded impact interfaces, the model suggested that load intensity and it's distribution over the knee play a role in defining the fracture load as well as the site (patella or femur) of patellofemoral joint injury.